11 Milling Operation

11.1 Basic concept of milling operation

The milling operation is used mainly for production of
plane surfaces or slots of different shapes. The tool
(milling cutter) makes the main rotary motion while
the secondary motion is made by the workpiece which
is mounted on the machine table. The milling
operations can be classified into plane milling and face
milling.

Main motion (n)

Main motion (n)

t

WP Secondary motion (s)

n
t

WP Secondary motion (s)

s
n

Plane milling Face milling

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11.2 Milling machines

Vertical milling machine

Horizontal milling machine

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11.3 Workpiece clamping on milling machines
The workpiece can be clamped on the milling machines using the following devices
1- vises which is mounted on the machine table

2- Dividing head which is used for holding a workpiece using a universal chuck for dividing it
into a number of equally spaced angular divisions (square blocks, gears).

3- Special fixtures for complex shape workpieces and mass production.

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11.4 Cutting forces when plane milling
Main motion (n) Main motion (n)

PZ
PV P
PH
PH P
PZ Y
P
Y

Feed (s) PV
Feed (s) P

The following table shows a comparison between up and down milling.

Up milling Down milling

Feed  Opposing teeth rotation  In the same direction of teeth rotation
Chip thickness  The load on tooth grows gradually  Chip area starts with maximum value
as chip starts with zero value and and then decreases to zero at the end
increases gradually leading to leading to decreasing in the tool life
increasing in tool life (T). (T). It is used for high chip removal.
Vertical force  PV which is in upper direction leads  PV tends to push workpiece against
to lift the workpiece from its clamping to decrease the vibration
clamps leading to vibrations, bad and obtain a good surface roughness.
accuracy and surface roughness.
11.5 Classification of milling cutters
11.5.1 According to method of production
1- Milled cutters Flank Flank
2- Relieved cutters m.c.e.



In relieved tooth, = 0 and  = constant



=0
sharpening is carried on tooth



face only as the flank is a part Face Face

of spiral with  = constant.
Relieved tooth
The relieved tooth is used in Milled tooth

form cutters utilized for gear

production.

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11.5.2 According to method of cutter clamping
1- Shank cutter 2- Shell cutter

Shell cutter
Taper shank

WP

Milling arbor

Shank cutter WP

WP

Clamped on vertical milling machine for face It is used in horizontal milling machine for
milling plane milling

11.5.3 Other types of milling cutters

1- Plain milling cutters

Helical plain milling cutter

The cutting teeth are only on their outer part

(the circumference) or periphery. They are
used on horizontal milling M/c. The cutters
are mounted on a special shaft called arbor.

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 2- Side milling cutters:
They are similar to plain milling cutters; however they also have cutting edges on the sides.
They are used to produce slots and grooves and keyways.

Plain side milling cutters

They have straight teeth on the periphery and
Side milling cutter for machining a keyway
both sides.
3- Saw milling cutters
They are used for ordinary cut off operations and cutting narrow slots.
They are available with different values of width and diameters.

4- Form milling cutters

They are used for cutting curved surfaces or surfaces with irregular shape.

A convex milling cutter

A concave milling cutter

Corner rounding milling cutters

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 5- Angular milling cutters
They are used for machining V-notch, grooves, dovetail

Milling a dovetail on a vertical milling M/c A single angle milling cutter

A double angle milling cutter

6- End milling cutters
They are used for milling slots, shoulders, curved edges, keyways and pockets. They are
mounted on vertical milling machines. The shell end mills have no shank which are mounted
on special arbors.

Slots and steps Keyway Half round slot

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 Pocket milling facing using end mill in 3 passes
7- T-slot cutters
They are used for milling T-slots such as those on milling machine tables. The narrow portion
of the T-slot is machined first using a side milling cutter or an end mill. The wide portion is cut
with the T-slot cutter.

producing the narrow portion

Producing the T-slot

8- Gang (straddle) milling operation
This way of cutter mounting is used to cut two working areas simultaneously at the workpiece to
reduce the machining time. The cutters are positioned using distance rings.

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11.6 Gear manufacturing
Gears can be manufactured by casting, forging, extrusion, drawing, powder metallurgy, and
blanking sheet metal (for making thin gears, such as those used in watches and small clocks). The
standard nomenclature for involutes spur gear is shown in the following figure.

The dimensional accuracy and surface finish required for gear teeth depend on its intended use.
Poor gear-tooth quality contributes to inefficient energy transmission and noise and adversely
affects the gear's frictional and wear characteristics. Submarine gears, for example, have to be of
extremely high quality so as to reduce noise levels, helping the submarine avoid detection.
There are two basic gear manufacturing methods: form cutting and generating.
11.6.1 Form Cutting
In form cutting, the cutting tool is similar to a form-milling cutter
made in the shape of the space between the gear teeth. The gear-tooth
shape is reproduced by cutting the gear blank around its periphery.
The cutter travels axially along the length of the gear tooth at the
appropriate depth to produce the gear tooth profile. After each tooth
is cut, the cutter is withdrawn, the gear blank is rotated (indexed), and
the cutter proceeds to cut another tooth. The process continues until
all teeth are cut.
Each cutter is designed to cut a range of number of teeth. The precision of the form-cut tooth
profile depends on the accuracy of the cutter and on the machine and its stiffness. Although
inefficient, form cutting can be done on milling machines, with the cutter mounted on an arbor and
the gear blank mounted in a dividing head.

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Form cutting is a relatively simple process and can be used for cutting gear teeth with various
profiles; however, it is a slow operation, and some types of machines require skilled labor.
Consequently, it is suitable only for low-quantity production.
Methods of dividing:
1- direct
2- indirect
40
n
z
where z is the number of divisions (no. of teeth in gears)
Example:
The indexing plate of the dividing head has the following number of holes:
15, 16, 17, 18, 19, 20, 21, 23, 27, 31, 33, 37, 39, 41, 43, 47, 49.
Calculate the number of revolutions (n) of the index crank when the spur gear with number of teeth
z = 33 will be produced by milling.
40 40 7
n  1
z 33 33
The index crank will perform 1 revolution and will turn by 7 divisions (holes) on the circle with 33
holes.
11.6.2 Gear Generating
The pinion-shaped cutter can be considered as one of the gears in a conjugate pair and the other as
the gear blank. It is used on machines called gear shapers. The cutter has an axis parallel to that of
the gear blank and rotates slowly with the blank at the same pitch-circle velocity in an axial
reciprocating motion. A train of gears provides the required relative motion between the cutter
shaft and the gear-blank shaft.
Cutting may take place at either the down stroke or the upstroke of the machine.

11.7 Attainable accuracy

IT (Grade of accuracy) = 8 – 12

Surface roughness Ra = 1.6 – 12.5 µm

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