Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine Tools 6 th Edition Threads and Thread Cutting Unit 55

55-2 Objectives Recognize and state the purposes of six common thread forms Set up a lathe to cut inch external Unified threads Set up an inch lathe to cut metric threads Set up a lathe and cut internal threads Set up a lathe and cut external Acme threads

55-3 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

55-4 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

55-5 Thread Terminology Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-6 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

55-7 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

55-8 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)

55-9 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

55-10 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

55-11 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-12 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

55-13 Table 55.1 ISO metric pitch and diameter combinations NominalThreadNominalThread DiameterPitchDiameterPitch (mm)(mm)(mm)(mm) Portion of table taken from textbook

55-14 ISO Metric Thread 60º included angle, crest = times pitch D (depth of thread) is times pitch Root is 1/4 th of pitch, Flat on root (FR) wider than crest (FC) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-15 ISO Metric Thread Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-16 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

55-17 American National Standard Thread Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-18 British Standard Whitwork (BSW) Thread Application is same as for American National form thread Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-19 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.

55-20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-21 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.

55-22 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.

55-23 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.

55-24 International Metric thread Standardized thread used in Europe F = 0.125P R = P (maximum) = 0.054P (minimum) D = P (maximum) = P (minimum) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-25 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

55-26 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.

55-27 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 the external thread (2A) =.9168 in. Minimum pitch diameter of the external thread (2A) =.9100 in. Tolerance =.0068 in. The tolerance for a 1 in.—8 UNC Class 2A thread is:

55-28 Limits Maximum and minimum dimensions of part Maximum pitch diameter of the external thread (2A) =.9168 in.. Minimum pitch diameter of the external thread (2A) =.9100 in. The limits for a 1 in.—8 UNC Class 2A thread are:

55-29 ISO Metric Tolerances and Allowances ISO metric screw thread tolerance system provides for allowances and tolerances defined by tolerance grades, tolerance positions, and tolerance classes Tolerance grades specified numerically –Medium tolerance indicated by number 6, number below 6 indicates finer tolerance and number above 6 indicates greater tolerance –Tolerance for thread at pitch line and major diameter may be shown on drawing using the following symbols: External threads: e indicates large allowance g indicates small allowance h indicates no allowance Internal threads: G indicates small allowance H indicates no allowance

55-30 Tolerance Example: An external metric thread may be designated as follows: PitchOutside NominalDia.Dia. Metric SizePitchToleranceTolerance M 6 X g6g The thread fit between mating parts is indicated by internal thread designation followed by the external thread tolerance: M 20 X 2 - 6H/5g 6g

55-31 Symbols Used to Indicate Allowance For external threads: –e indicates a large allowance –g indicates a small allowance –h indicates no allowance For internal threads: –G indicates a small allowance –H indicates no allowance

55-32 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

55-33 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 thread P = pitch Calculate pitch, depth, minor diameter, and width of flat for a ¾—10 UNC thread.

55-34 Thread Calculations: Example 2 P = pitch = 1 mm D = 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?

55-35 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

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

55-37 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.

55-38 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

55-39 Procedure to Set Up a Lathe for Threading (60º Thread) 1.Set lathe speed to ¼ speed used for turning 2.Set quick-change gearbox for required pitch in threads per inch or in millimeters 3.Engage lead screw 4.Secure 60º threading toolbit and check angle using thread center gage 5.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 9.Arrange apron controls to allow split-nut lever to be engaged

55-41 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

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

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

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

55-45 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.

55-46 Table 55.3: Depth setting for cutting 60° ISO metric threads Compound Rest Setting Pitch (mm) (mm) 0°30°29° Portion of table taken from textbook

55-47 Table 55.2 Depth settings for cutting 60° national form threads* Compound Rest Setting tpi0°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 16.Remove burrs from top of thread with file 17.Check thread with master nut and take further cuts

55-49 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

55-50 To Reset a Threading Tool Must reset when need to remove partly threaded work from lathe, tool removed for regrinding, or work slips under lathe dog Procedure 1.Set up lathe and work for thread cutting 2.Start lathe, toolbit clear of work, engage split-nut lever on correct line 3.Allow carriage to travel until toolbit opposite any portion of unfinished thread

Stop lathe, leaving split-nut lever engaged 5.Feed toolbit into thread groove using only compound rest and crossfeed handles until right-hand edge of toolbit touches rear side of thread 6.Set crossfeed graduated collar to zero 7.Back out threading tool using crossfeed handle, disengage split-nut lever, and move carriage until toolbit clears start of thread

