Báo cáo thí nghiệm về screw thread

Measuring Screw Thread Parameter
with Universal Microscopy
Nguyễn Khánh Hiệp-group 3-20100290
Hanoi University of Science and Technology
Abstract
In the industrial production, as an important transmission part, the screw
thread is applied extensively in many automation equipments. In this paper, as a
new kind of real-time and non-contact measurement method, a screw thread
parameter measurement system based on image processing method is developed to
accurately measure the outside diameter, inside diameter, pitch diameter, pitch,
thread height and other parameters of screw thread. The experimental results show
that the measurement uncertainty of screw thread average diameter can reach 1 μm.
I. Introduction
Thread pieces as a connecting are
commonly used in manufacturing and
aerospace industries, particularly in
the case of some aircraft systems,
many components are connected to a
thread. With the development of
modern technology, requires high
reliability and interchangeability to
thread, and the accurate detection of
thread parameters for improving
interchangeability and thread
reliability is of great significance.
Universal Microscopy is one of the
most important measurement
instruments for dimensional
parameters measurement. It is very
universal instrument with wide
equipment for two axis contact-less

measurement in horizontal plane. This
instrument is suitable for measurement
of distances, diameters, angles,
threads, radii and evaluation of its
geometrical parameters. Big advantage
of this kind of optomechanical
instruments is its long lifetime in the
case of right handling.
II. Main approach
1. Terminology:
Screw thread is a helical ridge
produced by forming a continuous
helical groove of the uniform section
on the external or internal surface of a
cylinder or cone. A screw thread
formed on a cylinder is known as
straight or parallel. Screw thread,
while the one formed on a cone or
frustum of a cone is known as tapered
screw thread. In the present
experiment, parameters of a straight
1
screw thread of small magnitude are
measured. Parameters of small screws
such as screws used in watches,
electrical plugs, and toys cannot be
measured by instruments like vernier
calipers or micrometers. Unless they
are magnified it is not possible to
measure all the parameters.Some

parameters of a straight screw thread
are defined as follows:
Crest of a thread is defined as the
prominent part of thread. Root of a
thread is defined as the bottom of the
groove between two flanks of thread.
Flanks of thread are straight edges
which connect the crest with the root.
Angle of the thread also known as
included angle is the angle between
the flanks of the thread measured in
axial plane.
The pitch of a thread is the distance
measured parallel to the axis of the
thread, between corresponding points
on adjacent thread forms in the same
axial plane and on the same side of
axis.
Major diameter is the diameter of an
imaginary cylinder Co-axial with the
screw which just touches the crests of
an external thread or roots of an
internal thread.
Minor Diameter is the diameter of an
imaginary cylinder co-axial with the
screw which just touches the roots of
an external thread or the crest of an
internal thread.
2. Learning objectives:
- To understand the working of optical

profile projector.
- To measure the parameters of gear
threaded component.
3. Experiment program:

Fig 1.Universal Microscopy in C8-209
Hanoi University of Science &
Technology
a. Determination of the
major diameter
To measure pitch of the thread, align
the intersection of two diagonal lines
at the point on the flank which is
between of crest and root. At this
2
position take the x value of
micrometer . Next move the
intersection of two diagonal lines
along the x axis that the next point is
set in the line which have the same
direction with old flank. Again note-
down the reading of the x value.
Difference of the two readings gives
the pitch of the thread. In order to get
more exactly value of pitch, we repeat
all step with another point, and take
the average value of two times.
b. Determination of Pitch
To measure pitch of the thread, align
the intersection of two diagonal lines

at the point on the flank which is
between of crest and root. At this
position take the y value of
micrometer . Next move the
intersection of two diagonal lines
across 3 (or 5) crests along the y axis
that the next point is set in the line
which have the same direction with
old flank. Again note-down the
reading of the y value. Difference of
the two readings gives the pitch of the
thread. In order to get more exactly
value of pitch, we repeat all step with
another point, and take the average
value of two times.
3
c. Determination of the
angle
To measure the included angle of the
thread coincide one of the lines of the
cross lines with the flank of the thread
as below figure.
Note-down the reading of the
protractor. Now rotate the protractor
in turn the cross lines in such a way
that same line coincides with the
opposite flank as shown in below.
Note-down the reading of the
protractor. Difference of the two
reading gives the included angle of the

thread.
III.Experiment results
Experimental data and calculated
result of the survey
1. Measure the average
diameter
Times 1 2 3 4
A 7.175 0.099 0.098 7.178
B 7.186 0.099 0.096 7.190
C 7.177 0.078 0.086 7.162
D 7.185 0.096 0.094 7.170
Times 1 2 3 4
( ) ( )
2
2
A B D C
x x x x
d
− + −
=
7.07
8
7.095 7.088 7.083
Average diameter:
2
1
2
7.078 7.095 7.088 7.083
7.086
4

n
i
i
d
d
n
=
+ + +
= = =
∑
2. The average measured step
Times Y
E
Y
F
Y
I
Y
K
1 0.672 3.169 0.040 2.543
2 0.091 2.581 0.019 2.518
3 5.630 8.131 6.281 8.752
4 1.908 4.382 2.545 5.032
Times
( ) ( )
( )
2
2
.
F E K I

y y
P
m
y y
m
− + −
=
=
1 1.250
4
2 1.247
3 1.243
4 1.240
Average
Step:
1
1.250 1.247 1.243 1.240
1.245
4
n
i
i
P
P
n
=
+ + +
= = =
∑
3. Half-page thread angle

60
2 2 2
19”tr ph
α α α
= = °∆ −
1 0
30 4 ”
2
1tr
α
α α
°= − =
0 2
29 3 ”
2
8ph
α
α α
°= − =
α
1
=

329°19”
α
0
=0°(360°)
α
2

= 29°38”
Conclusion
As a new kind of real-time and non-
contact measuring system, the screw
thread parameter measurement system
based on image method is developed
for accurately measuring inside
diameter, outside diameter, pitch
diameter, pitch, and thread height of
screw thread. The system has some
advantages, such as easy and quick
operation, high measuring accuracy
and fast speed of error analysis. The
two-dimensional coordinate data
obtained by scanning the axis section
of thread teeth shaped contours can
reflect the geometrical shape error of
screw thread completely. Experiment
result shows that measuring resolution
is 0.5 μm, with error’s percentage is
about 0.4%.

Reference
[1]. Wu Jingchun, Li chunmei, Feng Hutian. Non-contact measurement system
design of ball screw thread surface parameters. Modular machine tool and
automation processing technology, (11), 52-54(2011).
[2]. Yuan Pengbin, Yuan Zongming, OuYang Zhiying. Non-contact easurement of
thread parameters based on machine vision systems. Pipeline technology and
equipment, (4), 21-22(2008).
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