Introduction to Precision Manufacturing

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 Content

 What is precision manufacturing?

 Part Accuracy
 Static Stiffness & Accuracy
 Dimensioning & Tolerancing: Linear & Geometrical
 Basic Elements of Work Setting
 Concept of Tool Design

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Precision Manufacturing
What is precision manufacturing?

Manufacturing parts in a way that is precise, stable, and can be

repeated (precision) with consistency and accuracy.

 Accuracy
 Degree of Accuracy
 Precision

Note: Precision manufacturing assures to produce the parts with

desired level of accuracy over and over again.

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Precision machine:
Defined positional accuracies can be achieved consistently,
throughout the production time, while producing large number
of components.

Precision machines are key in modern manufacturing to attain

process controls and tolerances that function under extremely
tight specifications. (i) Repeatability and (ii) well-controlled
tolerances are the characteristics of precision machining.

Using precision machine is a first step or factor in order to

produce high quality parts consistently.

What are the factors that affect the part accuracy?

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Part Accuracy
What does it mean by part accuracy?

 Dimensional accuracy
 Geometrical form accuracy
 Positional accuracy
 Surface profile accuracy
Machine accuracy vs. Part accuracy
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Fish bone diagram: Factors affecting part accuracy

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Part Accuracy: Dimensional, Form and Surface Characteristics

ELS – Elastically Linked Systems (http://dx.doi.org/10.1016/j.cirp.2013.03.100)

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Static Stiffness & Accuracy
Stiffness: M/c Tool, Tool & Tool Holder, Work & Work holder

Static stiffness is the ratio

between a static force and the
resulting static deflection.
Static stiffness determines its ability to produce
dimensionally & geometrically accurate parts.

Dynamic stiffness is the

frequency dependant ratio
between a dynamic force and the
resulting dynamic displacement.
Dynamic stiffness affects the quality of the
component’s surface finish and the metal removal
rates (MRR) that can be achieved.
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Elastic modulus of work material (Steel & Titanium)
Steel: 210 GPa Titanium: 110 GPa

Stiffness of work block – Geometry

Cutting forces & Direction

Rough cutting and Finish cutting: Part accuracy

Static and dynamic stiffness of machining system
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Dimensioning & Tolerancing: Linear & Geometrical
Liner dimensioning and tolerances

FIT - Condition of looseness or tightness between two mating

parts being assembled together.
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Geometric Dimensioning & Tolerances (GD&T):
Form, Profile, Orientation, Location & Run-out

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Datum or Reference Surface on Part:
A datum is a plane, axis or point location that tolerances are
referenced to and also from where the other features are
referenced from.

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Basic Elements of Work Setting
Reference surface, Hole/Dowel pins, Dial gauges, Clamps, Jaw
chucks, Vices, Angle plates, Face plates, V-blocks, T-bolts

https://www.youtube.com/watch?v=oxxvlx1FObQ 14
Concept of Tool design
Tool design is a specialized area of manufacturing engineering
comprising the analysis, planning, design, construction and
application of tools, methods, and procedures necessary to in
crease manufacturing productivity.

 The main objective of the tool designer is to increase production

while maintaining quality (accuracy and precision) and lowering costs.

 Design tools to aid higher production rate – Quick Loading/unloading,

High MRR

 Design tools to consistently manufacture parts with the required

accuracy and precision.

Jigs & Fixtures, Limit gauges, Cutting & Press tools, Moulds & Dies etc.

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Tool designers’ responsibility is to select/develop:

Machine tools (must be familiar with their capability)

Cutting tools, tool holders and cutting fluids

Jigs and fixtures

Gauges and measuring instruments

Dies for sheet-metal cutting and forming

Dies for forging, upsetting, cold finishing, and extrusion

Next Topic: 2. Overview of Tool Design

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Overview of Tool Design 1
Content
• The Design Process
• Cutting Tools
• Locating and Clamping Methods
• Limit Gauges
• Metal Forming Dies
• Selection of Tooling Materials
• Heat Treatment

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The Design Process
Comprises the planning, designing, developing, and
analysis of tools, methods and procedures
necessary to increase efficiency and productivity.

 Statement and analysis of the problem

 Analysis of the requirements
 Development of initial ideas
 Development of possible design alternatives
 Finalization of design ideas
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1. Statement and analysis of the problem
 The first step in the design of any tool is to define the
problem or objective as it exists without tooling.

2. Analysis of the requirements

 Perform specific functions
 Meet certain minimum precision requirements
 Keep costs to a minimum
 Be available when the production schedule requires it
 Be operated safely
 Meet various other requirements such as adaptability to
the machine
 Have an acceptable working life
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3. Development of initial ideas

 Initial design ideas are normally conceived after an

examination of the preliminary data.
 In many cases, the designer and planner (responsible for tool
request) work together in a team environment to develop
the initial design parameters.

4. Development of possible design alternatives

 During the initial concept phase of design, many ideas will

occur to the designer and/or the team. There are always
several ways to do any job.

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5. Finalization of design ideas

 Once the initial design ideas and alternatives are determined,

the tool designer must analyze each element to determine
the best way to proceed toward the final tool design.

 Rarely is one tool alternative a clear favourite. The tool

designer must evaluate the strong points of each alternative
and weigh them against the weak points of the design.

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Cutting Tools

 Feature to be machined
 Work material and material condition
 Tool material and coatings
 Cutting tool/insert geometry
 Tool/insert holder (size/shape)
 Use of cutting fluids
 Inspection of rotating tools (Runout)

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Locating and Clamping
 Locating the work piece - The dimensional and positional
relationship between the work piece and the cutting tool
used on the machine.

 Generally, 9 out of 12 degrees of freedom need to be

restricted using locators.

 3 degrees of freedom (+X, +Y & +Z) are free to load the

component on the machine table.

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 Clamping the work piece - During machining operation,
clamping is a way to counter the cutting forces and keep the
workpiece in the located position.

 After loading the component, remaining degrees of

freedom are going to be arrested using clamps.

Manual work setting

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 Jigs & Fixtures are designed based on the locating &
clamping principles.
 If rightly designed they ensure faster loading and ensures
produce the parts at high production rate with high
precision & accuracy.

JIG Fixture

 JIGs – Ensures the drilling (position) of the hole at right

place by guiding the drill bit.
 FIXTUREs – Means for holding and locating the workpiece
during machining operation.
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Limit Gages
Gauge (Go/No-Go) are inspection tools of rigid design, without a
scale, which used to check the maximum and minimum material
limits of the manufactured components (accept/reject).

Gauge is not recommended if the tolerances on the components

are less than 20 µm, as the tolerances become so close on the
gauges (10% of work tolerances), and difficult to manufacture
and use them because of alignment and wear related issues.

 Gauges are designed based on Taylor’s principles and prime

objective is a gauge should not accept the part which is
manufactured outside its limits.
 Limits on the No-Go gauge are critical as the part rejected will
be scrapped as it checks the minimum material limit.
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Sheet Metal Forming Dies
 A die is a specialized tool used in manufacturing industries
to cut or shape material mostly using a press.

