EXPERIMENT REPORT

Experiment 6

MEASURING SCREW THREAD PARAMETER WITH OPTICAL PROFILE PROJECTOR

Group 9:

Le Xuan Dat (Leader)

Hoang Nam

Tran Anh Tu (Writer)

Pham Thanh Tung

Overview

This experiment is designed to measure screw thread parameter, based on the theory of optical profile measurement and using an inspection instrument. The device has exterior system of measurement path, transverse displacement measurement system, axial displacement measurement system, and a module to deal with, control and assess the data in the upper system. After measuring, we establish the relation between measured spot and image, including the image length of receiving lens, focal length, optical system, and analyze some main factor affected measuring precision. The experimental results show that the measurement uncertainty of screw thread average diameter is up to 1 μm.

I. Objective

- Learn more about the theory of screw, thread

- Understand the working of optical profile projector

- Know how to measure thread component using projector

II. Background knowledge

1. Introduction

Thread pieces are widely used in mechanical systems as connecting parts to other components, and with the development of modern technology, the reliability and high accuracy parameter of thread become essential.

People have invented many ways to measure thread, like using thread micrometer, three wire method, thread triangle method… All of these are good, but they are often used in normal thread, with less requirement of accuracy. For complex thread parts with high accurate requirement, the traditional methods are not suitable. Therefore the method of non-contact thread parameter inspection based on optical measurement principle is presented, which can reduce errors in measurement, increase detection accuracy and achieve thread parameters automatic measurements.

2. Screw thread definition and classification

Screw thread is a helical structure used to convert between linear and rotational movement. It is produced by forming continous helical groove on the surface of a cylinder or a cone. There are many forms of thread, which are divided by the following types:

- Hand of thread: Threads are normally left-handed or right-handed. Left-handed thread gets advanced when turn counter clockwise, while right-handed one is clockwise in advantage.

- Forming surface: A thread with helical ridge running outside is a male thread, while the one with helical groove formed on the internal surface is a female thread.

- A thread formed on a cylinder is called straight or parallel thread, while the one formed on a cone is known as tapered thread.

- Shape: Based on shape, threads can be divided into many forms, some of the most popular ones are shown in the figure below:

Figure 1: Some popular types of thread

3. Thread parameters

a. Pitch:

- Thread pitch is the axial distance from one thread groove to the next.

- The thread pitch can be measured with a steel rule, calliper or comparator, and determined by dividing the count into the length.

For example, there are 5 pitches in 1 in., so the thread pitch is 0.200 in.

Figure 2: Thread pitch measurement example

b. Diameter:

- 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 (e.g. the external diameter of a bolt or nut). It can be measured with a micrometer, caliper or steel rule. Major diameters are generally the first numbers found in thread designations.

- Minor diameter: Minor diameter is the lower extreme diameter of the thread. (e.g. the internal diameter of a nut). 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.

- 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. While thread wires is complicated to use with lots of steps and mathematics, the optical comparator is a better choice.

Figure 3: Different types of diameter of a thread

c. Angle:

- Thread angle is the angle between two threads.

- In ANSI Standard, most of V-thread angle is 60o, whithworth thread angle is 55o, acme and worm thread is 29o, and other angle can be used for different kinds of threads.

- Thread angles are commonly easily known from the name of thread.

Figure 4: Thread angle

III. Experimental procedure

1. Instrument

Optical profile projector is the device which can display the magnified image of the components. The result image can be compared with some standard for its dimensions, form…

An optical profile projector consists of:

- A projector having a light source, a condensor or collimating lens.

- Work holding table.

- Projection system having mirrors and lenses

- Screen on which image of the workpice is projected.

- Measuring devices.

The light source may be a filament lamp. A strong beam of light consisting of concentrated bundle of parallel right rays is produced by optical means. The image of the object may be produced either on horizontal or vertical screen. Surface illumination produces a reflected image of the object on the screen.

The objects to be tested are placed on the work table. The light beam after passing the object to be projected passes into the projection system comprising lenses and mirrors. The lenses are used to obtain desired magnification and mirrors to direct the beam of light on the screen. Normally 10x, 20x, 50x, 100x magnification is done.

The work table is provided with means for measurement like micrometer. Angles can be measured by rotating the screen. The screen is dived into 360 degree with 90 degree cross line marking. Various charts can be fixed on the screen with the help of four clamps.

Figure 5: Optical profile projector at C8-209 Hanoi University of Science and Technology

Figure 6: Light source for the optical profile projector

Figure 7: Image of thread seen from the optical profile projector

Figure 8: Co-ordinate value read on the screen

2. Process

Step by step measurement:

1. Switch on the optical profile projector.

2. Place the clean workpice on the glass of the table.

3. Set zero for digital indicator of machine.

4. Focus it properly by moving focusing wheel and moving the work table to obtain correct magnified image of the object. Horizontal (y axis) measurement can be taken by right hand side micrometer and the vertical measurement can be taken from front side micrometer.

5. To measure the average diameter, locate the position of the centre of one upper and lower right flanks parallel to each other, then calculate the distance between two points. Do the same for the next left flanks, the average diameter is the average distance of the two measurements. Measure each position 4 times.

Figure 9: Average diameter measurement

6. To measure pitch, take the distance between two similar points on adjacent flanks 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. Normally we measure the distance between n pitches and then divide the result by n to get the distance between one pitch (n = 3÷5). Measure each distance 4 times.

Figure 10: Pitch measurement

7. To measurement thread angle, match the reference line between flanks and note down the angle.

Figure 11: Thread angle measurement

3. Result

a, Measure the average diameter

Times A B C D d2

1 - 0,038 7,066 7,029 - 0,018 7,0755

2 - 0,031 7,071 7,033 - 0,026 7,0805

3 - 0,039 7,076 7,043 - 0,028 7,093

4 - 0,026 7,080 7,043 - 0,026 7,0875

While d2 is the average diameter d2=¿¿

d2=∑i=1

n

d2 i

n=7,0755+7,0805+7,093+7,0875

4=7,084

b, Measure thread pitch

Choose n = 3

Times YE YF YI YK P

1 1,331 5,039 5,650 1,930 1,238

2 1,320 5,044 5,666 1,939 1,242

3 1,315 5,040 5,669 1,924 1,245

4 1,322 5,052 5,671 1,934 1,245

While P is the average distance of one pitch P=¿¿

P=∑i=1

m

Pi

m=1,238+1,242+1,245+1,245

4=1,243

c, Measure thread angle

α1 α0 α2

α2=|α

2tr−α

2ph| = 300o6' 30o2' 0 330o8'

α2

tr=α1−α0=30o 2' α2

ph=α 0−α 2=330o 8'

IV, Conclusion

Experiment result shows that the measuring resolution is 1µm. The principle and implementing method of non-contact internal and external thread parameter inspection based on optical measurement principal are presented in this article, which can carry out the automated measurement of thread parameter with high precision since it avoids friction, edge deformation and other problems caused by contact measurement. However, errors can occur due to the accuracy of the machine, and by the skill of the observer.