Metrology subject-Mechanical engineering

1. Unit V
Screw Thread Measurement

2. Thread Terminology
• Thread: the helical grooves opened to
inner and outer surfaces.
• External thread (screw): A thread on the
external surface of a cylinder.
• Internal thread (nut): A thread on the
internal surface of a cylinder.
• Major diameter: The largest diameter of
a screw thread.
• Minor diameter: The smallest diameter
of a screw thread.

3. • Pitch diameter: The diameter of an imaginary cylinder, the
surface of which cuts the thread forms where the width of the
thread and groove are equal.
• Crest: The edge or surface that joins the sides of a thread and is
farthest from the cylinder or cone from which the thread
projects.
• Root: The edge or surface that joins the sides of adjacent thread
forms and coincides with the cylinder or cone from which the
thread projects.
• Thread Depth: The distance between _ crest and root.
• Pitch: The distance between corresponding points on adjacent
thread forms measured parallel to the axis.

4. • Right-hand thread: A thread that when viewed axially winds in a
CW and receding direction. Threads are RH unless otherwise
specified.
• Left-hand thread: A thread that when viewed axially winds in a
CCW and receding direction. These threads are designated LH.

5. Multiple Threads
• Lead: The distance that a threaded part moves axially in one
complete revolution.
• Lead or Helix Angle: The angle made by the pitch _ helix,
defined as

6. • Multiple thread: A thread combination having the same form
produced on two or more helices where the lead is an integral
multiple of the pitch (e.g. on a double thread, lead is twice the
pitch). A multiple thread permits a more rapid advance without a
coarser (larger) thread form.
• Single thread: A thread having the form produced on only one
helix of cylinder. On a single thread, the lead and pitch are
equivalent. Threads are always single unless otherwise specified.

7. • There are several thread forms used for specific applications.
Selection of the appropriate thread form depends upon the
functionality, size and purpose of the required job.
• The threads are designated in Metric or British system as shown
below.
Thread Types and Designation

9. • The threads can be measured using fixed limit gauges such as
adjustable thread ring and plug gauges, which provide GO
(green) or NOT-GO (red) type of measurement.
• Screw pitch gauge consisting of a metal case having several
leaves can also be used. Each leaf has teeth corresponding to a
definite pitch. By matching the teeth with the thread on work,
the correct pitch can be read directly from the leaf.
• Thread rolls with various forms and known dimensions are
used to measure internal threads.
Thread Gauges

11. Thread Micrometers
• Screw thread micrometers having various type and size of
interchangeable spindle and anvils are used for measuring the
pitch diameter of external threads
• The pitch diameter of internal threads is measured by inside
thread micrometers with various type and dimensions for
different applications.

13. Three-Wire Method
• Three-wire method is one of the most accurate and versatile
ways of measuring the pitch diameter of a thread by using three
lapped and polished wires and a micrometer
• Wires touching the thread at the pitch diameter are known as
“Best Size Wires”. Such wires are used since the
measurements of pitch diameter are least affected by errors that
may be present in the angle of the thread.

15. • The table gives the pitch diameter for some thread types having
lead angle from 0°to 5.

16. Measurement of pitch
Pitch errors
Progressive pitch error
The reason for this error is that the cutting feed in threading is
constant but not correct. Mainly due to pitch error in the lead screw
of the lathe.
Periodic pitch error
In this error the cutting feed is not constant. It is caused by pitch
errors in the gears or by an axial movement of the lead screw.
Drunken threads
The pitch is correct, but the thread helix is not a straight line but a
curve.
Measurement of pitch errors
Pitch errors are commonly measured by a profile projector or special
Pitch measuring machine

17. Illustration of the three pitch errors, (from top to
bottom) progressive error, periodic error and thread
drunkenness. Solid lines represent the theoretical
profile

18. Definitions
The geometrical elements for a V-form thread are illustrated in the
figure:

19. Flank angle q: The angle between the straight portion of the
thread flank and a line normal to the thread axis, for metric
threads 2q = 600, for Withworth threads 2q = 550.
Minor diameter: The diameter of an imaginary cylinder containing
all points on the root of the thread.
Major diameter: The diameter of an imaginary cylinder containing
all points on the crests of the thread.
Simple effective diameter Ed: The diameter of an imaginary
cylinder co-axial with the thread axis, which cuts the thread at
half the pitch
Pitch p: The distance measured at a line parallel to the thread axis
between the similar points of two flanks

20. Measurement of the major diameter
It is most conveniently measured by means of a micrometer (see
below), but other instruments are also applicable, for example
instrumental microscope, profile projector.
Measurement of the minor diameter
Three methods are used in practice:
a) Projecting the thread profile on a screen (profile projector), or
viewing through an instrumental microscope.
b) Special measuring machine and two V-pieces:
C = S + (d1-d2)
The machine is the same as shown below in measuring the effective
diameter.

