Threading - MMM

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PowerPoint to accompany

Krar • Gill • Smid

Technology of Machine Tools6th Edition

Thread-Cutting Tools and Procedures

Unit 24

24-2

Objectives

• Calculate the tap drill size for inch and metric taps

• Cut internal threads using a variety of taps

• Know the methods used to remove broken taps from a hole

• Cut external threads using a variety of dies

24-3

Hand Taps

• Cutting tools used to cut internal threads• Made from high quality tool steel, hardened

and ground• Two, three, or four flutes cut lengthwise

across threads to form cutting edges– Provide room for chips– Admit cutting fluid to lubricate tap

• End 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

½ in.—13 UNC½ in. = major diameter of tap13 = number of threads per inchUNC = Unified National Coarse (type of thread)

24-5

Hand Taps: Sets of Three


• Taper– Tapered from end six threads – Used to start thread easily– Used for tapping hole that goes through

work as well as blind hole• Plug

– Tapered for three threads– Tread hole through workpiece

• Bottoming tap– Not tapered but chamfered at end– Used for threading to bottom of blind hole

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

24-7

Tap Drill Size

• Hole drilled to correct tap drill size – Leave proper amount of

material for tap to cut thread (75%)


A = body sizeB = tap drill sizeC = minor diameter

N

1 - D TDS =

TDS = tap drill sizeD = major diameter of tapN = number of threads per inchin. 764.111.875.

9

1 -

8

7 TDS

=−=

=

Find tap drill size for a 7/8 in. –9NC tap

24-8

Metric Taps• Several thread forms and standards in metric

thread system• International 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 Taps

Tap 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

24-10

Tapping a Hole

• Operation of cutting internal thread using tap and tap wrench

• Taps hard and brittle

• Easily broken– Extreme care used to prevent breakage– Broken tap in hole difficult to remove and

often results in scrapping work

24-11

To Tap Hole By Hand

1. Select correct taps and tap wrench for job

2. Apply suitable cutting fluid to the tap• No fluid required for tapping brass or cast iron

1. Place 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

24-12

4. Remove tap wrench and check tap for squareness

• Check two positions at 90º to each other

4. If tap not entered squarely, remove from hole and restart it by applying pressure in direction from which tap leans

5. When tap properly started, feed it into hole by turning tap wrench

6. Turn tap clockwise one-quarter turn, and turn it backward about ½ turn to break the chip (must turn with steady motion)

24-13

Removing Broken Taps

• Several 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 Tap Extractor

1. Select proper size extractor for tap

2. Slide collar, to which fingers attached, down body so fingers project well below end of body

3. Slide 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

4. Slide collar down until it rests on top of work

• Also provides support for fingers

4. Apply wrench to square end of body top5. Turn wrench gently in counterclockwise

direction

Note: Do not force extractor – will damage fingers

24-16

To Remove Broken Tap (Carbon Steel) By Drilling

1. Heat broken tap to bright red color and allow it to cool slowly

2. Center-punch tap close to center3. Using drill smaller than distance between

opposite flutes, carefully drill holl through broken tap

4. Enlarge hole to remove as much of metal between flutes as possible

5. Collapse remaining part with punch and remove pieces

24-17

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

1. Dilute one part nitric acid with five parts water

2. Inject mixture into hole; acid will act on steel and loosen tap

3. Remove tap with extractor or pliers

4. Wash remaining acid from thread with water to neutralize acid

24-18

Threading Dies

• Used to cut external threads on round work• Most 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

24-19

Threading 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 die

24-20

To Thread With a Hand Die

1. Chamfer end of workpiece with file or on grinder

2. Fasten work securely in vise

3. Select proper die and die stock

4. Lubricate tapered end of die with suitable cutting lubricant

5. Place 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 restart9. Turn die forward one turn and reverse it

approximately one-half turn to break chip10. 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

Krar • Gill • Smid

Technology of Machine Tools6th Edition

Threads and Thread Cutting Unit 55

55-23

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

55-24

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

55-25

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

55-26

Thread Terminology


55-27

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

55-28

• 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, toolbitadvanced from left to right


55-29

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

55-30

ISO Metric Thread• 60º included angle, crest = 0.125 times pitch

D (depth of thread) is 0.6143 times pitchRoot is 1/4th of pitch,Flat on root (FR) wider than crest (FC)


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 1/8th the pitch

• Used in fabrication, machine construction

55-32


NPF

NPD

.125or x .125

.6134or x 6134.

=

=


55-33

British Standard Whitwork(BSW) Thread

• Application is same as for American National form thread

NPR

NPD

.1373or x .1373

.6403or x 6403.

=

=


55-34

Unified Thread• Developed by U.S., Britain, and Canada for

standardized thread system• Combination of British Standard Whitworth and



55-35

NPF

NPF

NPD

NPD

.250or x .250 thread)(internal

.125or x .125 thread)(external

.5413or x 5413. thread)(internal

.6134or x 6134. thread)(external

=

=

=

=


55-36

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)


55-37

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


55-38

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


55-39

International Metric thread

• Standardized thread used in EuropeF = 0.125PR = 0.0633P (maximum) = 0.054P (minimum)

D = 0.7035P (maximum) = 0.6855P (minimum)


55-40

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

55-41

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.

55-42

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:

55-43

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:

55-44

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

55-45

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

55-46

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

55-47

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


55-48

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

55-49

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

6. Set toolbit at right angles to work, using thread center gage

7. Arrange apron controls to allow split-nut lever to be engaged

55-50

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

55-51

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

55-52

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

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

55-54

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


55-55

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

55-56

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

55-57

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 dog

Procedure1. Set up lathe and work for thread cutting2. Start lathe, toolbit clear of work, engage

split-nut lever on correct line3. Allow carriage to travel until toolbit

opposite any portion of unfinished thread

55-59

4. 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

55-60

8. 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

55-61

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

55-62

The Three-Wire Method of Measuring 60º Threads


55-63

To Calculate theMeasurement over the Wires

NGDM

5155.13 −+=

where M = measurement over the wiresD = major diameter of the threadG = diameter of the wire size usedN = number of tpi

Any of the following formulas can be used to calculate G:

PN

PN

PN

.505or .505

ireSmallest w

.57735or .57735

wiresize-Best

1.010or 010.1

reLargest wi

=

=

=

Threading - MMM

Engineering

Transcript of Threading - MMM