Screw thread measurement

most of the methods of measurement of thread terminology is covered in this presentation

1. Screw Thread Measurement

2. Measuring elements of screw thread Elements that is to be measured to determine the accuracy of a screw thread is as follows: Major diameter Minor diameter Effective diameter or pitch diameter Pitch Flank angle Thread form

3. Measurement of major diameter Major diameter is the imaginary diameter of thread which would touch the crests of an internal and root of external thread. For measuring the major diameter of external threads following methods are to be used: By using the ordinary micrometer By using the bench micrometer For measuring the major diameter of internal threads is usually done by the thread comparator fitted with ball ended styli.

4. Using ordinary micrometer In this the micrometer is used as a comparator . This micrometer is first set over the cylinder standard having approx. same dimension. This standard is called setting gauge . After taking reading the micrometer is set on the major diameter and reading is taken and then the diameter is measured by following equation: This micrometer is used gently during reading because their might come error by applying extra force which will form deformation of crest of the thread .    

5. Using Bench Micrometer

6. Using bench Micrometer Bench micrometer is designed by the NPL to remove deficiencies inherent in the hand micrometer. In this the fiducial micrometer is used to ensure that all the readings are taken at the same pressure. The instrument has a micrometer head having Vernier scale to read to the accuracy of 0.002mm. This instrument is also used as the comparator to avoid the pitch errors of micrometer threads, zero error setting etc. Then the process is same as of the ordinary micrometer.

7. Using Bench Micrometer

8. Measurement of minor diameter Minor diameter is the imaginary diameter of thread which would touch the roots of the external and crest of the internal threads. For measuring minor diameter of external threads following methods are used: Two V pieces method By projecting the thread on the screen For measuring minor diameter of internal thread following methods are used: Using taper parallels By using rollers and slip gauges

9. Two V pieces method Minor diameter of a V-screw thread is measured by a comparative method using floating carriage diameter measuring and two small V-pieces. This will make contact with the root of the thread. The threaded workpiece is mounted between the centre of the instrument. Then the reading of the micrometer is taken after this threaded workpiece is replaced by the standard cylinder setting gauge of diameter approx. equal to the core diameter of screw. Then the diameter is calculated from the following: Minor diameter of the thread  

10. By projecting thread on screen If the threads are very sharp or have no radius at the root. The measurement of minor diameter is done by projecting the thread form on a screen. This projected form is compared with the use of the Tool Makers Microscope.

11. Using taper parallels For the internal thread of the minor diameter of diameter less than 200mm is measured using the taper parallels. The taper parallels are the pairs of the wedges having parallel outer edges. The taper parallels are inserted inside the thread and adjusted until firm contact is not established with the minor diameter. Then the diameter of the outer edges of the taper parallels is measured using the micrometer.

12. By using rollers and slip gauges This method is used for the diameter having more than 20mm diameter. The precision rollers are inserted inside thread and proper slip gauges inserted between the rollers so that firm contact is established. Then the length of the slip gauges plus twice the diameter of the rollers is the minor diameter.

13. Measurement of effective diameter Effective diameter is the imaginary diameter in between major and minor diameter . It is also known as P itch Diameter . For measuring effective diameter of external threads following methods are to be used: Thread micrometer Wire methods For measuring effective diameter of internal thread is usually measured on a horizontal comparator fitted with detachable small ball ended styli .

14. Thread micrometer This method is simple and rapid. The thread micrometer is same as ordinary micrometer except that it has special contact points to suit the end screw threads form that is to be checked. The contact points are selected on the basis of the types of the thread and the pitch of the thread to be measured. The male contact point is fixed at the anvil and the corresponding female contact point is fitted in to the spindle of the thread micrometer.

