Experiment 6Measuring Screw Thread Parameter with Optical Profile Projector

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I. Abstract.

The experiment is about a technique and system of measuring screw thread

parameter based on the theory of optical profile measurement, which can be carried out

the automated measurement of screw thread parameter. An inspection instrument was

designed and produced, which included exterior imaging system of optical path,

transverse displacement measurement system, axial displacement measurement system,

and a module to deal with, control and assess the data in the upper system. The relation

between measured spot and image is established, including the image length of receiving

lens, focal length, optical system and some main factor affected measuring precision are

analyzed. The experimental results show that the measurement uncertainty of screw

thread average diameter can reach 1 μm.

II. Content:

1. Principle

Thread pieces as a connecting are commonly used in manufacturing, 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 is great significance.

The measurement of screw thread parameter in traditional method is the use of

thread gauge by contact-type or the use powerful tools microscopes by men, low

efficiency. For complex screw thread parts in the shape of geometry, traditional testing

methods were not applied. Therefore, the principle and implementing method of non-

contact thread parameter inspection based on optical measurement principle were

presented, which can reduce measurement errors in measurement, can increase detection

accuracy and achieve thread parameters automatic measurements.

It is the instrument used to measure geometrical parameters of gear. Screw threads

and compare the profiles with the standard templates by projecting the image of the

object on a screen.

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Concept Structure

Appendix

Terminology:

Screw thread: a screw thread is the helical ridge produced by forming a continuous

helical groove of uniform section on the external or internal surface of a cylinder or a

cone. A screw thread formed on a cylinder is known as straight or parallel screw thread,

while the one formed on a cone is known as tapered threads.

Screw Threads

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Thread form: There are many different thread forms in use today. The forms

most widely used for power transmission screw threads are illustrated at Figure below.

Identifying screw threads:

Six basic elements in identifying screw threads:

Thread Pitch:

- The thread pitch is the axial distance from one thread groove to the next.- The thread pitch can be measured with a steel rule or a caliper or comparator can be used.- The pitch can be determined by dividing the count into the length.In the example shown in Figure on the right, there are 5 pitches in 1 in. So the thread pitch is 0.200 in.

.

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Hand Of The Thread:

- The hand of the thread can be easily determined by visual inspection

- Most threads are right hand and Left hand threads are common on manual drives

where clockwise handle rotation raises, tightens, extends, or creates motion away from

the operator.

Major Diameter:

- Major diameter is the diameter of an imaginary co-axial cylinder which touches the

crests of an external thread or the roots of an internal thread.

- Major diameter is an external /outer diameter of bolt or nut. The major diameter can be

measured with a micrometer, caliper or steel rule. Major diameters are generally the first

numbers found in thread designations. Care must be taken to measure the major diameter

on a section of the screw thread that is not worn.

Minor Diameter: The minor diameter can be determined by direct measurement on an

optical comparator or by measuring the depth of the thread with a depth micrometer and

subtracting twice the measured depth of thread from the major diameter.

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Pitch Diameter:

- The pitch diameter is the diameter at which the thread tooth and the thread space are

equal.

- To accurately measure the pitch diameter requires an optical comparator or thread

wires.

- The optical comparator is the easiest to use as the measurement can be directly made

and no mathematics are necessary.

Angles of thread:

- The angle of thread is known from the name of the thread.

- All National form and Unified threads have a 60o angle.

- Acme and Worm threads have a 29o angle

- Whitworth threads have a 55o angle

The screw thread parameters can be measured contact-type with thread gauge,

micrometer, caliper or steel rule… but it is very difficult and low efficiency. This

experiment approach new technology measurement is the optical profile projector with

very high accuracy.

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2. Equipment

2.1. M21 machine

2.2. Specimen : Screw thread sample

3. Practice

3.1. Switch on the optical profile projector.

3.2. Fixed sample on the jig of Machine ( M21)

3.3. Set zero for Digital Indicator of Machine.

3.4. Focus it properly by moving focusing wheel and moving the work table

to obtain correct magnified image of the object.

3.5. Horizontal (Y axis) measurement can be taken by right hand side

micrometer and the vertical (X axis) measurement can be taken from

front side micrometer.

3.6. Adopt similar procedure for the measurement of the average diameter

of the screw.

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3.7. To measure pitch take the distance between two similar points on

adjacent flank or crest to crest or root to root on adjacent threads,

parallel to the axis of the screw thread. For achieving this adjust the

reference line on the projector to the corresponding points and note the

micrometer readings between these points.

3.8. For measurement of thread angle match the reference line between

flanks and note down the angle.

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4. Results

Experimental data and calculated result of the survey

4.1. Measure the average diameter

Times A B C D

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2

3

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Average diameter:

4.2. The average measured step

Times YE YF YI YK

1

2

3

4

Average Step: (m=3)

3. Half-page thread angle

1 = 0 = 2 =

III. Discussion, Analysis and Conclusion

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