Reverse engineer threads?

[LVLAaron]

Rebuilding some of the parts on my PM1440's chinesium QCTP for something a bit more rigid

How can I figure out what thread(s) are on the threaded post?

 

[Ischgl99]

Measure the OD of the thread and then use a thread gauge to check the pitch. The gauge will fit in all the threads if using the correct pitch. If you don’t have a thread gauge, measure across the top of several threads and then divide by number of threads (minus the first one) to get the pitch.

 

[LVLAaron]

The long end is:
15.75mm (1.5 pitch)

Small end is:
17.75 (1.5 pitch)

Sounds like M16 and M18 both 1.5 pitch

 

[Ischgl99]

I should have mentioned, the thread diameter measured will usually be slightly lower than the nominal size. For a 12mm thread, you will probably measure somewhere closer to 11.9 than to 12. If it is an imperial thread, then the same applies, but it should be slightly less than the fractional size.

 

[woodchucker]

what's not rigid about that threaded bar?

 

[Ischgl99]

That’s most likely what they are. The diameters have a tolerance range with the maximum at the nominal at the upper end, and the stuff out of China tends to often be at the lower end.

 

[LVLAaron]

what's not rigid about that threaded bar?

The bar is fine. It's more just an exercise in re-creating parts than anything.

 

[LVLAaron]

That’s most likely what they are. The diameters have a tolerance range with the maximum at the nominal at the upper end, and the stuff out of China tends to often be at the lower end.

Looks right according to: http://theoreticalmachinist.com/Threads-MetricMProfile.aspx

 

[mksj]

Common is a M16 2.0, I have made several QCTP with metric threads for the foot and imperial on the top. That was using the stock base, if you make your own base/foot and post it comes down to what you have the tooling to cut the threads. I also put a shoulder on the base thread so it locks and does not extend beyond the bottom of the foot.

 

[Ischgl99]

Mark brings up a good point that the top thread could be imperial. Your measurement is within the tolerance of a 5/8” thread, but your pitch is off for that. The fine thread for a 5/8” thread is 18 tpi, which is 1.41mm. This is one of the gray areas where some metric and imperial threads are close enough that it’s hard to tell which it is. In those cases, I use the thread pitch since that is more important than the diameter to a point.