5 Milling and Related Operations

IPE 331: Production Processes Lecture sheet-5

MILLING & RELATED OPERATIONS

Introduction

Milling machine is one of the most versatile conventional machine tools with a wide range of metal cutting capability. Milling is capable of producing simple two dimensional flat shapes to complex three dimensional surface configurations. The following figure shows some milling products.

Fig. Typical parts and shapes produced by Various Milling processes

The Process:

Typically uses a multi-tooth cutter

Work is fed into the rotating cutter

Capable of high MRR

Well suited for mass production applications

Cutting tools for this process are called milling cutters


Milling Machines

Milling machines are among the most versatile and useful machine tools due to their capabilities to perform a variety of operations. They can be broadly classified into the following types:

Column and Knee type milling machines: Used for general purpose milling operations, column and knee type milling machines are the most common milling machines. The spindle to which the milling cutter is may be

Horizontal Milling Machine

Vertical Milling Machine.

Horizontal Milling Machine

Column: The column houses the spindle, the bearings, the gearbox, the clutches, the shafts, the pumps, and the shifting mechanisms for transmitting power from the electric motor to the spindle at a selected speed.

Knee: The knee mounted in front of the column is for supporting the table and to provide an up or down motion along the Z axis.

Saddle: The saddle consists of two slideways, one on the top and one at the bottom located at 90 to each other, for providing motions in the X or Y axes by means of lead screws.

Table: The table is mounted on top of the saddle and can be moved along the X axis. On top of the table are some T-slots for the mounting of workpiece or clamping fixtures.

Arbor: The arbor is an extension of the spindle for mounting cutters. Usually, the thread end of an arbor is of left hand helix.


Vertical Milling Machine

Column: The column houses the spindle, the bearings, the gearbox, the clutches, the shafts, the pumps, and the shifting mechanisms for transmitting power from the electric motor to the spindle at a selected speed.

Knee: The knee mounted in front of the column is for supporting the table and to provide an up or down motion along the Z axis.

Saddle: The saddle consists of two slideways, one on the top and one at the bottom located at 90 to each other, for providing motions in the X or Y axes by means of lead screws.

Table: The table is mounted on top of the saddle and can be moved along the X axis. On top of the table are some T-slots for the mounting of workpiece or clamping fixtures.

Milling head: The milling head consisting the spindle, the motor, and the feed control unit is mounted on a swivel base such that it can be set at any angle to the table.

Ram: The ram on which the milling head is attached can be positioned forward and backward along the slideways on the top of the column.

Milling Methods

Up Milling: In up cut milling, the cutter rotates in a direction opposite to the table feed as illustrated in the following Figure. It is conventionally used in most milling operations because the backlash between the lead screw and the nut of the machine table can be eliminated.

Down Milling: In down cut milling, the cutter rotates in the same direction as the table feed as illustrated in the following Figure. This method is also known as Climb Milling and can only be used on machines equipped with a backlash eliminator or on


a CNC milling machine. This method, when properly treated, will require less power in feeding the table and give a better surface finish on the workpiece.

Milling Operations

Milling operations are classified into two major categories

Peripheral Milling

Generally in a plane parallel to the axis of the cutter

Cross section of the milled surface corresponds to the contour of the cutter

Face Milling

Generally at right angles to the axis of rotation of the cutter

Milled surface is flat and has no relationship to the contour of the cutter

Combined cutting action of the side and face of the milling cutter

(a)

(b)

Fig. (a) Peripheral milling (b) Face milling

Peripheral Milling

Slab milling: A milling operation that uses a cylindrical mill on an axis parallel to the worktable to create a flat surface.

Slotting: A milling operation that cuts a narrow ridge into the surface of a workpiece.


Slitting: A rotary tool-steel cutting tool with peripheral teeth, used in a milling machine to remove material from the workpiece through the relative motion of workpiece and cutter.

(a)

(b)

(c)

Fig. (a) Slab milling (b) Slotting (c) Slitting

Side milling: Milling with a side-milling cutter to machine one vertical surface.

Form milling: A milling operation that uses a mill with a unique shape to create that shape into the workpiece.

Straddle milling: Face milling of two parallel vertical surfaces of a workpiece simultaneously by using two side-milling cutters.

(a)

(b)

(c)

Fig. (a) Side milling (b) Form milling (c) Straddle milling

Face Milling

Conventional milling: The diameter of the cutter is greater than the workpart width, so that the cutter overhangs the work on both sides.

Partial face milling: The cutter overhangs the work on only one side.

End milling: The cutter diameter is less than the work width, so a slot is cut into the part.

Surface contouring: A ball-nose cutter (rather than square end cutter) is fed back and forth across the work along a curvilinear path at close intervals to create a three-dimensional surface form.


Fig. Various types of face milling

Pocket milling: Another form of end milling, this is used to mill shallow pockets into flat parts.

Profile milling: This is form of end milling in which the outside periphery of a flat part is cut.

Cutting Tools-Milling cutter


Cutting Tools Milling

Milling Cutter Types

Fig. Various types of milling cutters

Cutting Tools for Horizontal Milling

Slab Mills: For heavy cutting of large and flat surfaces.

