Screw thread measurements

National Institute of Technology Sikkim
Department of Mechanical Engineering

1. Department of Mechanical Engineering National Institute of Technology Sikkim Screw Thread and Gear Measurement

2. Terminology of screw threads

3. Screw thread- definition 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.

4. Types of T hread External thread : a thread formed on outside of a work piece is known as external thread. Example: on bolts or studs etc. Internal thread : a thread formed on inside of a work piece is known as internal thread. Example: on a nut or female screw gauge.

5. Screw T hread - U se Screw threads are used: To hold parts together-act as fastners (ex: V-threads) To transmit motion & power ( Square, Acme threads)

6. Screw Thread T erminology P i t ch C r e s t R oo t Flank T h r e a d Angle Pitch line Axis of thread Axial thickness Addendum D ed e ndu m F la n k a n g l e Major dia Pitch dia Minor dia EXTERNAL THREAD TERMINOLOGY

7. Screw Thread T erminology Pitch: The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis. Lead : The distance a screw thread advances in one turn. For a single start threads, lead=pitch, For double start, lead=2xpitch, & so on. Thread Form : The cross section of thread cut by a plane containing the axis. Major Diameter : This 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 threads. It is also called as ‘Nominal diameter’

8. Screw Thread T erminology Minor diameter : This 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. This is also referred to as ‘root’ or ‘core diameter’ . Effective diameter or Pitch diameter : It is the diameter of an imaginary cylinder coaxial with the axis of the thread and intersects the flanks of the thread such that width of the threads & width of spaces between threads are equal. Flank: It is the Thread surface that connects crest with root. Depth of thread : It is the distance between crest and root measured perpendicular to axis of screw.

9. Screw Thread T erminology Angle of thread : Included angle between sides of thread measured in axial plane. He l ix angle : Ang l e th a t the th r ead m a k es with p l ane perpendicular to thread axis. Flank angle : It is half the included angle of the thread. Addendum : It is the distance between the crest and the pitch line measured perpendicular to axis of the screw. Ded e ndu m : It i s t h e di s t a nc e b e t w een the p i t c h li n e & the root measured perpendicular to axis of the screw.

10. Measurement of Various Elements Of Thread To find out the accuracy of a screw thread it will be necessary to measure the following: Major diameter. Minor diameter. Effective or Pitch diameter. Pitch Thread angle and form

11. Measurement of M ajor D iameter Bench micrometer The instruments which are used to find the major diameter .

12. Ordinary micrometer The ordinary micrometer is quite suitable for measuring the external major diameter. It is first adjusted for appropriate cylindrical size (S) having the same diameter (approximately).This process is known as ‘ gauge setting’ . After taking this reading ‘ R the micrometer is set on the major diameter of the thread, and the new reading is ‘R2

13. Measurement by Bench M icrometer: C la m p Fiducial I nd i cato r M e a s u r i n g Anvils Holding centres Micrometer head Supports BENCH MICROMETER

14. Measurement by Bench M icrometer For getting the greater accuracy the bench micrometer is used for measuring the major diameter. In this process the variation in measuring Pressure, pitch errors are being neglected. The fiducial indicator is used to ensure all the measurements are made at same pressure. The instrument has a micrometer head with a vernier scale to read the accuracy of 0.002mm. Calibrated setting cylinder having the same diameter as the major diameter of the thread to be measured is used as setting standard. After setting the standard, the setting cylinder is held between the anvils and the reading is taken

15. Measurement by Bench M icrometer Then the cylinder is replaced by the threaded work piece and the new reading is taken

16. Measurement by Bench M icrometer:

17. Measurement by Bench M icrometer: Holding centre Measuring anvil Holding centre Measuring anvil Standard Cylinder Screw Thread Measurement of Major D iameter

18. Measurement of the M ajor D iameter of an Internal T hread An indirect approach of measuring internal dia is obtained by obtaining the cast of the Thread. The main art thus lies in obtaining a perfect cast.

19. Measurement of the Major Diameter of an Internal Thread

20. Measurement of Minor D iameter 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.

21. Measurement of Minor D iameter

22. The threaded work piece is mounted between the centres 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 D iameter

23. Measurement of Minor D iameter of Internal T hreads The Minor diameter of Internal threads are measured by Using taper parallels Using Rollers. 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.

