lathe
DESCRIPTION
Lathe MachinesTRANSCRIPT
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The Lathe
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
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Technology of Machine Tools6th Edition
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
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Indicated by the swing and the length of the bed
Lathe Size
Parts of the Lathe
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Bed
Headstock
QuickChangeGearbox
Tailstock
Carriage
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Setting Speeds on a Lathe
• Speeds measured in revolutions per minute– Changed by stepped pulleys or gear levers
• Belt-driven lathe– Various speeds obtained by changing flat belt
and back gear drive
• Geared-head lathe– Speeds changed by moving speed levers into
proper positions according to r/min chart fastened to headstock
Safety Note!! NEVER change speeds when lathe is running.
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
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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 isapplied to feed rod
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Technology of Machine Tools6th Edition
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
46-16
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 shank60º centers, steel with carbide tips
• Care to adjust and lubricate occasionally
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46-17
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
46-18
Three-jaw Universal Chuck• Holds round and hexagonal work• Grasps work quickly and accurate within
few thousandths/inch• Three jaws move
simultaneously whenadjusted by chuck wrench– Caused by scroll plate into
which all three jaws fit
• Two sets of jaw: outside chucking and inside chucking
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46-19
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
46-20
Headstock Spindles
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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
46-21
Headstock Spindles
3. Cam-lock spindle nose• Held by tightening cam-locks using T-wrench• Chuck aligned by taper
on spindle nose
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Registration lines on spindle nose
Registration lines on cam-lock
Cam-locks
Cam-lock mating stud on chuck or faceplate
46-22
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
46-23
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|
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.
46-24
Types of Lathe Dogs
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• 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
46-25
Types of Lathe Dogs
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• 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
46-26
Left-Hand Offset Toolholder
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• Offset to the right
• Designed for machining work close to chuck or faceplate and cutting right to left
• Designated by letter L
46-27
Right-Hand Offset Toolholder
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• 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
46-28
Straight Toolholder
• General-purpose type
• Used for taking cuts in either direction and for general machining operations
• Designated by letter S
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46-29
Toolholders for Indexable Carbide Inserts
• Held in holder by cam action or clamps
• Types available– Conventional
– Turret-type
– Heavy-duty toolposts
46-30
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
46-31
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
46-32
Super Quick-Change Toolpost
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Technology of Machine Tools6th Edition
Cutting Speed, Feed, and Depth
of Cut Unit 47
47-34
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
47-35
Lathe Cutting Speeds in Feet and Meters per Minute Using High-Speed Steel Toolbit
Turning and Boring
Rough CutFinish Cut Threading
Material ft/min m/min ft/min m/min ft/min m/min
Machine steel 90 27 100 30 35 11
Tool steel 70 21 90 27 30 9
Cast iron 60 18 80 24 25 8
Bronze 90 27 100 30 25 8
Aluminum 200 61 300 93 60 18
47-36
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)
D
4 x CS
D
12 x CS r/min
47-37
Example:
D
4 x CS r/min
Calculate r/min required to rough-turn 2 in. diameter piece of machine steel (CS 90):
1802
4 x 90 r/min
D
320 x CS r/min Metric Formula
47-38
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
47-39
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)
47-40
Feeds for Various Materials(using high-speed steel cutting tool)
Rough Cuts Finish Cuts
Material in. mm in. mm
Machine steel .010–.020 0.25–0.5 .003–.010 0.07–0.25
Tool steel .010–.020 0.25–0.5 .003–.010 0.07–0.25
Cast iron .015–.025 0.4–0.65 .005–.012 0.13–0.3
Bronze .015–.025 0.4–0.65 .003–.010 0.07–0.25
Aluminum .015–.030 0.4–0.75 .005–.010 0.13–0.25
47-41
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.
47-42
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Example: Depth of cut on a lathe
47-43
Factors Determining Depth of Rough-Turning Cut
• Condition of machine
• Type and shape of cutting tool used
• Rigidity of workpiece, machine, and cutting tool
• Rate of feed
47-44
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
47-45
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On machines where the workpiece revolves,the cutting tool should be set in for only half the amount to be removed from the diameter.
47-46
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On machines where the workpiece does not revolve, the cutting tool should be set in for
the amount of material to be removed.
47-47
Hints on Graduated Collar Use
1. Make sure collar is secure before setting a depth of cut
2. All depths of cut must be made by feeding cutting tool toward workpiece
3. If graduated collar turned past desired setting, must be turned backward half-turn and fed into proper setting to remove backlash
4. Never hold graduated collar when setting depth of cut
47-48
5. 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
47-49
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The compound rest is set at 84º16' for making fine settings.
