Unit1 Screw Thread

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Unit1 Screw Thread

  1. 1. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/1 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c UNIT 1 SCREW THREAD OBJECTIVES General Objective: To understand the methods of testing and measuring elements of ISO and BSW screw threads. Specific Objectives: At the end of the unit you will be able to : Ø Identify the methods of measuring major diameter, minor diameter and mean diameter. Ø Measure and calculate major diameter, minor diameter and mean diameter of a screw thread. Ø To check the thread form by using the optical comparator.
  2. 2. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/2 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c INPUT 1.0 INTRODUCTION All elements of the thread influence the strength and interchange ability of screw thread, but the pitch, angle and effective diameter are much more important than the other elements 1.1 ELEMENTS OF A THREAD To understand and calculate the thread elements, the following definition relating to screw threads should be known (Fig. 1.1). root pitch major diameter mean diameter minor diameter thread angle Figure 1.1 Screw thread terminology
  3. 3. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/3 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 1.1.1. Major Diameter It is the largest diameter of the thread. This is the distance between the crests of the thread measured perpendicular to the thread axis. 1.1.2. Pitch/Mean Diameter The diameter of the thread used to establish the relationship, or fit, between an internal and external thread. The pitch diameter is the distance between the pitch points measured perpendicular to the thread axis. The pitch points are the points on the thread where the thread ridge and the space between the threads are of the same width. 1.1.3. Minor Diameter It is the smallest diameter of the thread. This is the distance between the roots of the thread measured perpendicular to the thread axis. 1.1.4. Thread Angle This is the included angle of the thread form. 1.1.5. Pitch It is the distance between the same points on adjacent threads. This is also the linear distance the thread will travel in one revolution. 1.1.6. Root The surface of the thread that joins the flanks of adjacent threads. The distance between the roots on opposite sides of the thread is called the root, or minor diameter.
  4. 4. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/4 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 1.2. MEASURING THE MAJOR DIAMETER To measure major diameter of the screw, a micrometer, with anvils of a diameter sufficient to span two threads, may be used,( Fig. 1.2). To eliminate the effect of errors in the micrometer screw and measuring faces, it is advisable first to check the instrument to a cylindrical standard of about the same diameter as the screw. For such purposes a plug gauge or a set of ‘Hoffman’ rollers is useful. anvil Figure 1.2 Checking the major diameter with a micrometer 1.3. MEASURING THE MINOR/CORE DIAMETER The diameter over the roots of a thread may be checked by means of a special micrometer adapted with a shaped anvils, (Fig. 1.3) or a micrometer may be used in conjunction with a pair of vee pieces ( steel prisms ). The second method is recommended ( Fig.1.5). The steel prisms on the micrometer are pressed into the thread groove. The ends of the prisms are slightly curved and parallel to the root thread. It is important , when making the test, to ensure that the micrometer is positioned at right angles to the axis of the screw being measured, and when a large amount of such work is to be done, a special ‘floating bench micrometer’ ( Fig. 1.4 ) is used. It is because, it supports the screw and incorporates the micrometer elements correctly located, as well as providing means for suspending the vee prisms.
  5. 5. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/5 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Fig. 1.3 Checking the core diameter of a thread with an shaped anvil micrometer Fig. 1.4. A Floating Micrometer
  6. 6. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/6 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c The prism values are stated as, Dm = W – 2T Note: Dm - mean diameter W - distance between two prism T - prism height (known) T prism W Figure 1.5 Checking minor diameter by using a micrometer and prisms 1.4. MEASURING THE MEAN/PITCH/EFFECTIVE DIAMETER The three-wire method is recognized as one of the best methods of checking the pitch diameter because the results are least affected by any error which may be present in the included thread angle. For threads which require an accuracy of 0.001 in. or 0.02 mm, a micrometer can be used to measure the distance over the wires. For threads requiring greater accuracy an electronic comparator should be used to measure the distance over the wires. In the three-wire method, three wires of equal diameter are placed in the thread; two on one side and one on the other side (Fig. 1.6). The wires used should be hardened and lapped to three times the accuracy of the thread to be inspected. A standard micrometer may then be used to measure the distance over the wires. For greatest accuracy, the best size wire should be used.
