The Lathe

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Transcript The Lathe - Dr. Jivraj Mehta Institute of Technology, Anand

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Technology of Machine Tools

6 th Edition

Krar • Gill • Smid

Section 11 History

• • Lathe forerunner of all machine tools First application was potter's wheel – Rotated clay and enabled it to be formed into cylindrical shape • Very versatile (many attachments) – Used for turning, tapering, form turning, screw cutting, facing, drilling, boring, spinning, grinding and polishing operations • Cutting tool fed either parallel or right angles

Special Types of Lathes

• • Engine lathe – Not production lathe, found in school shops, toolrooms, and jobbing shops – Basic to all lathes Turret lathe – Used when many duplicate parts required – Equipped with multisided toolpost (turret) to which several different cutting tools mounted • Employed in given sequence

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid

Engine Lathe Parts

Unit 45

Engine Lathe

• • • Accurate and versatile machine Operations – Turning, tapering, form turning, threading, facing, drilling, boring, grinding, and polishing Three common – Toolroom – – Heavy-duty Gap-bed

Lathe Size and Capacity

• • Designated by largest work diameter that can be swung over lathe ways and generally the maximum distance between centers Manufactured in wide range of sizes – Most common: 9- to 30- in. swing with capacity of 16 in. to 12 feet between centers – – Typical lathe: 13 in. swing, 6 ft long bed, 36 in.

Average metric lathe: 230-330 mm swing and bed length of 500 – 3000 mm

Lathe Size

Quick Change Gearbox Headstock

Parts of the Lathe

Setting Speeds on a Lathe

• Speeds measured in revolutions per minute – Changed by stepped pulleys or gear levers •

Safety Note!!

–

NEVER change speeds

Various speeds obtained by changing flat belt • Geared-head lathe – Speeds changed by moving speed levers into proper positions according to r/min chart fastened to headstock

Shear Pins and Slip Clutches

• • • Prevents damage to feed mechanism from overload or sudden torque Shear pins – – Made of brass Found on feed rod, lead screw, and end gear train Spring-loaded slip clutches – – Found only on feed rods When feed mechanism overloaded, shear pin will break or slip clutch will slip causing feed to stop

Shear pin in end gear train prevents damage to the gears in case of an overload Spring-ball clutch will slip when too much strain is applied to feed rod

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Lathe Accessories

Unit 46

46-15

Lathe Accessories

• Divided into two categories – Work-holding, -supporting, and –driving devices • • • Lathe centers, chucks, faceplates Mandrels, steady and follower rests Lathe dogs, drive plates – Cutting-tool-holding devices • • • Straight and offset toolholders Threading toolholders, boring bars Turret-type toolposts

Lathe Centers

• Work to be turned between centers must have center hole drilled in each end – Provides bearing surface • • Support during cutting Most common have solid Morse taper shank 60º centers, steel with carbide tips • Care to adjust and lubricate occasionally

46-16

Chucks

• Used extensively for holding work for machining operations – Work large or unusual shape • Most commonly used lathe chucks – Three-jaw universal – Four-jaw independent – Collet chuck

46-17

Three-jaw Universal Chuck

• • • • Holds round and hexagonal work Grasps work quickly and accurate within few thousandths/inch Three jaws move simultaneously when adjusted by chuck wrench – Caused by scroll plate into which all three jaws fit Two sets of jaw: outside chucking and inside chucking

46-18

Four-Jaw Independent Chuck

• Used to hold round, square, hexagonal, and irregularly shaped workpieces • Has four jaws – Each can be adjusted independently by chuck wrench • Jaws can be reversed to hold work by inside diameter

46-19

Headstock Spindles

Universal and independent chuck fitted to three types of headstock spindles 1.

– Threaded spindle nose Screws on in a clockwise direction 2.

– Tapered spindle nose Held by lock nut that tightens on chuck

46-20

Headstock Spindles

• • Cam-lock spindle nose Held by tightening cam-locks using T-wrench Chuck aligned by taper on spindle nose Registration lines on spindle nose Registration lines on cam-lock Cam-locks Cam-lock mating stud on chuck or faceplate

46-21

Collet Chuck

• • • Most accurate chuck Used for high-precision work Spring collets available to hold round, square, or hexagon-shaped workpieces • Each collet has range of only few thousandths of an inch over or under size stamped on collet

46-22

Collet Chuck

| Special adapter fitted into taper of headstock spindle, and hollow draw bar having internal thread inserted in opposite end of headstock spindle. It draws collet into tapered adapter causing collet to tighten on workpiece.

