Unit-1

S.No Question
1 How does the structure of NC/CNC machine tools differ from conventional machine tools?

1. Operation method
The CNC system consists of software and hardware. The operator uses the software to monitor the
machine tool and create programs. The NC program is a list of instructions that instructs the machine
how to handle the parts. The NC hardware reads the program and tells the machine how to operate.

Traditional machining means that the operator directly controls all movements of the machine tool.
The operator can choose when to move the equipment. He also decided to stop the feed, when and
where to stop. Operators use special rulers and gauges embedded in the control rod to improve
accuracy.

2. Cost and output

The conventional machining cost is low, and it is usually used for small batch projects. CNC
machining is used for mass production and is not cost-effective for smaller projects.

3. Replication
CNC – After correct programming, the CNC machine can produce the same final result as many times
as needed. The high production quota greatly benefits from CNC functions.

Due to human factors, traditional machining (very suitable for making a single customized part), it is
impossible to make an accurate copy using traditional machining.

4. Labor Required
CNC Machining- With proper training, CNC operators can supervise multiple machines at once. After
the machines are programmed, they can do the work on their own-almost without human intervention.

Conventional Machining – Mastering conventional processing technology requires a wealth of

knowledge, training and talent-with different skills (usually multiple people) required to complete
different parts of the process. In manual lathes, each machine must have a skilled technician.

5. Accuracy
CNC Processing- Since production is software-based, CNC machines can manufacture parts that are
almost impossible to produce by hand or traditional machining methods. Eliminating the factor of
human error means that it can be cut precisely every time, greatly reducing the chance of error,
reducing product waste, and ensuring quality!

Conventional Processing-The final result of conventional processing depends entirely on the skill
level and experience of the technicians.

6. Processing Technology
In ordinary machining process, whether it is positioning datum, clamping method, tool, cutting
method, etc. can be simplified, but the data processing process is more complicated, and these factors
need to be fully considered. The same processing for tasks, the CNC machining process can have
multiple schemes, which can arrange multiple machining parts and machining tools as the main line.
The process has diversified characteristics. This is the difference between the CNC machining process
and the traditional machining process.

2 Classify CNC machines tools on the basis of : (i) Types of motion control (ii) According to
programming Method.(iii) According to types of controllers
CNC Machining Types in Terms of CNC Motion Types
(1) Point-To-Point CNC Machining (Rapid Motion) - The numerical control device can
only control the CNC machine to accurately move the component from one position to
another, that is, only control the axis value of the end point of the stroke, and do not perform
any cutting process during the movement. The speed and route of movement between the two
related points depends on productivity. In order to have the highest possible productivity on
the basis of accurate positioning, the movement between the two related points is first to move
quickly to a new axis, and then to a speed of 1-3 level, so that it slowly approaches the
positioning point, guaranteeing its positioning accuracy. It is used to minimize non-productive
time during the machining cycle. Common uses for rapid motion include positioning the tool
to and from cutting positions, moving to clear clamps and other obstructions, and in general,
any non-cutting motion during the program.
(2) Linear CNC machining (Straight Line Motion) - When machining parts, it is necessary
to control not only the position between the two related points but also the moving speed and
route between the two related points. The route is generally composed of straight segments
parallel to each axis. The difference between it and the point-controlled CNC machine is that
when the tools of CNC Machine move, it can cut a part in the direction of one coordinate axis
(usually cutting along a 45° oblique line, but not along a straight line of any slope). And its
auxiliary functions are more than the Rapid Motion, for example, to increase the spindle speed
control, cycle feed machining, tool selection and other functions. Straight line motion can be
used any time a straight cutting movement is required, including when drilling, turning a
straight diameter, face or taper, and when milling straight surfaces.
(3) Contour CNC Machining (Circular Motion) - This type of CNC lathe machining motion
can continuously control two or more axes at the same time. The machining not only controls
the starting point and the end point, but also controls the speed and position of each point in
the whole machining process, so that the CNC machining can process complex shaped turned
parts that meet the requirements of the drawings. Its auxiliary functions are also relatively
complete.
There are three basic CNC machine programming methods — manual, conversational, and CAM
system programming. With manual programming, the operator inputs code manually, which can be
time-consuming and somewhat tedious. It also requires intimate knowledge of the programming
language. However, manual programming is a valuable foundational skill set to have on hand, and it’s
an effective way to make quick modifications to CAM outputs and optimize code.
CNC Machining Types in Terms of CNC Servo Control Systems
(1) CNC Open loop Control System - In open loop control, there is no detection feedback
device for CNC machining. The flow of the signal from the numerical control device is one-
way, so there is no system stability problem. It is also because of the one-way flow of the
signal that it does not test the actual position of the moving parts of the CNC machine, so the
machining accuracy of the CNC lathe is not high, and its accuracy mainly depends on the
performance of the servo system. The working process is as follows: The input data is
distributed by the numerical control device to distribute the command pulse, and the
controlled worktable is moved by the servo mechanism (the servo element is often a stepping
motor).
(2) CNC Closed loop Control System - Since the accuracy of open loop control can not meet
the requirements of precision CNC lathes and large CNC machines, it is necessary to detect its
actual working position. To this end, a detection feedback device is added to the open-loop
control CNC machining to detect the movement of the CNC lathe during processing. The
position of the component is matched to the position required by the numerical control device
in order to achieve high machining precision.
(3) CNC System-Hybrid Servo Control System - The characteristics of the open-loop
control CNC lathe processing and the closed-loop control CNC lathe processing are
selectively concentrated, and a hybrid control scheme can be formed. Large CNC lathe
machining requires high feed rates and return speeds, and requires considerable precision. If
only full-closed control is used, the CNC lathe drive chain and the workbench are all placed in
 the control link. The factors are very complicated, although the installation and
commissioning are often difficult. In order to avoid these contradictions, a hybrid control
method can be adopted.

