By Design: Molded threads, Part 1

A classic example of a system that benefited from standardization.

The Greek philosopher Archimedes (287-212 BC) is credited with discovering the screw. The Archimedes screw took the form of an inclined cylinder with a continuous screw on its inside surface. Rotating the bottom of the cylinder in water raised that liquid to a higher level. This simple pumping mechanism is still in use today.

There is no documentation of when the water pumping screw morphed into the inside and outside threads that we know today. Early man did, however, recognize the advantages of these spiraling incline planes. The screw jacks for lifting heavy loads or for clamping purposes were early applications. Using threads for precision linear adjustments on all kinds of machines and on instruments, such as micrometers, or for controlling the amount of fuel entering an engine were much later adaptations. The use of nuts, bolts, and screws for assembling parts is a comparatively new development.

The British “flying shuttle” weaving loom, patented in 1733, started the industrial revolution. A profusion of machines followed. These machines were built by toolmakers. Threads were widely used for assembly, adjustment, clamping, and for converting rotary into linear movement. Each individual toolmaker had his own ideas on what constituted the ideal thread form and pitch (threads per inch). There was no standardization. If a threaded component failed, a replacement had to be machined. This was good for the toolmakers, but the machine’s owners suffered losses while the new part was being made. This inefficient method of working continued for a century.

The first standardized thread

In 1841 the British machine toolmaker, Joseph Whitworth, proposed the first standardization of threads. Over the next 20 years his standard 55° thread profile spread across the world.

Not to be outdone, the American William Sellers promoted an improved thread standard. He changed Whitworth’s 55° thread profile to 60° and laid out a systematic approach for thread pitch, form, and depth with rules for proportioning hex nuts and bolts up to 6 inches in diameter.

Back in those days, the United States did not have a Bureau of Standards. The Franklin Institute was the only organization that America had for establishing standards in the art and science of mechanical engineering. The Institute endorsed Sellers’ threading system in 1864.

Just like today, it took time for anything new to be accepted. Toolmakers were reluctant to change what had been working for years. However, the U.S. Army and Navy quickly adopted the new system. The Bureau of Steam Engineering followed suit. By the 1880s the system was accepted. Products as diverse as steam engines and farming machinery were produced with interchangeable threaded components.

The Whitworth and Sellers standards were intended to be applied to metal threads. The first man-made plastic material did not appear until John Wesley Hyatt invented celluloid in 1868. There were, however, fabricated plastic parts with machined threads in the late 1800s. It would be another 40 years before the plastics industry became sophisticated enough to produce molded threads.

The Sellers, or American Standard, and the British Whitworth thread profile are shown above. Both thread profiles were designed for machined metal parts. Both profiles have proven to be successful. For obvious reasons, the United States adopted and promoted the American Standard profile.

In the mid-19th century, the toolmaking industry was not as sophisticated as it is today. The 60º thread profile with a flat at the root and crest of the thread was the easier of the two for machinists to produce. The Whitworth thread, with a radius at the root of the thread, is a far superior design for plastic parts that have to be heavily loaded. Eliminating sharp corners on threads is an important but lengthy topic that must be saved for another day.

Over the years the American standards expanded to include what is identified as the unified national coarse and the unified national fine series of threads. For example, a 1?2-inch-diameter bolt could be a coarse series with 13 threads/in, or a fine series with 20 threads/in.

Pitch limitations

Nuts, bolts, and screws have a specific pitch or number of threads per inch for a given diameter. Other kinds of molded parts do not have these limitations. Any number of threads per inch can be specified on any diameter. The fine series goes up to 80 threads/in. It is generally considered impractical to mold more than 32 threads/in except in small diameters.

The limitation with large numbers of threads/in is that the thread’s cross sections are small, making them difficult to fill with hard-flow or fiber-reinforced materials. The threads also provide very little engagement between the inside and outside threads. With 32 threads/in, the engagement is only .020 in/side.

I once undertook a lens holder project that required molding a 21?8-inch-diameter acetal part with 40 threads/in. That thread provided an engagement of only .016 in/side. A ±.004-inch tolerance on both mating parts used up half of that engagement. In addition to being weak and susceptible to stripping, these very shallow threads were difficult to engage with the mating part without cross threading.

Experience has convinced me that it is not wise to mold more than 32 threads/in. On the other hand, 40 threads/in is standard in the optics field. If you want to be a part of that marketplace you have to be able to mold 40 threads/in.

The 40 threads/in required on the lens holder became impractical due in part to the molding process’s inability to maintain close tolerance on the 21?8-inch diameter. The crystalline, high-mold-shrinkage-factor, acetal material also contributed to the problem. A lower-mold-shrinkage-factor, amorphous material would have been better for that project.

A smaller diameter would also have been helpful. For example, a 4-40 NC machine screw with 40 threads/in on a .112-inch outside diameter is a practical part for the injection molding process. The thread cavity profiles are still small and difficult to fill, but the small outside diameter could be easily held to a tolerance of ±.0005 inch. The thread engagement is still only .016 in/side, but the molding tolerance would not significantly reduce that thread engagement.