46-23

Types of Lathe Dogs

• Standard bent-tail lathe dog – Most commonly used for round workpieces – Available with square-head setscrews of headless setscrews • Straight-tail lathe dog – – Driven by stud in driveplate Used in precision turning

46-24

Types of Lathe Dogs

• Safety clamp lathe dog – Used to hold variety of work – Wide range of adjustment • Clamp lathe dog – Wider range than others – Used on all shapes

46-25

Left-Hand Offset Toolholder

• • Offset to the right Designed for machining work close to chuck or faceplate and cutting right to left • Designated by letter L

Right-Hand Offset Toolholder

• • • Offset to the left Designed for machining work close to the tailstock and cutting left to right – Also for facing operations Designated by letter R

Straight Toolholder

• • General-purpose type Used for taking cuts in either direction and for general machining operations • Designated by letter S

Toolholders for Indexable Carbide Inserts

• • Held in holder by cam action or clamps Types available – Conventional – Turret-type – Heavy-duty toolposts

46-29

Cutting-Off (Parting) Tools

• • • Used when work must be grooved or parted off Long, thin cutting-off blade locked securely in toolholder by either cam lock or locking nut Three types of parting toolholders – – Left-hand Right-hand – Straight

46-30

Threading Toolholder

• Designed to hold special form-relieved thread-cutting tool • Has accurately ground 60º angle – Maintained throughout life of tool • Only top of cutting surface sharpened when becomes dull

46-31

Super Quick-Change Toolpost

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Cutting Speed, Feed, and Depth of Cut

Unit 47

Cutting Speed

• • Rate at which point on work circumference travels past cutting tool • Always expressed in feet per minute (ft/min) or meters per minute (m/min) Important to use correct speed for material – Too high: cutting-tool breaks down rapidly – Too low: time lost, low production rates

47-34

47-35 Lathe Cutting Speeds in Feet and Meters per Minute Using High-Speed Steel Toolbit Material

Machine steel

ft/min m/minft/min m/minft/min m/min

90 27 100 30 35 11 Tool steel

Turning and Boring Rough Cut Finish Cut Threading

70 21 90 27 30 9 Cast iron Bronze Aluminum 60 200 18 90 61 80 27 300 24 100 93 25 30 60 8 25 18 8

Calculating Lathe Spindle Speed

• • • Given in revolutions per minute Cutting speed of metal and diameter of work must be known Proper spindle speed set by dividing CS (in/min) by circumference of work (in)

r/min



CS x 12



D



CS x 4 D

47-36

Example:

Calculate r/min required to rough-turn 2 in. diameter piece of machine steel (CS 90):

r/min r/min



CS x 4 D



90 x 2 4



180

Metric Formula

r/min



CS x 320 D

47-37

Lathe Feed

• • Distance cutting tool advances along length of work for every revolution of the spindle Feed of engine lathe dependent on speed of lead screw for feed rod – Speed controlled by change gears in quick-change gearbox

47-38

Two Cuts Used to Bring Diameter to Size

• • Roughing cut – Purpose to remove excess material quickly – Coarse feed: surface finish not too important • .010- to .015-in. (0.25- to 0.4-mm) Finishing cut – Used to bring diameter to size – Fine feed: Produce good finish • .003- to .005-in (0.07- to 0.012-mm)

47-39

47-40

Feeds for Various Materials (using high-speed steel cutting tool)

Rough Cuts Finish Cuts Material

Machine steel Tool steel Cast iron Bronze Aluminum

in.

mm in.

.010

–.020 0.25–0.5

.003

–.010 0.07

.010

–.020 0.25–0.5

.015

–.025 0.4–0.65

.003

–.010 0.07–0.25 .005

–.012 0.13–0.3

.015

–.025 0.4–0.65

.015

–.030 0.4–0.75

.003

–.010 0.07

.005

–.010 0.13–0.25

Depth of Cut

• • Depth of chip taken by cutting tool and one half total amount removed from workpiece in one cut Only one roughing and one finishing cut – Roughing cut should be deep as possible to reduce diameter to within .030 to .040 in. (0.76 to 1 mm) of size required – Finishing cut should not be less than .005 in.

