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Manufacturing Technology: Metal Cutting and Machine Tools - P.N.Rao

Ch. 7/1

MILLING

The Milling Process is characterised by

Interrupted cutting:Each of the cutting edges removesmaterial for only part of the rotation of the milling cutter. Asa result, the cutting edge has time to cool before itremoves material again. Thus the milling operation ismuch more cooler compared to turning.

Small size of chips: though the size of the chips is small, in

view of the multiple cutting edges in contact a largeamount of material is removed and as a result thecomponent is generally completed in a single pass only,and

Variation in chip thickness: This contributes to the non-steady state cyclic conditions of varying cutting forcesduring the contact of the cutting edge with the chipthickness varying from zero to maximum size or vice

versa.

The varieties of milling machines available are:

a) Knee and Column typeHorizontalVerticalUniversal

Turret type

Machinedsurface

Feed

Workpiece

Chip

Peripheral milling

Cutter axis of rotation


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b) Production (Bed) typeSimplexDuplexTriplex

c) Plano millersd) Special type

Rotary tableCopy milling (Die sinking machines)Key way milling machinesSpline shaft milling machines

Horizontal Knee and column type milling machine

Spindle

Head

Movement ofthe head

Column

Spindle nose

T-slot for clamping

Saddle

Accurate screwfor moving table

Knee


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Some of the Milling Operations normally carried out on vertical axis

machines

Simplex bedtype milling machine

Ways

Base

Column

Spindle

Workpiece

Table

(a) Top view

Workpiece

Face milling cutter

Quill

Column

Head

(b) Side view


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Duplex bedtype milling machine

Milling cutters are classified into various typesBased on construction:

SolidInserted tooth type

Based on mounting:Arbor mountedShank mountedNose mounted

Based on rotationRight hand rotation (Counter clockwise)Left hand rotation (Clockwise)

Based on helixRight hand helixLeft hand helix

Ways

Base

Column

Spindle

Workpiece

Table

(a) Top view

Workpiece

Face milling cutter

Column

Head

Quill

(b) Side view


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(a) Slab milling cutter (b) Slab milling cutter (c) Slitting sae

(d) Side and face cutter (f) Staggered tooth cutter (g) Side and face cutter (e) Two side and face cutter

(a) Angle milling cutter

(d) Form relieved circular cutter

(b) Angle milling cutter

(e) Form relieved circular cutter

(c) Shell end mill

(f) Wood ruff key cutter


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(a) Four flute end mill

(b) Two flute end mill

(c) Multi flute end mill

(d) Ball end mill

(e) End mill uses for making a slot

(f) End mill

(g) Slot drilll

Length overall

Length of cut

Dia of cutDia of shank

Shak dia

Shankdia

Dia

Dia

Radius

Two flute

End Mill

Section A-A

A

A


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Hand of cut:This refers to the direction in which the cutter is rotated. Whenviewed towards the spindle, when the cutter is moving counter clockwise it iscalled right hand rotation while the opposite is called the left hand rotation.

Hand of helix:In case of helical milling cutters, when viewed from the endif the flutes move in a clockwise direction it is called the right hand helixwhile the opposite is called the left hand helix. The axial cutting forcedirection depends upon the hand of the helix. If two milling cutters ofdifferent helices are arranged side by side in a gang milling operation, thenet axial force can be reduced to zero depending upon the cut taken byeach of the milling cutters.

(a) T-Slot Cutter (b) Dove Tail Cutter

(a) Peripheral miling (b) Face milling

Axialdimension

w

w

d

d

Radialdimension


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Straight teeth

Helix angle

Radial roke angle

Radial rake angle

Clearance angle

Clearance angle

Helical teeth

Force

Force

Force variation

Force variation

Time

Time

Helix angle = 0

Helix angle 0

(a)

(b)

Trailing edgeLeading edge


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Up and Down milling

Based on the directions of movement of the milling cutterand the feeding direction of the workpiece,

Up milling (conventional milling)

Down milling (Climb milling)

Advantages of Climb milling:1 Suited to machine thin and hard-to-hold parts since the workpiece is

forced against the table or holding device by the cutter.2 Work need not be clamped as tightly.3 Consistent parallelism and size may be maintained, particularly on

thin parts.4 It may be used where breakout at the edge of the workpiece could

not be tolerated.5 It requires upto 20% less power to cut by this method.6 It may be used when cutting off stock or when milling deep, thin

slots.