Side and Face Cutters: This type of cutters has cutting edges on the periphery and sides of the teeth for cutting shoulders and slots.

Slitting Saws: For cutting deep slots or for parting off


(a)

(b)

(c)

Fig. (a) Slab mill (b) Side and Face cutter (c) Slitting saw

Note: Horizontal milling cutters are specified by the name, the material, the width, the diameter, and the hub size of the cutter. For example: Side and face cutter, High Speed Steel, Cutter size : 10 X 100, Hub size: 25

Cutting Tools for Vertical Milling

Rough Cut End Mills: For rapid metal removal.

End Mills: Commonly used for facing, slotting and profile milling.

Face Milling Cutters: For heavy cutting.

Slot Drills: For producing pockets without drilling a hole beforehand.

(a)

(b)

(c)

(d)

Fig. (a) Rough cut end mill (b) End mill (c) Face milling cutter (d) Slot drill

Note: Most vertical milling cutters are of end mill types and are specified by the material, the diameter, the length, the helical angle, the types of shank and the shank diameter. For face milling cutter, only the diameter of the cutter and the types of carbide inserts are required.

Milling Process

Milling is a metal removal process by means of using a rotating cutter having one or more cutting teeth.

Cutting action is carried out by feeding the workpiece against the rotating cutter. Thus, the spindle speed, the table feed, the depth of cut, and the rotating direction of the cutter become the main parameters of the process. Good results can only be achieved with well balanced settings of these parameters.


Spindle speed, N = 1000V

D

Where N = R.P.M. of the cutter V = Linear cutting speed of the material in m/min. (as shown in Table-1 ) D = Diameter of cutter in mm Table Feed rate, f = ft . N . n Material Removal Rate (MRR) = w.d.f Where, f = Table feed rate in mm/min ft = feed per tooth of cutter in mm or chip load in mm/tooth (as shown in Table-1 ) n = No. of teeth of cutter and N = R.P.M. of the cutter Fig. 2.38 Milling process w = Width of cut d = Depth of cut

Table-1: Cutting speed and Feed rate for some common material Tool Material High Speed Steel Carbide

Material Cutting speed

(v) Feed (f)

Cutting speed (v)

Feed (f)

Mild Steel 25 0.08 100 0.15 Aluminium 100 0.15 500 0.3

Hardened Steel --- --- 50 0.1

Time Calculation

dDd2

d2

D2

2

DA

milling HorizontalFor

length.approach isA Here,

rate feed Table

2ALengthmachiningfor Time

cut of width theis W where,

2

Dfor W WDWA

D upto

2

Dfor W

2

DA

milling alFor vertic


Example problems

Q 1. A C 50 steel flat surface of 150*100 mm is to be produced on a horizontal milling machine. An HSS slab mill of 100mm dia and 150 mm width is to be used for the purpose. The milling cutter has 8 teeth. Calculate the machining time assuming that the entire stock can be removed in one depth of 2 mm. here, cutting speed is 20 m/min and feed is 0.13 mm/tooth.

Solution:

Q 2. A surface 115 mm wide and 250 mm long is to be rough milled with a depth of cut of 6 mm by a 16 tooth cemented carbide face mill 150 mm in diameter. The work material is alloy steel (200 BHN) . Estimate the machining time. Here cutting speed is 60 m/min and feed is 0.18 mm/tooth.

Solution:

min633.26580.13

142501

rate feed Table


65rev/min63.66100

201000N speed, Spindle

1420012A

milling HorizontalFor

min 1.06512160.18

752250

rate feed Table


rev/min 125127.32150

601000N speed, Spindle

]2

D W[ mm 75

2

150A distance,Approach

milling alFor vertic


Dividing (Indexing) Head

Indexing is the operation of dividing the periphery of a workpiece into any number of equal parts. For example if we want to make a hexagonal bolt. Head of the bolt is given hexagonal shape. We do indexing to divide circular workpiece into six equal parts and then all the six parts are milled to an identical flat surface. If we want to cut n number of teeth in a gear blank, the circumference of gear blank is divided into n number of equal parts and teeth are made by milling operation one by one. The main component used in indexing operation is universal dividing (indexing) head, one of the more important attachments for milling machine. It is also used to rotate workpiece at predetermined ratio to table feed rate which is needed for making helical grooves.

Methods of indexing:

There are different indexing methods in popularity. These are :

(a) Direct indexing (b) Simple indexing (c) Compound indexing (d) Differential indexing


Simple indexing

Consists in turning the spindle through the required angle by rotating the index crank. The kinematic balance equation for determining the number of revolution of index crank is composed, providing that per n revolution of the index crank the spindle turn 1/z revolutions, where z is the number of parts into which a circumference is to be divided. The basic displacements are:

Q 3. Do indexing for cutting 37 teeth on the gear blank. Three indexing plates are available. These plates have concentric circles of holes with their different numbers as described below:

Plate No. 1 15 16 17 18 19 20

Plate No. 2 21 23 27 29 31 33

Plate No. 3 37 39 41 43 47 49

Solution:

To cut 37 teeth on this blank, an index plate having a circular row with 37 holes should be mounted on the head. The index crank is positioned against the selected circular row and after each tooth is cut, it is turned through one full revolution and three spaces between the index plate holes. For this kind of indexing it is convenient to use the adjustable sector.

rev37

31

37

40n

spindle theof rev. 1

crankindex theof rev. N

z

z

40n

400

z and 1 Zhead dividing available In the

Zz

0z

n

teethwheel- wormofnumber theis 0

z

andstart wormofnumber is Zwhere,

1

0z

Zn.1.