Set crossfeed handle back to zero and take trial cut without setting compound rest 9.Set depth of cut using compound rest handle and finish thread to required depth

55-53 To Convert an Inch-Designed Lathe to Metric Threading Metric threads may be cut on standard quick- change gear lathe by using a pair of change gears having 50 and 127 teeth respectively. Lead screw has inch dimensions so necessary to convert pitch in millimeters to centimeters and then into threads per inch 1 in. = 2.54 cm which gives ration 1:2.54 (spindle or drive gear) (lead screw)

55-54 Procedure to Cut a 2-mm Metric Thread on Standard Quick-Change Gear Lathe 1.Mount the 127-tooth gear on lead screw 2.Mount the 50-tooth gear on spindle 3.Convert the 2-mm pitch to threads per centimeter: 4.Set quick-change gearbox to 5 tpi

Set up lathe for thread cutting 6.Take light trial cut 7.Reverse spindle rotation until cutting tool has just cleared start of threaded section 8.Check thread with metric screw pitch gage 9.Cut thread to required depth Note: Never disengage the split nut until the thread has been cut to depth.

55-56 Procedure to Cut a Left-Hand Thread (60º) Basically same as for right-hand threads, with few exceptions: 1.Set lathe speed and quick-change gearbox for pitch of thread to be cut 2.Engage feed-direction lever so that lead screw will revolve in opposite direction to that for right-hand thread

Set compound rest to 29º to the LEFT 4.Set up left-hand threading tool and square it with work 5.Cut groove at left end of section to be threaded Gives cutting tool starting point 6.Proceed to cut thread to same dimensions as for right-hand thread

55-58 Cutting a Thread on a Tapered Section Use same setup as for regular thread cutting Most important that threading tool be set at 90º to axis of work –Do NOT set it square with tapered surface

55-59 Common Methods of Measuring Threads 1.Thread ring gage 2.Thread plug gage 3.Thread snap gage 4.Screw thread micrometer 5.Optical comparator 6.Three-wire method

55-60 Three-Wire Method of Measuring Threads Recommended by Bureau of Standards and National Screw thread Commission Three wires of equal diameter placed in thread, two on one side and one on other side –Wire used should be hardened and lapped to three times accuracy of thread to be inspected Standard micrometer used to measure distance over wires (M) Different sizes and pitches of threads require different sizes of wires

55-61 The Three-Wire Method of Measuring 60º Threads Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-62 To Calculate the Measurement over the Wires where M = measurement over the wires D = major diameter of the thread G = diameter of the wire size used N = number of tpi Any of the following formulas can be used to calculate G:

55-63 Multiple Threads Used when necessary to obtain increase in lead and deep, coarse thread cannot be cut May be double, triple or quadruple, depending on number of starts around periphery of work Pitch: distance from point on one thread to corresponding point on next thread Lead: distance nut advances lengthwise in one complete revolution –Single-start: pitch and lead equal –Double-start: lead twice the pitch –Triple-start: lead three times pitch

55-64 Single Thread Double Thread Triple Thread Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-65 Multiple Threads May be Cut on a Lathe 1.Using accurately slotted driveplate or faceplate 2.Disengaging intermediate gear of end gear train and rotating spindle desired amount 3.Using thread-chasing dial Only for double-start threads with odd-number lead

55-66 Procedure to Cut an 8-tpi Double Thread 1.Set up lathe and cutting tool as for cutting single-start thread 2.Set quick-change gearbox to 4 tpi 3.Cut first thread to half depth required for 4 tpi 4.Leave crossfeed handle set to depth of thread and not reading on compound rest graduated collar 5.Withdraw threading tool from work using compound rest handle

Revolve work exactly ½ turn Remove work from lathe with lathe dog attached Replace work in lathe with tail of dog in slot exactly opposite one used for first thread 7.Reset crossfeed handle to same position as when cutting first thread 8.Cut second thread, feeding compound rest handle until graduated collar at same setting as for first thread

55-68 Thread-Chasing Dial Method of Cutting Multiple Threads Double-start threads with odd-numbered lead may be cut using thread-chasing dial 1.Take one cut on thread by engaging split nut at numbered line on chasing dial 2.Without changing depth of cut, take another cut at unnumbered line on chasing dial (second thread will be exactly in middle of first thread) 3.Continue cutting thread to depth, taking two passes for every depth-of-cut setting

