Milling Grinding

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Unit 6- Milling and Grinding Machines.

Milling and Grinding Machines.

Milling is a machining process in which the metal cutting takes place with the help of a

rotating multi-point cutter called milling cutter. Here the job is held stationary and fed

against a rotating tool. The cutter has multiple cutting edges and it rotates at high speed.The machining takes place at a much faster rate and generally a good surface finish is

obtained.

The machine tools employed for various milling operations are called milling machines.

Milling machines are quite versatile and can do several operations like making flat

surfaces, grooving, thread and gear cutting.

Working Principle of Milling Machine

The milling cutter is attached to a horizontal rotating shaft known as arbor. The work

piece which clamped on the table is fed in the direction opposite to the milling cutter(up

milling) or in the same direction of the milling cutter (down milling) The metal is

removed by advancing the workpiece during each revolution of the rotating cutter in the

form of chips. The milling operation is extensively used in machining flat surfaces

contoured surfaces external & internal threads, helical surfaces of various cross sections.

Depending upon relative feed direction of worktable and rotation of cutter two different

methods of milling is possible

1. Conventional milling or Up milling

2. Climb milling or Down milling

Dept. of Mechanical Engineering, Srinivas School of Engineering. 1


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1. Conventional milling or Up milling

The milling cutter is attached to a horizontal rotating shaft known as arbor. The work

piece which clamped on the table is fed in the opposite direction of the milling cutter. The

metal is removed by advancing the workpiece during each revolution of the rotating

cutter in the form of chips.

In up-milling, the chip thickness varies from a minimum at the tooth entrance to a

maximum at the tooth exit. The forces produced by the cutting tool tend lift the work

piece up from the table. Hence conventional milling process requires heavy work holding

devices. Up milling leads to poor surface finish, due to vibrations developed by cutting

forces of the cutter.

Down Milling

In down milling, the metal is removed by the rotating cutter fed in the direction of

movement of the workpiece In down milling, the chip thickness varies from a maximum

near the tooth entrance to a minimum near the tooth exit. Thus the cutting tooth is

subjected to a maximum load from the very beginning. The cutting forces in down

milling tend to act downwards, forcing the workpiece into the fixture or the vice. Hence

the down milling process doesn't require heavier work holding devices This type of

milling produces higher surface finish compared to up milling.



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Classification of Milling Machines

On the basis of the type of construction the milling machines may be classified as:

1. Column and Knee-Type Milling Machine: It is a general-purpose milling machine. The

worktable is mounted on a knee which slides in the guide ways of the column.2. Vertical Milling Machine: The spindle of this machine is held in the vertical position.

Universal Milling Machine: This a kind of horizontal milling machine in which the table

can be swiveled on a horizontal plane.

3. Bed-Type Milling Machine: This is used as a production machine.

4. CNC Milling Machines: They are computer numerical controlled, here all the

movements of tool and work are controlled by the computer program entered by the

operator/ engineer.

5. Special-Purpose Milling Machines: They are manufactured for performing some

specific type of machining operation.

1. Column and Knee-Type (Horizontal) Milling Machine:

It is a general-purpose milling machine. The worktable is mounted on a knee which

slides in the guide ways of the column. It can be adjusted to a desired height. Figure

above shows the block diagram of a column and knee-type milling machine with

horizontal arbor.

Following are its principal parts:

Column with base: It is the main structural body of the milling machine to support otherparts.



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Arbor: It holds and provides rotary motion to the cutter.

Ram: It is also known as over arm. It supports the arbor and can be adjusted to

accommodate different arbor lengths.

Machine table: The job and its holding devices are mounted on the machine table. It can

move longitudinally to provide the feed motions to the job.

Power drive with gear boxes: It provides power and motions to the tool and work. Bed: It moves vertically upward and downward and accommodates the various drive

mechanisms.

Working Work piece is mounted directly on table using machine vice.

Cutter of required shape and size are mounted over the arbor which is

driven by spindle.

Feed in all 3 axes can be given by using knee elevating handle, cross slide

handle, table movement handle.

Vertical Milling Machine

In vertical milling machine, the axis of the spindle is perpendicular to the work table.

The work piece can be moved both in vertical and horizontal plane.



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The spindle head can be moved up and down over the guide ways. The saddle is mounted

on a knee which can be moved up and down over the guide ways provided on the column

face. The worktable mounted on the saddle can be moved longitudinally over the guide

ways provided on the top of the saddle. The machine is used to machine grooves, slots

and flat surfaces.

Base and column base is the foundation for the machine and column is the

vertical part houses motor, transmission system.

Spindle head mounted on front face of the column at the top. It has a vertical

spindle run by motor to which milling tool is fitted to end.

