When you have a long way to go, and a short time to get there, there’s a thread option available. Multi-start, also known as multiple-lead taps create a thread that offers quick assembly and disassembly. Most of us take these type of threads for granted, but see their use daily. From the caps on water and juice bottles, to the handles on our bathroom faucets, multiple-lead threads make life a little bit easier.
For the purpose of visualization, thread lead can be described as the linear screw travel accomplished in one revolution. Thread pitch is the distance from any point on a screw thread to the corresponding point on the next thread, measured parallel to the axis. With a single-start thread, lead and pitch are equal. A double-start thread’s lead is twice the pitch measurement.
Using 1/4-20 thread as an example, the screw must turn 20 revolutions to advance 1” in the part. A 1/4-20 double-start thread has the same number of threads per inch, but with two starting points, 180 degrees apart, needs just ten revolutions to advance 1”. A triple-start thread, with starts 120 degrees apart, will advance 3 times faster than a single lead thread. The result continues to change appropriately as additional starts are added.
The result of multiple-lead threads takes form in a water faucet that requires just 1/4-turn to go from Off to full On, or a water bottle cap that screws on and off in one turn. Adjustment or assembly mechanisms that require rapid movement with minimal effort are potential applications for multiple-start threads. Multiple-start threads are frequently used for focusing rings on cameras, telescopes, and binoculars to create effective linear movement with fewer rotations of the ring.
Single-start threads are much more common, and are more useful for general fastening needs. They offer more resistance to loosening than multi-start threads, due to the lower helix angle of the thread. As you can see though, multi-start threads offer advantages for those of us who are rotationally challenged.
Tapping multiple-lead threads offers an additional challenge, even though the amount of material removed is the same. Tap chamfer length may need to be increased to accommodate the spaced starting points of each lead. Feeds rates in machining must also be increased to match the higher helix of multiple-lead thread. Because of the increased feed rate, clearance may be an issue when stopping and reversing the tap.