 A stamping die, one-of-a-kind precision tool that cuts and

forms sheet metal into a desired shape or profile.

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Tooling Materials and Heat Treatment

It is important to consider proper selection of materials and

heat treatment at the design stage itself.

Material selection
Heat treatment
Hardness requirement
Distortion control

Material for a tool is determined by the mechanical & other

properties necessary for that tool’s proper operation, i.e.,
elastic modulus, wear resistance and dimensional stability.

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Tooling Materials and Heat Treatment

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Tooling Materials and Heat Treatment
The principal materials used for tools can be divided into
three major categories:
Ferrous metals, Nonferrous metals, Non-metallic materials
For most applications, more than one type of material will be
satisfactory, and a final choice normally will be governed by
material availability and economic considerations.

 Heat Treatment
Tool steels are high in alloying elements, not only helps in
attaining required material properties (hardness, strength,
toughness etc.), also allows air quenching during heat
treatment, helps in avoiding distortions.

Next Topic: 3. Introduction to Jigs & Fixtures 15

Introduction to Jigs & Fixtures

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Jigs & Fixtures: Elements and their Function

 Jig/Fixture Body
 Locating Elements
 Clamping Elements
 Tool guide(jigs bushing)

Fig. Typical Jig

Fig. Typical Fixture 2

Jigs & Fixtures: Elements and their Function
 Body- Main purpose is to support and house the job

 Locating elements- The pins of various design and made of

hardened steel are the most common locating devices used
to locate/position a work piece in a jig or fixture.

 Clamping elements- The purpose of the clamping is to exert

a pressure to press a work piece against the locating
surfaces and hold it there in a position to the cutting forces.

 Tool guide (in case of jigs)- To locate the tool relative to the
work, using jigs bushing and templates. Also useful when
the stiffness of the cutting tool may be in sufficient.

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Classification and Types of Jigs

Classes of Jigs:
 Drilling Jigs
 Boring Jigs

Fig. Drilling Jig

Fig. Boring Jig

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Types of Jigs:
Drill jigs are divided into two general types, open and closed,
refer to how the tool is built.

 Open jigs are for simple operations where work is done on

only one side of the part.
 Closed or box, jigs are used for parts that must be machined
on more than one side.

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Template Jigs- Normally used for accuracy rather than speed.
This type of jig fits over on the work and usually not-clamped.

Plate jigs - Similar to templates. The only difference is that

plate jigs have built-in clamps to hold the work.

Fig. Template Jig Fig. Plate Jig 6

 Sandwich jigs: Form of plate jig with a back plate. This type
of jig is ideal for thin or soft parts that could bend or warp
in another style of jig.

 Angle-plate jigs: Used to hold parts that are machined at

right angles to their mounting locators. Pulleys, collars, and
gears are some of the parts that use this type of jig.

Fig. Sandwich Jig Fig. Angle-plate Jig

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 Leaf jigs (small box jigs) with a hinged leaf to allow for easier
loading and unloading . The main differences between leaf
jigs and box jigs are size and part location.
 Indexing jigs are used to accurately space holes or other
machined areas around a part.

Fig. Leaf Jig Fig. Indexing Jig 8

Classification and Types of Fixtures
Fixtures are normally classified by the type of machine on
which they are used, for example milling fixture, turning
fixture, welding fixture, assembly fixture etc.

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 Plate fixtures are the simplest form of fixture. The basic
fixture is made from a flat plate that has a variety of clamps
and locators to hold and locate the part.

 The angle-plate fixture is a variation of the plate fixture. With

this tool, the part is normally machined at a right angle to its
locator.

Fig. Angle-plate Fixture

Fig. Plate Fixture 10
Vise-jaw fixtures: For machining small parts. With this type of
tool, the standard vise jaws are replaced with jaws that are
formed to fit the part.

Indexing fixtures: Similar to indexing jigs. These fixtures are

used for machining parts that must have machined details
evenly spaced (milling of gear tooth, splines etc.).

Fig. Vice-jaw Fixture

Fig. Indexing Fixture
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 Multistation fixtures: Primarily for high-volume production
runs, where the machining cycle must be continuous. Duplex
fixtures are the simplest form of multistation fixture.

 Profiling fixtures: To guide tools for machining contours that

the machine cannot normally follow. These contours can be
either internal or external.

Fig. Duplex Fixture Fig. Profiling Fixture

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Fundamental Aspects of Jigs
 Unique location of components with respect to the jig.

 Ease of loading and unloading the components.

 Clamping of the components so as to impart adequate

clamping force and also to have ease in operation.

 Guiding the drills/reamers

 Provision for swarf removal.

 Proper fastening methods to hold the jig to the table.

 Provision for replacement of bushes, in case different tools

like reaming subsequent to drilling.
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Fundamental Aspects of Fixtures
 Unique & foolproof location of components on the fixture.

 Clamping techniques to be adopted to deploy adequate

forces without damaging the component.

 Techniques for the ease of clamping like quick acting screws,

cam clamps, hydraulic clamps, etc.

 Provision for easy loading and unloading of components.

 Provision for swarf removal .

 Fastening of fixture to the machine table or chuck or collet.

 Design of tenons at the bottom of the fixture so as to locate

the fixture with respect to the machine table.
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Fig. Fixture holding a component (Inspection)
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Following aspects should also be taken into account
in the design of jigs and fixtures:

 Consideration of sequence of operations given in the

process planning chart.

 Study of the detailed drawing of the component critically,

i.e., the dimensions which are provided with tolerances.

 Consideration of the manufacturing defects such as

(a)shrinkages, (b) blow holes, (c) inclusions as in the cast
bodies, (d) distortions as in the case of welding and
fabricating fixture body or frame.

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Features: Locating/Datum Surface
 A datum feature is an important functional surface where all
other dimensions will be specified with reference to it.

 A Datum is a perfect point, line, plane or surface but only exists

theoretically. However a Datum Feature is a tangible surface,
point or axis on a part where that theoretical datum is located.

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Loading/Unloading
and Clamping (12 DOF)

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Tolerancing on Fixtures: Significance

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Next Topic: 4 SupportingFig.and Locating
Basic Principles
steps of fixture design
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Supporting and Locating Principles
Jigs and fixtures must accurately & consistently
position the workpiece relative to the cutting tool,
part after part. The locators must ensure that the
workpiece is properly referenced and the process
is repeatable.