47-41

Example: Depth of cut on a lathe

Factors Determining Depth of Rough-Turning Cut

47-43

• • • Condition of machine Type and shape of cutting tool used Rigidity of workpiece, machine, and cutting tool • Rate of feed

Inch System

• Circumference of crossfeed and compound rest screw collars divided into 100-125 equal divisions – Each has value of .001 in.

• Turn crossfeed screw clockwise 10 graduations, cutting tool moved .010 in. toward work • Lathe revolves, so .010 depth of cut taken from entire work circumference reducing diameter .020 in.

• Check machine for its' graduations

47-44

On machines where the workpiece revolves, the cutting tool should be set in for only half the amount to be removed from the diameter.

On machines where the workpiece does not revolve, the cutting tool should be set in for the amount of material to be removed.

Hints on Graduated Collar Use

Make sure collar is secure before setting a depth of cut All depths of cut must be made by feeding cutting tool toward workpiece If graduated collar turned past desired setting, must be turned backward half-turn and fed into proper setting to remove backlash Never hold graduated collar when setting depth of cut

47-47

• • • Graduated collar on compound rest can be used for accurately setting depth of cut • • • Shoulder turning Compound rest set at 90º to cross-slide Lock carriage in place Spacing of shoulders to within .001 in. accuracy • Facing Compound rest swung to 30º, amount removed from length of work = ½ amount of feed on collar • • Machining accurate diameters Set compound rest to 84º16' to the cross-slide .001 in movement = .0001-in. infeed movement

47-48

The compound rest is set at 84º16' for making fine settings.

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Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Lathe Safety

Unit 48

Safety

• • • Be aware of safety requirements in any area of shop Always attempt to observe safety rules Failure results in: – Serious injury – Resultant loss of time and pay – Loss of production to company

48-51

Safety Precautions

• • Lathe hazardous if not operated properly Important to keep machine and surrounding area clean and tidy • Accidents usually caused by carelessness

48-52

Safety Precautions

• • Always wear approved safety glasses Rollup sleeves, remove tie and tuck in loose clothing • Never wear ring or watch

48-53

48-54

Safety Precautions

• • • • Do not operate lathe until understand controls Never operate machine if safety guards removed Stop lathe before measure work or clean, oil or adjust machine Do not use rag to clean work or machine when in operation – Rag can get caught and drag in hand

Safety Precautions

• Never attempt to stop a lathe chuck or driveplate by hand • Be sure chuck or faceplate mounted securely before starting – If loose, becomes dangerous missile • Always remove chuck wrench after use – Fly out and injure someone – Become jammed, damaging wrench or lathe

48-55

Safety Precautions

• Move carriage to farthest position of cut and revolve lathe spindle one turn by hand – Ensure all parts clear without jamming – Prevent accident and damage to lathe • Keep floor around machine free from grease, oil, metal cuttings, tools and workpieces – Oil and grease can cause falls – Objects on floor become tripping hazards

48-56

Safety Precautions

• • Avoid horseplay at all times Always remove chips with brush – Chips can cause cuts if use hands – Chips become embedded if use cloths • Always remove sharp toolbit from toolholder when polishing, filing, cleaning, or making adjustments

48-57

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Mounting, Removing, and Aligning Lathe Centers

Unit 49

Objectives

• Mount and/or remove lathe centers properly • Align lathe centers by visual, trial-cut, and dial-indicator methods

49-59

Lathe Centers

• • Work machined between centers turned for some portion of length, then reversed, and other end finished Critical when machining work between centers that live center be absolutely true – Concentric work

49-60

To Mount Lathe Centers

• • • • • • Remove any burrs from lathe spindle, centers, or spindle sleeves Clean tapers on lathe centers and in headstock and tailstock spindles Partially insert cleaned center in lathe spindle Force center into spindle Follow same procedure when mounting tailstock center Check trueness of center

49-61

To Remove Lathe Centers

• • Live center – Use knockout bar pushed through headstock spindle (slight tap) – Use cloth over center and hold to prevent damage Dead center – Turn tailstock handwheel to draw spindle back into tailstock • End of screw contacts end of dead center, forcing it out of spindle

49-62

Alignment of Lathe Centers

• • Parallel diameter produced when lathe center aligned Three common methods used to align 1.

Aligning centerlines – on back of tailstock with each other Only a visual check and not too accurate

49-63

Alignment of Lathe Centers

Using the trial-cut method where small cut taken from each end of work and diameters measured with a micrometer 3.

