Threads

Machine Screw Diameters
By H. J. Coventry

The Miniature Locomotive, July-August 1954

Machine screws are numbered as distinct from Cap Screws, Bolts, etc, and range usually from #0 to #12. A convenient method of finding the exact diameter if a table is not handy is as follows:


 * Multiply the number of 13 and add sixty thousandths. The answer will be thousandths.

Examples:


 * #0 -- 0 multiplied by 13 equals 0, plus 0.060 equals 0.060
 * #3 -- 3 multiplied by 13 equals 39, plus 0.060 equals 0.099
 * #5 -- 5 multiplied by 13 equals 65, plus 0.060 equals 0.125
 * #10 -- 10 multiplied by 13 equals 0.130, plus 0.060 equals 0.190

Pipe Threads
Ciszewski posted on PracticalMachinist.com:


 * You can tap NPT without taper reaming. Morse Cutting Tools recommends:


 * 1/4 NPT - drill with a 7/16 bit to a minimum depth of 0.78125 & tap to a depth of 0.72525
 * 3/8 NPT - drill with a 37/64 bit to a minimum depth of 0.8125 & tap to a depth of 0.7565
 * 1/2 NPT - drill with a 45/64 bit to a minimum depth of 1.03125 & tap to a depth of 0.96025


 * Following these recommendations will only give you a full thread for the first 2 to 3 threads.