OBJECTIVES
 Referencing and basic rules of locating
 Identify the types of locators and supports
 Specify the use of locators and supports

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Principles of Referencing
 Referencing- process of positioning the workpiece relative
to the workholder, and the workholder to the cutting tool.
 Referencing the workholder to cutting tool is performed by
the guiding or setting devices (drill jigs- using drill bushings,
fixtures- using keys, feeler gages, and/or probes).
 Referencing the workpiece to the workholder, on the other
hand, is done with locators.
 Poor design of the locators lead to improper location of the workpiece
and the part will be machined incorrectly.
 Likewise, if a cutter is improperly positioned relative to the fixture
leads to incorrect machining.
 So, in the design of a workholder, referencing of both the workpiece
and the cutter must be considered and simultaneously maintained.
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Repeatability: Locating the work
 Repeatability is the ability of the workholder to consistently
produce parts within tolerance limits, and is directly related
to the referencing capability of the tool.
 The location of the workpiece relative to the fixture and of
the fixture to the cutter must be consistent.
 The workholder must be designed to accommodate the
workpiece's locating surfaces.
 The ideal locating point on a workpiece is a finely machined
surface. Machined surfaces permit location from a
consistent reference point.
 Cast, forged, or sawed surfaces can vary greatly from part to
part, and will affect the accuracy of the location.
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The Mechanics of Locating
A workpiece free in space can move in any direction. This motion
can be broken down into 12 "degrees of freedom”, which must
be restricted to ensure proper referencing.

Crucial element in workholder design: locators, not clamps,

must hold the workpiece against the cutting forces.

 The devices that restrict a workpiece's

movement are the locators.

 The locators must be rigid enough to

maintain the position of the workpiece
and to resist the cutting forces.
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Locator and workpiece

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 Locators provide a positive stop for the workpiece. Placed
against the stop, the workpiece cannot move.
 Clamps rely only upon friction between the clamp and the
clamped surface to hold the workpiece. Sufficient force
could move the workpiece.

 It should be noted that clamps are intended to hold the

workpiece against the locators.

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Forms of Location
Following are the three general forms of location: (i) Plane, (ii)
Concentric, and (iii) Radial
Plane locators locate a workpiece from any surface. The surface
may be flat, curved, or have an irregular contour.
 Concentric locators locate a workpiece from a central axis. This
axis may or may not be in the center of the workpiece.

Radial locators restrict the

movement of a workpiece
around a concentric locator.

Mostly, locating is performed

by a combination of all these
locational methods.
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Basic Rules for Locating
(i) Positioning the locators, (ii) Part tolerance, (iii) Foolproofing, and (iv) Duplicate location

(i) Positioning the locators:

 Locator should contact the work on a machined surface. This
ensures the accurate placement of the work in the tool and
ensures repeatability.
 Accurate location is important to achieve the repeatability.
 Locators should be placed as far apart as possible to permit
the use of fewer locators which ensures complete contact.

To avoid the chip

clogging and
jamming, relief
may be provided
on the locators.
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(ii) Part Tolerance:
 As a general rule, the tool tolerance should be between 20
and 50 percent of the part tolerance.

 Specifying tool tolerances closer than 20 percent increases

the cost of the tool and adds little to the quality of the part.

 Locators must be designed to fit the part at any size within

the part limits.

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(iii) Foolproofing:
Foolproofing means by which the tool designer ensures that
the part will fit into the tool only in its correct position.
Generally a pin will be provided to prevent the part from
being loaded incorrectly.

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(iv) Duplicate Locators:
 The use of duplicate locators must be avoided. Locator
duplication implies more than one reference and causes
position related issues.

 Locational inaccuracies develop because of the difference in

position and location tolerances between the tool and work.

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Restricting Planes of Movement
An object is free to revolve around or move parallel to any
axis in either direction (total 12 DOF).
To accurately locate a part in a jig or fixture, movements
must be restricted. This is done with locators and clamps.
Using pin- or button-type locators minimizes the error by
limiting the contact area and raising part above chips.

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3-2-1 Locating Method (using external flat surfaces)
Placing the part on a 3-pin base, five directions of movement
are restricted. 2-more pins restrict another three directions
and 1-more pin restrict another one direction.

The remaining three directions will be restricted by a clamping

device. This 3-2-1, or 6-point locating method is most common
external locating method for square or rectangular parts.

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Locating using a Hole: Primary and Secondary
 The holes on a part provide an excellent method of locating.
One of the hole is used as a primary locator, and other holes
is used as a secondary locator.
 Primary locator is a round pin (9 DOF), and the secondary
locator is a diamond pin (2 DOF, alignment purpose).

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Locating the work: Types of Locators
Locating from a Flat Surface:
 There are three primary methods of locating work from
a flat surface: (i) solid supports, (ii) adjustable supports,
and (iii) equalizing supports.
 The above locators set the vertical position of the part,
support the part, and prevent distortion during the
machining operation.

 Solid supports are the easiest to use. They can be either

machined into the tool base or installed.

 This type of support is normally used when a machined

surface acts as a locating point.
Fig. Solid Supports
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 Adjustable supports are used in combination with solid
supports, when the surface is uneven, e.g. cast parts.
 There are many styles of adjustable supports, more
common are the threaded , spring, and push types.
 The threaded style is the easiest and most economical, and
it has a larger adjustment range than the others.

Fig. Adjustable Supports (threaded, push and spring)

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 Equalizing supports provide balanced equal support through
two connected contact points.
 As one point is depressed, the other raises and maintains
contact, especially necessary on uneven cast surfaces.

 Before choosing a support, designer must consider the

shape and surface of the part and type of clamping device.
 Support must be strong enough to resist clamping pressure
and cutting forces. Clamps should be positioned directly
over the supports to avoid distorting the part.
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Locating the work: Types of Locators
Locating from an Internal Diameter:
 When large holes locate the work, fasten the internal locator
with both screws and dowels. Under normal conditions, two
dowels and two screws are needed to hold the locator.
 With shank-type locators, it is a good practice to use the press-
fit locator rather than the threaded locator for accuracy.

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 Pin-type locators are used for smaller holes and for aligning
members of the tool. Special bushings should also be used so
that they can be replaced when they wear.
 Pins used for part location are made with either tapered ends
or rounded ends, allowing loading and unloading simple.

 The main difference between the pins used for location and the
pins used for alignment is the amount of bearing surface.
Alignment pins usually have a longer area of contact.

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 Another style of pin is the diamond pin (relieved locators) ,
which is normally used along with the round pin. It is easier to
locate a part on one round pin and one diamond pin.

 Relieved locators reduce the area of contact between the

workpiece and the locator. Decreasing the contact area has
little or no effect on the overall locational accuracy.

 Reducing the contact area helps make the jig or fixture easier
to load and unload.

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 The split contact locator is used for thick workpieces. Here,
small split area used rather than using the complete thickness
of the part for location.
 This design provides full location and makes the locator less
likely to bind (jam) in the workpiece.

 The raised contact locator reduces the contact area and raises
the point from base where the locator and workpiece touch.

Fig. Split contact Fig. Raised contact 23

 Spherical contact locator will not bind in any locating hole. A
spherical locator greatly reduces the contact area by removing
all the material not directly in contact with the workpiece.

 Spherical locators are impossible to bind because, unlike with

cylindrical locators, the distance between the opposite sides of
the contact area is always same.