Working Work piece is mounted on table and cutter is mounted on vertical spindle.

Feed is given by movement of knee, saddle and table and also vertical

movement of tool.

Milling Operations Various milling operations can be performed on a milling machine

to produce flat, vertical, inclined surfaces, grooves, slots, keyways, gear teeth etc. Some

of the most commonly operations are, plain or slab milling, angular milling, face milling,

form milling, slot milling, and straddle milling.

1. Plain or slab Milling:

This milling process is employed for machining a flat surface parallel to the axis of cutter

by using a slab milling cutter.

Process used to mill flat surfaces of the work piece.

Milling cutter axis is parallel to the surface that is being milled.



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2. End milling:

This milling process is employed for machining a flat surface perpendicular to the axis of

cutter by using a face end milling cutter. The cutting comes from the combined action of

cutting edges located at the face of the cutting tool as well as the edges on the periphery.

Process of milling that is used to mill slots, pockets and keyways.

Axis of milling cutter is perpendicular to the surface of the work piece.

3. Slot Milling:

A large variety of slots can be machined using end-milling cutters. Following are some of

the common types of slots for which slot milling is used:

Plain slot;

T-slot;

dovetail slot;

V groove.

Cutter tool slot drill

Axis of spindle is horizontal or parallel to surface of work. To produce rectangular slots, T-slots, dovetail slots.



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4. Angular millng

To produce angular surfaces like dovetail, grooves and chamfers.

Surface is neither parallel nor perpendicular to the milling cutter axis.

5. Form Milling:

It is used to produce curved profiles of any desired shape using specially designed form

milling cutters. This process is generally employed for machining different types of gears.

This is a process used to machine special forms / contours consisting of curves. form milling cutter is the cutting tool which is shaped exactly to the contour to

be milled.

6. 5. Straddle Milling:



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This method is employed for producing two parallel vertical surfaces simultaneously by

using a pair of side-milling cutters.

Used to machine two parallel vertical surfaces at a single time.

It has two cutters mounted parallel on the arbor separated by a calculated spacing.

Also used to mill sides of hexagon.

6. Gang Milling:

When more than two milling cutters are mounted on the same arbor for machining

horizontal and vertical surfaces simultaneously, the process is called gang milling.

Similar to straddle milling but machining is done with several types of milling

cutters according to shape of desired work.

7. Angular Milling:



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In this method, one or more angular-milling cutters are used for machining inclined

surfaces or grooves.

8. Keyway Milling: This process is employed for machining keyways and splines in the

shafts using side and face-milling cutters or end-milling cutters.

Specification of Milling Machines.

Milling machines are specified by the following basis

1. Table size (l*b)

2. Traverse for the job (Longitudinal, Vertical, Cross)

3. Spindle Speed .Rpm

4. Power of motor or capacity of machine .KW

GRINDING

Grinding is a machining process in which the material is removed in the form of fine

chips by the mechanical action of abrasive particles. The abrasive particles are bonded

togethermeans of bonding materials. The abrasive particles after bonding take the shape

of a wheel.

Principle of Grinding



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The material is removed from the workpiece when it is fed against rotating abrasive

wheel. The material removed is due to the mechanical action of abrasive particles.Thickness of the chips removed during grinding ranges between 0.25 to 0.50 microns.

(0.00025-0.0005 mm)

Purpose of grinding

Grinding operation is done for the following purposes.

1. To obtain better surface finish.

2. To remove small amount of metal from the workpiece so as to bring its

dimensions within close tolerances of the order of 0.000025mm.

3. To machine sliding parts to avoid friction

4. To machine hardened surfaces

5. To sharpen the cutting tools

Abrasives

The grinding wheels are made of abrasive particles The Abrasives are mineral crystals

with hardness much higher than that of the workpiece, bonded together by means of

suitable bonding materials.

Abrasives used for grinding wheels are basically two types.

1. Natural Abrasives

These are readily available in nature. They include sandstone or solid quartz, emery,

diamond and garnets.

2. Artificial (Man made) AbrasivesThese are manufactured artificially and are not available in nature. Ex, Silicon carbide

and aluminum oxide, etc.

Depending on their strengths and composition of base metal artificial abrasives are

broadly classified as follows.

a. Silicon Carbide Abrasives

Silicon carbide stands second in the order of hardness after diamond. e.g Carborundum,

crystolon, carbolon etc. Silicon carbides are used to grind low tensile materials.

b. Aluminium oxide Abrasives

Aluminium oxide is tough and hence it is used to grind high tensile materials like' carbon

steels e.g. Aloxite, alundum, borolon etc.

Dept. of Mechanical Engineering, Srinivas School of Engineering.

