Threads and

Thread Cutting

Session 11

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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
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Thread Terminology

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 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

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Thread Terminology
Major Diameter
Commonly known as the outside diameter .
On a screw thread, the major diameter is the
largest diameter of the thread on the screw or
nut.

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Thread Terminology
Minor Diameter
Called the root diameter, the minor diameter is
the smallest diameter of the thread on the
screw or nut.

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Thread Terminology
• 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

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Thread Terminology
Number of Threads
The number of threads per inch.

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Thread Terminology
Pitch
The distance from a given point on one thread
to a corresponding point on the very next thread

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Thread Terminology
Lead
The distance a screw thread advances
in one revolution.
• The lead and the pitch of a single lead
thread are the same.
• On double lead threads, the lead is twice
the pitch.
•
A double lead thread has two start
points.
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Thread Terminology

Single Lead Double Lead

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Thread Terminology
Root
• Bottom surface joining sides of two
adjacent threads
• External thread on minor diameter
• Internal thread on major diameter

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Thread Terminology
Crest
• Top surface joining two sides of thread
• External thread on major diameter
• Internal thread on minor diameter

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Thread Terminology
Flank
• Thread surface that connects crest
with root

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Thread Terminology
Depth of thread
• Distance between crest and root measured
perpendicular to axis

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Thread Terminology
Angle of thread
• Included angle between sides of
thread measured in axial plane

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Thread Terminology
Helix angle
• Angle that thread makes with plane
perpendicular to thread axis

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Thread Terminology
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

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Thread Terminology
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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 Thread Forms
• ISO Metric
• American National Standard Thread
• British Standard Whitworth (BSW) Thread
• Unified Thread
• American National Acme Thread
• Square Thread
• Brown & Sharpe Worm Thread
• International Metric thread
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ISO Metric Thread

60° included angle

D (Internal) = 0.6143 x pitch
Crest = .125 x pitch
Root = .250 x pitch

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ISO Metric Thread

D (external)  0.54127 x P
FC  .125 x P
FR  .250 x P

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ISO metric pitch and diameter
combinations
Nominal Thread Nominal Thread
Diameter Pitch Diameter Pitch
(mm) (mm) (mm) (mm)
1.6 0.35 20 2.5
2 0.4 24 3
2.5 0.45 30 3.5
3 0.5 36 4
3.5 0.6 42 4.5
4 0.7
Portion 48 taken
of table 5
5 0.8 from textbook
56 5.5
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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)

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Common Thread Forms
Coarse Thread Series UNC
The one most commonly used in the mass
production of bolts, screws, nuts and other
general fastening applications.
Less of a tendency for cross-threading
than fine threads.
It is also used on quick assembly or
disassembly, or if corrosion or slight damage
is possible.

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Common Thread Forms
Fine Thread Series UNF
When used on external threads have greater
tensile stress area than coarse threads of the
same size.
Resists stripping out better than coarse threads
in areas where the external and mating internal
threads are subjected to loads equal to or
greater than the capacity of the screw or bolt.
Used were the length of engagement is
limited
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Common Thread Forms

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Common Thread Forms
Taper Pipe Series NPT
The taper of the external thread forms a joint
with the taper of the internal thread.
When used with a sealer (such as pipe dope or
teflon tape) will produce a pressure tight joint.

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American National
Standard Thread
.6134
D  .6134 x P or
N
.125
F  .125 x P or
N

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British Standard Whitworth
(BSW) Thread
Application is same as D  .6403 x P or .6403
for American N
National form thread .1373
R  .1373 x P or
N

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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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Unified Thread
.6134
D (external thread)  .6134 x P or
N
.5413
D (internal thread)  .5413 x P or
N
.125
F (external thread)  .125 x P or
N
.250
F (internal thread)  .250 x P or
N

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Acme Screw Thread
• Replacing square thread in many cases
• Used for feed screws, jacks, and vises
D = minimum .500P F = .3707P
= maximum .500P + 0.010 C = .3707P - .0052
(for maximum depth)

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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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Square Thread
• Being replaced by Acme thread because
of difficulty in cutting it D = .500P
• Often found on vises F = .500P
and jack screws C = .500P + .002

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International Metric thread
• Standardized thread used in Europe
D = 0.7035P (maximum) F = 0.125P
= 0.6855P (minimum) R = 0.0633P (maximum)
= 0.054P (minimum)

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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
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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”- 8 UNC Class 2A and 2B

Minimum pitch diameter of the

internal thread (2B) = .9188 in.
Maximum pitch diameter of the
external thread (2A) = .9168 in.
Allowance = .002 in. 38
Tolerance
• Variation permitted in part size
• 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) = .9168 in.
Minimum pitch diameter of the
external thread (2A) = .9100 in.
Tolerance = .0068 in.
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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) = .9168 in..
Minimum pitch diameter of the
external thread (2A) = .9100 in.

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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
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Tolerance Example:

An external metric thread may be

designated as follows:
Pitch Outside
  Nominal Diameter Diameter
Metric Size Pitch Tolerance Tolerance
    M 6 X 0.75 - 5g 6g
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
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Allowance Symbols
• 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

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Classes 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

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Classes Of Thread Fits
¼-20 UNC 2A
In this particular case the class of fit
would be a 2. The symbol A indicates an
external thread.

¼-28 UNF 3B
In this particular case the class of fit
would be a 3. The symbol B indicates
an internal thread.

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Thread Calculations

Calculate pitch, depth, minor diameter, and

width of flat for a ¾”-10 UNC thread.
D = single depth of thread
P = pitch
Minor dia  Major dia - ( D  D)
 .75 - (.061  .061)
1 1
P   .100 in.  .628 in.
tpi 10
P 1 1
D  .61343 x P Width of flat   x
8 8 10
 .61343 x .100  .061 in.  .0125 in.
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Thread Calculations

What is the depth, minor diameter, crest

and root for an M 6.3 X 1 thread?
P = pitch = 1 mm Minor dia  Major dia - ( D  D)
D = 0.54127 x 1  6.3 - (.54  .54)
= 0.54 mm
 5.22 mm
Width of root  0.25 x P Width of crest  0.125 x P
 0.25 x 1  0.125 x 1
 0.25 mm  0.125 mm

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

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Three-Wire Measuring
• Three wires of equal diameter placed
in thread, two on one side and one on
other side
• Standard micrometer used to measure
distance over wires (M)
• Different sizes and pitches of threads
require different sizes of wires

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The Three-Wire Method of
Measuring 60º Threads

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Measurement with Wires
1.5155
M  D  3G 
N
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 1.010

Largest wire  or 1.010P
following N
.57735
formulas can Best - size wire  or .57735P
N
be used to .505
Smallest wire  or .505P
calculate G: N 52
Multiple Threads
• 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
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 Double

Single

Triple

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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

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Internal Threads

The compound rest is set at 29º to the left

for cutting right-hand internal threads.
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