2_machining processes and machine tools

8/7/2019 2_Machining Processes and Machine Tools

1/69

Turning and Related Operations

Turning is a machining process in which a single point toolremoves material from the surface of a rotating cylindricalworkpiece; the tool is fed linearly in a direction parallel to the axisof rotation.

Turning is traditionally carried out on a machine called Lathe.


2/69

Operations related to turning

Facing

The tool is fed radially into the rotating work on one end to create aflat surface on the end.


3/69


Taper Turning

Instead of feeding the tool parallel to the axis of rotation of thework, the tool is fed at an angle, thus creating a tapered cylinder orconical shape.


4/69


Contour Turning

The tool follows a contour that is other than straight line, thuscreating a contoured form in the turned path.


5/69


Form Turning

In this operation some times called forming, the tool has a shapethat is imparted to the work by plunging the tool radially into thework.


6/69


Cutoff or Parting

The tool is fed radially into the rotating work at some location alongits length to cut off the end of the part. The operation sometimesreferred to as parting.


7/69


Threading

A pointed tool is fed linearly across the outside surface of therotating workpart in a direction parallel to the axis of rotation at alarge effective feed rate, thus creating threads in the cylinder.


8/69


Boring

A single point tool is fed linearly, parallel to the axis of rotation, onthe inside diameter of an existing hole in the part. The purpose is toachieved a desired diameter.


9/69


Drilling

Drilling can be performed on a lathe by feeding the drill into therotating work along its axis.


10/69


Knurling

This is not a machining operation because it does not involvecutting of material. Instead it s a metal cutting operation used toproduce a regular cross hatched pattern in the work surface.


11/69

The Engine Lathe

The basic lathe used for turning and related operations is anengine lathe. It is a versatile machine tool, manually operated andwidely used in low and medium production. The term engine datesfrom the time when these machines were driven by steam engines.


12/69

The Engine Lathe


13/69

The headstock contains the drive unit to rotate the spindle, which rotates the work.

Opposite the headstock is the tail stock, in which a center is mounted to support the other

end of the workpiece.


14/69

The cutting tool is held in a tool post fastened to the cross-slide, which is assembled to the

carriage.

The carriage is designed to slide along the ways of the lathe in order to feed the tool

parallel to the axis of rotation.


15/69

The ways are built into the bed of the lathe, providing a rigid frame work.

The carriage is driven by leadscrew that rotates at the proper speed to obtain the desired

feed rate.

The cross slide is designed to feed in a direction perpendicular to the carriage movement.


16/69

By moving the carriage, the tool can be fed parallel to the work axis to perform straight

turning; or by moving the cross slide, the tool can be fed radially into the work to perform

facing, form turning and cutoff operations.


17/69

On most lathes, the tool is mounted on the compound rest. The compound rest can rotate

and translate with respect to the cross slide, permitting the positioning of the tool with

respect to the work.

The apron attached to the front of the carriage, has the controls for providing manual andpowered motion for the carriage and the cross slide.


18/69

Size of a Lathe

The size of a lathe is designated by swing and maximum distance betweencenters.

The swing is the maximum workpart diameter that can be rotated in thespindle.

It is determined as twice the distance between the centerline of the spindleand the ways of the machine.

The maximum distance between centers indicates the maximum length of aworkpiece that can be mounted between headstock and tailstock centers.


19/69

Methods of Holding the Work in a Lathe

Four common methods

(i) Mounting the work between centers

(ii) Chuck

(iii) Collet

(iv) Face plate


20/69


Mounting the work between centers

This refers to the use of two centers, one in the headstock and the other in the

tailstock.At the headstock, a device called a dogis attached to the outside of the workand is used to drive the rotation from the spindle.

The tailstock center has a cone-shaped point which is inserted into a taperedhole in the end of the work.

The tailstock center is either a live center or a deadcenter.

Live center


21/69


Chuck

Chuck is available in severaldesigns, with three or four jaws to grasp the cylindricalworkpart on its outside orinside diameter.

A self-centering chuck has a

mechanism to move the jawsin or out simultaneously, thuscentering the work at thespindle axis.

Chucks can be used with orwithout a tailstock center.


22/69


Collet

A collet consists of a tubularbushing with longitudinal slitsrunning over half its lengthand equally spaced around itscircumference.

Inside diameter of the colletused to hold work as barstock.

Owing to the slits, one endof the collet can be squeezedto reduce its diameter andprovide a secure graspingpressure against the work.