• Using parallel test bar and dial indicator Fastest and most accurate method

To Align Centers by Adjusting the Tailstock

49-65

Loosen tailstock clamp nut or level Loosen on of the adjusting screws, depending on direction tailstock must be moved and tighten other until line on top aligns with line on bottom half Tighten screw to lock both halves in place Make sure tailstock lines still aligned Lock tailstock clamp nut or lever

To Align Centers by Trial-Cut Method

Take a light cut (~.005 in.) to true diameter from section A at tailstock end for .250 in. long Stop feed and note reading on graduated collar of crossfeed handle Move cutting tool away from work with crossfeed handle Bring cutting tool close to headstock end

49-66

To Align Centers by Trial-Cut Method

Return cutting tool to same graduated collar setting as at section A Cut a .500-in (13 mm) length at section B and stop lathe Measure both diameters with micrometer

49-67

To Align Centers by Trial-Cut Method

If both diameters not same size, adjust tailstock either toward or away from cutting tool ½ difference of two readings Take another light cut at A and B at same crossfeed graduated collar setting. Measure diameters and adjust tailstock.

49-68

To Align Centers Using Dial Indicator and Test Bar

49-69

Clean lathe and work center, mount test bar Adjust test bar snugly between centers and tighten tailstock spindle clamp 3.

– Mount dial indicator on toolpost or lathe carriage Indicator plunger should be parallel to lathe bed and contact point set on center

To Align Centers Using Dial Indicator and Test Bar

– Adjust cross-slide Indicator registers approximately .025 in at tailstock, indicator bezel to 0 5.

Move carriage by hand so indicator registers on diameter at headstock end and not indicator reading 6.

If both indicator readings not same, adjust tailstock with adjusting screws until indicator registers same at both ends

49-70

To Align Centers Using Dial Indicator and Test Bar

Tighten adjusting screw that was loosened Tighten tailstock clamp nut 9.

Adjust tailstock spindle until test bar snug between lathe centers 10.

Recheck indicator readings at both ends and adjust tailstock, if necessary

49-71

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Grinding Lathe Cutting Tools

Unit 50

Grinding Lathe Cutting Tool

• Wide variety of cutting tools for lathe – All have certain angles and clearances regardless of shape

To Grind a General-Purpose Toolbit

Dress face of grinding wheel Grip toolbit firmly, supporting hands on grinder toolrest • Hold toolbit at proper angel to grind cutting edge angle Tilt bottom of toolbit toward wheel and grind 10º side relief or clearance angle

Cutting edge ~ ½ In long and extend over ¼ width of toolbit 10º side relief or clearance angle

• While grinding, move toolbit back and forth across face of wheel Prevents grooving wheel 5.

• • • Toolbit must be cooled frequently during grinding

Never

overheat toolbit!

Never

quench stellite or cemented-carbide tools

Never

grind carbides with aluminum oxide wheel

50-76

50-77

Grind end cutting edge so it forms angle of – a little less than 90º with side cutting edge Hold tool so that end cutting edge angle and end relief angle of 15º ground at same time

Using toolbit grinding gage, check amount of end relief when toolbit is in toolholder

Hold top of toolbit approximately 45º to – axis of wheel and grind side rake to approximately 14º • Do not grind below top of toolbit Creates a chip trap

50-80

Grind slight radius on point of cutting tool, being sure to maintain same front and side clearance angle 10.

With oilstone, hone cutting edge of toolbit slightly • • Lengthen life of toolbit Enable it to produce better surface finish on workpiece

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Machining Between Centers

Unit 52

Machining Between Centers

• • Training programs (schools) – Remove and replace work in lathe many times before completed – Need assurance that machined diameter will run true with other diameters • Machining between centers saves time in setting up Common operations – Facing, rough and finish-turning, shoulder turning, filing and polishing

52-82

Setting Up a Cutting Tool

Move toolpost to the left-hand side of the T-slot in the compound rest Mount tool holder in toolpost so setscrew in toolholder 1 in.

beyond toolpost

52-83

Setting Up a Cutting Tool: cont.

Insert proper cutting tool into toolholder, having tool extend .500 in. beyond toolholder and never more than twice its thickness 4.

• Set cutting-tool point to center height Check it against lathe center point 5.