3 State advantages and limitations of CNC machine tools.

ADVANTAGES OF CNC MACHINING

This process provides a number of benefits to users and their customers:

Variety of Materials

CNC machining can be used to manufacture products using a diversity of materials.

These include aluminum, bronze, gold, silver, and steel. Polymers including ABS,
PTFE, and polycarbonate can be utilized with this machine too.

Accurate Component Machining

Computer Aided Design (CAD) is the software used to produce the final component’s
3D designs. When this is fed into the computer, the machine carves out the product
with the exact specifications. Even complex shapes can be created in little time.

Repeatability

When a design is fed into the machine’s computer, the machine can repeat the
manufacturing process several times. Plus, you will always get a high level of intricacy
and precision in the component.

LIMITATIONS

• COST
CNC machines cost more than manually operated machines. The initial investment for
these machines is considered too high. Purchasing parts, such as tooling and machines,
requires extensive justification.
They also require high maintenance costs and can be expensive to repair. CNC
machinery does not eliminate the need for expensive tools.
• ERRORS
CNC machines are not completely error-free. Operators can still push the wrong
buttons, make improper alignments and wrongly locate parts on a jig.

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4 Explain clearly the difference between NC and CNC machine

S.no NC Machine CNC Machine

CNC stands for Computer Numerical

1. Here NC stands for Numerical Control
Control.
 It is defined as the machine which is It is defined as the machine which is used to
controlled by the set of instructions in control the motions of the workpiece and tool
2. the form of numbers, letters and with the help of prepared program in
symbols. The set of instructions is called computer. The program is written in
as program. alphanumeric data.

In CNC machines the programs are fed

In NC machine the programs are fed into
3. directly into the computer by a small
the punch cards.
keyboard similar to our traditional keyboard.

4. Modification in the program is difficult. Modification in the program is very easy.

5. A high skilled operator is required. A less skilled operator is required.

6. The cost of the machine is less. The cost of the CNC machine is high.

7. Maintenance cost is less The maintenance cost is high.

In CNC machines, the programs can be

The programs in the NC machine cannot
8. stored in the computer and can be used again
be stored.
and again.

It offers less flexibility and It offers additional flexibility and

9.
computational capability. computational capability.

The accuracy is less as compared with

10. It has high accuracy.
the CNC.

It requires more time for the execution It takes very less time in the execution of the
11.
of the job. job.

It can be run continuously for 24 hours of a

12. It is not possible to run it continuously.
day.

5 Explain the feedback system of NC/CNC machine.

FEEDBACK DEVICES IN CNC MACHINE

Introduction:

Feedback devices are that device which sends the information back to the controller in close loop
system.

Description:
Encoder is a device that used to convert linear or rotational position information in to electrical
output signal.