21. (a) The anvil set-up, d1 is the reading over setting gauge,
(b) the measurement, d2 is the reading over the thread

22. c) Micrometer for measuring screw threads.
This is an external micrometer with interchangeable inserts and can
be used to measure all diameters using appropriate inserts for
different measurements.
Screw thread micrometer

23. Measurement of the effective diameter
Special measuring machine and two cylinders
(a) The anvil set-up, d1 is the reading over setting gauge,
(b) The measurement, d2 is the reading over the thread

24. The effective diameter is
estimated as
Ed = D + P + (d1 - d2)
where P is the difference
between the effective
diameter Ed and diameter
under the cylinders,
Floating carriage diameter
measuring machine
for metric threads defined as
P = 0.866p - 1.483d
where d is the diameter of cylinders.

25. Measurement over three wires of diameter d
If a measuring machine is not available,
the effective diameter may be determined by measuring over three
wires.
The diameter d of wires is chosen so that they pitch approximately at
the effective diameter (so called best size wires).
The best wire size can be determined as
d = p/2cosq
For metric threads d = 0.57735p.

26. Wires are available in sets for
different pitches.
Measurement by means of O-Vee
spring gauges
The O-Vee gage inserted onto the
thread
The method is illustrated in the
figure. The O-Vee gage is first
inserted and then a reading is taken
over coils by micrometer.
Data plate attached shows correct
tolerances for the specific fit.

27. Measurement of thread form and angle
The simplest way is to use optical profile projectors or microscope.
To avoid interference caused by the helix angle, the thread should be
turned at the helix angle, as shown in the figure.
Turning thread through helix angle avoids interference but shortens
length and distorts profile

29. Gauging of screw threads
• The major diameters of external threads and the minor diameters
of internal threads are checked with plain ring and plug gages
respectively.
(Left) Go and not-go ring gages for checking a screw thread, (right)
Screw thread plug gages

30. • The effective diameters of screw threads are checked by limit
gages of special design.
• There are no limit gages for checking pitch and thread form
because the pitch or form errors change the effective diameter.
• Thus a correct effective diameter means that both the pitch and
thread form are also correct.
• The form gage shown in the figure is not a limit gage but serves
for easy checking the thread pitch.

31. Form gages for checking the pitch of a screw thread.

32. Screw Thread Major and Minor Diameters - parts vs. gages
The minor diameter of a screw thread ring gage is easily
measurable as is the major diameter of a screw thread work plug
gage. People frequently check these diameters and then they have
questions as to apparent discrepancies in the gages compared to the
part sizes. The three questions raised are:
1. Given that on a screw thread ring gage the NOGO pitch diameter
is smaller than the GO pitch diameter, why is the NOGO minor
diameter larger than the GO minor diameter?
2. Given that on a screw thread plug gage the NOGO pitch
diameter is larger than the GO pitch diameter, why is the NOGO
major diameter of a screw thread work plug gage smaller than the
GO minor diameter?
3. Given that the screw thread pitch diameters are the same as the
screw thread gage pitch diameters, why are the screw thread gage
minor and major diameters so much different than the product
screw thread minor and major diameters?

33. Taylor’s principle: ‘Go’ and ‘No Go’ gauges should be designed to
check maximum and minimum material limits.
Taylor’s principle states that the
‘Go’ gauges should check both geometric features (full form and
full length of thread) and size (maximum diameter of thread),
while ‘No Go’gauge
(i) A gauge for the major diameter
(ii) A separate gauge for the effective diameter
And one which is not influenced by pitch error.
It should check only one linear dimension at a time.

34. Limit gauges for internal threads
a. Full form GO gauge: External screw threaded plug gauge,
Gauging length equal to work length, Made to low limits of size
b. Effective Diameter NOT-GO gauge: Effective diameter is high
limit, restricted contacts with work threads
c. Minor Diameter NOT-GO gauge: Plain cylindrical plug gauge
manufactured to the higher limit of the minor diameter.

35. Limit gauges for external threads
a. Full form GO gauge: Caliper type gauges having rollers or edge
type anvils, made to high limit of size. A gauge of this type
incorporates both GO and NOT GO anvils.
b. Effective Diameter NOT-GO gauge: Caliper type gauges
Effective diameter is low limit, restricted contacts with work
threads
c. Major Diameter NOT-GO gauge: Plain caliper or gap gauge,
manufactured to the lower limit of the major diameter.