Tighten toolpost securely to prevent it from moving during a cut

52-85

Purposes of a Trial Cut

• • • – Produce accurate turned diameter Measured with micrometer Set cutting-tool point to the diameter Set crossfeed micrometer collar to the diameter

52-86

Procedure to Take a Trial Cut

Set up workpiece and cutting tool as for turning Set proper speeds and feed to suit material Start lathe and position toolbit over work approximately .125 in. from end Turn compound rest handle clockwise ¼ of a turn to remove any backlash

52-87

52-88

feed toolbit into work by turning crossfeed handle clockwise until light ring appears around entire circumference of work Do NOT move crossfeed handle setting Turn carriage handwheel until toolbit clears end of workpiece by about .060 in.

Turn crossfeed handle clockwise about .010 in. and take trial cut .250 in. along length of work Disengage automatic feed and clear toolbit past end of work with carriage handwheel

10.

Stop the lathe 11.

Test accuracy of micrometer by cleaning and closing measuring faces and then measure trial-cut diameter 12.

Calculate how much material must still be removed from diameter of work 13.

Turn crossfeed handle clockwise ½ amount of material to be removed

52-89

14.

Take another trial cut .250 in. long and stop the lathe 15.

Clear toolbit over end of work with carriage handwheel 16.

Measure diameter and readjust crossfeed handle until diameter is correct 17.

Machine diameter to length

52-90

Rough Turning

• • • Removes as much metal as possible in shortest length of time Accuracy and surface finish are not important in this operation – .020- to .030-in. feed recommended Work rough-turned to – Within .030 in. of finished size when removing up to .500 in. diameter – Within .060 in. when removing > .500 in.

52-91

Procedure for Rough Turning

Set lathe to correct speed for type and size of material being cut 2.

• Adjust quick-change gearbox for a .010 to .030-in. feed Depends on depth of cut and condition of machine 3.

Move toolholder to left-hand side of compound rest and set toolbit height to center

52-92

52-93

Tighten toolpost securely to prevent toolholder from moving during machining Take light trial cut at right-hand end of work for a .250 in. length 6.

Measure work and adjust toolbit for proper depth of cut 7.

Cut along for .250 in., stop lathe, and check diameter for size Diameter .030 in. over finish size 8.

Readjust depth of cut, if necessary

Finish Turning

• • Follows rough turning Produces smooth surface finish and cuts work to an accurate size • Factors affecting type of surface finish – Condition of cutting tool – Rigidity of machine and work – Lathe speeds and feeds

52-94

Procedure For Finish Turning

Make sure cutting edge of toolbit free from nicks, burrs, etc.

Set toolbit on center; check it against lathe center point Set lathe to recommended speed and feed

52-95

• • • Take light trial cut .250 in. long at right hand end of work Produce true diameter Set cutting tool to diameter Set graduated collar to diameter 5.

Stop lathe and measure diameter Set depth of cut for half amount of material to be removed Cut along for .250 in., stop lathe, check 8.

Readjust depth of cut and finish-turn

52-96

Filing in a Lathe

• • • • • Only to remove small amount of stock, remove burrs, or round off sharp corners Work should be turned to within .002 to .003 in. of size For safety, file with left hand so arms and hands kept clear of revolving chuck Remove toolbit from toolholder before filing Cover lathe bed with paper before filing

52-97

Procedure to File in a Lathe

Set spindle speed to twice that for turning Mount work between centers, lubricate, and carefully adjust dead center in work Move carriage as far to right as possible and remove toolpost Disengage lead screw and feed rod Select 10- or 12-in. mill file or long-angle lathe file

52-98

Start lathe Grasp file handle in left hand and support file point with fingers of right hand

52-100

Apply light pressure and push file forward to its full length; release pressure on return stroke • Move file about half width of file for each stroke and continue filing until finished Use 30-40 strokes per minute 10.

• • • • • Safety precautions Roll sleeves above elbow Remove watches and rings Never use file without properly fitted handle Never apply too much pressure Clean file frequently with file brush

Procedure for Polishing in a Lathe

• • • Select correct type and grade of abrasive cloth for finish desired Piece about 6 – 8 in. long and 1 in. wide Use aluminum oxide abrasive cloth for ferrous metals Use silicon carbide abrasive cloth should be used for nonferrous metals 2.

Set lathe to run at high speed Disengage feed rod and lead screw

52-101

Remove toolpost and toolholder Lubricate and adjust dead center Roll sleeves up above elbows and tuck in any loose clothing Start lathe 7.

Hold abrasive cloth on work With right hand, press cloth firmly on work while tightly holding other end of abrasive cloth with left hand 10.