Rotary optical encoder

1. Outer track is used to determine position.

2. Middle track is used to measure direction of rotation.

3. Inner track is used to indicate a complete revolution.

4. Integrated counter circuits are used to count leading or falling edges of pulses.

Incremental Encoder

1. Produces equally spaced pulses from one or more concentric tracks on the code disk.

2. Position is determined by counting pulses Incremental Encoder must be calibrated.

3. Power loss will lead to loss of position information unless backup power source is used.
 Absolute Encoder

1. It produces a binary number that uniquely identifies each position on the code disk.

2. Absolute encoders do not have to be recalibrated after a power loss.

3. Absolute encoders are more expensive than incremental encoders

Resolver:

A resolver is a rotary transformer that produces an output signal that is a function of the rotor position.

Unit-2

S.No Question
1 Explain linear and circular interpolations used in turning. Draw a neat sketch of circular
interpolation
INTERPOLATION

Interpolation, which is necessary for any type of programming, consists of generating data
points between given coordinate axis positions. Within the Machine Control Unit (MCU), a
device called an interpolator causes the drives to move simultaneously from the start to the
end of the command. The interpolator is either an electronic hardware device for a NC
system, or a software program for a CNC system. An interpolator provides two functions:

It calculates individual axis velocities to drive the tool along the programmed path at the
given feed rate.
It generates thousands of intermediate coordinate points along the programmed path
between the start point and the end point of the cut.

During positioning, all programmed axes move simultaneously at the specified feed rates
until each axis has reached its destination. All drives start together, but without an
interpolator individual destinations are reached successively according to the path traveled.
However, an interpolator coordinates these axis motions in such a way that the programmed
path is constantly maintained from the beginning to the end of the movement.

Linear and circular interpolation are most commonly used in CNC programming
applications:
Linear interpolation is used for straight-line machining between two points.
Circular interpolation is used for circles and arcs.
Helical interpolation, used for threads and helical forms, is available on many CNC
machines.
Parabolic and cubic interpolation are used by industries that manufacture parts having
complex shape such as aerospace parts.
 

2 What are canned cycles? Discuss how a canned cycle is useful in writing a part program?
Canned cycle

It is a process that can be executed with one command instead of programming a series of
individual moves.

• When you machine a hole, you perform a group of well defined motion.

• Canned cycle performs these repetitive motion with a simple set of codes.

Canned or fixed cycles are programming aids that simplify programming.

• Canned cycles combine many programming operations and are designed to shorten the
program length, minimize mathematical calculations, and use minimal tool motions.

• Examples: drilling, peck drilling, tapping, boring, back spot

Canned cycle :

The advantages of writing a part program with these structures are:

• Reduced lengths of part program.

• Less time required developing the program.

• Easy to locate the fault in the part program.

• No need to write the same instructions again and

again in the program.

• Less memory required in the control unit.

Canned cycle :

• G74 Left hand tapping cycle

 • G76 Fine boring with no drag line

• G80 Cancel canned cycle

• G81 Drilling cycle

• G82 Counter boring cycle

• G83 Deep hole Peck drilling cycle

• G84 Right hand tapping cycle

• G85 Reaming cycle

• G86 Boring cycle

• G87 Back boring cycle

• G88 Boring cycle

• G89 Boring cycle with dwell

3 Explain cutter radius compensation and tool length compensation.

What is Cutter Compensation?

Cutter compensation (cutter comp), or Cutter Diameter Compensation (CDC), allows

programmed path adjustments on the CNC machine to impart greater precision and
compensate for the machine, tool, or material characteristics.

By shifting the centerline from the tool shaft to the cutter's edge, the program can use
geometry to determine the offset direction instead of the tool's center point on the shaft.

Cutter compensation also acts as an additional guard for precision. Many CAD programs will
automatically calculate the toolpaths for complex parts. While this is done accurately with
CAD software, it doesn’t allow the operator to use a different tool to save time in tool
changes. It also may not allow for compensation of tool wear to ensure the part is cut
accurately, even with a slightly worn tool.

Cutter compensation uses cutter comp G code to define the actual offset to be performed.
This may be cutter diameter compensation or cutter radius compensation, depending on the
cut and the tool size. The centerline is always set based on the tool's radius - any less, and the
piece would be undercut.

