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Thread-Cutting Tools and Procedures

发布时间:2013-12-04 13:02:25  

PowerPoint to accompany

Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

24-2

Objectives

?

?

?

?Calculate the tap drill size for inch and metrictapsCut internal threads using a variety of tapsKnow the methods used to remove broken taps from a holeCut external threads using a variety of dies

24-3

Hand Taps

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?

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?Cutting tools used to cut internal threadsMade from high quality tool steel, hardened and groundTwo, three, or four flutes cut lengthwise across threads to form cutting edges–Provide room for chips–Admit cutting fluid to lubricate tapEnd of shank square so tap wrench can be used to turn tap into hole

24-4

Hand Taps

?Inch tap markings–Major diameter

–Number of threads per inch–Type of thread

? in.—13 UNC? in. = major diameter of tap13 = number of threads per inch

UNC = Unified National Coarse (type of thread)

24-5–Tread hole through workpiece

?Bottoming tap

–Not tapered but chamfered at end

–Used for threading to bottom of blind holeCopyright ? The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24-6

Another Method of

Identifying Taps

?Use of annular ring(s) cut around top of shank

One –taper tap

Two –plug tap

Three –bottoming tap

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

24-7

71TDS = tap drill sizeTDS ? - D = major diameter of tap89N = number of threads per inch ?.875?.111?.764 in.

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

24-8

Metric Taps

?

?

?Several thread forms and standards in metric thread systemInternational Standards Organization (ISO) has adopted standard metric thread–Will be used in US, Canada and other countries–Have only 25 thread sizes (1.6 to 100 mm)Available in sets of three: taper, plug and bottoming taps

–Identified by letter M followed by nominal diameter of thread in mm times pitch in mm

24-9

Tap Drill Sizes for Metric TapsTap drill size for metric taps calculated in same manner as for U.S. Standard threads

TDS = major diameter (mm) –pitch (mm)Example:

Find the tap drill size for a 22 –2.5 mm thread

TDS = 22 –2.5

= 19.5 mm

?

?

?24-10Tapping a HoleOperation of cutting internal thread using tap and tap wrenchTaps hard and brittleEasily broken

–Extreme care used to prevent breakage–Broken tap in hole difficult to remove and often results in scrapping work

1.

2.

3.24-11To Tap Hole By HandSelect correct taps and tap wrench for jobApply suitable cutting fluid to the tapNo fluid required for tapping brass or cast ironPlace tap in hole as vertically as possible;

press downward on wrench, applying equal pressure on both handles; turn clockwise (for right-hand thread) for two turns?

4.

5.

6.

7.24-12Remove tap wrench and check tap for squarenessCheck two positions at 90o to each otherIf tap not entered squarely, remove from hole and restart it by applying pressure in direction from which tap leansWhen tap properly started, feed it into hole by turning tap wrenchTurn tap clockwise one-quarter turn, and turn it backward about ? turn to break the chip (must turn with steady motion)?

?24-13Removing Broken TapsSeveral methods may be used to remove broken tap (some successful, others not!)–Tap Extractor

?Tool with four fingers that slip into flutes of broken tap; wrench fitter to extractor and turned to extract–Drilling

?Drill through broken tap

–Acid Method

?Acid acts on steel and loosens tap for extraction–Tap Disintegrators

?Uses electrical discharge principle to cut through tap

24-14

To Remove Broken Tap Using a 1.

2.

3.Tap ExtractorSelect proper size extractor for tapSlide collar, to which fingers attached, down body so fingers project well below end of bodySlide fingers into flutes of broken tap, making sure they go down into hole as far as possible

24-15

4.Slide body down until it rests on top of

broken tap

?Give maximum support to fingers

5.Slide collar down until it rests on top of

work

?Also provides support for fingers

6.Apply wrench to square end of body top

7.Turn wrench gently in counterclockwise

direction

Note: Do not force extractor –will damage fingers

24-16

To Remove Broken

Tap (Carbon Steel) By Drilling1.

2.

3.

4.

5.Heat broken tap to bright red color and allow it to cool slowlyCenter-punch tap close to centerUsing drill smaller than distance between opposite flutes, carefully drill holl through broken tapEnlarge hole to remove as much of metal between flutes as possibleCollapse remaining part with punch and remove

pieces

24-17

To Remove Broken Tap (High-speed Steel) Using the Acid Method1.

2.

3.

