Patent GB-947402-A (GB)
Title: Method and apparatus for inspecting threads
IPC: G01B7/28
Filed: 1960-10-20
Abstract: 947,402. Gauging screw threads; measuring electrically. AMERICAN IRON & MACHINE WORKS CO. Oct. 20,1960 [Oct. 30, 1959], No. 35995/60. Headings G1M and G1N. A method of measuring the deviations in the thread lead of a threaded member 22 includes aligning the thread axis in a direction parallel to or collinear with the axis of a reference screw 9, rotating the member 22 and the screw 9 synchronously and taking continuous indications of the relative movement, in the axial direction, between a stylus 130 and a probe 10 tracking the threads of the member and the screw respectively. General construction. As shown in Fig. 1 the device is used for testing the thread lead and the pitch radius of a taper-thread tool-joint 20 for oil drilling equipment. This is mounted inside a drum 3 on a vertically adjustable platform 23 and is held by set screws 30. A shaft 7 inclined at the nominal angle of the thread 22 is mounted on a frame 8 which can swing about a vertical axis on bearings 124, and a carriage 6 is free to slide on this shaft. The carriage carries a ballended stylus 130 spring-urged against the thread 22 and a stylus 131 spring-urged upwardly against the under-surface 112 of a nut 10 engaged with the reference screw 9 and fixed against rotation. Recording-apparatus. Movements of the styli are sensed by linear variable differential transformers and recorded in a recorder 93 driven by gearing at a rate proportional to the speed of rotation of the thread 22 and screw 9. From the recordings of the movements of the stylus 130 the variations in pitch radius are determined and from the movements of the stylus 131 the variations in lead are determined. To determine the actual pitch radius at a given point, the readings given by a known plug gauge at a similar point are compared with those given by the member 22. A formula is given which indicates the error in the pitch radius from these results. Similarly an internally threaded member can be compared with a ring gauge. A third stylus 199 (Fig. 2) is used to check the planeness of the shoulder 24 on the member 22 and also its perpendicularity to the axis. This stylus is mounted on a member 12 which can pivot about an axis 202 to enable the workpiece to be inserted prior to gauging. The recordings from this stylus are obtained on the same sheet as those from the stylus 131, the two L. V. D. T.'s being alternately plugged into a common transformer. Methods are described for measuring the amplification of the L. V. D. T.'s, finding their zero positions and checking that the amplification is unchanged. The L. V. D. T.'s are each provided with shielded two lead cables, the shield serving as a common input and output lead. Setting-up. A handle 271 (Fig. 1) can be screwed into the bore of the screw 9 and used to adjust this in order to obtain a zero reading from the stylus 131 when the workpiece 22 has been inserted and before it has been connected to the screw 9 by a shaft 11. This is inserted from the top of the machine and is provided with a key engaging a keyway in the bore of the screw 9. The lower end of the shaft 11 is secured to the driving spindle 42 (operated by a handle 4) by a Morse taper 115. If a part is being inspected which cannot receive the shaft 11, the screw and the part may be rotated in synchronism by means of a parallel shaft coupled to the screw and the spindle 42 by similar sets of gears. To align the axis of the threaded member 22 with the axis of the shaft 42 a pair of rings 214, 215 (Fig. 9) are used. These have surfaces 216 which rest on the crests of the threads. The position of the member 22 within the drum 3 is adjusted until dial gauges 220 mounted on a plate 229 forming part of the machine frame indicate constant readings during rotation of the member 22. Testing internally threaded members, also plug and ring gauges. A device is shown (Fig. 6) for enabling an internal thread to be tested. In this case an adaptor 186 is attached to the stylus 130 which is biased to the left by a spring 149. The adaptor 186 bears against one arm of a lever 183, the other arm of which acts against a plate 176 carrying a stylus 182 which bears against the thread 166. The plate 176 and the lever 183 are mounted on a member 168 attached to the carriage 6 and provided with a tubular opening large enough to prevent it fouling the drive shaft 11 at either end of the travel of the carriage 6. Attachments are also described, Figs. 11 and 12 (not shown) to permit the testing of plug and ring gauges.