February 16, 2014 3 Comments
This is especially for those who cut and thread spokes. Be it with a magnificent Morizumi or a venerable Phil Wood machine, thread rolling is the game. While industrially commonplace, it's not well understood by those who do it daily.
How Things Work
Quite a few of you like to figure how things work. Here is a glimpse into the nature of thread rolling. The die set (a pair of flat plates with grooves) that makes spoke threads (by rolling a spoke between ribbed surfaces) has grooves spaced at the thread pitch.
So there should be a die size for each spoke gauge, right? The gauges:
Here is the conundrum: the thread pitch for bicycle spokes is the SAME regardless of spoke gauge. Huh? One of our brilliant predecessors ordained they shall all be 56 threads per inch (metrically speaking, 0.45mm pitch).
Isn't that Illegal?
Name another fastener where the pitch does not become finer as the diameter goes down. That's to minimize breakage and it's universal practice. What good's a World Order if not to standardize things that work better?
Except bicycles. Basically, we (cycling) make our own rules. We discovered this short cut works. Theory out the window. Let's make bicycles simpler to understand and to make. From a wheel builder's perspective, thread pitch determines the feel of wheel truing. 1/4 turn of a nipple makes a similar dimensional change to length regardless of spoke gauge. That is the idea behind a universal thread pitch.
One More Logical Step
Since we have adopted the same pitch for all these spoke sizes can we use the same dies to make the threads? Just run them farther apart for thicker wires? All 56 threads per inch? This is a challenge to the laws of physics.
A die has small grooves that make peaks and valleys of the thread form. These grooves are set at an angle specific to the diameter of the spoke. As the spoke rotates, grooves are created simultaneously on each side, that eventually meet up to create a continuous thread.
For a smaller diameter spoke, the angle is greater (from perpendicular) because a small circumference means a groove gets around more quickly (and must not run into itself). A larger diameter spoke (12g is 30% greater circumference than 14g) means a greater distance for each rotation. The groove angle must be smaller so the thread is continuous.
The actual change in angle from 15G to 12G sounds like it would be significant (according to the circumference difference), but it's tiny—about 1º.
In practice, there is plenty of tolerance (wiggle room) and dies optimized for smaller gauges (15/14G) can roll perfect (to-the-eye) threads on larger gauges (12/13G). For this practical benefit we encourage all bicycle gauges be threaded with the same dies. It works!
Now go forth and thread like guru's!
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