The Lathe Section 11.

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

Engine Lathe Parts Unit 45

Engine Lathe Accurate and versatile machine Operations Three common
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: mm swing and bed length of 500 – 3000 mm

Indicated by the swing and the length of the bed
Lathe Size Indicated by the swing and the length of the bed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Parts of the Lathe Headstock Tailstock Quick Bed Change Gearbox
Carriage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Lathe Accessories Unit 46

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

Headstock Spindles Universal and independent chuck fitted to three types of headstock spindles Threaded spindle nose Screws on in a clockwise direction Tapered spindle nose Held by lock nut that tightens on chuck Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Cam-lock mating stud on chuck or faceplate
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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Types of Lathe Dogs Safety clamp lathe dog Clamp lathe dog
Used to hold variety of work Wide range of adjustment Clamp lathe dog Wider range than others Used on all shapes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Straight Toolholder General-purpose type
Used for taking cuts in either direction and for general machining operations Designated by letter S Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Toolholders for Indexable Carbide Inserts
Held in holder by cam action or clamps Types available Conventional Turret-type Heavy-duty toolposts

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

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

Super Quick-Change Toolpost

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

Lathe Cutting Speeds in Feet and Meters per Minute Using High-Speed Steel Toolbit
Turning and Boring Rough Cut Finish Cut Threading Material ft/min m/min ft/min m/min ft/min m/min Machine steel Tool steel Cast iron Bronze Aluminum

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)

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

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

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 mm)

Feeds for Various Materials (using high-speed steel cutting tool)
Rough Cuts Finish Cuts Material in mm in mm Machine steel .010– – – –0.25 Tool steel .010– – – –0.25 Cast iron .015– – – –0.3 Bronze .015– – – –0.25 Aluminum .015– – – –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.

Example: Depth of cut on a lathe

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

Inch System Circumference of crossfeed and compound rest screw collars divided into 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

On machines where the workpiece revolves, the cutting tool should be set in for only half the amount to be removed from the diameter. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

On machines where the workpiece does not revolve, the cutting tool should be set in for the amount of material to be removed. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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 = in. infeed movement

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

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

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

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

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

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

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

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

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

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

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

Alignment of Lathe Centers
Parallel diameter produced when lathe center aligned Three common methods used to align Aligning centerlines on back of tailstock with each other Only a visual check and not too accurate Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Alignment of Lathe Centers
Using the trial-cut method where small cut taken from each end of work and diameters measured with a micrometer Using parallel test bar and dial indicator Fastest and most accurate method Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

To Align Centers by Adjusting the Tailstock
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

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

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.

To Align Centers Using Dial Indicator and Test Bar
Clean lathe and work center, mount test bar Adjust test bar snugly between centers and tighten tailstock spindle clamp Mount dial indicator on toolpost or lathe carriage Indicator plunger should be parallel to lathe bed and contact point set on center

Adjust cross-slide Indicator registers approximately .025 in at tailstock, indicator bezel to 0 Move carriage by hand so indicator registers on diameter at headstock end and not indicator reading If both indicator readings not same, adjust tailstock with adjusting screws until indicator registers same at both ends

Tighten adjusting screw that was loosened Tighten tailstock clamp nut Adjust tailstock spindle until test bar snug between lathe centers Recheck indicator readings at both ends and adjust tailstock, if necessary

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 Shape and Dimensions of General-purpose Lathe Toolbit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Cutting edge ~ ½ In long and extend over ¼ width of toolbit
10º side relief or clearance angle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

While grinding, move toolbit back and forth across face of wheel
Prevents grooving wheel Toolbit must be cooled frequently during grinding Never overheat toolbit! Never quench stellite or cemented-carbide tools Never grind carbides with aluminum oxide wheel

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 70º to 80º Point Angle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

Side rake ground the length of the cutting edge
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 Side rake ground the length of the cutting edge Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

With oilstone, hone cutting edge of toolbit slightly
Grind slight radius on point of cutting tool, being sure to maintain same front and side clearance angle With oilstone, hone cutting edge of toolbit slightly Lengthen life of toolbit Enable it to produce better surface finish on workpiece

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

Setting Up a Cutting Tool
Move toolpost to the left-hand side of the T-slot in the compound rest Mount toolholder in toolpost so setscrew in toolholder 1 in. beyond toolpost Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Heavy Cuts: Set toolholder at right angles to work

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 Set cutting-tool point to center height Check it against lathe center point Tighten toolpost securely to prevent it from moving during a cut

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

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

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

Stop the lathe Test accuracy of micrometer by cleaning and closing measuring faces and then measure trial-cut diameter Calculate how much material must still be removed from diameter of work Turn crossfeed handle clockwise ½ amount of material to be removed

Take another trial cut .250 in. long and stop the lathe
Clear toolbit over end of work with carriage handwheel Measure diameter and readjust crossfeed handle until diameter is correct Machine diameter to length

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.