Fig. Cylindrical contact Fig. Spherical contact

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Locating the work: Types of Locators
Locating from an External Profile:

 Locating work from an external profile, or outside edge, is the

most common method in the early stages of machining.

 Nesting locators position a part

by enclosing it in a recess, of the
same shape as the part.

 The most common type is the

ring nest, which is normally used
for cylindrical profiles. The full
nest is for non cylindrical parts.

Fig.Ring
Fig. Full Nest
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 The partial nest is a variation of the full nest and encloses
only a part of the workpiece.
 Vee locators are used mainly for round work. They can locate
flat work with rounded or angular ends and flat discs.

Fig. Partial Nest Fig. Vee locators 26

 Adjustable-stop locators keep
the cost to a minimum.

 Sight locators align rough parts

for approximate machining.

Fig. Adjustable-stop locators Fig. Sight Locators 27

 Ejectors are used to remove work from close-fitting locators,
such as full nests or ring nests.

 Ejectors speed up the unloading of the part from the tool,

which reduces the in-tool time and increases production rate.

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Spring-stop buttons and spring-locating pins, while not
locating devices, do aid in properly locating a part.

These devices are used to push the part against the locators
to ensure proper contact during the clamping operation.

Fig. Spring-locating Pins Fig. Spring-stop Buttons 29

Summary: Supporting and Locating Principles
To achieve proper location, the locators must properly
reference the part and ensure the repeatability.
– Referencing is the process of properly positioning the part
with respect to the cutter or other tool.
– Repeatability is the feature of location that permits the parts
to be made within their stated tolerances, part after part.

Critical aspects of locating a part: Position, locational

tolerances, foolproofing, and avoiding duplicate location.

Locators positioned under a part are referred to as supports.

Locators at part edges are called locators or stops.

Next Topic: 05 Clamping and Work Holding Principles

Clamping and Work Holding Principles

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Basic principles and functionality of clamping
The basic function of a clamp is to hold a part against the
locators during the machining operation.
The clamp must be strong enough to hold the part and to
resist the movement.
The clamp must not damage or deform the part.
The clamp must be fast-acting and allow rapid loading and
unloading of parts.
Clamps are also positioned so they do not interfere with the
tool or machine tool.
To be effective and efficient, the clamps must be planned
into the tool design (important element in tool design).
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Basic Rules of Clamping
Positioning the clamps, Tool forces and Clamping forces

Positioning the clamps:

 Clamp must be in contact with the work at its most rigid
point to prevent bending or damaging the part.
 The part must be supported if the work is clamped at a
point where the force could bend the part.

Fig. Wrong fixture design Fig. Acceptable fixture design

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Basic Rules of Clamping
Tool Forces:
 Tool forces generated by the cutting action. A properly
designed tool can use the cutting forces to its own advantage.
 To clamp a part correctly, the tool designer must know how
tool forces, or cutting forces act in reference to the tool.

Fig. Using cutting force to hold a part

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Basic Rules of Clamping
Clamping Forces:
 Clamping force is to hold the part against the locators.
 Clamping prevents the part from shifting or being pulled from
the jig or fixture during the machining operation.
 The type and amount of clamping force needed to hold a part
is usually determined by the tool forces.

Clamping force calculations: Click Here

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 *Locators not shown

Fig. Wrong clamping configuration Fig. Acceptable clamping configuration

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TYPES OF CLAMPS
The type of clamp the tool designer chooses is determined by
(i) shape and size of the part, (ii) type of jig or fixture being
used, and (iii) work to be done.
Strap Clamps:
 Strap clamps are the simplest clamps used for jigs and
fixtures. Their basic operation is the same as that of a lever.
 Strap clamps can be grouped into three Classes.

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 Most strap clamps use the third-class lever arrangement. When
these strap clamps are used, the spacing of the three elements
is also important.
 The distance between the fastener (effort) and the workpiece
should always be less than that between the fastener and the
heel pin (fulcrum). 8
Types of Strap Clamps:
 Common types are the hinge clamp, the sliding clamp, and
the latch clamp.

 The fulcrum is positioned so that the clamp bar is parallel to

the base of the tool at all times.

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Strap Clamp Elements:

The fulcrum is positioned so that the

clamp bar is parallel to the base.

To make up the differences, spherical

washers/nuts are used to reduce
stresses in the threaded members. Spherical Nuts and Washers

Strap clamps operated by either manual

devices or power-driven devices.

Manual devices include hexagonal nuts,

hand knobs, and cams.
Power Clamping System
Power devices include hydraulic systems
and pneumatic systems. Mechanical Holding Devices
10
Screw Size and Holding Force:

 Although standard high-strength fastening devices

may be used for many workholding tasks, the
commercially available jig and fixture studs, bolts,
nuts, washers, and other hardware should be used
whenever possible.

 At no time should the long threaded rods available

from a hardware store be used for workholding.
These rods do not have sufficient strength to be used
for jig and fixture applications.

11
Other Types of Clamps:
 Screw clamps offer the tool designer almost unlimited application
potential and lower costs, limited by slow operating speeds.
 Screw clamp uses the torque developed by a screw to hold a part
in place—either by direct pressure or by action on another clamp.
 Swing clamps combine the screw clamp with a swinging arm that
pivots on its mounting stud for quick loading.

Fig. Screw Clamp Fig. Swing Clamp

12
Hook Clamps:
 Similar to swing clamps but much smaller. Useful in close
places or where several small clamps must be used.
 Modified hook clamp is made to be operated from the
opposite side of the tool. Useful where a safety hazard exists.
Quick-Acting Knobs:
Quick-acting knobs are useful for increasing the output of low-
cost tools. These knobs are made so that when pressure is
released, they can be tilted and slid off a stud.

Fig. Hook clamps Fig. Quick-acting knob 13

Cam-action Clamps
 Cam-action clamps, when properly selected and used, provide
a fast, efficient, and simple way to hold work. Direct clamping
is less effective and can loosen up in case of vibrations.

 Commercial cam clamp assemblies use cam action rather than

screw threads to bind strap clamps. This indirect clamping has
all the advantages of cam action.

14
Wedge Clamps
 Wedge clamps apply the basic principle of the inclined plane
to hold work in a manner similar to a cam.
 These clamps are normally found in two general forms, flat
wedges and conical wedges.
 Wedges having a slight angle, from 1 to 4 degrees, normally
hold the work without additional attachments. This type of
wedge is considered to be self-holding.

15
Power Clamping Systems
 Normally operate under hydraulic power or pneumatic power, or
with an air-to-hydraulic booster.
 The air-to-hydraulic system is preferred as it can operated from
the regular shop line pressure and no extra pumps are needed.
 Advantages: better control of clamping pressures, less wear on
moving parts of the clamp, and faster operating cycles.
 Disadvantage is cost, however it can be easily compensated by
increased production speeds and higher efficiency.