Move cloth slowly back and forth

52-102

Shoulder

• Shoulder: the change in diameters, or step, when turning more than one diameter on a piece of work • Three common types of shoulders – Square – Filleted – Angular or Tapered

Three Types of Shoulders

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Knurling, Grooving, and Form Turning

Unit 53

Knurling

• Process if impressing a diamond-shaped or straight-line patter into the surface of the workpiece – Improve its appearance – Provide better gripping surface – Increase workpiece diameter when press fit required

53-106

Knurling

• Diamond- and straight-pattern rolls available in three styles – Fine – Medium – Course

53-107

Knurling Tool

• Toolpost-type toolholder on which pair of hardened-steel rolls mounted Knurling tool with one set of rolls in self-centering head

53-108

Knurling tool with three sets of rolls in revolving head

Universal Knurling Tool System

53-109

• Dovetailed shank and as many as seven interchangeable knurling heads that can produce wide range of knurling patterns • Combines in one tool – Versatility – Rigidity – Ease of handling – Simplicity

Procedure to Knurl in a Lathe

53-110

• Mount work between centers and mark required length to be knurled If work held in chuck for knurling, right end of work should be supported with revolving tailstock center 2.

Set lathe to run at one-quarter speed required for turning Set carriage feed to .015 to .030 in.

53-111

Set center of floating head of knurling tool even with dead-center point 5.

Set knurling tool at right angles to workpiece and tighten it securely

53-112

Start machine and lightly touch rolls against work to check tracking Move knurling tool to end of work so only half the roll face bears against work Force knurling tool into work approximately .025 in. and start lathe OR Start lathe and then force knurling tool into work until diamond pattern come to point

53-113

Stop lathe and examine pattern 10.

Once pattern correct, engage automatic carriage feed and apply cutting fluid to knurling rolls 11.

• Knurl to proper length and depth Do not disengage feed until full length has been knurled; otherwise, rings will be formed on knurled pattern 12.

If knurling pattern not to point after length has been knurled, reverse lathe feed and take another pass across work

Grooving

• • • Done at end of thread to permit full travel of nut up to a shoulder or at edge of shoulder for proper fit Also called recessing, undercutting, or necking Rounded grooves used where there is strain on part

53-114

Procedure to Cut a Groove

Grind toolbit to desired size and shape of groove required Lay out location of groove Set lathe to half the speed for turning Mount workpiece in lathe Set toolbit to center height

53-115

53-116

Locate toolbit on work at position where groove is to be cut Start lathe and feed cutting tool toward work using crossfeed handle until toolbit marks work lightly Hold crossfeed handle in position and set graduated collar to zero 9.

Calculate how far crossfeed screw must be turned to cut groove to proper depth 10.

Feed toolbit into work slowly using crossfeed handle

53-117

11.

Apply cutting fluid to point of cutting tool • To ensure cutting tool will not bind in groove, move carriage slightly to left and to right while grooving • Should chatter develop, reduce spindle speed 12.

Stop lathe and check depth of groove with outside calipers or knife-edge verniers

Safety note: Always wear safety goggles when grooving on a lathe

PowerPoint to accompany

Technology of Machine Tools

6 th Edition

Krar • Gill • Smid Threads and Thread Cutting

Unit 55

Threads

• • Used for hundreds of years for holding parts together, making adjustments, and transmitting power and motion • Art of producing threads continually improved Massed-produced by taps, dies, thread rolling, thread milling, and grinding

55-119

Threads

• • Thread – Helical ridge of uniform section formed on inside or outside of cylinder or cone Used for several purposes: – Fasten devices such as screws, bolts, studs, and nuts – – – Provide accurate measurement, as in micrometer Transmit motion Increase force

55-120

Thread Terminology

55-121

Thread Terminology

• • Screw thread – Helical ridge of uniform section formed on inside or outside of cylinder or cone • External thread – Cut on external surface or cone Internal thread – Produced on inside of cylinder or cone

55-122

55-123

• • • Major diameter – Largest diameter of external or internal thread Minor diameter – Smallest diameter of external or internal thread Pitch diameter – Diameter of imaginary cylinder that passes through thread at point where groove and thread widths are equal – Equal to major diameter minus single depth of thread – Tolerance and allowances given at pitch diameter line