55-69 Square Threads Found in vise screws, jacks, and other devices where maximum power transmission required Square threading tool looks like short cutting- off tool Both sides of square threading tool must be ground at angle to conform to helix angle of thread Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-70 Square Threading Tool Helix angle of thread (angle of square threading tool) depends on two factors: –Changes for each different lead on given diameter (greater lead, greater angle) –Changes for each different diameter of thread for given lead (larger diameter, smaller angle) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-71 To Calculate the Helix Angles of the Leading and Following Sides of a Square Thread

55-72 Clearance Square toolbit must be provided with approximately 1º clearance on each side –Makes it thinner at bottom For leading side of tool –Add 1º to calculated helix angle On following side –Subtract 1º from calculated angle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-73 Procedure to Cut Square thread 1.Grind threading tool to proper leading and following angles Width of tool should be ~.002 in. wider than thread groove 2.Align lathe centers and mount work 3.Set quick-change gearbox for required number of tpi 4.Set compound rest at 30º to right, which will provide side movement if necessary to reset cutting tool

Set threading tool square with work and center 6.Cut right-hand end of work to minor diameter for approximately.060 in. long 7.If work permits, cut recess at end of thread to minor diameter 8.Calculate single depth of thread 9.Start lathe and just touch tool to work diameter 10.Set crossfeed graduated collar to zero

Set.003 in. depth of cut with crossfeed screw and take trial cut 12.Check thread with thread pitch gage 13.Apply cutting fluid and cut thread to depth, moving crossfeed in from.002 to.010 in. for each cut Depth of cut will depend on thread size and nature of workpiece Note: Since thread sides are square, all cuts must be set using crossfeed screw

55-76 Acme Thread Gradually replacing square thread because stronger and easier to cut with taps and dies Used extensively for lead screws because 29º angle formed by its sides allows split nut to be engaged during thread cutting Provided with.010-in. clearance for both crest and root on all sizes of threads

55-77 Procedure to Cut an Acme Thread 1.Grind toolbit to fit end of Acme thread gage 2.Grind point of tool flat until it fits into slots of gage Indicates number of threads per inch to be cut Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Set quick-change gearbox to required number of threads per inch 4.Set compound rest 14 ½º to right 5.Set Acme threading tool on center and square it with work using gage 6.At right-hand end of work, cut section.060 in. long to minor diameter 7.Cut thread to proper depth by feeding cutting tool, using compound rest

55-79 Measuring Acme Threads One Wire Method Single wire or pin of correct diameter placed in thread groove and measured with micrometer Thread correct size when micrometer reading over wire is same as major diameter of thread and wire is tight in the thread Diameter of wire to be used can be calculated as follows: Wire diameter =.4872 x pitch Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

55-80 Internal Threads Cutting threads in a hole –Operation performed on work held in chuck or collet or mounted on faceplate –Threading tool similar to boring toolbit except shape ground to form of thread to be cut Procedure 1.Calculate tap drill size of thread

Mount work to be threaded in chuck or collet or faceplate 3.Drill hole approximately 1⁄16 in. smaller than tap drill size in workpiece 4.Mount boring tool in lathe and bore hole to tap drill size 5.Recess start of hole to major diameter of thread for.060-in. length 6.If thread does not go through workpiece, a recess should be cut at end of thread to major diameter; recess should be wide enough to allow threading tool to “run out” and permit time to disengage split-nut lever

55-82 The compound rest is set at 29º to the left for cutting right-hand internal threads. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Set compound rest at 29° to left, to right for left-hand threads 8.Mount threading toolbit into boring bar and set it to center 9.Square threading tool with thread center gage 10.Place mark on boring bar, measuring from threading tool, to indicate length of hole to be threaded 11.Start lathe and turn crossfeed handle out until threading tool just scratches internal diameter

Set crossfeed graduated collar to zero 8.Set.003-in. depth of cut by feeding compound rest out and take trial cut 9.At end of each cut on internal thread, disengage split-nut lever and feed crossfeed handle in to clear thread 10.Clear threading tool from hole and check pitch of thread 11.Return crossfeed handle back to zero and set depth of cut by turning compound rest out desired amount 12.Cut thread to depth; check fit with a screw or threaded plug gage