  7. 7. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/7 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Figure 1.6 Three wire method The hard round bars (wire) with the same size are positioned opposite to the screw thread groove shown in the diagram above. The distance is measured between the outside of the round bars. The most suitable wire size is 0.57735p. In Fig. 1.7 P is the pitch of the screw thread. The suitable wire size is quite hard to get, usually a size bigger than 0.57735p wire size will be used. Fig. 1.7. Conditions when measuring with wires
  8. 8. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/8 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 1.4.1. Best Size Wires. Wires which touch the thread at the pitch diameter are known as "Best Size" Wires. Such wires are used because the measurements of pitch diameter are least affected by errors that may be present in the angle of the thread. The above analysis for the distance over wires holds good provided the wire touches the thread somewhere on its right side, and provided the thread angle is correct. The extremes of wire sizes which touch on the straight sides and which can be measured are shown at (a) and (c), Fig.1.9. For ISO metric, unified and Whitworth threads these limiting sizes are given in Table 1.1 Table 1.1. Wire sizes for thread measurement ( p = pitch of thread) Thread Max. Min. ‘Best Size range for Form Wire Wire Wire’ Best wire ISO metric and 1.01p 0.505p 0.557p 0.534p Unified 0.620p Whitworth 0.853p 0.506p 0.564p 0.535p 0.593p
  9. 9. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/9 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Pitch (P) A h r W C a H B DE a 60o 2 D E P/2 Figure 1.8. Three-wire measurement Note: W = Distance over wires DE = Pitch/ Effective Diameter Dw = Wire diameter a = 600 From the Fig. 1.8, mean/pitch diameter can be calculated by applying the following formula; a a AD = AB cosec = r cosec 2 2 a P a H = DE cot = cot 2 2 2 P a CD = 0.5H = cot 4 2 a P a h = AD – CD = r cosec – cot 2 4 2
  10. 10. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/10 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c and distance over wires (W) = DE + 2h + 2r a P a = DE + 2 {r cosec – cot } + 2r 2 4 2 a P a = DE + 2r cosec - cot + 2r 2 2 2 a P a = DE +2r ( 1 + cosec ) – cot 2 2 2 and, since 2r = d (the diameter of the wire), a P a W = DE + d ( 1 + cosec ) – cot (1) 2 2 2 From this general formula we may apply the special adaptation for common threads. Figure 1.9. a) ISO metric and unified b) Whitworth
  11. 11. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/11 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c (a) ISO metric and unified Fig. 1.9 (a) The effective diameter lies 0.3248p inside the crest of the thread, Hence DE = D – 0.6496p a a = 60° and cosec =2 2 a cot = 1.732 2 a P a W (over wires) = DE + d (1 + cosec ) – cot 2 2 2 P =D – 0.6496p + d(3) – (1.732) 2 = D +3d- 1.5156p (2) (b) Whitworth Fig. 1.9(b) Depth of thread = 0.64p, so that DE = D – 0.64p a a a = 55° and cosec = 2.1657 cot = 1.921 2 2 a P a Hence W ( over wires) = DE + d { 1 + cosec } - cot 2 2 2 P = D -0.64p + d 3.1657) - (1.921) 2 = D + 3.165d - 1.6 p (3)
  12. 12. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/12 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 1.5. OPTICAL COMPARATOR An optical comparator or shadowgraph (Fig. 1.10a and 1.10b) projects an enlarge shadow onto a screen where it may be compared to lines or to a master from which indicates the limits of the dimensions or the contour of the part being checked. The optical comparator is a fast, accurate means of measuring or comparing the work piece with a master. It is often used when the work piece is difficult to check by other method. Optical comparators are particularly suited for checking extremely small or odd-shaped parts, which would be difficult to inspect without the use of expensive gauges. Optical comparators are available in bench and floor models, which are identical in principle and operation. Light from a lamp passes through a condenser lens and is projected against the work piece. The shadow caused by the work piece is transmitted through a projecting lens system, which magnifies the image and casts it onto a mirror. The image is then reflected to the viewing screen and is further magnified in this process. The extent of the image magnification depends on the lens used. Interchangeable lenses for optical comparators are available in the following magnifications: 5 x, 10 x, 31.25 x, 50 x, 62.5 x, 90 x, 100 x, and 125 x. A comparator chart or master form mounted on the viewing screen is used to compare the accuracy of the enlarged image of the work piece being inspected. Charts are usually made of translucent material, such as cellulose acetate or frosted glass. Many different charts are available for special jobs, but the most commonly used are linear-measuring, radius, and angular charts. A vernier protractor screen is also available for checking angles. Since charts are available in several magnifications, care must be taken to use the chart of the same magnification as the lens mounted on the comparator. Many accessories are available for the comparator, increasing the versatility of the machine. Some of the most common ones are tilting work
  13. 13. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/13 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c centres, which permit the work piece to be tilted to the required helix angle for checking threads; a micrometer work stage, with permit quick and accurate measuring of dimensions in both direction; and gauge blocks, measuring rods, and dial indicators used on comparators for checking measurement. The surface of the work piece may be checked by a surface illuminator, which lights up the face of work piece adjacent to the projecting lens system and permits this image to be projected onto the screen. 1.5.1. To check the angle of a 60o thread using an optical comparator 1. Mount the correct lens in the comparator. 2. Mount the tilting work centres on the micrometer cross-slide stage. 3. Set the tilting work centres to the helix angle of the thread. 4. Set the work piece between centres. 5. Mount the vernier protractor chart and align it horizontally on the screen. 6. Turn on the light switch. 7. Focus the lens so that a clear image appears on the screen. 8. Move the micrometer cross-slide stage until the thread image is centralized on the screen. 9. Remove the vernier protractor chart to show a reading of 30o. 10. Adjust the cross-slides until the image coincides with the protector line. 11. Check the other side of the thread in the same manner.