• • • Number of threads per inch – Number of crests or roots per inch of threaded section (Does not apply to metric threads) Pitch – Distance from point on one thread to corresponding point on next thread, measured parallel to axis – Expressed in millimeters for metric threads Lead – Distance screw thread advances axially in one revolution (single-start thread, lead = pitch)

55-124

• • • Root – Bottom surface joining sides of two adjacent threads – External thread on minor diameter – Internal thread on major diameter Crest – – – Top surface joining two sides of thread External thread on major diameter Internal thread on minor diameter Flank – Thread surface that connects crest with root

55-125

• • • Depth of thread – Distance between crest and root measured perpendicular to axis Angle of thread – Included angle between sides of thread measured in axial plane Helix angle – Angle that thread makes with plane perpendicular to thread axis

55-126

• • Right-hand thread – Helical ridge of uniform cross section onto which nut is threaded in clockwise direction – When cut on lathe, toolbit advanced from right to left Left-hand thread – Helical ridge of uniform cross section onto which nut is threaded in counterclockwise direction – When cut on lathe, toolbit advanced from left to right

55-127

Thread Forms

• April, 1975 ISO came to an agreement covering standard metric thread profile – Specifies sizes and pitches for various threads in new ISO Metric Thread Standard – Has 25 thread sizes, range in diameter from 1.6 to 100 mm – Identified by letter M, nominal diameter, and pitch

M 5 X 0.8

55-128

55-129

American National Standard Thread

• • • Divided into four main series, all having same shape and proportions – National Coarse (NC) – – National Fine (NF) National Special (NS) – National Pipe (NPT) Has 60º angle with root and crest truncated to 1/8 th the pitch Used in fabrication, machine construction

American National Standard Thread

 .

6134 x

or .6134

N F

 .125

or .125

Unified Thread

• • Developed by U.S., Britain, and Canada for standardized thread system Combination of British Standard Whitworth and American National Standard Thread

(external thread)  .

6134 x

or .6134

N D

(internal thread)  .

5413 x

or .5413

N F

(external thread)

(internal thread)  .125

or .125

 .250

or .250

55-133

American National Acme Thread

• • Replacing square thread in many cases Used for feed screws, jacks, and vises

= minimum .500

P F

= .3707

= maximum .500

+ 0.010

= .3707

.0052

(for maximum depth)

Brown & Sharpe Worm Thread

• Used to mesh worm gears and transmit motion between two shafts at right angles to each other but not in same plane

= .6866

P F

= .335

P C

= .310

Square Thread

• • Being replaced by Acme thread because of difficulty in cutting it

= .500

Often found on vises and jack screws

= .500

P C

= .500

+ .002

55-136

International Metric thread

• Standardized thread used in Europe

= 0.7035

(maximum) = 0.6855

(minimum)

= 0.125

P R

= 0.0633

(maximum) = 0.054

(minimum)

Thread Fits and Classifications

• • • Fit – Relationship between two mating parts – Determined by amount of clearance or interference when they are assembled Nominal size – Designation used to identify size of part Actual size – – Measured size of thread or part Basic size: size from which tolerances are set

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Allowance

• • Permissible difference between largest external thread and smallest internal thread Difference produces tightest fit acceptable for any given classification The allowance for a 1 in.—8 UNC Class 2A and 2B fit is: Minimum pitch diameter of the internal thread (2B) = .9188 in.

Maximum pitch diameter of the external thread (2A) = .9168 in.

Allowance = .002 in.

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Tolerance

• • • • Variation permitted in part size May be expressed as plus, minus, or both Total tolerance is sum of plus and minus tolerances In Unified and National systems, tolerance is plus on external threads and minus on internal threads The tolerance for a 1 in.—8 UNC Class 2A thread is: Maximum pitch diameter of the external thread (2A) = .9168 in.

Minimum pitch diameter of the external thread (2A) = .9100 in.

Tolerance = .0068 in.

Limits

• Maximum and minimum dimensions of part The limits for a 1 in.—8 UNC Class 2A thread are: Maximum pitch diameter of the external thread (2A) = .9168 in..

Minimum pitch diameter of the external thread (2A) = .9100 in.

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Three Categories of Unified Thread Fits

• • • • External threads classified as 1A, 2A, and 3A and internal threads as 1B, 2B, 3B Classes 1A and 1B – Threads for work that must be assembled – Loosest fit Classes 2A and 2B – Used for most commercial fasteners – Medium or free fit Classes 3A and 3B – Used where more accurate fit and lead required – No allowance provided

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Thread Calculations: Example 1

To cut a correct thread on a lathe, it is necessary first to make calculations so thread will have the proper dimensions. Calculate pitch, depth, minor diameter, and width of flat for a ¾—10 UNC thread.