24. 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. 2. Using rollers:

25. Pitch M easurement The most commonly used methods for measuring the pitch are Pitch measuring machine Tool makers microscope Screw pitch gauge

26. Tool M akers M icroscope

27. Hollow base Lamp Collimator lens B a s e C ol u m n Eye piece Optical head Mirror work table with carriage Tool Makers Microscope

28. Tool M akers M icroscope

29. Worktable is placed on the base of the instrument. The optical head is mounted on a vertical column it can be moved up and down. Work piece is mounted on a glass plate. A light source provides horizontal beam of light which is reflected from a mirror by 90 degree upwards towards the table. Image of the outline of 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. The screen can be rotated through 360°. Different types of graduated screens and eyepieces are used . Tool M akers M icroscope

30. Pitch M easuring M achine When the pointer is accurately placed in position, the micrometer reading is noted. The stylus is then moved along into the next thread space, by rotation of the micrometer, and a second reading taken. The difference between the two readings is the pitch of the thread. Readings are taken in this manner until the whole length of the screw thread has been covered. Spring loaded head permits the stylus to move up the flank of the thread and down into the next space as it is moved along. Accurate positioning of the stylus between the two flanks is obtained by ensuring that the pointer T is always opposite to its index mark when readings are taken.

31. Screw P itch G auge

32. Measurement of screw thread angle (Flank angle)

33. Measurement of effective diameter Effective diameter measurement is carried out by following methods. T wo wires method T hree wires method. Micrometer method.

34. Two W ire M ethod The effective diameter can not be measured directly but can be calculated from the measurements made. Wires of exactly known diameters are chosen such that they contact the flanks at their straight portions. If the size of the wire is such it contacts the flanks at the pitch line, it is called the ‘ best size ’ of wire which can be determined by geometry of screw thread. The screw thread is mounted between the centers & wires are placed in the grooves and reading M is taken. Then the effective diameter E =T+P where T =M-2d, & P is a value which depends on diameter of wire, pitch & angle of the screw thread.

35. Two W ire M etho d M M-Dimension over the wire

36. Two W ire M ethod

37. Two W ire M ethod

38. Two W ire M ethod

39. P AP=O P - OA Two W ire M ethod

40. Three Wire method The three-wire method is the accurate method. In this method three wires of equal and precise diameter are placed in the groves at opposite sides of the screw. In this one wire on one side and two on the other side are used. The wires either may held in hand or hung from a stand. This method ensures the alignment of micrometer anvil faces parallel to the thread axis.

41. Three Wire method

42. This method is more accurate than two wire method as it ensures alignment of micrometer faces parallel to the thread axis. Here, three wires of exactly known diameters are used, one on one side & the two on the other side. The wires may be held in hand or hung from a stand. From the fig, M=diameter over the wires E= effective diameter (to be found) d= diameter of wires, h=height of wire center above the pitch line, r=radius of wire, H=depth of thread, D=major diameter of the thread. Three Wire method

43. E M H B C D  A h E M Dia 'd' P  E Three Wire method

44. 2 2 2 2 4 H  DE cot   P cot  and CD  H  P cot  2 2 2 2 4 2 Further h  ( AD  CD )    d cosec       P cot     2 2   4 2  Distance over the wires, M  E  2h  2r the triangl e ABD , AD  AB cosec   d cosec  2 2 2 F r o m  2   Or M  E  d   cosec    P cot   1    2  2      P    P cot    2 r  E  2 r  1  cosec  cot i.e. M  E  2  rcosec  2 Three Wire method

45. 2 2 M  D  3.1657d  1.605P where D is the major diameter of the thread. For Metric threads, Depth of thread  0.6495P  E  D  . 649 5 P ,   6 o , c o s e c   2 , c o t   1. 732 2 2  M  D  3d  1.5155P We can measure the valueof M practically & then compare with the theoretical valuesusing formulae derived above. After finding the correct value of M, as d is known, E can be found out.  E  D - 0.64P, cosec   2.1657, and cot   1.921 F o r W h i t w o r t h t h r e a d ,   5 5 o , d e p th o f t h r e a d  . 6 4 P Three Wire method

46. Best Wire Size     A P / 4 P Pitch line P / 2 BEST SIZE OF WIRE B

47. P where P is the pitch of the thread. 4 2 A B s e c . 2 cos 2 AB  sin  90 - 2  The best size wire is the one which makes contact at the pitch line or effective diameter of the screw thread. In other words, as shown in fig OB is perpendicular to flank portion of the thread at the pitch line. b  D  2 sec   P sec  4 2 2 2 b  P b But OB  radius of wire  1  dia of best size wire (D )  i.e. D  2  OB  2  ABsec . Alsosince AB lies on the pitch line, AB 2         O B     A B   AB  In the triangle OAB, Sin  BO A   , or sin  90 -   OB 2 OB  AB     Best Wire Size