This standard originated in England around 1820-1830. Though originally for wrought iron pipe, it was also adopted by the makers of brass and copper tubing. It became the standard for pipe used in gas lighting, and from there, for the threaded pipe in your 21st century table lamp. In that context, it is referred to as “1/8 IP”, the “IP” standing for “iron pipe.”
|Diameter of tube, inches||Thickness
As with most standards intended to define pipe that makes threaded, liquid-tight joints, the Briggs standard also defined a tapered thread
Tapered thread on a nominal 2½-inch ID pipe
The thread employed has an angle of 60°; it is slightly rounded off both at the top and at the bottom, so that the height or depth of the thread, instead of being exactly equal to the pitch, is only four-fifths of the pitch, or equal to 0.8 1/n, if n be the number of threads per inch. For the length of tube-end throughout which the screw-thread continues perfect, the empirical formula used is
(0.8 D + 4.8) × 1/n,
where D is the actual external diameter of the tube throughout its parallel length, and is expressed in inches. Further back, beyond the perfect threads, come two having the same taper at the bottom, but imperfect at the top. The remaining imperfect portion of the screw-thread, furthest back from the extremity of the tube, is not essential in any way to this system of joint; and its imperfection is simply incidental to the process of cutting the thread at a single operation. From the foregoing it follows that, at the very extremity of the tube, the diameter at the bottom of the thread,
D - [(2 × (0.8D + 4.8))/32n + (2 × 0.8)/n] = D - (0.05 D + 1.9) × 1/n.
Report of the Committee on Standard Pipe and Pipe Threads.
Transactions of the American Society of Mechanical Engineers, vol. 8, 1887.
Copyright © 2018 Sizes, Inc. All rights reserved.
Last revised: 2018.