16
NON-MECHANICAL CLAMPING
Workholding devices used to hold parts by means of other than
direct mechanical mechanisms.

Mainly where the clamping forces must be applied evenly across

the entire part to minimize any possible workpiece distortion.

(i) Magnetic clamping & (ii)Vacuum clamping

 Magnetic chucks are most often used to

hold ferrous metals or workpieces made
from other magnetic materials.

 However, there are also some magnetic

chuck setups that can be used for
holding nonmagnetic workpieces.
17
 Vacuum chucks are another style of chuck used for nonmagnetic
materials or when parts that must be clamped uniformly.
 Vacuum chucks, like magnetic chucks, equalize the clamping pressure
over the entire clamping surface.
 In operation the holding forces are generated by a vacuum pump that
draws out the air between the chuck face and the workpiece.

 It increases the process reliability when clamping large, flat metal-

workpieces with smooth bottom surfaces on CNC machining centers.

18
 Vacuum Clamping Technology: Link

Clamping with vacuum technology is

efficient and universal. Vacuum clamping
guarantees very short set-up times.

Even components which are difficult to

clamp mechanically can be easily and
quickly clamped without any distortion.
19
Multiple Clamping Devices
Many times production operations call for making more than one
part at a time. The tool designer must know how to design
clamps that are capable of holding several parts.
Using the basic ideas and rules for single-part clamping, the tool
designer can easily design clamps to hold any number of parts.

The main points to remember in

multiple clamping are that the
clamping pressure must be equal
on all parts and that the clamp
must have only one operating, or
locking, point.

Next Topic: 06 Design of Jigs & Fixtures

20
Design Aspects of Jigs and Fixtures

1
Considerations of Design Economics
 The demand for maximum productivity at minimal cost
are a challenge to the tool designer.
 In addition to developing designs for efficient and
accurate jigs and fixtures, the tool designer is
responsible for finding ways to keep the cost of special
tools as low as possible.

 Design economy begins with the tool designer’s ideas

and is carried through to the completion of the tool.

 Design details should be carefully studied to find ways to

reduce costs and still maintain part quality.

2
Preformed Materials:
 Preformed materials can greatly reduce tooling costs by
eliminating many machining operations.
 Whenever practical, preformed materials such as drill rods,
structural sections, pre-machined brackets, tooling plate, and
precision-ground flat stock, should be specified in the design.

Standard Components:
 Commercially available standard jig and fixture components
can greatly improve tooling quality.

 Standard components, such as clamps, locators, supports,

drill bushings, pins, screws, bolts, nuts, and springs, should be
planned into the design to reduce expenses.
3
Secondary Operations:
 Grinding, heat-treating, and machining, should be limited to
areas that are necessary for efficient tool operation.

 Hardening should be limited to areas that are subjected to

wear, such as supports, locators, and moving parts.

Tolerance and Allowance:

 Generally, the tolerance of a jig or fixture should be
between 20 and 50 percent of the part tolerance.

 Overly accurate tooling is economically wasteful and no

more valuable than tooling within the required tolerance.
The only effect on the part is higher cost.
4
ECONOMIC ANALYSIS
 The tool designer must furnish management with an idea of
how much the tooling will cost and how much the production
method saves over a specific run.
 Above information is generally furnished in the form of a
tooling estimate, which includes the estimated cost of the tool
and projected savings over alternate methods.

 The estimate also includes any special conditions that may

justify the cost of the tooling, such as close tolerances or high-
volume production.

 For a valid estimate, the tool designer must accurately

estimate the cost and productivity of the design in terms of
materials, labor, and the number of parts per hour.
5
Estimating Tool Cost and Productivity
 The cost of a tool design is to add the total costs of material
and labor needed to fabricate the tool, must be done
carefully so that no part or operation is forgotten.

 One method is to label each part of the tool and list the
materials in a separate parts list.

 Then, using a cost work sheet, list each part and calculate
the material and labor for each operation. The final expense
added is the cost of designing the tool.

 The next step in estimating is calculating the number of

parts per hour the tool will produce. The simplest method is
to divide 1 hour by the single-part time.
6
Calculating the Cost Per Part
A comparison of tool costs or labor
expenses cannot give the tool designer
enough information to determine the
true economic potential of a design.

Calculating Total Savings

To determine the most economical
production method, the tool
designer must compare production
alternatives.

Problem: A flange-plate adapter costs Rs.24/- per part to mill

without a fixture and Rs. 10/- per part when a fixture is used.
Assuming the fixture costs Rs. 12800/-, how much will the fixture
save over a production run of 1500 parts?
7
Calculating the Break-Even Point
The break-even point is the minimum number of parts a tool
must produce to pay for itself.
Any number less than this minimum results in a loss of money;
any number more results in a profit.
It is logical to assume that the lower the break-even point, the
higher the profit potential.

Problem: A lathe fixture costs Rs. 15, 000/- to build and

produces parts at a cost of Rs. 20/-. How many parts
must it produce to pay for itself when compared to an
alternate method that requires no special tooling and is
capable of making the parts at a cost of Rs. 40/- each?
8
COMPARATIVE ANALYSIS
 The tool designer must consider and evaluate several options
before making a tooling recommendation to management.
 By comparing each method, tool designer can see the tooling
requirements in terms of costs versus savings.

 Then the method that returns the most for each rupee spent
can be selected.

 When preparing this comparison, the tool designer must

weigh all the economic factors in relation to expenses and
productivity.

Typical elements for comparison: Lot Size, Tool Cost, Parts/Hour,

Labor/Hour, Labor/Lot, Cost/Part
9
Problem: Using the listed alternatives, prepare a comparative
analysis for the following tooling problem: A total of 950 flange
plates require four holes accurately drilled 90 degrees apart to mate
with a connector valve. Which of the listed alternatives is the most
economically desirable?

a. Have a machinist who earns Rs.10/- per hour lay out and drill
each part at a rate of 2 minutes per part.

b. Use a template jig, capable of producing 50 parts per hour and

costing Rs.18/-, in the production department, where an operator
earns Rs.6.5/- per hour.

c. Use a duplex jig, which costs Rs. 37.5/- and can produce a part
every 26 seconds, in the production department, where an operator
earns Rs. 6.5/- per hour.
10
Developing the Initial Design
PREDESIGN ANALYSIS
All tool design ideas begin in the mind of the tool designer. A
great deal of planning and research is needed to turn tooling
ideas into practical hardware.

The first step in designing a tool is organizing all relative

information.
Part drawings and production plans are carefully studied to find
exactly what tool is required.
Preliminary plans for the tool are developed, usually by means
of sketches.
The tool de-signer must develop alternatives that are practical
and cost-effective.
Finally, tool drawings are made from the tools that can be built.
11
Overall Size and Shape of the Part
The tool designer must consider how the size and shape of the part influence
the bulk and mass of the tool.