  14. 14. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/14 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Note: If the threaded angle is not correct or square with the centre line, adjust the vernier protractor chart to measure the angle of the thread image. Other dimensions of the threads, and width of flats, may be measured with micrometer measuring stages or devices such as rods, gauge blocks and indicators. helix angle Figure 1.10 (a). Checking a thread form on an optical comparator
  15. 15. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/15 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Figure 1.10 (b) Principle of the optical projector
  16. 16. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/16 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c ACTIVITY 1 TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT…! 1.1. Draw and label a schematic drawing of how you would check the core diameter of an external V-thread. 1.2. Using ‘best’ wire sizes determine the distance of the wire for M 20 x 2.5 ISO metric thread. 1.3. Why is the three-wire method is one of the best method of measuring the pitch diameter of a V thread? 1.4. With the aid of a labelled diagram, briefly explain how you would use an optical comparator to check the thread angle of 60o
  17. 17. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/17 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c FEEDBACK ON ACTIVITY 1 1.1 T prism W Dm = W – 2T ; T = prism height (known) 1.2. 20 mm x 2.5 mm pitch Best wire diameter = 2.5 x 0.577 = 1.443 mm. From formula W = D + 3d – 1.5156P = 20 + 3 (1.443) – 1.5756 (2.5) = 20.54 mm 1.3. The results are least affected by any error which may present in the included thread angle.
  18. 18. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/18 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 1.4. Using an optical comparator to check the thread angle of 60o. To check the angle of a 60o thread using an optical comparator 1. Mount the correct lens in the comparator. 2. Mount the tilting work centres on the micrometer cross-slide stage. 3. Set the tilting work centres to the helix angle of the thread. 4. Set the work piece between centres. 5. Mount the vernier protractor chart and align it horizontally on the screen. 6. Turn on the light switch. 7. Focus the lens so that a clear image appears on the screen. 8. Move the micrometer cross-slide stage until the thread image is centralized on the screen. 9. Remove the vernier protractor chart to show a reading of 30o. 10. Adjust the cross-slides until the image coincides with the protector line. 11. Check the other side of the thread in the same manner.
  19. 19. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/19 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Note: If the threaded angle is not correct or square with the centre line, adjust the vernier protractor chart to measure the angle of the thread image. Other dimensions of the threads, and width of flats, may be measured with micrometer measuring stages or devices such as rods, gauge blocks and indicators.
  20. 20. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/20 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c SELF-ASSESSMENT 1 1. Calculate the effective diameter of M35 x 5.5 threads by using three wire method. The distance between wires is 35.60 mm. Used formula E = M – 3d + 0.866P ; when d = 0.577P and P = pitch. Sketch the measurement setup. 3 2. Using the ‘best’ wire sizes, determine the distance over wires for (a) in 4 Whitworth, (b) M 20 x 2.5 ISO metric threads.
  21. 21. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/1/21 he he k k lic lic SCREW THREAD C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c FEEDBACK OF SELF-ASSESSMENT 1 1. E = 23.263 mm Three wire method 2. (a) 0.0564 in, (b) 1.4425 mm

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