This document provides information on measuring various geometric features of screw threads and gears. It discusses measuring the major diameter, minor diameter, pitch, and other elements of threads using instruments like micrometers, thread gauges, and comparators. For gears, it describes measuring runout, pitch, profile, backlash, tooth thickness, and alignment using devices like dial indicators, involute measuring machines, and angular measurement techniques. The document also defines common terminology for screw thread and gear geometry.

1. Form Measurement
Unit 3

2. Syllabus
Measurement of screw threads-Thread
gauges, floating carriage micrometer-
measurement of gears-tooth thickness-
constant chord and base tangent method-
Gleason gear testing machine - radius
measurements-surface finish, straightness,
flatness and roundness measurements

3. • Screw thread measurement
• Gear measurement
• Radius measurement
• Surface Finish measurement
• Straightness measurement
• Flatness and roundness measurements.

4. Screw thread

5. Terminology
Crest
It is top surface joining the two sides of
thread.
Flank
Surface between crest and root.
Root
The bottom of the groove between the two
flanks of the thread

6. Contd.,
Pitch
The distance measured parallel to the axis from a
point on a thread to the corresponding next point.
Helix angle:
The helix is the angle made by the helix of the
thread at the pitch line with the axis.
Flank angle
Angle made by the flank of a thread with the
perpendicular to the thread axis

7. Contd.,
Depth of thread
The distance between the crest and root of the
thread
Major diameter:
Diameter of an imaginary co-axial cylinder
which would touch the crests of external or internal
thread.
Minor diameter (Root diameter or Core diameter):
Diameter of an imaginary co-axial cylinder
which would touch the roots of an external thread.

8. Contd.,
• Addendum
– Radial distance between the major and pitch
cylinders For external thread.
– Radial distance between the minor and pitch
cylinder For internal thread.
• Dedendum
– Radial distance between the pitch and minor
cylinder = For external thread.
– Radial distance between the major and pitch
cylinders = For internal thread

9. Errors in Screw Thread
• Major diameter error
– reduction in the flank contact
– interference with the matching threads
• Minor diameter error
– Interference
– reduction of flank contact

10. Contd.,
• Effective diameter error
• Pitch error
– the total length of thread engaged will be either
too high or too small
• Flank angles error
• Crest and root error

11. Pitch Errors Classification
• Progressive error
• Periodic error
• Drunken error
• Irregular error

12. Errors in Thread
1) Progressive error:
The pitch of the thread is uniform but is longer or shorter
its nominal value and this is called progressive error.

13. Contd.,
2. Periodic error
These error repeats itself at regular intervals
along the thread.
Causes of periodic error:
•Teeth error in gears.
•Lead screw error.
•Eccentric mounting of the gears.

14. Contd.,
Length of thread
Cumulativepitcherror
Length of thread
Cumulativepitcherror
ERRATIC ERRORSPROGRESSIVE ERROR
Fig (c) Fig (d)

15. Contd.,
3) Drunken error:
Drunken errors are repeated once per turn of the thread in
a Drunken thread.
In Drunken thread the pitch measured parallel to the thread
axis. If the thread is not cut to the true helix the drunken
thread error will form

16. Contd.,
4) Irregular errors:
It is vary irregular manner along the length of
the thread.
Irregular error causes:
1. Machine fault.
2. Non-uniformity in the material.
3. Cutting action is not correct.
4. Machining disturbances

17. MEASUREMENT OF VARIOUS
ELEMENTS OF THREAD
1) Major diameter
2) Minor diameter
3) Effective or Pitch diameter
4) Pitch
5) Thread angle and form angle

18. Major Diameter
• Ordinary micrometer
• Bench micrometer

19. Measurement by Bench
micrometer:
Clamp
Fiducial
Indicator
Measuring
Anvils Holding centres
Micrometer head
Supports
BENCH MICROMETER

20. Measurement by Bench micrometer:
Then the cylinder is replaced by the threaded work piece and the new
reading is taken

21. Measurement by Bench micrometer:

22. Measurement by Bench micrometer:
Holding centre
Measuring anvil
Holding centre
Measuring anvil
StandardCylinder
ScrewThread
Measurement of Major diameter

23. Measurement of the major diameter of an
Internal thread:
The major diameter is usually measured by thread
comparator fitted with ball-ended styli.
First the Instrument is set for a cylindrical reference
having the same diameter of major diameter of internal
thread and the reading is taken.
 Then the floating head is retracted to engage the tips of
the styli at the root of spring under pressure.
For that the new reading is taken,

24. Measurement of the major diameter of an
Internal thread:

25. Measurement of Minor diameter
The minor diameter is measured by a
comparative method by using floating carriage
diameter measuring machine and small ‘ V pieces
which make contact with the root of the thread.
These V pieces are made in several sizes,
having suitable radii at the edges.
 V pieces are made of hardened steel.
The floating carriage diameter-measuring
machine is a bench micrometer mounted on a
carriage.

