Designing the Pitch of a Screw: History of Whitworth
Designing the Pitch of a Screw: A Whitworth History
Andrew Prestridge | September 10, 2020
"There are too many screws!"
It’s a common refrain heard from the designer, to marketer, to the parent trying to put together their kid’s toy at 3am to avoid ruining yet another Christmas. But putting aside questionable design (and life) decisions, we’ll address a variant of the oh-so-common remark: “there are too many screw options.”
Visiting your local hardware store, popping open the Machinery’s Handbook, or flipping through a McMaster-Carr catalog, you’ll quickly see there are thousands of consumer options for screws and bolts. From the tiniest #0000 (only 0.021” across!) to a 2.5” heavy-duty bolt, there’s a size for every occasion, and a screw pitch to match. Or rather, several pitches. A quarter-inch screw might have 20 threads per inch, or 28, or 32. Metric sizes are no escape either, as M8 screws come in your choice of 1.25mm, 1.00mm, and 0.75mm pitch.
An assortment of different machine screws
Choosing between three or more options, for something that has absolutely no impact on so many designs, feels like an unnecessary headache in the design process.
Having said that, there was a time with simultaneously more and fewer options. Prior to the 1840s there was no universal standard for thread pitch - every manufacturer was left to their own devices. In a way, any screw pitch you could make, you could use and no one would have grounds to complain. At the same time, the product designers of the time had no readily available standards to pull from or common choices to pick between.
Naturally this led to severe fragmentation and millions of incompatible screws, nuts, and bolts - each slightly different. Larger companies narrowed in and developed their own internal guidelines, but these lacked the reach and accessibility of a singular standard. That was the case until Joseph Whitworth took the stage in 1841.
Joseph Whitworth - Father of Modern Screw Pitch
Portrait of Joseph Whitworth, 1846. Artist unknown. [1]
Joseph Whitworth developed a screw thread design that was adopted by major English railroad companies and quickly spread across the country. His papers on screw threads exposed the “evils” of figuring out thread pitches without a uniform standard. While Whitworth admits that any standard would be based on largely arbitrary decisions, and there would always be special cases that don’t fit the standard, he cautions that there is more to be lost by delaying and urges the attention of engineers across the nation. With the support of the railroads, then the Royal Dockyards and shipping companies, Whitworth’s 55-degree thread, with rounded edges and simple table of pitches spread nationally.
|
Diameter (in) |
1/4 |
5/16 |
3/8 |
7/16 |
1/2 |
5/8 |
3/4 |
7/8 |
1 |
... |
4 |
|
Threads per inch (TPI) |
20 |
18 |
16 |
14 |
12 |
11 |
10 |
9 |
8 |
… |
3 |
While these pitches are familiar and survive today, the thread shape has not. Excerpt from Joseph Whitworth's "A Paper on An (sic) Uniform System of Screw Threads," 1841. [2]
From there, the British Standard Whitworth thread spread to the US and Canada, and further globally. However, by the 1860s Whitworth’s admittedly arbitrary decisions were already being challenged. In 1864 William Sellers proposed changing to an easier-to-produce 60-degree angle with flat-topped threads instead of rounded.
"Graphic Representation of Formulas for the pitches of Threads of Screw Bolts," William Sellers, 1864. [3]
The Modern Screw Pitch Standard
These changes saw the creation of United States Standard (USS) thread and opened the proverbial floodgates for competing standards. Soon after, thread standards were extended to National Coarse (NC), National Fine (NF), and a litany of metric thread alternatives. As the rate of manufacturing increased and more and more products were developed under each thread standard the age-old argument against change grew more powerful: it was too expensive. Tooling to make screws of each thread were expensive, and once it reached a critical mass, manufacturers were reluctant to spend more capital to upgrade to the latest “standard.” Over the years, international efforts have helped rein in the number of options (3 doesn’t seem as bad as the 20+ that have been developed), but nothing is quite as sweet as the two decades in the mid-1800s that started to converge on thread universality.
References
[1] Retrieved from Art UK: https://artuk.org/discover/artworks/joseph-whitworth-18031887-179915/search/keyword:joseph-whitworth
[2] Whitworth, Joseph. A Paper on An Uniform System of Screw Threads, 1841. Retrieved from Wikisource: https://en.wikisource.org/wiki/Miscellaneous_Papers_on_Mechanical_Subjects/A_Paper_on_an_Uniform_System_of_Screw_Threads
[3] Sellers, William. Graphic Representation of Formulas for the Pitches of Threads of Screw Bolts, 1864. Retried from Wikimedia: https://commons.wikimedia.org/wiki/File:JFIScrewThread300.png
Whitworth and Screw Pitch
Chaos
Screws were custom-made by individual machinists for local applications
Direction
Whitworth proposed the first standardization to unify screw threads
Order
While there are still varieties (inch v. metric, coarse v. fine) screws are generally standard