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Technology of Machine Tools6th Edition
Lathe Safety Unit 48
48-51
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
48-52
Safety Precautions
• Lathe hazardous if not operated properly
• Important to keep machine and surrounding area clean and tidy
• Accidents usually caused by carelessness
48-53
Safety Precautions
• Always wear approved safety glasses
• Rollup sleeves, remove tie and tuck in loose clothing
• Never wear ring or watch
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
48-55
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
48-56
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
48-57
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
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Technology of Machine Tools6th Edition
Mounting, Removing, and Aligning Lathe
Centers Unit 49
49-59
Objectives
• Mount and/or remove lathe centers properly
• Align lathe centers by visual, trial-cut, and dial-indicator methods
49-60
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
49-61
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
49-62
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
49-63
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 checkand not too accurate
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49-64
Alignment of Lathe Centers
2. Using the trial-cutmethod where smallcut taken from eachend of work anddiameters measured with a micrometer
3. Using parallel test bar and dial indicator• Fastest and most accurate method
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49-65
To Align Centers by Adjusting the Tailstock
1. Loosen tailstock clamp nut or level2. 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
3. Tighten screw to lock both halves in place4. Make sure tailstock lines still aligned5. Lock tailstock clamp nut or lever
49-66
To Align Centers by Trial-Cut Method
1. Take a light cut (~.005 in.) to true diameter from section A at tailstock end for .250 in. long
2. Stop feed and note reading on graduated collar of crossfeed handle
3. Move cutting tool away from work with crossfeed handle
4. Bring cutting tool close to headstock end
49-67
To Align Centers by Trial-Cut Method
5. Return cutting tool to same graduated collar setting as at section A
6. Cut a .500-in (13 mm) length at section B and stop lathe
7. Measure both diameters with micrometer
49-68
To Align Centers by Trial-Cut Method
8. If both diameters not same size, adjust tailstock either toward or away from cutting tool ½ difference of two readings
9. Take another light cut at A and B at same crossfeed graduated collar setting. Measure diameters and adjust tailstock.
49-69
To Align Centers Using Dial Indicator and Test Bar
1. Clean lathe and work center, mount test bar
2. 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
49-70
To Align Centers Using Dial Indicator and Test Bar
4. 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
49-71
To Align Centers Using Dial Indicator and Test Bar
7. Tighten adjusting screw that was loosened
8. 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
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Technology of Machine Tools6th Edition
Grinding Lathe Cutting Tools
Unit 50
50-73
Grinding Lathe Cutting Tool
• Wide variety of cutting tools for lathe– All have certain angles and clearances regardless
of shape
Shape and Dimensions of General-purpose Lathe Toolbit
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50-74
To Grind a General-Purpose Toolbit
1. Dress face of grinding wheel2. Grip toolbit firmly, supporting hands on grinder
toolrest3. Hold toolbit at proper
angel to grind cuttingedge angle
• Tilt bottom of toolbittoward wheel and grind 10º side reliefor clearance angle
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50-75
Cutting edge ~ ½ In long and extend over ¼ width of toolbit
10º side relief or clearance angle
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50-76
4. 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
50-77
6. 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
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70º to 80º Point Angle
50-78
7. Using toolbit grinding gage, check amount of end relief when toolbit is in toolholder
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50-79
8. 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
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Side rake ground the lengthof the cutting edge
50-80
9. 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
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Technology of Machine Tools6th Edition
Machining Between Centers
Unit 52
52-82
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
52-83
Setting Up a Cutting Tool
1. Move toolpost to the left-hand side of the T-slot in the compoundrest
2. Mount tool-holder intoolpost sosetscrew intoolholder 1 in.beyond toolpost
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52-84
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Heavy Cuts: Set toolholder at right angles to work
52-85
Setting Up a Cutting Tool: cont.
3. 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
52-86
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
52-87
Procedure to Take a Trial Cut
1. Set up workpiece and cutting tool as for turning
2. Set proper speeds and feed to suit material
3. Start lathe and position toolbit over work approximately .125 in. from end
4. Turn compound rest handle clockwise ¼ of a turn to remove any backlash
52-88
5. feed toolbit into work by turning crossfeed handle clockwise until light ring appears around entire circumference of work
6. Do NOT move crossfeed handle setting7. Turn carriage handwheel until toolbit
clears end of workpiece by about .060 in.8. Turn crossfeed handle clockwise
about .010 in. and take trial cut .250 in. along length of work
9. Disengage automatic feed and clear toolbit past end of work with carriage handwheel
52-89
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
52-90
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
52-91
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.
