Con rod threads for cap screws.
Con rod threads for cap screws.
(OP)
On another thread it was proposed that threaded holes with long engagement lengths are often thread milled with a progressive pitch. When a bolt with a constant thread pitch is installed it creates a more even stress distribution among the engaged threads.
I was thinking the variation in pitch on a 3/8"-24 bolt could only be on the order a thousandth of an inch ( 0.001" ) over 3/4 inch engagement lest the threads that touch first be severely strained by the time the first threads take up load.
Is anyone familiar with that technique.
I was thinking the variation in pitch on a 3/8"-24 bolt could only be on the order a thousandth of an inch ( 0.001" ) over 3/4 inch engagement lest the threads that touch first be severely strained by the time the first threads take up load.
Is anyone familiar with that technique.





RE: Con rod threads for cap screws.
It's normally done with a cutter that looks like a tap with one flute but multiple teeth, and completes a nominal threaded length roughly equal to the toothed length of the cutter, in one helical trip around and along one pitch of the finished thread path, with the spindle turning very fast.
You could do a threaded hole with progressive pitch, but you'd need to use a cutter with just one tooth, and you'd lose the speed advantage of thread milling by having to move that cutter along the complete helix of the generated thread, not just one turn of the helix. I haven't heard of anyone doing it that way, but it's not impossible.
Mike Halloran
Pembroke Pines, FL, USA
RE: Con rod threads for cap screws.
je suis charlie
RE: Con rod threads for cap screws.
They don't pull LeMans winners' names out of a hat.
I can attest those tri-lobed bolts had a LOT of prevailing torque.
It was not uncommon for folks to switch to a socket head cap ("Allen") screw.
If that was to be attempted today I'd want the solid shank length to closely match the stock bolt.
RE: Con rod threads for cap screws.
Here is a link to a short article that describes the fatigue benefits of "over-pitching" internal threads.
RE: Con rod threads for cap screws.
RE: Con rod threads for cap screws.
RE: Con rod threads for cap screws.
Thread milling with progressive pitch, using a single point tool as in the sectioned illustration provided by tbuelna, should still be a little faster than tapping, as the head orbits helically down the hole while making the thread, then half-orbits to a centered position and withdraws fast, again with the spindle turning fast all the time.
Tapping still makes sense where you have to produce the exact same array of tapped holes forever, like the 4/5/6 holes around a cylinder bore, or the 20-ish holes around an oil pan, and can afford to maintain a multi-spindle tapping head, and have already bought the tooling.
If you were starting fresh from scratch, you might decide differently.
Mike Halloran
Pembroke Pines, FL, USA
RE: Con rod threads for cap screws.
http://www.datron.com/blog/thread-milling-vs-tappi...
"The greatest advantage of tapping is speed. High-speed tapping centers set up with a rigid tap can thread holes in a fraction of the time it would take to thread mill the same holes."
Tapping can be quite a bit faster than thread milling, particular on a machine set up for rigid tapping.
http://www.cnccookbook.com/CCCNCTappingFeedsSpeeds...
RE: Con rod threads for cap screws.
Obviously, a conventional tap or multi-point thread mill would not work for machining variable pitch threads.
Here is a paper describing the Spiralock threadform. It uses a constant pitch internal thread with a unique cross section to achieve a more even distribution of preload between the engaged threads.