15. Thread Micrometer Then the anvils are then made to contact the thread to be checked and the reading is taken, which will give the pitch diameter or effective diameter. In this the actual reading is the measurement of the major diameter on one side and minor diameter of the other side which gives us the effective diameter. If the thread is of the whithworth thread the relation between the outer diameter and the pitch is as follow:  

16. Wire methods There are mainly three types of wire methods. They are as follows: One wire method Two wire method Three wire method This methods are based on the size of the wire . The size of the wire whose diameter makes the contact with the flank of the thread on the effective diameter this size of wire is known as Best Size of Wire . This size is decided by the following equation: Where p= pitch and θ = thread angle  

17. One wire method In this only one wire is used. It is placed between two threads at one side and the anvil contacts the crest on the other side. First the reading is noted on a standard gauge whose dimension is approx. same as that we are going to be obtained. Then the setting gauge is replaced by the thread that is to be measured and the reading is taken. The effective diameter If < then use + ve sign then use – ve sign  

18. Two wire method In this method the two wires of identical diameter is placed between the flanks of the thread. In this method also first of all the reading of the standard cylinder is taken and then the actual thread that is to be measured is kept. The diameter of the standard = S The diameter under the wires = T The effective dimeter of screw = E Then E=S+P where T= S-( - ). Where P is depended upon the diameter of wire and pitch of the thread.  

19. Three wire method In this method three wires of equal and precise diameter are placed in the thread grooves at opposite sides of the screw and measuring the distance M over outer surfaces of wires with the micrometer. Out of the three wires the set of two wires is placed at one side and the one wire is placed on the other side. This three wires are either held in hand or secured in the grooves by applying grease in the thread. This method ensures the alignment of micrometer anvil faces parallel to the thread axis. This method is more accurate then other methods.

20. Three Wire Method

21. Three wire method The effective diameter can be calculated with the following relations: The reading of micrometer M=E+Q The wire diameter The thread angle , and Thus effective diameter E=M-Q  

22. Measurement of pitch Pitch is the axial distance between the two corresponding crests or roots. The methods used to measure pitch is as follows: Pitch measuring machine Tool makers microscope Screw pitch gauge

23. Pitch measuring machine A pitch measuring machine provides a relatively simple and accurate method of measuring he pitch of the screw thread. The threaded component whose pitch is to be measured is mounted between the centers on the pitch measuring machine.

24. Tool makers microscope Work piece is mounted on a glass plate. A light source provides horizontal beam of light which is reflected from a mirror by 900 upwards towards the table. Image of the outline contour of the work piece passes through the objective of the optical head . The image is projected by a system of three prisms to a ground glass screen . The measurements are made by means of cross lines engraved on the ground glass screen .

25. Measurement of flank Angle Flank Angle is the angle formed by a flank and a perpendicular to the thread axis in an axial plane. It is also called the half thread angle . For this measurement we have to measure the thread angle . To measure the thread angle the following methods is used: Optical Projection

26. Optical Projection The modern projectors are equipped with the work holding fixtures, the projection lamp and the lenses situated on the top of the cabinet, and the screen at the front. Most of the projectors are fitted with the Vernier protectors, which gives the reading to one minute of arc. The thread is viewed at a screen then the angle is measured by means of the protractor.

27. Measurement of Thread Form Thread Form is the shape of the contour of one complete thread as seen in axial section . It may be in a shape of square, triangle, trapezoidal or other shapes . The thread form is also measured or identified by the Optical projection method .

28. Optical projection The modern projectors are equipped with the work holding fixtures, the projection lamp and the lenses situated on the top of the cabinet, and the screen at the front. The projected image of the thread form is enlarged on the ground glass over which the drawing or the template of the form is made. Then this two forms are compared with each other.

29. Reference The textbook of metrology by M. Mahajan( Dhanpat rai co.) Mechanical measurement and instrumentation by Er. R. K. Rajput Mechanical Measurement and Metrology by ( B ooks India Publication) https:// en.wikipedia.org/wiki/Screw_thread https:// law.resource.org/pub/in/bis/S01/is.10587.1983.pdf https://me-mechanicalengineering.com/screw-threads-terminology /

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