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 by 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

NPT American National Standard Tapered Pipe Thread
NPT is a North American tapered pipe standard. From Wikipedia:


 * The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered (0.75 in/ft or 62.5 mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal (no clearance remains between the crests and roots of the threads because of the taper). This means that NPT fittings should be burr-free and lubricated using a lubricating material like lubricating paste or tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.

MTP Model Taper Pipe
MTP is a North American tapered pipe standard designed specifically for small live steam models. The standards governing the dimensioning of MTP taps and dies are based directly on applicable ANSI/ASME B 1.20.1 - 1983 standards. This thread system is a miniature version of American National Taper Pipe thread with the same taper of 3/4 inch per foot and the same (60 degree) thread form.

The 5/16-27 MTP is functionally and dimensionally identical to 1/16-27 National Pipe Thread.

MTP (Model Taper Pipe) should not be confused with MPT, which can mean Male Pipe Thread or Mechanical Pipe Thread (a non-tapered variety) and is a subset of NPT for male connections.

METP Model Engineer Taper Pipe
METP is the British equivalent of the North American MTP.

ME Model Engineering (British Straight Thread)
ME is a British, non-tapered (straight) thread, based on the Whitworth thread with a pitch of 32 and 40 TPI. The flank angle is 55°.

UNS American National Standard Unified Special
UNS is a non-tapered (straight) thread.

From Wikipedia:


 * The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It is the main standard for bolts, nuts, and a wide variety of other threaded fasteners used in these countries. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

From a PM Research web page:


 * For many years we have made our model pipe fittings with straight threads in 3/16-40 UNS (Unified Special) with a basic pitch diameter of .171 and 1/4-40 UNS with a basic pitch diameter of .234. Taps and dies for UNS or ME straight threads are readily available and inexpensive. These fittings work well with the use of thread sealant such as Loctite PST565.

UST Universal Straight and Taper
UST is a proprietry tapered thread designed by PM Research. From their website:


 * We have designed the tapered thread to be interchangeable with our earlier manufactured straight threads, and interchangeable with UNS or ME straight threads. We have designated our pipe thread as UST (Universal Straight and Taper). Our 5/16-27 pipe fittings have the same thread as 1/16 NPT (National Pipe Thread) and are also the same as 5/16- 27 MTP (Model Taper Pipe).

Comments
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.