This document provides information about thread cutting tools and procedures. It discusses various types of taps used to cut internal threads, including hand taps, tap sizes and sets of taps. It also discusses tap drill sizes, how to tap holes, and methods for removing broken taps. Additionally, it covers threading dies used to cut external threads and the procedures for using hand dies. The objectives are to understand thread cutting tools and processes for both internal and external threads in inch and metric systems.

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Permission required for reproduction or display.
PowerPoint to accompany
Krar • Gill • Smid
Technology of Machine Tools
6th
Edition
Thread-Cutting Tools
and Procedures
Unit 24

2. 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

3. 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

4. 24-4
Hand Taps
• Inch tap markings
– Major diameter
– Number of threads per inch
– Type of thread
½ in.—13 UNC
½ in. = major diameter of tap
13 = number of threads per inch
UNC = Unified National Coarse (type of thread)

5. 24-5
Hand Taps: Sets of Three
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• 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

6. 24-6
Another Method of
Identifying Taps
• Use of annular ring(s) cut around top of shank
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One – taper tap
Two – plug tap
Three – bottoming tap

7. 24-7
Tap Drill Size
• Hole drilled to correct tap
drill size
– Leave proper amount of
material for tap to cut
thread (75%)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A = body size
B = tap drill size
C = minor diameter
N
1
-DTDS =
TDS = tap drill size
D = major diameter of tap
N = number of threads per inchin.764.111.875.
9
1
-
8
7
TDS
=−=
=
Find tap drill size for a
7/8 in. –9NC tap

8. 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

9. 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

10. 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

11. 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

12. 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)

13. 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

14. 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

15. 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 top
5. Turn wrench gently in counterclockwise
direction
Note: Do not force extractor – will damage fingers

16. 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 center
3. 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

17. 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

18. 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 damaged
threads
• May be driven by suitable wrench
• Not adjustable

19. 24-19
Threading Dies
• Adjustable split die
– Has adjusting screw that permits adjustment
over or under standard depth of thread
– Fits into die stock
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• Adjustable screw plate die
– Two die halves held in
collet by threaded plate
– Adjusting screws against
each die half
– Greater adjustment than split die

20. 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

21. 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)

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

23. 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

24. 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

25. 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

26. 55-26
Thread
Terminology
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Permission required for reproduction or display.

27. 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

28. 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
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Permission required for reproduction or display.

29. 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

30. 55-30
ISO Metric Thread
• 60º included angle, crest = 0.125 times pitch
D (depth of thread) is 0.6143 times pitch
Root is 1/4th
of pitch,
Flat on root (FR) wider than crest (FC)
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Permission required for reproduction or display.

31. 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

32. 55-32
American National Standard Thread
N
PF
N
PD
.125
orx.125
.6134
orx6134.
=
=
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.

33. 55-33
British Standard Whitwork
(BSW) Thread
• Application is same
as for American
National form thread
N
PR
N
PD
.1373
orx.1373
.6403
orx6403.
=
=
Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.

34. 55-34
Unified Thread
• Developed by U.S., Britain, and Canada for
standardized thread system
• Combination of British Standard Whitworth and
American National Standard Thread
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Permission required for reproduction or display.

35. 55-35
N
PF
N
PF
N
PD
N
PD
.250
orx.250thread)(internal
.125
orx.125thread)(external
.5413
orx5413.thread)(internal
.6134
orx6134.thread)(external
=
=
=
=
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Permission required for reproduction or display.

36. 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)
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Permission required for reproduction or display.

37. 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
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Permission required for reproduction or display.

38. 55-38
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 + .002
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Permission required for reproduction or display.

39. 55-39
International Metric thread
• Standardized thread used in Europe
F = 0.125P
R = 0.0633P (maximum)
= 0.054P (minimum)
D = 0.7035P (maximum)
= 0.6855P (minimum)
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Permission required for reproduction or display.

40. 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

41. 55-41
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.

42. 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 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:

43. 55-43
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:

44. 55-44
Thread Calculations: Example 2
P = pitch = 1 mm
D = 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?
mm125.0
1x125.0
x.1250crestofWidth
mm22.5
.54)(.54-6.3
)(-diaMajordiaMinor
=
=
=
=
+=
+=
P
DD
mm25.0
1x0.25
x25.0rootofWidth
=
=
= P

45. 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

46. 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

47. 55-47
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
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48. 55-48
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

49. 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

50. 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

51. 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

52. 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

53. 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

54. 55-54
When tool is fed in at 29º, most of the cutting
is done by the leading edge of toolbit.
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55. 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

56. 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

57. 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

58. 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
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

59. 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

60. 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

61. 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

62. 55-62
The Three-Wire Method of
Measuring 60º Threads
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63. 55-63
To Calculate the
Measurement over the Wires
N
GDM
5155.1
3 −+=
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:
P
N
P
N
P
N
.505or
.505
ireSmallest w
.57735or
.57735
wiresize-Best
1.010or
010.1
reLargest wi
=
=
=