52-92
Procedure for Rough Turning
1. 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
52-93
4. Tighten toolpost securely to prevent toolholder from moving during machining
5. 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
1. Diameter .030 in. over finish size
8. Readjust depth of cut, if necessary
52-94
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
52-95
Procedure For Finish Turning
1. Make sure cutting edge of toolbit free from nicks, burrs, etc.
2. Set toolbit on center; check it against lathe center point
3. Set lathe to recommended speed and feed
52-96
4. 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 diameter6. Set depth of cut for half amount of
material to be removed7. Cut along for .250 in., stop lathe, check8. Readjust depth of cut and finish-turn
52-97
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
52-98
Procedure to File in a Lathe
1. Set spindle speed to twice that for turning2. Mount work between centers, lubricate,
and carefully adjust dead center in work3. Move carriage as far to right as possible
and remove toolpost4. Disengage lead screw and feed rod5. Select 10- or 12-in. mill file or long-angle
lathe file
52-99
6. Start lathe
7. Grasp file handle in left hand and support file point with fingers of right hand
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52-100
8. Apply light pressure and push file forward to its full length; release pressure on return stroke
9. Move file about half width of file for each stroke and continue filing until finished
• Use 30-40 strokes per minute10. 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
52-101
Procedure for Polishing in a Lathe
1. 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 speed3. Disengage feed rod and lead screw
52-102
4. Remove toolpost and toolholder5. Lubricate and adjust dead center6. Roll sleeves up above elbows and tuck in
any loose clothing7. Start lathe8. Hold abrasive cloth on work9. 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
52-103
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
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52-104
Three Types of Shoulders
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Krar • Gill • Smid
Technology of Machine Tools6th Edition
Knurling, Grooving,
and Form Turning Unit 53
53-106
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
53-107
Knurling
• Diamond- and straight-pattern rolls available in three styles– Fine
– Medium
– Course
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53-108
Knurling Tool
• Toolpost-type toolholder on which pair of hardened-steel rolls mounted
Knurling tool withone set of rolls inself-centering head
Knurling tool with three sets of rollsin revolving head
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53-109
Universal Knurling Tool System
• 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
53-110
Procedure to Knurl in a Lathe
1. 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
3. Set carriage feed to .015 to .030 in.
53-111
4. Set center of floating head of knurling tool even with dead-center point
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5. Set knurling tool at right angles to workpiece and tighten it securely
53-112
6. Start machine and lightly touch rolls against work to check tracking
7. Move knurling tool to end of work so only half the roll face bears against work
8. 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
9. Stop lathe and examine pattern10. 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
53-114
Grooving
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Square
Round
V-shaped
• 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
53-115
Procedure to Cut a Groove
1. Grind toolbit to desired size and shape of groove required
2. Lay out location of groove
3. Set lathe to half the speed for turning
4. Mount workpiece in lathe
5. Set toolbit to center height
53-116
6. Locate toolbit on work at position where groove is to be cut
7. Start lathe and feed cutting tool toward work using crossfeed handle until toolbit marks work lightly
8. 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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
PowerPoint to accompany
Krar • Gill • Smid
Technology of Machine Tools6th Edition
Threads and Thread Cutting
Unit 55
55-119
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
55-120
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
55-121
Thread Terminology
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-122
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
55-123
• 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
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• 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)
55-125
• 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
55-126
• 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
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• 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, toolbitadvanced from left to right
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-128
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
55-129
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 1/8th the pitch
• Used in fabrication, machine construction
55-130
American National Standard Thread
NPF
NPD
.125or x .125
.6134or x 6134.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-131
Unified Thread• Developed by U.S., Britain, and Canada for
standardized thread system• Combination of British Standard Whitworth and
American National Standard Thread
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-132
NPF
NPF
NPD
NPD
.250or x .250 thread)(internal
.125or x .125 thread)(external
.5413or x 5413. thread)(internal
.6134or x 6134. thread)(external
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55-133
American National Acme Thread
• Replacing square thread in many cases• Used for feed screws, jacks, and vises
D = minimum .500P
= maximum .500P + 0.010
F = .3707P
C = .3707P - .0052 (for maximum depth)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-134
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
D = .6866P
F = .335P
C = .310P
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-135
Square Thread
• Being replaced by Acme thread because of difficulty in cutting it
• Often found on vises and jack screws
D = .500PF = .500PC = .500P + .002
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-136
International Metric thread
• Standardized thread used in EuropeF = 0.125PR = 0.0633P (maximum) = 0.054P (minimum)
D = 0.7035P (maximum) = 0.6855P (minimum)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-137
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
55-138
Allowance
• Permissible difference between largest external thread and smallest internal thread
• Difference produces tightest fit acceptable for any given classificationThe allowance for a 1 in.—8 UNC Class 2A and 2B fit is:
Minimum pitch diameter of theinternal thread (2B) = .9188 in.Maximum pitch diameter of theexternal thread (2A) = .9168 in. Allowance = .002 in.