Type and Condition of Material

 Parts from soft materials, such as aluminum, magnesium, or plastic, are easier
and faster to cut than harder materials.
 Since cutting forces are reduced for these materials, the design of the tool is
directly affected.
 Reduced cutting forces allow lighter, less rigid tools, but the higher production
rate requires faster tool operation.
 The condition of the part material also affects how the part is held and
located. Rolled or extruded bar-stock is more uniform in size than cast parts
and is normally easier to locate.
 Cast parts are sometimes more fragile than solid sections, and clamping
pressure must be reduced to prevent breaking or cracking the casting.
12
Type of Machining Operation
 The particular machining operation to be done specifies the type of tool
to be made.
 In some cases, multipurpose tools can be designed for more than one
operation, such as the drill jig/milling fixture.
 Drill jig for large holes must be made stronger than a jig for small holes.
 Increased cutting forces require added tool strength and rigidity.

Degree of Accuracy Required

The effect accuracy has on the design is
usually reflected in the tool tolerances. The
general rule of tolerance is that 20 to 50
percent of the part tolerance is applied to
the tool. The degree of required accuracy
determines this tolerance.
13
Number of Pieces to be Made
 As a rule, larger production runs justify more detailed and
expensive tooling than do smaller runs.

 This is because the tool will be in service longer and production

speeds are generally higher.

 Longer production runs also require replaceable parts to be

used in making the tool.

 Bushings are included, along with liners and lock screws, in

tools that are used in longer production runs.

 Details, such as locators and clamps, are also affected by the

size of production runs.
14
Locating and Clamping Surfaces
The part drawing must be studied to find the best surfaces to locate and clamp
the part. The order of preference is as below:

1. Holes
2. Two machined surfaces that form a right angle
3. One machined and one unmachined surface that form a right angle
4. Two unmachined surfaces that form a right angle

 One of the prime requirement for a locating surface is repeatability. Parts

must be positioned identically, within the tolerance limits, part after part.

 Clamping surfaces must be rigid and capable of holding the part without
bending. Bending can distort the machining operation.

 If the clamping surface can bend, it must be supported. If a finished surface

is used to hold the part, the clamp should have a cap or pad to prevent
damage to the finished surface.
15
Type and Size of Machine Tool
 The process planning engineer normally selects the machine tool for each
operation.

 However, if a better tool could be used, the tool designer should consult the
process engineer before beginning the design.

 For example, when holes are drilled with a drill jig, a drill press should be
used. Little is gained from using a vertical mill or a jig borer, since the accuracy
is built into the jig, not into the machine tool.

Type and Size of Cutters

 Normally, the type and size of cutters are specified by the process engineer,
but occasionally the tool designer may do this.

 Before the tool designer selects the cutters, every detail about the tools
being used must be known to ensure that the part is properly referenced to
the tool and that enough tool clearance is provided.
16
Sequence of Operations
Quite often the tool designer must design more than one tool for a part.
When this is the case, the sequence of operations must be determined as
well as which tool to design first.
For example, if a drill jig for a part is designed first, then the holes provide an
excellent location for the milling fixture that is needed in the next operation.

PREVIOUS MACHINING OPERATIONS

 This phase of design is closely related to the sequence of operations.

 The tool designer must know what operations, if any, take place before the
operation being planned.

 In this way, locators and clamps can be positioned to take advantage of the
existing machined surfaces.

 This is important when more than one person is designing tools for the
same part.
DESIGNING AROUND THE HUMAN ELEMENT
Design Ergonomics
 Ergonomics is a science that studies the human body and uses what
it learns about how the human body works to determine the best
design of objects, systems, and environmental systems for human
interaction.

 It is important that the designer consider ergonomics during the

design phase and make good use of information that is provided
from the machining technician as well as from industrial engineers
involved in basic motion and time study.

 The designer’s job is a onetime event. Once the design is complete

and any necessary revisions are completed, the design is released
for build and journeys onto the production floor. But the machining
technician will load and unload parts continuously for manufacture
for extended periods.
The following partial list of questions provides a starting point
for the tool designer to consider when planning a tool design.
• Is the operation of the tool smooth and rhythmic?
• Can both hands be used at the same time?
• Are hands clear and free of moving parts?
• Do both hands start and stop together?
• Does the intended motion minimize operator fatigue?
• Can feet be used to lessen hand and arm fatigue?
• Is the tool height appropriate?
• Are controls and clamps within easy reach of the operator?
• Are handles designed to reduce hand and finger fatigue?
• Is safety designed into the tool with respect to the operation of
the supporting equipment?
Safety as Related to Tool Design
The following checklist should be consulted during every step of the
design to ensure that the tool is completely safe to operate.

• Is the tool clear of the cutters during the loading and unloading
operations?
• Are any operator movements required close to a moving or revolving
tool?
• Are chip guards needed to protect the operator and others nearby?
• Are all sharp edges on the tool chamfered?
• Are attached accessories (pins, feeler gauges, wrenches, etc.) far enough
away to prevent tangling in the tool?
• Is the entire operation visible from the operator’s position?
• Could the part be pulled from the tool?
• Is the tool body rigid enough to resist all cutting forces?
• Could the clamping device loosen during the machining cycle?
TOOL DRAWINGS VERSUS PRODUCTIONDRAWINGS

Tool drawings are used to transfer detailed instructions from

the tool designer to the toolmaker.

The form and specifications of these drawings are normally

established within each company to meet particular needs.

However, there are standards and conventions that all

companies follow.

Tool drawings differ from standard production drawings in the

amount of detail shown.

Toolmakers are highly skilled technicians. Therefore, they

require less detailed information on drawings.
See and understand: Fig. 8-30, 8-31 (Book by Edward G. Hoffman)
Materials for Jigs & Fixture Elements

1
PROPERTIES OF TOOL MATERIALS
 Properties of the interest are those that directly influence the
behavior of the material while in use (tool).

 Adaptability, durability, and economy must be considered

before a material is selected for a particular tool.

 Before any choice can be made, the designer must have a

working knowledge of the properties and characteristics of
the materials common to tool construction.

 The properties of tool materials that concern the tool designer

are hardness, toughness, wear resistance, machinability,
brittleness, tensile strength, and shear strength.
CarrLane Tooling Materials: https://www.carrlane.com/engineering-
resources/material-and-finish-information/material-technical-information 2
PROPERTIES OF TOOL MATERIALS
 Hardness is the ability of a material to resist indentation. The
harder the material, the greater its tensile strength. Both these
properties are desirable for fixtures elements.

 Toughness is the ability of a material to absorb sudden applied

loads or shocks repeatedly without permanent deformation.

 In steels, hardness controls (correlated with) toughness to

approximately Rockwell C44-48, > material become brittle.

 Brittleness is the opposite of toughness. Brittle materials have

the tendency to fracture when sudden loads are applied.

 Materials that are very hard are also very brittle. So tool
designer need to be extra careful while imparting hardness.
3
 Wear resistance is the ability of a material to resist abrasion
against the counter body during service.
 Hardness is also a prime factor in wear resistance. Wear
resistance usually increases with hardness.
 Machinability is the measure of ease of a material can be
machined. Factors concerning machinability are cutting speed,
tool life, and surface finish.

