Acme and Square-Threaded Taps Machinery's Handbook, 31st Edition
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maximum diameter = 0.4 + (0.05 × 0.1) = 0.405 drill selected = letter X or 0.3970 + 0.0046 (probable oversize) = 0.4016 Diameters of Acme Centralizing Screw Threads of Classes 2C, 3C, and 4C may be cal culated from pitch = 1/number of threads per inch, and: minimum diameter = basic major diameter - 0.9 × pitch maximum diameter = minimum minor diameter + 0.05 × pitch Example: 1 ⁄ 2 -10 Acme 2C, pitch = 1 ⁄ 10 = 0.1 minimum diameter = 0.5 - (0.9 × 0.1) = 0.41 maximum diameter = 0.41 + (0.05 × 0.1) = 0.415 drill selected = 13 ⁄ 32 or 0.4062 + 0.0046 (probable oversize) = 0.4108. Diameters for Acme Centralizing Screw Threads of Classes 5C and 6C: These classes are not recommended for new designs, but may be calculated from: minimum diameter = [basic major diameter - (0.025 √ basic major dia.)] - 0.9 × pitch maximum diameter = minimum minor diameter + 0.05 × pitch pitch = 1/number of threads per inch Example: 1 ⁄ 2 -10 Acme 5C, pitch = 1 ⁄ 10 = 0.1 minimum diameter = [0.5 - (0.025 √ 0.5)] - (0.9 × 0.1) = 0.3923 maximum diameter = 0.3923 + (0.05 × 0.1) = 0.3973 drill selected = 25 ⁄ 64 or 0.3906 + 0.0046 (probable oversize) = 0.3952 Tapping Square Threads.— If it is necessary to tap square threads, this should be done by using a set of taps that will form the thread by a progressive cutting action, the taps vary ing in size in order to distribute the work, especially for threads of comparatively coarse pitch. From three to five taps may be required in a set, depending upon the pitch. Each tap should have a pilot to steady it. The pilot of the first tap has a smooth cylindrical end from 0.003 to 0.005 inch (0.076-0.127 mm) smaller than the hole, and the pilots of following taps should have teeth. Collapsible Taps.— The collapsing tap shown in the accompanying illustration is one of many different designs that are manufactured. These taps are often used in turret lathe practice in place of solid taps. When using this particular style of collapsing tap, the adjustable gage A is set for the length of thread required. When the tap has been fed to this depth, the gage comes into contact with the end of the work, which causes the chasers to collapse automatically. The tool is then withdrawn, after which the chasers are again expanded and locked in position by the handle seen at the side of the holder.
A
Collapsing Tap Collapsible taps do not need to be backed out of the hole at the completion of the thread, reducing the tapping time and increasing production rates.
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