Disadvantages:

1 It cannot be used unless the machine has a backlash eliminator andthe table jibs have been tightened.

2 It cannot be used for machining castings or hot rolled steel, since thehard outer scale will damage the cutter.

Direction of rotation

Workpiece

Depth of cut

Feed Feed

(a) Up Milling (b) Down Milling

d dh h

h1 h1


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Thin workpiece

Parallel strips(a) (b)

Work

T-bolt Clamp

Step blockY-block

Milltable

Finger clamp(double end) Finger clamp

(single end) U-clamp

Plain slotted clampGooseneck clamp

(c)

Correct

Correct

Correct

Incorrect

Incorrect

Incorrect

Block

Block

Block

Block

Work

Work

Work

Work


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Gang milling

Typical process sequence in milling

Side and face milling cutter

Slab mill

Finishedpart

Slab millSlot mill

Shell endmill

Endmill

Anglemill

1 2

3

4

5


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Dividing head construction

Indexing method of the Divid ing head

Worm wheel

Spindle

WormBevel gear Sector

Index plate

Clamp nut

Index pin

Index clamp

Hole in index plate

Sector armIndex pin

Index crank

Sector armIndex plate

Worm shaft Worm singlethread

Worm wheel40 teeth

Index head spindle


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Index plate no. 1 of Brown and SharpeDividing head

The index plates available with the Brown and Sharpe milling machinesare

Plate no. 1: 15, 16, 17, 18, 19, 20 holesPlate no. 2: 21, 23, 27, 29, 31, 33 holesPlate no. 3: 37, 39, 41, 43, 47, 49 holes

The index plate used on Cincinnati and Parkinson dividing heads is

Plate 1: Side 1 24, 25, 28, 30, 34, 37, 38, 39, 41, 42 and 43 holesSide 2 46, 47, 49, 51, 53, 57, 58, 59, 62 and 66 holes

It is also possible to get additional plates from Cincinnati to increasethe indexing capability as follows:

Plate 2:Side 1 34, 46, 79, 93, 109, 123, 139, 153, 167, 181, 197 holes Side 2 32, 44, 77, 89, 107, 121, 137, 151, 163, 179, 193 holes

Plate 3:Side 1 26, 42, 73, 87, 103, 119, 133, 149, 161, 175, 191 holes Side 2 28, 38, 71, 83, 101, 113, 131, 143, 159, 173, 187 holes


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Compound indexing using the Index plate no. 1 of Brown and SharpeDividing head with 5 holes in 20-hole circle minus 1 hole in 15-hole circle

Differential Indexing

The change gear set available is

24, 24, 28, 32, 40, 44, 48, 56, 64, 72, 86 and 100

Workpiece

Dog

Change gear mounted at

back end of the spindle

Spindle

Change gears,compound gearing

Idlergear

Millingmachinetable

Change gear

Shaft which rotatesthe index plate

Milling machine tablefeed screw

Center

Lock for the index plate in an unlockedposition. The index plate is rotatedby gearing from the spindle


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A few points to be remembered during the differential indexing is

(a) Use the hole circles for indexing which will easily factorise with theavailable gear set. For example, in the case of Brown & Sharpe, 18,

20, 21 and 27 hole circles should be used.(b) The difference from the actual to the approximate indexing should

be a small value such that the change gear set can accommodatethis ratio. For example a total difference of 0.5 to 1.5 will be mostconvenient.

(c) The idler gear has to be provided when the index plate has to movein the opposite direction to that of the crank movement.

7.8 Machining Time Estimation

1000

ND=V

Where, V = cutting speed (surface), m/minD = diameter of the milling cutter, mmN = rotational speed of the milling cutter, rpm

Approach distance, A =

22

22

d

DD= )( dDd

Where D = diameter of the slab milling cutter

Feed, f

A

N

d

D


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d = depth of cut

Time for one pass =NZf

Al + 2minutes

Where Z = number of teeth in the milling cutterf = feed per tooth, mm

Face milling operation

Approach distance for the face milling case is given as

A =2

Dfor W =

2

Dupto D

A = )( WDW for W

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