4.Dilute one part nitric acid with five parts waterInject mixture into hole; acid will act on steel and loosen tapRemove tap with extractor or pliersWash remaining acid from thread with water to neutralize acid

??24-18Threading DiesUsed to cut external threads on round workMost common threading dies–Adjustable split die

–Adjustable screw plate die–Solid die

?Used for chasing or recutting damagedthreads

?May be driven by suitable wrench?Not adjustable

24-19Threading Dies?Adjustable split die–Has adjusting screw that permits adjustment over or under standard depth of thread–Fits into die stock

?Adjustable screw plate die

–Two die halves held in

collet by threaded plate

–Adjusting screws against

each die half

–Greater adjustment than split dieCopyright ? The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

24-20

To Thread With a Hand Die1.

2.

3.

4.

5.Chamfer end of workpiece with file or on grinderFasten work securely in viseSelect proper die and die stockLubricate tapered end of die with suitable cutting lubricantPlace tapered end of die squarely on workpiece

24-21

6.Press down on die stock handles and turn clockwise several turns

7.Check die to see that it has started squarely with work

8.If not square, remove die and restart

9.Turn die forward one turn and reverse it approximately one-half turn to break chip

10.During threading process, apply cutting fluid frequently

If thread must be cut to shoulder, remove die and restart it with tapered side of die facing up (complete the thread)

PowerPoint to accompany

Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-23

Objectives

?

?

?

?

?Recognize and state the purposes of six common thread formsSet up a lathe to cut inch external Unified threadsSet up an inch lathe to cut metric threadsSet up a lathe and cut internal threadsSet up a lathe and cut external Acme

55-24

Threads

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?

?Used for hundreds of years for holding parts together, making adjustments, and transmitting power and motionArt of producing threads continually improvedMassed-produced by taps, dies, thread rolling, thread milling, and grinding

55-25

Threads

?

?Thread–Helical ridge of uniform section formed on inside or outside of cylinder or coneUsed for several purposes:

–Fasten devices such as screws, bolts, studs, and nuts

–Provide accurate measurement, as in micrometer–Transmit motion

–Increase force

55-26

Thread

Terminology

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

?

?

?55-27Thread TerminologyScrew thread–Helical ridge of uniform section formed on inside or outside of cylinder or coneExternal thread–Cut on external surface or coneInternal thread

–Produced on inside of cylinder or cone

55-28?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.

?55-29Thread FormsApril, 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 pitchM 5 X 0.8

55-30ISO Metric Thread?

60o included angle, crest = 0.125 times pitch

D (depth of thread) is 0.6143 times pitch

Root is 1/4thof pitch,

Flat on root (FR) wider than crest (FC)

Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-31

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 1/8ththe pitchUsed in fabrication, machine construction

55-32American National Standard Thread

.6134D ?.6134 x P N

.125F ? .125 x P N

Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-33British Standard Whitwork(BSW) Thread?Application is same

as for American

National form thread.6403D?.6403 x P N.1373R ? .1373 x P NCopyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-34Unified 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.

55-35

.6134D (external thread)?.6134 x P N

.5413D (internal thread)?.5413 x P N

.125F (external thread)? .125 x P N

.250F (internal thread)? .250 x P N

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

55-36American National Acme Thread?Replacing square thread in many cases?Used for feed screws, jacks, and visesD=minimum .500PF=.3707P

C=.3707P-.0052

(for maximum depth)=maximum .500P+0.010

Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-37Brown & 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.

55-38Square Thread?Being replaced by Acme thread because of difficulty in cutting itD= .500P?Often found on vises F = .500Pand jack screwsC= .500P + .002Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-39International Metric thread?Standardized thread used in EuropeD= 0.7035P (maximum)

= 0.6855P (minimum)F = 0.125PR= 0.0633P (maximum)

= 0.054P(minimum)

Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-40

Thread Fits and Classifications?

?

?Fit–Relationship between two mating parts–Determined by amount of clearance or interference when they are assembledNominal size–Designation used to identify size of partActual size

–Measured size of thread or part

–Basic size: size from which tolerances are set

55-41

Allowance

?

?Permissible difference between largest external thread and smallest internal threadDifference 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.

55-42

Tolerance

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?

?

?Variation permitted in part sizeMay be expressed as plus, minus, or bothTotal tolerance is sum of plus and minus tolerancesIn Unified and National systems, tolerance is plus on external threads and minus on internal threadsThe tolerance for a 1 in.—8 UNC Class 2A thread is:

Maximum pitch diameter of the

external thread (2A) = .9168 in.