4
5
6
FERROUS TOOL MATERIALS
Ferrous tool materials include cast iron, carbon steel, alloy
steel, and tool steel.

Ferrous metals make up the largest group of tool material in

common use. Cast iron, alloy steel, and tool steel are general
terms that cover a wide range of materials.

(i) Cast Iron

Cast iron is used for tool bodies and some commercial jig and
fixture components.

It is being replaced by other materials that are less expensive

and take less time to fabricate into tools.
7
(ii) Carbon Steels
 Carbon steel is the primary material of fixture tooling. Ease of
fabrication, low cost made them popular for tool construction.

 Machinability and weldability of these materials decreases

with increased carbon content.

 The three main types of this steel are low carbon, medium
carbon, and high carbon.

8
LOW CARBON STEELS are used mainly for structural parts of a
fixture. They should be used only in areas where mass is required
and no wear or stress will occur, such as base plates or supports.

The carbon content is between 0.05 and 0.3 percent. If required

can be case-hardened to resist wear for low-production tools. It is
also easily welded or joined by any standard process.

MEDIUM CARBON STEELS are used in much the same way as low
carbon steels, but in areas of tooling that require more strength.
Medium carbon steels work well as clamps, studs, nuts, and in
almost any area where toughness is required.

The carbon content of medium carbon steel is between 0.3 and

0.5 percent, which allows the material to be hardened by surface
hardening or other conventional hardening processes.
9
 High carbon steels are limited to tool construction in areas that
are subject to the most wear. Parts such as drill bushings,
locators, wear pads, and supports can be made of this material.

 High carbon steels have a carbon content between .50 and 2.0
percent. They are easily hardened by the conventional processes,
but do not resist wear as well as most tool steels do.

10
Iron > Carbon > CW > HT vs. Yield Strength

(6.89 X MPa)
11
Steel Crystal Structures: • Ferrite – BCC iron with
carbon in solid solution
(soft, ductile, magnetic)
• Austenite – FCC iron with
carbon in solid solution
(soft, moderate strength,
non-magnetic)
• Cementite – Compound of
carbon and iron FE3C
(Hard and brittle)
• Pearlite – Alternate layers
of ferrite and cementite.
• Martensite – iron – carbon
w/ body centered
tetragonal – result of heat
HT: Ferrite then Austentite then Martensite
treat and quench 12
 Fe-Fe3C Equilibrium Diagram
FCC has 8 and 4 octahedral and
tertrahedral voids per unit cell AUSTENITE 1130c
respectively, whereas BCC has Normalizing
12 and 6 respectively.
Annealing
1000c
Hardening
910c
900c
FERRITE
723c
AC1 lower critical temp
600c
500c Spheroidising
Process annealing
High temp. tempering
PEARLITE + CEMENTITE
FERRITE + PEARLITE
Low temp. tempering
Hypoeutectoid steel

% of Carbon 0.8 2.0 13

 Heat Treatment of Steels - Facts
• Must have a carbon content of at least 0.4%
(ideally 0.6%) to heat treat.
• Must heat to austenitic (FCC) temperature (910c).
• Must rapid quench to prevent formation of
equilibrium products.
• The FCC can hold more carbon (more voids) and on
rapid cooling the crystal structure wants to return
to its BCC structure.
• It cannot due to trapped carbon atoms. The net
result is a distorted crystal structure called body
centered tetragonal called martensite.
14
Time Temperature Transformation (TTT) Diagram

15
(iii) Alloy Steels
 Alloy steels are not generally used for tool construction
because of their added cost.
 Alloying elements change the properties of the steels
significantly, will be used as and when required.

The most common alloying elements and their effects are:

16
As an aid in identifying and standardizing the great variety of
carbon and alloy steels, a universal numbering system has been
adopted that was developed by

(a) Society of Automotive Engineers (SAE)

(b) American Iron and Steel Institute (AISI)

 It is used throughout the manufacturing industry as a standard

for metals identification.

 The basis of this numbering system is a four digit code that

indicates specific information about the metal.

17
 The first digit indicates the type of metal. For example, 1
indicates carbon steel; 2 nickel steel; 3 nickel-chromium steel;
4 molybdenum; 5 chromium….

 The second digit indicates either the percentage of major

alloy in the metal or a code to denote a specific alloy.

The last two

digits when
read together
express the
carbon content
in hundredths
of a percent.

18
19
In cases where there is more
than 0.99 percent carbon, a
fifth digit is added.

20
(iv) Tool Steels
 Tool steels are steels that are
made to exact standards for
specific types of service.
 Tool steels are used in jig and
fixture work for parts that are
highly stressed or higher wear
resistance.
Tool steels are identified by
a letter and number code
developed by the AISI.

This system establishes the

standard composition and
Read here for more details:
assigns each a number.
https://www.azom.com/article.aspx?ArticleID=6138
21
22
(v) NONFERROUS TOOL MATERIALS
 Nonferrous tool materials such as aluminum and magnesium
alloys are metals that have a base metal other than iron.

 Not as widely used for jigs and fixtures as ferrous materials

are, nonferrous tool materials have the advantages of weight,
stability, and workability.

Aluminum Alloys
 Aluminum is the most widely used nonferrous tool material
because of its machinability, adaptability, and weight.

 Aluminum is available in a wide variety of forms, which

further increases its usefulness.

 The primary forms used for jig and fixture work are
aluminum tool plates and extrusions. 23
24
25
Magnesium Alloys
 Magnesium is is very lightweight and versatile, and it has a high
strength-to-weight ratio.

 Magnesium alloys are identified by a system developed by the

American Society for Testing Materials (ASTM) that uses the
alloy composition as the basis for identification.

 The first one or two letters tell which alloys are used. The
numbers indicate the approximate percentage of each alloy.

26
27
NONMETALLIC TOOL MATERIALS
 Nonmetallic tool materials have become an important part of
jig and fixture work.
 Tools that are intended for a limited production run can be
made faster, less expensively, and better with such materials.
 The materials used in jig and fixture work are urethane, and
epoxy or plastic resins.

3D Printed Jigs & Fixtures: A Powerful Solution

for the Production Floor

Strong plastics used in 3D printing processes are

an excellent alternative to conventional metals.
Light-weight, 3D printed jigs and fixtures
produce the same, or better, functionality while
improving ease of use.
28
Urethane
 Urethane is used in jig and fixture work mostly for secondary
clamping purposes.
 Its main benefit is its controllable deflection. To hold the part
firmly without damaging the surface.
 A urethane pad will transfer the force while preventing the
clamp from scratching the part.
 Secondary action clamps use the deflection capabilities of
urethane to good advantage.