23/69


Faceplate

A faceplate is a work holding device that fastens to the lathespindle and is used to grasp parts with irregular shapes.

The faceplate is equipped with custom-designed clamps for theparticular geometry of the part.

The work can be bolted or clamped directly on the faceplate or canbe supported by an auxiliary fixture that is attached to the faceplate.


24/69

Types of Lathe and Turning Machines

Engine Lathe

Most frequently used in manufacturing

Power drive for all movements except the compound slide.

They are commonly range in size from 12 to 50 in. swing and from2 to 12 ft. centre distances.

Most Engine lathes are equipped with chip pans and a built incoolant circulating system.

Toolroom Lathe

Toolroom lathes have somewhat greater accuracy, and usually a

wider range of speeds and feeds than ordinary engine lathe.

They have continuously variable spindle speed.

Shorter beds as compared to engine lathes since they aregenerally used for machining small parts.


25/69


Vertical Turning Centre

Vertical lathes are an excellent alternative to large horizontal

lathes.Gravity aided seating of large heavy work pieces allows a highdegree of accuracy.

Advantages: Smaller footprint, lower initial cost and increasedproductivity.

Vertical CNC

turning Centre


26/69


Turret Lathe

In a turret lathe, the tailstock is replaced by a turret that holds up to

six cutting tools. These tools can be rapidly brought into action against the work oneby one by indexing the turret.

The conventional tool post used on an engine lathe is replaced by afour-sided turret that is capable of indexing up to four tools into

position.The turret lathe is used for high production work that requires asequence of cuts to be made on the part.


27/69


Chucking Machine

Chucking machine uses a chuck in its spindle to hold the workpart.

No tailstock; parts cannot be mounted b/w centers.

For short, light-weight parts.

Tool feed is automatic.

Bar Machine

Similar to chucking machine except that a collet is used.

Collet permits long bar stock to be fed through the headstock intoposition.

At the end of each machining cycle, cutoff operation separates the

new part. The bar stock is then indexed forward to present stock for the nextpart.

Feeding the stock as well as indexing and feeding the cutting toolsis accomplished automatically

Also called automatic barmachine orautomatic screwmachine.


28/69


Multiple spindle bar machines

These machines have more than one spindle, so multiple parts are

machined simultaneously by multiple tools.A six spindle automatic bar machine works on six parts at a time.

At the end of each machining cycle, the spindles are rotated to thenext position.

High production rates.


29/69

Boring Machines or Boring Mills

Boring Mills can be horizontal or vertical. The designation refers tothe orientation of the axis of rotation of the machine spindle orworkpart.

Horizontal boring operation: Two setup

The first setup is one in which the work is fixed to a rotating spindle,and the tool is attached to a cantilevered boring bar that feeds intothe work.

The second possible setup is one in which the tool is mounted to aboring bar and the boring bar is supported between centers. Thework is fastened to a feeding mechanism that feeds its past the tool.


30/69

Boring Machines or Boring Mills

Vertical boring machine is used for large, heavy workparts withlarge diameters.

The part is clamped to a worktable that rotates relative to themachine base.

The tools are mounted on tool heads that can be fed horizontally

and vertically relative to the worktable.


31/69

Drilling and Related Operations

Drilling is a machining operation used to create round hole in aworkpart.

This contrasts with boring, which can be used to enlarge an existinghole.

Drilling is usually performed with a rotating cylindrical tool that has two

cutting edges on its working end. The tool is called a drillordrillbit.

A twist drill


32/69

Drilling Process

There are four major actions taking place at the point of a drillA small hole is formed by the web.

Chips are formed by the rotating lips.

Chips are removed from the hole by the screw action of the helicalflutes.

Drill is guided by land that rubs against the walls of the hole.


33/69

Nomenclature of Twist drill

A twist drill


34/69

Cutting Parameters in Drilling

Similar formulae for speed and feed.

Depth of cut is taken as D/2, D=dia of drill

AllowanceA=D/2

MRR=3DVfr


35/69

Drilling related operations

CounterboringCounter boring provide an enlarged cylindrical hole with a flat bottom

so that the bolt head or a nut will have a smooth bearing surface that is

normal to the axis of the hole.

The depth may be sufficient so that the entire bolt head should be belowthe surface of the part.

The pilot on the end of the tool fits into the drilled hole helps to ensure

concentricity with the original hole.