But in complex part geometries or contours, the tool shape and programming may not cut
with the required precision. This is where cutter compensation comes in. If the cut is equal to
or less than the tool's radius, the tool radius is used as the offset. If the size of the tool and the
cut required is greater than the radius, the tool diameter offset can utilize the center line to
determine the best path.

G-Code G43, G44 and G49 (TOOL LENGTH COMPENSATION)

In an CNC Programming Tool length compensation Code is used to adjust for differences in
length between different tools, without worrying about those differences in your part
program.
This standard length is the Reference Tool. In general, you load the Reference Tool, jog the
Z axis down until that tool touches some surface, and set the Z Reference position there. The
control memorizes this position of its Z axis. You then load each other tool, bring that tool
down until it touches the same surface, and tell the control to measure the tool. The control
 compares the Z axis position with this tool touching the surface to the previously stored Z
Reference position. The difference in Z axis positions is stored as the length offset for the
tool.

Clearly, to touch the same surface with a shorter tool, you have to move the Z axis down
further. This results in a negative offset. The shorter the tool, the more the negative offset.
To touch the same surface with a longer tool, you don't have to move the Z axis down as far.
This results in a positive offset. The longer the tool, the larger (or less negative) the offset.

4 Write a CNC program using appropriate G and M code to Center punch using the drilling
canned cycle with dwell

REFER FAGOR AUTOMATION MANUAL

5 Write a CNC program using appropriate G and M code for the following figure

REFER FAGOR AUTOMATION MANUAL

Unit-3

S.No Question
1 Describe the principle of working of an automatic pallet changer and automatic tool changer
Auto Tool Changer (ATC ) & Automatic Pallet Changer (APC) • ATC ( Auto Tool
Changer ) is a device which can automatically change the tool from the tool magazine to the
machine spindle as per the CNC programme. • Tool Magazine is a device which holds
number of tools and can automatically index to enable ATC to pick the right tool and to
replace the used tool.
• Automatic Pallet Changer (APC) is a device which can automatically change the pallet
to/from machine to pallet stand. • By this Mechanism ( i .e. APC ) the pallet with the finished
component and the pallet with a raw component could be exchanged automatically.

Automatic Pallet Changer (APC) Pallet is a transferable work table having `T’ slots or
tapped holes for component/fixture clamping. Used to avoid the machine waiting time during
loading & unloading of component. Pallet is held on the machine table by locating pins and
clamping mechanism to ensure repeatability and accuracy.
2 Explain in detail about various components in CNC machine tool
Components of a CNC machining system
A Computer Numerical Control (CNC) system offers advanced pro-combinational skill and
technology that is controlled and managed through a coded alpha-numeric system. At its
base, a CNC system consists of five components with functionality that is blended into a
synchronous system. Computer programmed data is then used to control and run the
operations of a CNC system. Every specialist of CNC machine parts in Toronto understands
the components and their essence in making the machining process a success.
Input devices
With a CNC system being controlled and managed through a specialty system, it is necessary
to have an effective way of coordinating the pre-programmed data with the designed system.
These input devices include flash disks, compact discs, and other modern data transfer
components.
The Machining Component
This is a high accuracy system that allows for the machining of the end products. It also
encapsulates the cutting spindles and the systems controlling the machining parts. As a
critical component of the CNC machine parts in Toronto, the processing component responds
to the pre-programmed (coded) computer aided control logic.
Machine Control Unit
At the heart of the CNC system is a high value logic control panel consisting of the data
processing unit (DPU) and a control loop unit (CLU). The DPU component allows for
effective reception and decoding of the programs. Once decoded into machine language, the
interpolator resting on the DPU calculates the exact positions of the cutting spindles. Once
interpreted, the decoded data is relayed to the CLU as electrical signals. The machining parts
of the device then initiates a motion in respect of the relayed signals.
Driving system
 Another critical component of a CNC machine is the driving system that responds to the pre-
programmed instructions by allowing for the cutting of the end products into shape and
design. In its most simple form, the driving system consists of several control motors whose
rotation determines the movement of the cutting spindle.
Feedback device
For a CNC system to operate effectively, there is need to relay feedback about the various
processes during and after every process is completed. The feedback devices include linear
transducers and direct position scales and encoders.

3 What is high speed machining and what are the various features of high speed machining.

High-speed machining, specifically milling, has the same variables as traditional milling.
There are speeds and feeds to set and a depth of cut to be determined. However, in a high-
speed machining operation, slow, heavy cuts are replaced by fast, lighter cuts.
While it may seem counterproductive to take lighter cuts when heavy cuts are possible, shops
that can make this switch in thinking will produce accurate parts faster.