:isequation balance kinematic The

spindle theof rev. 1

crankindex theof rev. N

z

z


Differential indexing

Is employed where simple indexing cannot be effected i.e. where an index plate with the number of holes required for simple indexing is not available. In differential indexing, the required turn of the dividing head spindle is obtained as a sum of two turns: a turn of index crank relative to the index plate and a turn of the index plate itself, which is driven from the spindle through the change gear quadrant a/b.c/d. in this indexing the index plate should be released by retracting the lock pin, which secure the plate to the head housing.

Depending on the setup, the index plate can rotate either in the same direction with the index crank or in the opposite direction. Therefore, when the index crank is rotated, its effective turn will be greater or smaller that its visible turn with respect to the index plate.

The number of revolution of index crank is set up in the same manner as in simple indexing, but not for the required z number of division. The number of divisions taken is zx,nearest to z.

As the effective turn of the index crank includes its turn relative to the index plate and the turn of the index plate itself, the kinematic balance equation for determining the ratio of the differential change gear quadrant will be the following.

Gearing can be simple and compound. For simple 1 idler for +ve rotation, 2 idler for ve rotation. For compound No idler for +ve rotation, 1 idler for ve rotation.

zd

c

b

a

d

c

b

a

z

d

c

b

a

z

d

c

b

a

z

x

x

x

x

o

x

o

zz

40.

formula foregoing thefrom

found isgear change theof ratio The

plateindex of revolution ofnumber ..1

plateindex the torelative

crank index of revolution ofnumber z

40 where,

z

40..

1

z

40

z

z..

1

z

z


Q 4. Select the differential change gears and the index plate, and determine the number of revolutions of the index crank for cutting a gear with 227 teeth.

Brown and sharpe

Plate No. 1 15 16 17 18 19 20 Plate No. 2 21 23 27 29 31 33 Plate No. 3 37 39 41 43 47 49


Q 5. Select the differential change gears and the index plate, and determine the number of revolutions of the index crank for cutting a gear with 57 teeth.

Brown and sharpe:

Plate No. 1

21 23 27 29 31 33

Plate No. 2

37 39 41 43 47 49

Solution:


Milling helical grooves

In milling helical groove, a complex helical movement is imparted to the workpiece, which involves a straight movement along the workpiece axis and rotation of the workpiece about the same axis. The workpiece receives the straight movement with the work table of the machine and rotation from the work table lead screw through change gears. The table is set to the spindle axis at an angle equal to the helix angle of the groove being cut. In milling a left hand groove, the table swiveled clockwise at an angle , and in right hand groove, counterlockwise.

The set angle for the table is,

Where D= diameter of the

Workpiece being cut,

Ph.g =lead of helical groove.

If the helix is determined by the lead angle ,

the table should be set at an angle of (90o- )

ghP

D

.

arctan


Q 6. To what angle must the milling machine table be swiveled to cut a helix having a lead of 250 mm on piece of work 40 mm in diameter?

Solution:

mm. screw, lead table- workof lead theisP where,

Pz

P

d

c.

b

a

obtain we1,with Z

PPd

c.

b

a.1.1.1

Z

z.1

cut. being groove theof

P lead the toequalamount an by travelsmachine the

of worktable the workpiece theof revolutionevery for

that providedout workedisd

c.

b

a gear train change up

settingfor chain thisofequation balance kinematic The

l.s

l.so

h.g

1

1

1

1

h.gl.s

1

1

1

1o

h.g

1

1

1

1

6315 angleHelix

2792.0

450

40 x 1416.3

helix of lead

D x 3.1416 anglehelix ofTangent


Q 7.It is required to mill six right-hand helical flutes with a lead of 600 mm; the blank diameter is 90 mm. if the pitch of the table lead screw is 7.5 mm, give complete information about the setup.

Brown and sharpe

Plate No. 1 15 16 17 18 19 20 Plate No. 2 21 23 27 29 31 33 Plate No. 3 37 39 41 43 47 49

Solution:


Industrial Applications

Fig. 2.39 Milling operation to produce industrial part

Milling machines are widely used in the tool and die making industry and are commonly used in the manufacturing industry for the production of a wide range of components. Typical examples are the milling of flat surface, indexing, gear cutting, as well as the cutting of slots and key-ways.

When equipped with digital readout, the machine is capable of producing more precise work for the manufacturing of plastic moulds, tool & dies, and jigs & fixtures.

5 Milling and Related Operations

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