36. Classes of fits for ISO threads
a. Fine
b. Medium
c. Coarse
Fits depends on two factors:
i. Tolerance grade (width of the Tol. band)
ii. Tolerance Position (fund. Deviation from basic or normal size)

37. Current day practice and requirement of easy assembly the following
tolerance positions are provided:
Nut threads : G and H.
Bolt threads : a, b, c, d, e, f, g, h.
Fundamental deviation for
nut and bolt threads for
various tolerance position
For nuts
EI(G) = +(15 + 11p)
EI(H) = 0
Tolerance position w.r.t. zero line

38. For bolts
es(a) = -(270 + 19p)
es(b) = -(185 + 19p)
es(c) = -(115 + 19p)
es(d) = -(65 + 19p)
: p up to 2.5 mm
es(e) = -(50 + 11p)
: p >= 0.5 mm
es(f) = -(30 + 11p)
es(g) = -(15 + 11p)
es(h) = 0
EI and es values are in micrometer p
is in millimeter
Tolerance position and grade for
ISO screw threads

39. Tolerances are given by
a. Tolerances for major diameter and bolt thread (TD)
TD(4) = 0.63 TD(6)
TD(8) = 1.6 TD(6)
b. Tolerances for minor diameter of nut thread (Td1).
i. For pitches from 0.2 to 0.8 mm
ii. For pitches of 1mm and coarser

40. Td1(4) = 0.63 Td1(6)
Td1(5) = 0.8 Td1(6)
Td1(7) = 1.25 Td1(6)
Td1(8) = 1.6 Td1(6)
Tolerance for pitch (effective) diameter of bolt thread (TE):
TE(01) = 0.2 TE (6), TE(0) = 0.25 TE (6), TE(1) = 0.315 TE (6)
TE(2) = 0.4 TE (6), TE(3) = 0. 5 TE (6), TE(4) = 0.63 TE (6)
TE(5) = 0.8 TE (6), TE(7) = 1.25 TE (6), TE(8) = 1.6 TE (6)
TE(9) = 2 TE (6).

41. Tolerance for pitch diameter of nut threads (TE1)
TE1(02) = 0.212 TE (6), TE1(01) = 0.265 TE (6),
TE1(0) = 0.335 TE (6),TE1(1) = 0.425 TE (6),
TE1(2) = 0. 53 TE (6), TE1(3) = 0.67 TE (6)
TE1(4) = 0.85 TE (6), TE1(5) = 1.06 TE (6),
TE1(6) = 1.32 TE (6), TE1(7) = 1.7 TE (6).
TE1(8) = 2.12 TE (6),
Tolerance grade for minor diameter of bolt thread (Td1) :
Td1 = ( TE +72 p) Td1 and TE are in micrometer and p is in mm

42. Screw gauge tolerances

43. To totally discuss the major and minor diameters of thread gages,
and the related inspection issues, let me state the following:
• The screw thread gage major and minor diameters are different
than the part major and minor diameters for specific reasons
based on one of Taylor’s Principles: The GO should gauge all
features simultaneously and the NOGO should gage each feature
independently.
• The GO ring gage major diameter is made as large as possible
and is tested to assure that it clears a specific diameter. By
design, the GO ring gage major diameter has no measurement
function. The part thread major diameter is to be measured via
alternate measurement methods.
• The GO ring gage minor diameter is set at the maximum material
condition. This tests the part to assure that the minor diameter
has not grown to a point that it would interfere with the mating
part.

44. • The GO plug gage major diameter is set at the maximum material
condition. This tests the part to assure that the major diameter has
not shrunk to a point that it would interfere with the mating part.
The part thread minor diameter is to be measured via alternate
measurement methods.
• The GO plug gage minor diameter is made as small as possible
and is tested to assure that it clears a specific diameter. By design,
the GO plug gage minor diameter has no measurement function.
• The NOGO ring gage major diameter is made as large as possible
and is tested to assure that it clears a specific diameter. By design,
the NOGO ring gage major diameter has no measurement
function.

45. • The NOGO ring gage minor diameter is set at or just below the
gage pitch diameter. This is to allow the NOGO ring gage to
properly measure the pitch diameter and exclude data from the
minor diameter. Once the minor diameter passes the GO gage it
is considered good.
• The NOGO plug gage major diameter is set at or just above the
gage pitch diameter. This is to allow the NOGO plug gage to
properly measure the pitch diameter and exclude data from the
major diameter. Once the major diameter passes the GO gage it
is considered good.
• The NOGO plug gage minor diameter is made as small as
possible and is tested to assure that it clears a specific diameter.
By design, the NOGO plug gage minor diameter has no
measurement function.