04 lathe operations

8/11/2019 04 Lathe Operations

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Lathe Operations

Dr. N. VENKAIAH

Assistant Professor

Mechanical Engineering DepartmentNIT Warangal !" !!#

Disclaimer

The content presente$ here is not entirel% m% o&n. 'ome portions are ta(en from

$ifferent so)rces &ith great regar$. This content is solel% for class room teaching an$

not for an% commercial )se.


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Straight Turning

FIGURE : Schematic illustration of a turning operation showingdepth of cut, d, and feed,f. cutting speed is the surface speed

of the workpiece at the tool tip.


3/31

FacingTool is fed radially inward


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Contour TurningInstead of feeding the tool parallel to the axis of rotation,

tool follows a contour, thus creating a contoured form


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ChamferingCutting edge cuts an angle on the corner of the cylinder, forming

a "chamfer"


6/31

Cut-off

Tool is fed radially into the rotating workto cut off the end of the part


7/31

Other Operations

Cutting with a form tool

Boring and internal groovingDrilling

Taper turning Knurling


8/31

Thread Cutting

ointed form tool is fed linearly across surface of rotatingworkpart parallel to axis of rotation at a large feed rate, thuscreating threads


9/31

Thread Cutting - Terminology

!ig #lements of external threads

!ig #lements of internal threads

It calls fairly high skill on the part of the

operator

Threads are formed on the principle of

helix

Calculation of proper change gears is

crucial

Tool tip should have an included angle

corresponding to the included angle of the

thread to $e produced

The chief elements of all the screw threads%

itch& 'a(or diameter& 'inor diameter& itch

diameter& Depth of threads& Thread angle


10/31

Thread Cutting Terminology

!ig )ingle )tart

!ig Dou$le )tart

'tart of threa$*

Denotes the num$er of separate thread grovesrunning parallel to each other along the surface of

the screw

A$+antage m)ltiple start threa$s

To o$tain more axial advancement for the same

amount of rotation of the screw as compared to

single start threads without weakening the thread

If the pitch in $oth cases remains the same, the

axial advancement for the same amount of rotation

will $ecome as many times of the single start as

the num$er of starts

The axial advancement in one rotation of the

screw is known as lea$ of the threa$. *hich is the

distance $etween two corresponding points on the

thread measured parallel to the axis,

+ote% Thelea$ scre& of a lathe is al&a%s single

starte$.LeadPitch =No. of Starts


11/31

Cutting R.H. and L.H. Threads

Right Hand threads:- hreads slope up to the right when

screw is held !erticall".

- o cut R.H. hreads, headstock spindle

and the lead screw should rotate in the

same direction.

#eft Hand threads:- hreads slope up to the left when

screw is held !erticall".- he spindle and the lead screw will

rotate in opposite direction.

This change of direction of rotation on

lathe is effected by tumbler gears or byusin ro er number o idlers.

!igure% ./ Threads

!igure% 0/ Threads


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Left Hand thread - pplications


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Thread Cutting !ear Trains

)imple gear train

Compound gear train

Intermediate gear has no effect on the

change gear ratio It simply acts as a

connection $etween the two, and

serves to keep the rotation of driver

and driven in the same direction

In a compound gear train, the stud

carries two wheels which are keyedtogether so that they rotate as a unit

Compound gear train is employed

when the gear ratio is not possi$le to

o$tain with a simple gear train out of the

given set of change gears

In modern lathes, ;uick change gears

are availa$le to o$tain the correct gear

ratio to cut a particular thread ;uickly

$y simply shifting the lever into different

positions which are marked on theinstruction plates of the machine


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Cutting metric thread on

"nglish standard lead scre#s or $ice $ersa

o cut metric threads on lathes ha!ing $ritish

standard lead screw:

o cut $ritish standard threads on a lathe

ha!ing lead screw of metric pitch:


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18/31

E%ercise

&. 'alculate change gears for cutting R.H. threads of ( mm pitch

on a lathe ha!ing lead screw of ) mm pitch.

Soln.:

0ike this, we can have num$er of solutions

and every solution will give the desired result


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20/31

E%ercise *. 'alculate change gears for cutting R.H.$.S.-. /0 threads of

&( 1I on a lathe ha!ing lead screw of 2 1I.

Soln.: 1Driver TPI of Screw to be cutChange gear ratio = =1Driven

TPI of Lead screw

TPI of lead screw=TPI of screw to be cut

8

1 != =


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E%ercise *3. 'alculate change gears for cutting R.H. threads of & mm pitch

on a lathe ha!ing lead screw of 2 mm pitch.

Soln.:

)uch a ratio with a gear of 3 ? 78& 1ear B ? @8&

and

1ear C ? A8& 1ear D ?


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E%ercise *+. 'alculate change gears to cut #.H. threads of (+ 1I on a lathe

ha!ing lead screw of ) 1I.

Soln.:

Compound 1ear Train with oneidler%

1ear > ? 8& 1ear B ? 388&

and

1ear C ? 9& 1ear D ? :9


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E%ercise *). 'alculate change gears to cut R.H. single start threads of 4.(+0

pitch on a lathe ha!ing a lead screw of 2 mm lead.

Soln.:

Compound 1ear Train with no idler%

1ear > ? 37:& 1ear B ? @8&

and

1ear C ? 98& 1ear D ? 388


24/31

Setting of Tools for Threading

!ig )etting threading toolfor internal threading

!ig )etting threading tool for

external threading

Cutting tool should $e set exactly atthe height of centers and normal to the

work piece axis

If it is incorrectly set, the thread angle

will not $e correct and the flanks formed

will not $e proper

This setting is essential $oth for external

and internal threads

> center ga)ge is always used for

setting the threading tool correctly asshown in figures


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Feeding the Tool in Threading

!ig a6 )traight Thread

!ig $6 Inclined feed

!ig c6 Inclined feed using a form tool

0arge amount of material is removed in the form of

chips and that leads to the (amming of the tool and

the flanks of the threads will $e rough

0imit only to the finishing on coarse threads

/owever, it can safely $e used for threads having

$elow 7 mm pitch

In this method 5!ig $6 tool is fed at an angle 5usually

half of the thread angle6 to the axis of the work

The tool is set, as usual, s;uare with the work axis

and the re;uired inclination is o$tained $y swiveling

the compound rest

'ost of the cutting is done $y the left edge and tip ofthe tool

*hen threading is to $e done on a large scale, the

tool form shown at !ig 5c6 is preferred

This tool is normally provided with the negative rake

Conse;uently, the chips flow easily on the tool faceand the digging2in tendency of the tool is reduced


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%ro&iding undercut

Endercuts are necessary whencutting threads on stepped work

It allows for the overrun of the tool

after one cut is over

In the a$sence of this undercut

there is always a likelihood of the

tool running into the larger diameter

causing $ending of the (o$,

$reaking of (o$ or $reaking of

tool

The undercut should $e wide enough to give sufficient time to the

operator to disengage the half nut and withdraw the tool after

finishing the cut

Its depth should always $e slightly more than the depth of the thread


27/31

Cutting S'uare and cme Thread

!ig Tool setting for s;uare threads

nceCircumferethreadofLead ..tan =


28/31

Cutting (ultiple start threads


29/31

Turn)uc*le

Back


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Lathe Operations

Figure: /arious cutting operations that can 5e performed on a lathe.


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Lathe Operations

Figure: /arious cutting operations that can 5e performed on alathe.

04 lathe operations

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