Counterboring tool


36/69


Countersinking

Countersinking provides a beveled section at the end of a drilled hole toprovide a proper seat for a flat-head screw or rivet.

The most common angles are 60, 82 and 90

Countersinking tools are similar to counterboring tools excepts that thecutting edges are the elements of a cone, and they dont have a pilotbecause the bevel of the tool causes them to be self centering.


37/69


Spot facing

Spot facing is done to provide a smooth bearing area on a rough surfaceat the opening of a hole normal to its axis.

Machining is limited to the minimum depth that will provide a smoothsurface

They are usually made with a multi-edge cutting tool that does not havea pilot


38/69

Drilling related operationsReaming

Reaming removes a small amount of material from the surface of theholes.

It is done for two purposes: to bring holes to a more exact size and to

improve finish of an existing hole.

Multiedge tools are used as shown.

No special machines are built for reaming

A Reamer


39/69

Milling

Milling is a machining operation in which a workpartis fed past a rotating cylindrical tool with multiplecutting edges.

The axis of rotation of the cutting tool is

perpendicular to the direction of feed.

The cutting tool in milling is called a milling cutterand the cutting edges are called teeth.

The machine tool that traditionally performs thisoperation is called milling machine.


40/69

Milling

The geometric form created by milling is a plane surface.

Other work geometries can be created either by means of thecutter path or the cutter shape.

Milling is an interrupted cutting operation; the teeth of themilling cutter enter and exit the workpiece during eachrevolution.

This interrupted cutting action subjects the teeth to a cycle of

impact force on every rotation.

The tool material and cutter geometry must be designed towithstand these conditions


41/69


42/69

Peripheral Milling

In peripheral millingalso called plain milling, the axis of the tool is parallelto the surface being machined, and the operation is performed by cuttingedges on the outside periphery of the cutter.

Types of peripheral milling are(a)Slab milling, the basic form of peripheral milling in which the cutter widthextends beyond the workpiece on both sides.

(b)Slotting, also called slot milling, in which the width of the cutter is lessthan the workpiece width, creating a slot in the work. When the cutter is verythin, this operation can be used to mill narrow slots or cut a workpart intotwo called sawmilling.

(c)Side milling, in which the cutter machines the side of the workpiece

(d) Straddle milling, the same as side milling, only cutting takes place onboth sides of the work.


43/69

Types of Peripheral Milling

Slab Milling Saw Milling

Slot Milling Straddle Milling


44/69

Up Milling and Down Milling

In peripheral milling, the rotation direction of the cutter distinguishes twoforms of milling: up millingand down milling.

In up milling, also called conventional milling, the direction of motion ofthe cutter teeth is opposite to the feed direction when the teeth cut into the

work. It is milling against the feed.In down milling, also called climb milling, the direction of cutter motion isthe same as the feed direction when the teeth cut the work. It is milling withthe feed.


45/69


In up milling, the chip formed by each cutter tooth starts out very thin andincreases in thickness during the sweep of the cutter.

In down milling, each chip starts out thick and reduces in thicknessthroughout the cut.


46/69


The cutting force direction is tangential to the periphery of the cutter for theteeth that are engaged in the work.

In up milling, this has the tendency to lift the workpart as the cutter teeth exitthe material.

In down milling, this cutter force direction is downward, tending to hold thework against the milling machine table.


47/69

Face Milling

In face milling, the axis of the cutter is perpendicular to thesurface being milled, and machining is performed by cutting edgeson both the end and periphery of the cutter.


48/69

Types of Face Milling

(a) Conventionalfacemilling, in which diameter of the cutter is greaterthan the workpart width, so the cutter overhangs the work on bothsides.

(b) Partialfacemilling, where the cutter overhangs the work on only one

side.(c) Endmilling, in which the cutter diameter is less than the work width,

so a slot is cut into the part.

(d) Profilemilling, a form of end milling in which the outside periphery of aflat work is cut

(e) Pocketmilling, another form of end milling, used to mill shallowpockets into flat parts.

(f) Surface contouring, in which a ball nose cutter (rather than squareend cutter) is fed across the work along a curvilinear path to create athree dimensional surface form.


49/69

Types of Face Milling

Conventional Face Milling

End Milling

Pocket Milling


50/69

Cutting conditions in Milling

Same formula for cutting speed.

