14.7.
The fixed or solid pattern plug and ring screw thread gauges possess certain disadvantages that may be enumerated briefly as follows :
(1) Gauge wear. This wear takes place more rapidly in screw thread gauges than in plain gauges. It is not uncommon for hardened ring—thread gauge to wear from 0.02 mm to 0.03 mm during a few days use. When full form GO and effective diameter NOT GO ring gauges are used, manufacturing tolerance of work is reduced by amount that must be allowed for wear in the full form ring gauges.
(2) Gauge making tolerances. Solid gauges, being unadjustable for wear, their use reduces considerably the tolerance or limit of accuracy within which the work must actually be made, thus increasing the cost of manufacture.
(3) Possible Inaccuracy. Owing to wear effects, which in time may take up the full amount of tolerance allowed, there is always the risk of a gauge being used beyond its designed capacity with the result that the work checked with the gauge will be inaccurate ; frequent checking of screw thread gauges is necessary in order to obviate this effect.
The minimum effective diameter of a thread hole must necessarily be greater than the maximum effective diameter of the screw. The allowance recommended by the B.S.I, for this is 0.002 inch (0.05 mm) to allow for the wear on the solid ring and plug gauges generally used.
In order to overcome the disadvantages of solid gauge wear, inaccuracy and increased cost of production of parts manufactured for this type of gauge ; the adjustable pattern screw thread gauge was evolved which is now widely employed for precision gauging purposes.
Before describing a typical gauge of this class it may be mentioned that when adjustable gauges are employed, although wear can be compensated for but the B.S. allowance of 0.002″ for solid gauges is reduced by about 0.0005″, and the difference of0.0005″ represents manufacturing tolerance when producing the same quality of work. The result of having this greater tolerance is to lower the manufacturing cost of the work; further, whatever the tolerance may be, the more closely to size the limit gauges are made and maintained the more cheaply can the work be produced.
14.7.1.
The Wickman
adjustable thread gauge shown in Fig. 14.5 can be used for inspecting both right and left hand threads. It is adjustable to 0.002 mm and will test for out of roundness or eccentricity. It can be sealed after adjustment and is temper proof.
The principal feature of this gauge is the combined fine adjustment and lock as indicated in the upper part of Fig. 14.5. To set the gauge, the anvils are first adjusted by means of the adjusting screws until the setting of master gauges held between them is just sufficiently right to support the weight of the gauge. If the locking screws are tightened by means of a special
key supplied then the ball will be slightly compressed; their elasticity enables the setting gauge to be gripped or loosened alternately. An intermediate adjustment, where the setting gauges can just slide through the anvils is the desirable setting. After adjustment, a monogram lead seal can be fixed over the adjusting screws.
The frame of the gauge is of cast iron with specially selected characteristics to ensure that it will break rather than take a permanent set in the event of an accident. The anvils are of hardened tool steel and made sliding fit in their holder. Hardened abutments fixed solidly
to the frame prevent the anvils from turning or tilting. The gauges are made to suit all standard screw thread forms.
The principle of the Wickman thread gauging system is to ensure truly interchangeable thread work of sound mechanical strength by controlling within the prescribed limits, the major, minor and effective diameters. As mentioned previously, the effective dia. is virtually increased by errors of pitch and angle so it must not be too large. For this reason the ‘front’ or ‘GO’ portion of the anvil is provided with a full form of thread of sufficient width to cover the engagement of the screw ;
Fig. 14.5. Wickman Adjustable Screw Thread Limit Gauge.
it is set to the nominal diameter. The rear anvil is of specially truncated form and consists of a few threads only, in order that contact may be made only on the central portion of the flanks of the thread and that no effects of pitch error are taken into account. In this way, the gauge certifies that the effective dia. is not too small. The threads on the anvils are relieved so that there is no interference with the helix of the thread being inspected ; thus inspection takes place on the axial line of the thread, where the true thread is located. As there is no interference with the helix angle, the same gauge can be used for both right and left hand threads.
14.7.2.
Roller type Adjustable Thread Gauge.
In this type of thread limit gauge, annular rollers replace the sectioned threads that are employed on the type of gauge described above. The wearing surfaces of the gauge are thus greatly increased and suitable form is, therefore, available for workshop and coarse tolerance inspection purposes.
The rollers, of which there are four, are not of helical thread but annular pattern of corrected form to compensate for helix angle interference. They are simple to adjust and can be sealed in position. In general, however, this type of gauge does not give the same accuracy as the Wickman Gauge.
It consists of a pair of full form GO rollers at the front having about three times as many threads as the rear NOT GO rollers. The latter are employed for the effective dia. and are truncated on the crest and root so that only flank contact is made.
14.7.3.
Gauging of Taps.
As the majority of internal threads in production work are tapped, it is necessary to employ a satisfactory limit for controlling the dimensions of taps. The Wickman gauge system for taps fulfills this requirement and comprises a pair of adjustable thread caliper gauges designed for four fluted taps. The four gauging positions of the pair of gauges are illustrated at (a), (6), (c) and (d) in Fig. 14.6. The first gauge controls the form of the thread, pitch and effective dia. and ensures that neither the full nor the core dia. is too large [(a) and (6), (c) and (d)]. The second gauge ensures that full and core dia. are not too small.
Fig. 14.6. Wickman Tapping Gauging System.
The full form GO front anvils of the first gauge are shown at (a) and the rear pair of NOT GO anvils for effective dia. at (6).
The NOT GO full dia. front anvils of the second gauge are shown at (c) and the NOT GO core dia. rear anvils at (rf).
Tap gauges are set in similar manner to thread gauges except that on the full and core dia. an allowance is made for wear on tap threads. The anvils on tap gauges are, therefore, made to special thread form, i.e. some dimension from the tip is etched and must be taken into account when calculating dia. of setting plug gauge.
ISO Metric Threads and Gauges for them
Bureau of Indian Standards recommends the use of ISO (International Organisation for Standardisation) metric screw threads for use in Indian industries and thus it is essential that these threads be studied in details from all the angles. It may be noted that ISO metric thread is closely related to Unified thread.