Minimum pitch diameter of the

external thread (2A) = .9100 in.

Tolerance = .0068 in.

55-43

Limits

?Maximum and minimum dimensions of partThe limits for a 1 in.—8 UNC Class 2A thread are:Maximum pitch diameter of the

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

external thread (2A) = .9100 in.

55-44

Thread Calculations: Example 2What 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=0.54127 x 1

= 0.54 mmMinor dia?Major dia - (D?D) ? 6.3 - (.54?.54) ?5.22 mm

Width of crest ? 0.125 x P

?0.125 x 1

?0.125 mm Width of root?0.25 x P ? 0.25 x 1 ?0.25 mm

55-45

Procedure to Set the Quick-Change Gearbox for Threading1.

2.

3.

4.Check drawing for thread pitch requiredFrom chart on quick-change gearbox, find whole number that represents pitch in threads per inch or in millimetersWith lathe stopped, engage tumbler lever in hole, which is in line with the pitchSet top lever in proper position as indicated on chart

5.6.

7.55-46Engage sliding gear in or out as requiredTurn lathe spindle by hand to ensure that lead screw revolvesRecheck lever settings to avoid errors

55-47Thread-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 numberedor unnumbered: not both

Copyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-48

Procedure to Set Up a Lathe for 1.

2.

3.

4.

5.Threading (60o Thread)Set lathe speed to ? speed used for turningSet quick-change gearbox for required pitch in threads per inch or in millimetersEngage lead screwSecure 60o threading toolbit and check angle using thread center gageSet compound rest at 29o to right; set to left for left-hand thread

6.

7.

8.

9.55-49Set cutting tool to height of lathe center pointMount work between centersMake sure lathe dog is tight on workIf work mounted in chuck, it must be held tightlySet toolbit at right angles to work, using thread center gageArrange apron controls to allow split-nut lever to be engaged??

55-50

Thread-Cutting OperationProcedure 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

4.

5.

6.

7.55-51Move carriage until point of threading tool near right-hand end of workTurn crossfeed handle until threading tool close to diameter, but stop when handle is at 3 o'clock positionHold crossfeed handle in this position and set graduated collar to zeroTurn compound rest handle until threading tool lightly marks work

8.

9.

10.

11.55-52Move carriage to right until toolbit clears end of workFeed 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 threadedAt end of cut, turn crossfeed handle

counterclockwise to move toolbit away from work and disengage split-nut lever

55-53

12.Stop lathe and check number of tpi with

thread pitch gage, rule, or center gage

13.After each cut, turn carriage handwheel to

bring toolbit to start of thread and return crossfeed handle to zero

14.Set depth of all threading cuts with compound

rest handle

?See Table 55.2 and Table 55.3

55-54When 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.

55-55

Table 55.2Depth settings for cutting

60°national form threads*

Compound Rest Setting

tpi0°30°29°

24.027.031.0308

20.0325.0375.037

18.036.0417.041

16.0405.0468.046

14.0465.0537.0525

13

11

55-56

15.Apply cutting fluid and take successive 16.

17.cuts until top (crest) and bottom (root) of thread are same widthRemove burrs from top of thread with fileCheck thread with master nut and take further cuts

55-57

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

55-58

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 dogProcedure

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

4.

5.

6.

7.55-59Stop lathe, leaving split-nut lever engagedFeed toolbit into thread groove using only compound rest and crossfeed handles until right-hand edge of toolbit touches rear side of threadSet crossfeed graduated collar to zeroBack out threading tool using crossfeed handle, disengage split-nut lever, and move carriage until toolbit clears start of thread

8.

9.55-60Set crossfeed handle back to zero and take trial cut without setting compound restSet depth of cut using compound rest handle and finish thread to required depth

55-61

Common Methods of Measuring Threads1.

2.

3.

4.

5.

6.Thread ring gageThread plug gageThread snap gageScrew thread micrometerOptical comparatorThree-wire method

55-62The Three-Wire Method of Measuring 60o

ThreadsCopyright ? The McGraw-Hill Companies, Inc.

Permission required for reproduction or display.

55-63To Calculate theMeasurement over the Wires

M=measurement over the wires

D=major diameter of the thread

G=diameter of the wire size used

N=number of tpiAny of the following formulas can be used to calculate G:

1.010Largest wire?or 1.010PN

.57735Best-size wire ? or .57735PN

.505Smallest wire ? or .505PNwhere 1.5155M?D?3G?N

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