Formula for feed (already studied in first chapter)

Material removal rate= (cross sectional area of the cut) x feed rate

(in/min or mm/min)MRR=wdfm

where w= width of cut, d=depth of cut, fm= feed rate (in/min ormm/min)

Machining timefor slab millng, Tm = (L+A)/ fm and A= d(D-d); D=dia of cutter

for face milling, Tm = (L+2A)/ fm and A= w(D-w); D=dia of cutter


51/69

Types of Milling Machines

Milling machines must provide a rotating spindle for the cutter anda table for fastening, positioning, and feeding the workpart.

Can be classified as horizontal (for slab, slotting, side andstraddle milling) and vertical (for face, end milling, surfacecontouring)

Mainly classified into the following types

(1) Knee-column

(2) Bed type

(3) Planer type

(4) CNC


52/69

Knee-and-column milling machine

Basic machine tool for milling

Two main components: knee and column

Column supports the spindle; kneesupports the worktable

A

vailable as both horizontalor a verticalmachine.


53/69

Horizontal Knee-and-column milling machine

In horizontal type, an arborsupports the cutter.

The arbor is basically a shaftthat holds the milling cutter and

is driven by the spindle.An overarm is provided tosupport the arbor.


54/69

Vertical Knee-and-column milling machine

On vertical Knee and columnmachines, milling cutters can bemounted directly in the spindlewithout an arbor.


55/69

Knee-and-column milling machine

One of the key features of the kneeand column milling machine is thecapability of the feed tablemovement in any of x- y- and z axes.

Worktable= longitudinal movement

Saddle=Transverse movement

Knee= Vertical movement


56/69

Special knee-and-column milling machinesTwo special knee and column millingmachines

(a)Universal milling machine

(b)Ram mill

Universal milling machine

It has a table that can be swiveled in a

horizontal plane about a vertical axis toany specified angle. This facilitates thecutting of angular shapes on workparts.

Ram mill

The tool head containing the spindle is

located on the end of a horizontal ram;the ram can be adjusted in and out overthe worktable to locate the cutter relativeto the work. The toolhead can beswiveled to achieve an angularorientation.


57/69

Bed type milling machine

Bed type milling machinesare designed for highproduction.

They are constructed withgreater rigidity than knee andcolumn machines.

Heavier feed rates and depthof cuts are possible

Work table is mounted

directly to the bedThis construction limits thepossible motion of the tableto longitudinal feeding of thework past the milling cutter.

Single spindle machine iscalled simplex mill

A triplex mill


58/69

Planer type milling machine

Planer type mills are the largestmilling machines

General appearance andconstruction are those of a largeplaner.

One or more milling heads aresubstituted for the single-pointcutting tools.

Motion of the work past the tool isa feed rate motion rather than a

cutting speed motion.Planer machine are used to builtvery large part.

The worktable and bed are heavyand relatively low to the ground.


59/69

BroachingThe broaching operation is similar to shaping with multiple teeth and is

used to machine internal and external surfaces such as holes of circular,

square, or irregular shapes, keyways, and teeth of internal gears.

A two station broaching operation


60/69

Broaching

A broach is a long multitooth cutting tool with successively

deeper cuts. Each tooth removes a predetermined amount ofmaterial in a predetermined location.

The total depth of material removed in one path is the sum ofthe depth of cut of each tooth.

Broaching is an important production process and canproduce parts with very good surface finish and dimensionalaccuracy.

Broaching competes favorably with other processes such asboring, milling, shaping and reaming. Although broaches tendto be expensive, the cost is justified because of their use forhigh production runs.

B hi


61/69

BroachingFor exterior surface broaching, the broach tool may be pulled or pushedacross a workpiece surface, or the surface may move across the tool.

Internal broaching requires a starting hole or opening in the workpiece sothe broaching tool can be inserted. The tool or the workpiece is thenpushed or pulled to force the tool through the starter hole.

Almost any irregular cross-section can be broached as long as all surfacesof the section remain parallel to the direction of broach travel.

Broached parts

Th B h


62/69

The Broach

Principal parts of round internal broach

B hi


63/69

Broaching

Chips filling the gullet during a broaching operation


64/69

Internal Broaching

A variety of forms that can be produced by internal broach


65/69

Types of Broaches

Push and Pull Broaches

Keyway broach

Burnishers

Surface broaches


66/69

Types of Broaching Machine

Vertical Boring Machine

Horizontal Boring Machine

Chain Boring Machine


67/69


Vertical Boring Machine with loading and

unloading conveyor


68/69


Two station horizontal boring machine


69/69


Continuous chain broaching machine

2_machining processes and machine tools

Documents