29
Epoxy and Plastic Resins
 Epoxy and plastic resins are used in jig and fixture work as they
are lightweight, strong, tough, and less expensive.
 Can be used as is or with fillers mixed in for strength or wear
resistance. Filler materials include glass beads, ground glass,
steel shot, steel filings, stones.
 Can be used as setting/mounting material for fixing
replaceable elements of the fixtures.

30
 Limits, FITS AND TOLERANCES

Ø Due to the inevitable inaccuracy of manufacturing

methods, a part cannot be made precisely to a
given dimension. The permissible variation on the
size is called tolerance.

Ø The two extreme permissible sizes on the

actual size are called limits.

Ø When two parts are to be assembled, the relation

resulting from the difference between their sizes
before assembly is called a fit.
1
FIT - Condition of looseness or tightness between
two mating parts being assembled together.
2
 Max Hole size – Basic Size = Upper Deviation
HOLE
Min Hole size – Basic Size = Lower Deviation

Max shaft size – Basic Size = Upper Deviation

SHAFT
Min shaft size – Basic Size = Lower Deviation
 CLEARANCE FIT

Maximum shaft dimension < Minimum hole dimension

 INTERFERANCE FIT

Maximum Hole size < Minimum Shaft size 5

 TRANSITION FIT

Obtained by overlapping of tolerance zones of

shaft and hole …… Does not guarantee neither
clearance nor interference fit.
To obtain different types of fits, it is practice to
vary tolerance zone of one of the mating parts

HOLE BASED SYSTEM

Size of hole is kept
constant, shaft size is
varied to get different fits.

SHAFT BASED SYSTEM

Size of shaft is kept
constant, hole size is
varied to get different fits.

7
 Representation of Fit

A fit is indicated by the basic size common to

both components, followed by symbol
corresponding to each component, the hole
being quoted first. E.g. 45 H8/g7
8
9
10
Fundamental Deviations on Shaft Size

11
12
 The selection of
letter freezes one Representation of
limit of hole / shaft Tolerance
1) Letter Symbol

H : lower deviation
of hole is zero

h : upper deviation
of shaft is zero

13
2) Number or Grade (ITG) = IT01, IT0, IT1,….IT16 gives
the tolerance value (T).

Units in μm
14
i =

15
The selection of
Tolerance grade number Representation of
freezes the other limit of
hole / shaft Tolerance
1) Letter Symbol

H : lower deviation
of hole is zero

h : upper deviation
of shaft is zero
16
Representation of Fit Together (Letter & Grade) on
both mating components
decide quality of fit

0.021

INTERFERENCE Φ30.035
FIT Φ30.022
Φ30.021
0.013
0.022
Φ30.000

H7 : Tol Grade 7 mean 21μ variation

(H means upper deviation zero)
p6 : Tol Grade 6 means 13μ variation 17
(p means upper deviation is 22 μ)
FITS APPLICATIONS
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20
Unilateral and Bilateral Tolerances

21
Compound Tolerance
60 ± 0.02 40 ± ???
0.08

100 ± 0.06

22
Accumulation of Tolerances

23
Progressive Dimensioning

24
 Limit Gauges
ü The term ‘limit gauges’ signifies the use of gauges
for checking the limits of the components.

ü GO gauge checks Maximum Material Limit (MML)

GO and NOT GO limits of plug gauge

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ü Limit gauges ensure that the components lie
within the permissible limits, but they do not
determine the actual size or dimensions.

GO and NOT GO limits of snap gauge

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28
 Taylor’s Principle
§ Taylor’s principle states that the GO gauge is
designed to check maximum metal
conditions, that is, LLH and HLS.

§ GO gauge can be designed to check more

than one dimension at a time.

§ NOT GO gauge is designed to check minimum

metal conditions, that is, HLH and LLS.

§ NOT GO gauge should check only one

dimension at a time. 29
§ Gauge Tolerance: Normal practice is to take gauge
tolerance as 10% of the work tolerance.

§ Component that is manufactured outside the limits

should not be accepted by gauges.

§ Component rejected by the GO gauge can be

reworked to maintain the limits because GO gauge
checks maximum material limit.

§ Component rejected by the NOT GO gauge is

permanently rejected because NOT GO gauge
checks minimum material limit. Hence close gauge
tolerances will be provided on NOT GO gauges.
30
Problems
Design the general type of GO and NOGO gauges for
components having 30 H7 f8 fit and represent the
tolerance zones of components and gauges (on same zero
line)? Gauge tolerance is 10% of the work tolerance and
wear allowance is 10% of the gauge tolerance.

31
 Surface Roughness

Real surfaces are combination of both ROUGHNESS AND WAVINESS

 Surface Roughness

Real surfaces are combination of both ROUGHNESS AND WAVINESS

 Surface Texture
Surface Texture - details that make up a surface - roughness,
waviness and lay pattern .

Roughness : Closely space irregularities (cutting tool marks etc.)

Waviness: More widely spaced irregularities (vibration and chatter)

Error of form: Long period or non-cyclic deviations

Stylus Instrument

The pickup comprises the stylus, stylus holding mechanism, measuring

transducer, and any signal conditioning associated with the measuring
transducer (LVDT or moving coil or capacitive or piezoelectric or
interference based optical measurement).

LVDT - linear variable differential transformer

 Stylus Traverse Direction
Lay Direction

Feed marks during turning operation – LAY Pattern

Traversing Length of the Stylus

Generally sampling length (l ) for machined

components = 0.8 mm
Evaluation Length = 0.8 x 5 = 4 mm
Traversing Length = 0.8 x 5 + 0.8 x 2 = 5.6 mm
Estimates for choosing roughness
sampling lengths for the measurement of
surface profiles, ISO 4288: 1996
Cut-off wavelength (sampling length )
 Important Surface parameters
Arithmetic average height ( Ra)
Ra is also known as centre line average (CLA) and it is the most
universally used surface parameter.
Definition:-
This is the arithmetic mean of the absolute ordinate values y(x)
within the sampling length.

Graphical representation of Ra Mathematical representation of Ra

 Root mean square roughness ( Rq)
Rq is also known as RMS and it represents the

standard deviation of the distribution of surface heights. Rq is

more sensitive than Ra to the large deviation from the mean

line. RMS line is the line which divides the profile so that sum

of squares of the deviations of the profile height from it is

equal to zero.
Maximum height of peak, Rp Maximum depth of valleys, Rv
The maximum height of the profile above The maximum depth of the profile below
the mean line with in sampling length. the mean line with in sampling length.

Graphical representation of Rp and Rv

Maximum height of profile (Rz): This is the sum of the height of the
largest profile peak height Zp and the largest profile valley depth Zv
within a sampling length.
Total height of profile (Rt): This is the sum of the height of the largest
profile peak height Zp and the largest profile valley depth Zv within the
evaluation length.

Maximum roughness depth (Rmax): The largest of the successive values

of Rti calculated over the evaluation length.
Bearing Ratio Curve, from ISO 4287: 1997
Representing the material ratio of the
profile as a function of level.