55-139
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
Maximum pitch diameter of theexternal thread (2A) = .9168 in.Minimum pitch diameter of theexternal thread (2A) = .9100 in. Tolerance = .0068 in.
The tolerance for a 1 in.—8 UNC Class 2A thread is:
55-140
Limits
• Maximum and minimum dimensions of part
Maximum pitch diameter of theexternal thread (2A) = .9168 in..Minimum pitch diameter of theexternal thread (2A) = .9100 in.
The limits for a 1 in.—8 UNC Class 2A thread are:
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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
55-142
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.
D = single depth of threadP = pitch
in. .061.100 x .61343
x 61343.
in. 100.10
11
PD
tpiP
Calculate pitch, depth, minor diameter, and width of flatfor a ¾—10 UNC thread.
in. 0125. 10
1 x
8
1
8 flat ofWidth
in. 628.
.061)(.061 - .75
)( - diaMajor diaMinor
P
DD
55-143
Thread Calculations: Example 2
P = pitch = 1 mmD = 0.54127 x 1 = 0.54 mm
What are the pitch, depth, minor diameter, width of crest and width of root for an M 6.3 X 1 thread?
mm 125.0
1 x 125.0
x .1250 crest ofWidth
mm 22.5
.54)(.54 - 6.3
)( - diaMajor diaMinor
P
DD
mm 25.0
1 x 0.25
x 25.0root ofWidth
P
55-144
Procedure to Set the Quick-Change Gearbox for Threading
1. Check drawing for thread pitch required
2. From chart on quick-change gearbox, find whole number that represents pitch in threads per inch or in millimeters
3. With lathe stopped, engage tumbler lever in hole, which is in line with the pitch
4. Set top lever in proper position as indicated on chart
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5. Engage sliding gear in or out as required
6. Turn lathe spindle by hand to ensure that lead screw revolves
7. Recheck lever settings to avoid errors
55-146
Thread-Chasing dial
• Lathe spindle and lead screw must bein same relative position for each cut– Thread-chasing dial
attached to carriage forthis purpose
• Dial has eight divisions– Even threads use any division– Odd threads either numbered
or unnumbered: not both
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55-147
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
55-148
Procedure to Set Up a Lathe for Threading (60º Thread)
1. Set lathe speed to ¼ speed used for turning2. Set quick-change gearbox for required pitch
in threads per inch or in millimeters3. Engage lead screw4. Secure 60º threading toolbit and check
angle using thread center gage5. Set compound rest at 29º to right; set to left
for left-hand thread
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6. 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
9. Arrange apron controls to allow split-nut lever to be engaged
55-150
Thread-Cutting Operation
Procedure to cut a 60º thread
1. 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
3. Mark length to be threaded by cutting light groove at this point with threading tool while lathe revolving
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4. Move carriage until point of threading tool near right-hand end of work
5. Turn crossfeed handle until threading tool close to diameter, but stop when handle is at 3 o'clock position
6. Hold crossfeed handle in this position and set graduated collar to zero
7. Turn compound rest handle until threading tool lightly marks work
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8. Move carriage to right until toolbit clears end of work
9. 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
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When tool is fed in at 29º, most of the cutting is done by the leading edge of toolbit.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
55-155
Table 55.2 Depth settings for cutting 60° national form
threads*Compound Rest Setting
tpi 0° 30° 29°
24 .027 .031 .0308
20 .0325.0375.037
18 .036 .0417.041
16 .0405.0468.046
14 .0465.0537.0525
13 .050 .0577.057
11 .059 .068 .0674
Portion of table taken from textbook
55-156
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
55-157
Six Ways to Check Threads
• Depends on accuracy required:
1. Master nut or screw
2. Thread micrometer
3. Three wires
4. Thread roll or snap gage
5. Thread ring or plug gage
6. Optical comparator