Procedure for Rough Turning
Set lathe to correct speed for type and size of material being cut Adjust quick-change gearbox for a to .030-in. feed Depends on depth of cut and condition of machine Move toolholder to left-hand side of compound rest and set toolbit height to center

Take light trial cut at right-hand end of work for a .250 in. length
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 Measure work and adjust toolbit for proper depth of cut Cut along for .250 in., stop lathe, and check diameter for size Diameter .030 in. over finish size 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

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

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 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 Readjust depth of cut and finish-turn

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

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

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

Remove toolpost and toolholder
Lubricate and adjust dead center Roll sleeves up above elbows and tuck in any loose clothing Start lathe Hold abrasive cloth on work With right hand, press cloth firmly on work while tightly holding other end of abrasive cloth with left hand Move cloth slowly back and forth

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Three Types of Shoulders

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

Knurling Diamond- and straight-pattern rolls available in three styles
Fine Medium Course Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Knurling Tool Toolpost-type toolholder on which pair of hardened-steel rolls mounted Knurling tool with one set of rolls in self-centering head Knurling tool with three sets of rolls in revolving head Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

Set knurling tool at right angles to workpiece and tighten it securely
Set center of floating head of knurling tool even with dead-center point Set knurling tool at right angles to workpiece and tighten it securely Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

Stop lathe and examine pattern
Once pattern correct, engage automatic carriage feed and apply cutting fluid to knurling rolls Knurl to proper length and depth Do not disengage feed until full length has been knurled; otherwise, rings will be formed on knurled pattern If knurling pattern not to point after length has been knurled, reverse lathe feed and take another pass across work

Grooving 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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 Calculate how far crossfeed screw must be turned to cut groove to proper depth Feed toolbit into work slowly using crossfeed handle

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 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

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

Threads Thread Used for several purposes:
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

Thread Terminology Screw thread External thread Internal 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

Major diameter Minor diameter Pitch 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)

Root Crest Flank 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

Depth of thread Angle of thread Helix angle
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

Right-hand thread Left-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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

American National Standard Thread

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.

American National Acme Thread
Replacing square thread in many cases Used for feed screws, jacks, and vises D = minimum .500P = maximum .500P F = .3707P C = .3707P (for maximum depth) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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.

Square Thread Being replaced by Acme thread because of difficulty in cutting it Often found on vises and jack screws D = .500P F = .500P C = .500P Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

International Metric thread
Standardized thread used in Europe D = P (maximum) = P (minimum) F = 0.125P R = P (maximum) = 0.054P (minimum) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Thread Fits and Classifications
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

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) = in. Maximum pitch diameter of the external thread (2A) = in. Allowance = .002 in.

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

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

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 = single depth of thread P = pitch

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 = pitch = 1 mm D = x 1 = 0.54 mm

Procedure to Set the Quick-Change Gearbox for Threading
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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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
Mount work between centers Make sure lathe dog is tight on work If work mounted in chuck, it must be held tightly Set toolbit at right angles to work, using thread center gage Arrange apron controls to allow split-nut lever to be engaged

Thread-Cutting Operation
Procedure to cut a 60º thread Check major diameter of work for size 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

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

Move carriage to right until toolbit clears end of work
Feed compound rest clockwise about .003 in. Engage split-nut lever on correct line of thread-chasing dial and take trial cut along length to be threaded At end of cut, turn crossfeed handle counterclockwise to move toolbit away from work and disengage split-nut lever

Set depth of all threading cuts with compound rest handle
Stop lathe and check number of tpi with thread pitch gage, rule, or center gage After each cut, turn carriage handwheel to bring toolbit to start of thread and return crossfeed handle to zero Set depth of all threading cuts with compound rest handle See Table 55.2 and Table 55.3

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

Table 55.2 Depth settings for cutting 60° national form threads*
Compound Rest Setting tpi 0° 30° 29° Portion of table taken from textbook

Apply cutting fluid and take successive cuts until top (crest) and bottom (root) of thread are same width Remove burrs from top of thread with file Check thread with master nut and take further cuts

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

The Lathe Section 11.

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The Lathe Section 11.

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