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Bonding Materials

Bonding material is a substance employed to hold abrasive grains together in the form of

grinding wheels. Different bonding processes are vitrified process, silicate process, shell

or resinoid process, rubber process and oxychloride bonding process.

1. Vitrified processIn the vitrified process clay and water are mixed with the abrasive grains. The' mixture is

moulded and allowed to dry. The dry mould is trimmed to the required size and shape, the

mould is heated in kiln till the clay vitrifies, fuses and forms a porcelain or glass like

substance.

A vitrified bonding wheel is denoted by the letter V'.

2. Silicate Process

In the silicate process, abrasive grains are mixed with silicate of soda or water glass. The

mixture is poured in moulds to required shape and size and allowed to dry. The mouded

shapes are baked in a furnace at a temperature of 260 deg C for several days. The silicate

bonded wheels are used to grind edged tools. They are also used in other operations

where heat generation rate during machining should be confined to a minimum.

A silicate bond wheel is denoted by the letter S.

3. Shellac Process or Elastic Process

In the shellac process, abrasive grains are mixed with shellac and then rolled or pressed

into heated moulds. The moulded shapes are baked for a few hours at a temperature of

about 150 deg C.

Very thin wheels can be manufactured by this process which can be used to grind thin

sections.

They are also used for finishing iron and steel rolls,alluminium pistons,cams and shafts.A shellac bond wheel is denoted by the letter 't".

4. Resinoid Process

In this process. abrasive grains are mixed with synthetic resins and other compounds. The

mixture is moulded into required shape and finally heated a temperature of about 200 deg

Celsius .

The synthetic resin may be bakelite and redmanol. The wheels are useful for grinding

cams and steel rolls which require very high surface finish. A resinoid bond wheel is

denoted by the letter 'B'.

5. Rubber Process

In this process, abrasive grains are mixed with pure rubber and sulphur. The mixture is

rolled into sheets and pressed out using a punch press and finally vulcanized.

The process is used to manufacture very thin wheels which are strong and tough. The

wheels are used to cut narrow slots and grooves.

A rubber bond wheel is denoted by the letter 'R'.

6. Oxychloride Process

In this process, abrasive grains are mixed with oxides and chlorides or magnesium and

the mixture is moulded and allowed to dry. The dry mould is trimmed to the required


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shape and baked in a kiln. The wheels produced are extensively used in disc-grinding

operations without employing coolants.

An oxychloride bond wheel is denoted by the letter '0'.

METHODS OF GRINDING

Grinding can be done by using different methods. They are

Bench grinding

Surface grinding

Cylindrical grinding

Centre less grinding.

Internal grinding

Special grinding machine.

Surface Grinding


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Surface grinding machines are mainly used to finish plane or flat surfaces. They are also

employed to grind irregular, curved, tapered, convex and concave surfaces. Fig shows a

horizontal spindle reciprocating table type surface grinding machine. The wok piece heldin a Magnetic chuck is brought in contact with the grinding wheel gradually so as to

produce a fine finish to the surface of the work piece.

It is machine basically used to grind flat surface.

Job is mounted to a table which moves longitudinally as well as in transverse

direction.

Manual feed or power feed.

Work piece can clamped in two ways

Manual clamps.

Magnetic chuck. Internal pump and piping arrangement for coolant.

Protective guard for safety.


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2. Cylindrical Grinding

Cylindrical grinding machines are mainly used to grind plain cylindrical parts and also

grinding contoured cylinders tapered surfaces shoulders. The work piece is mounted

between tailstock and headstock centers. For precision grinding, both the tail stock and

headstock centers are stationary work piece is made to revolve between the stationary

centers and the grinding wheel rotating in the opposite direction is brought into contact

with the workpiece.

It is a process of grinding curved surfaces.

Surface may be straight or tapered.

Work piece is mounted on two centers, one is tailstock centre and the other is

headstock centre.


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Head stock center may or may not revolve.

3. Centre less Grinding

Centre less grinding is used to grind exterior cylindrical, tapered surfaces on work piecesthat can not be held and rotated between the centers (head and tail stocks). In centre less

grinding, the work piece rests on a work rest blade and gets support from a back up wheel

called regulating wheel. The workpiece mounted on the work rest is fed forward along

with the regulating wheel, forcing the workpiece against the grinding wheel. The

regulating wheel rotates in the same direction as the grinding wheel and provides support

to the workpiece. The axial movement of the workpiece is obtained by tilting the

regulating wheel slightly at an angle to the horizontal.

It is used to grind curved surface work piece which are longandslender.

Work piece rests on a work-rest blade and is backed by a second wheel called as

regulating wheel.


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Grinding wheelpushes the work piece down the work-rest blade against the

regulating wheel.

Center less grinding machine


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Milling Grinding

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