26. Measurement of Minor diameter

27. The threaded work piece is mounted between the centers of
the instrument and the.V pieces are placed on each side of
the work piece and then the reading is noted.
After taking this reading the work piece is then replaced by
a standard reference cylindrical setting gauge.
Measurement of Minor diameter

28. Measurement of Minor diameter
of Internal threads:
The Minor diameter of Internal threads
are measured by
1. Using taper parallels
2. Using Rollers.

29. Measurement of Minor diameter
of Internal threads:
1. Using taper parallels:
 For diameters less than 200mm the use of Taper
parallels and micrometer is very common.
 The taper parallels are pairs of wedges having reduced
and parallel outer edges.
 The diameter across their outer edges can be changed
by sliding them over each other.

30. Measurement of Minor diameter
of Internal threads:
Using rollers:
 For more than 200mm diameter this method is
used. Precision rollers are inserted inside the thread
and proper slip gauge is inserted between the rollers.
The minor diameter is then the length of slip gauges
plus twice the diameter of roller.

31. Measurement of Effective Diameter
• One wire
• Two Wires
• Three wires method

32. One wire method

33. Two wire method
In this method wires of suitable size are
placed between the standard and the
micrometer anvils

34. Three Wire Method

35. Pitch measurement
• Pitch measuring machine
• Tool makers microscope
• Screw pitch gauge

36. Pitch measuring machine

37. Tool makers microscope

38. Thread form and flank angle measurement
• Thread gauge
• Ring gauge
• Optical projectors

39. Gear Measurement
Types of gears
– Spur gear
– Spiral gear
– Helical gears
– Bevel gears
– Worm and Worm wheel
– Rack and Pinion:

40. Gear tooth Terminology

41. • Tooth profile
– It is the shape of any side of gear tooth in its
cross section
• Pitch circle
– It is the imaginary circle of gear that rolls without
slipping over the circle of its mating gear.
• Addendum circle
– The circle coincides with the crests (or) tops of
teeth.
• Dedendum circle (or) Root circle
– This circle coincides with the roots (or) bottom on
teeth

42. • Module(m)
– It is the ratio of pitch circle diameter to the
total number of teeth
• Face
– Part of the tooth in the axial plane lying
between tip circle and pitch circle.
• Flank
– Part of the tooth lying between pitch circle and
root circle.
• Top land
– Top surface of a tooth

43. Gear Errors
• Profile error
– The maximum distance of any point on the tooth
profile form to the design profile.
• Pitch error
– Difference between actual and design pitch
• Cyclic error
– Error occurs in each revolution of gear
• Run out: -
– Total range of reading of a fixed indicator with the
contact points applied to a surface rotated,
without axial movement, about a fixed axis

44. Gear Errors
• Eccentricity: -
– Half the radial run out
• Wobble
– Run out measured parallel to. the axis of
rotation at a specified distance from the axis
• Radial run out
– Run out measured along a perpendicular to
the axis of rotation.

45. Gear Errors
• Eccentricity: -
– Half the radial run out
• Wobble
– Run out measured parallel to. the axis of
rotation at a specified distance from the axis
• Radial run out
– Run out measured along a perpendicular to
the axis of rotation.

46. Gear Errors
• Axial run out
– Run out measured parallel to the axis of
rotation at a speed
• Periodic error
– Error occurring at regular intervals

47. GEAR MEASUREMENT
1. Run out
2. Pitch
3. Profile
4. Back lash
5. Tooth thickness
6. Alignment

48. Run out
• In the testing the gears are placed in the
mandrel and the dial indicator of the tester
possesses special tip depending upon the
module of the gear and the tips inserted
between the tooth spaces and the gears
are rotated tooth by tooth and the variation
is noted from the dial! indicator

49. Pitch measurement
• Point to point measurement (i.e. One tooth
point to next toot point)
• Direct angular measurement

50. Tooth to Tooth measurement

51. Direct Angular Measurement
• Measuring the error by using set dial gauge
against a tooth in this method the position of a
suitable point on a tooth is measured after the
gear has been indexed by a suitable angle
• If the gear is not indexed through the angular
pitch the reading differs from the original
reading.
• The difference between these is the cumulative
pitch error

52. Profile checking
• Optical projection method
• Involute measuring machine

53. Involute measuring machine

54. Backlash
• Backlash is the distance through which a
gear can be rotated to bring its
nonworking flank in contact with the teeth
of mating gear
• The determination of backlash is, first one
of the two gears of the pair is locked, while
other is rotated forward and backward and
by the comparator the maximum
displacement its measured

55. Tooth thickness measurement
• Gear tooth vernier caliper method
(Chordal thickness method)
• Base tangent method

56. Gear tooth vernier caliper

57. Base tangent method