Defining high-speed machining is difficult because it can be one of many operations, or a

combination of them. It can be defined as:

 Machining at a high cutting speed (vc).

 Machining with a high spindle speed (n).
 Machining with a high feed rate (vf).
 Machining with a high removal rate (Q).

High speed machining is usually associated with any spindle speed above 15k rpm, but it is
much more than just a faster spindle. The whole machine must be considered when selecting
a good candidate machine for HSM. Thermal compensation, overall machine rigidity and
construction, positional feedback, the motion control system, tool retention, and many other
characteristics must be looked at, in detail, before determining that a particular machine is
suitable for the task.
Conventional machining differs greatly from High Speed Milling in a lot of aspects. When
using conventional machining techniques, the contact time between the tool and piece is
much greater than it is with HSM. Conventional machining also involves a much greater
cutting force.
Conventional Machining will typically lead to a less accurate workpiece and inferior
surface finish than could be achieved through high-speed machining. Another major
difference associated with high-speed machining is the material removal rate is much higher.
4 1) explain in detail about the drives used in a CNC machine.
2)
3) Drive System
4) • Drives are used to provide controlled motion to CNC elements
5) • A drive system consists of amplifier circuits, drive motors, and ball lead-screws.
6) • The MCU feeds the control signals (position and speed) of each axis to the amplifier
circuits.

There are several types of motors that can be used in a CNC (Computer Numerical Control)
machine, depending on the specific requirements of the machine and the application it is
being used for. Some common types of motors used in CNC machines include:

1. Stepper motors: These are motors that move in small, discrete steps, and are often
used in CNC machines because they can be precisely controlled and can hold their
position accurately.

2. Servo motors: These are motors that use feedback to accurately control their speed
and position. They are often used in CNC machines because of their high precision
and speed.

3. Brushless DC motors: These are motors that do not use brushes to transfer
electrical current to the rotor, which makes them more reliable and efficient than
other types of motors. They are often used in CNC machines because of their high
precision and long lifespan.

4. AC induction motors: These are motors that use an alternating current to generate
a magnetic field in the rotor, which causes the rotor to rotate. They are often used
in CNC machines because of their reliability and simplicity.

5. Hybrid stepper motors: These are motors that combine the features of stepper
motors and servo motors, and are often used in CNC machines because of their
high precision and speed.
7)
8)
5 9) State various automated programmed tool languages.

APT programming language is a language for defining tool paths on CNC machines. APT is
an acronym for Automatically Programmed Tool. It was the predecessor to today’s CAM
software, created in the late 1950s to define complex tool paths in the aerospace industry in
the US. It was developed in MIT labs, the same place where the first NC machine was
invented.

APT was created before graphical user interfaces were available, and so it is based on text to
specify the geometry and toolpaths needed to machine a part. It has English-like commands:
GOLFT for Go Left, TANTO for Tangent to, etc.

You first write a program in APT, software converts it into a series of tool coordinates called
Cutter Location Data (CLD). Another piece of software, a translator called a Postprocessor,
then converts the CLD into an NC program (the G and M code program) that is understood
by the machine. You feed the NC program into the machine and cut the part.
APT programming language example for milling around a part

$$ PART SHAPE DEFINITION

STRT = POINT/ -50,-20,100

P1 = POINT/0, 0, 0 P2 = POINT/60, 0, 0
P2 = POINT/30, 10, 0
L1 = LINE/P1,P2
L2 = LINE/P2,ATANGL,135
C1 = CIRCLE/CENTER, P2, RADIUS, 60
PL1 = PLANE/P1, P2, P3 PL2 = PLANE/PARLEL, PL1, ZSMALL, 5
PL3 = PLANE/PARLEL, PL1, ZLARGE, 5
$$ MOTION COMMANDS
SPINDL / 3000, CW
FEDRAT / 100, 0
COOLNT/ON
FROM /STRT
RAPID, GOTO/ L1, TO, PL3, PAST, C1
GOTO/ PL2
TLRGT, GOFWD / L1, PAST, L2
GOLFT / L2, PAST, C1
GOFWD / C1, PAST, L1
GOTO/PL3
COOLNT/OFF
GOTO / STRT
FIN