D P

= = single depth of thread pitch Minor dia  Major dia (

  .75

(.061

 .061)

)

P D

 1

tpi

 1 10  .

100 in.

 .

61343 x

 .61343

x .100

 .061

in.

 .

628 in.

Width of flat  

8 .

 0125 1 8 in.

1 x 10

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Thread Calculations: Example 2

What are the pitch, depth, minor diameter, width of crest and width of root for an M 6.3 X 1 thread?

P D

= = pitch

1 mm 0.54127 x 1 = 0.54 mm Width of root  0 .

25 x  0.25

x 1

 0 .

25 mm Minor dia  Major dia (

 6.3

(.54

 .54) 

)  5 .

22 mm Width of crest  0 .125

 0 .

125 x 1  0 .

125 mm

Procedure to Set the Quick Change Gearbox for Threading

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Check drawing for thread pitch required From chart on quick-change gearbox, find whole number that represents pitch in threads per inch or in millimeters With lathe stopped, engage tumbler lever in hole, which is in line with the pitch Set top lever in proper position as indicated on chart

Engage sliding gear in or out as required Turn lathe spindle by hand to ensure that lead screw revolves Recheck lever settings to avoid errors

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Thread-Chasing dial

• • Lathe spindle and lead screw must be in same relative position for each cut – Thread-chasing dial attached to carriage for this purpose Dial has eight divisions – – Even threads use any division Odd threads either numbered or unnumbered: not both

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Thread Cutting

• • • Produces a helical ridge of uniform section on workpiece Performed by taking successive cuts with threading toolbit of same shape as thread form required Work may be held between centers or in chuck

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Procedure to Set Up a Lathe for Threading (60º Thread)

Set lathe speed to ¼ speed used for turning Set quick-change gearbox for required pitch in threads per inch or in millimeters Engage lead screw Secure 60º threading toolbit and check angle using thread center gage Set compound rest at 29º to right; set to left for left-hand thread

Set cutting tool to height of lathe center point 7.

• • Mount work between centers Make sure lathe dog is tight on work If work mounted in chuck, it must be held tightly 8.

Set toolbit at right angles to work, using thread center gage Arrange apron controls to allow split-nut lever to be engaged

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Thread-Cutting Operation

Procedure to cut a 60º thread

Check major diameter of work for size 2.

Start lathe and chamfer end of workpiece with side of threading tool to just below minor diameter of thread Mark length to be threaded by cutting light groove at this point with threading tool while lathe revolving

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Move carriage until point of threading tool near right-hand end of work Turn crossfeed handle until threading tool close to diameter, but stop when handle is at 3 o'clock position Hold crossfeed handle in this position and set graduated collar to zero Turn compound rest handle until threading tool lightly marks work

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Move carriage to right until toolbit clears end of work Feed compound rest clockwise about .003 in.

10.

Engage split-nut lever on correct line of thread-chasing dial and take trial cut along length to be threaded 11.

At end of cut, turn crossfeed handle counterclockwise to move toolbit away from work and disengage split-nut lever

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12.

Stop lathe and check number of tpi with thread pitch gage, rule, or center gage 13.

After each cut, turn carriage handwheel to bring toolbit to start of thread and return crossfeed handle to zero 14.

Set depth of all threading cuts with compound rest handle • See Table 55.2 and Table 55.3

55-154 When tool is fed in at 29º, most of the cutting is done by the leading edge of toolbit.

55-155 Table 55.2 Depth settings for cutting 60 ° national form threads*

tpi

24 20 18 16 14 13 11

Compound Rest Setting 0 ° 30 ° 29 °

.027

.031

.0308

.0325 .0375 .037

.036

.0417 .041

.0405 .0468 .046

.0465 .0537 .0525

.0577 .057

.068

.0674

15.

Apply cutting fluid and take successive cuts until top (crest) and bottom (root) of thread are same width 16.

Remove burrs from top of thread with file 17.

Check thread with master nut and take further cuts

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Six Ways to Check Threads

• 1.

Depends on accuracy required: Master nut or screw Thread micrometer Three wires Thread roll or snap gage Thread ring or plug gage Optical comparator

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