Soft plastics don’t like fine threads, they have no strength so tightening to seal can be difficult. If design allows much better to produce a metal adaptor with the fine thread one end and a course thread on the other to fit your ptfe part.

I agree with Howard; I've just cleaned up a 2mm pitch thread in Delrin that had been crossthreaded for a friend. Only too easy to get it crossthreaded. So a finer thread in a soft plastic will be a nightmare.

Additionally PTFE is horrid to machine when trying to hold tolerances. It is soft and creeps rather than cuts. Cut, measure to size, and the next morining it will have moved.

Use the coarsest thread possible and ideally use glass filled PTFE. It is much more stable when machining.

Andrew

Josh could try for a 2.5 mm pitch thread, with the following set up:-

Fixed spindle gear 25 drives 25 on the 1st stud, that is linked to a 50 that drives a 70 on the leadscrew. As before, he will need an idler on the 2nd stud to complete the gear train and again a 40T gear might be suitable

The resulting pitch will be 2.445mm

Regards Brian

Existing gears would give 24tpi which is an imperial pitch and not too far off something like 1 1/8" BSB @ 26tpi or M28 x 1

 Put the 75T on the leadscrew and use the others as idlers

if there is a hole through the adaptor that may dictate pitch if the wall is thin

Edited By JasonB on 09/08/2023 19:33:32

Presumably distilling involves a fair range of temperature. PTFE has quite a high thermal expansion, roughly 1% over 0C to 100C, with a step change near room temperature due to a change of state. It would be better to redesign to avoid threading the PTFE, and to clamp it across a thin section to reduce the effect of the size change.

Edited By Macolm on 09/08/2023 21:05:28

Presumably, the thread does notb have to stand much axial load?

So DOES it have to be fine, or is it to mate with an already existing fine thread?

But, a fine thread In a relatively weak material, being shallow, will not be particularly strong.

Many threads in composites are coarse pitch, probably for that reason. If the fitting is purely for connection or sealing, a coarse thread may well be better, being stronger and more difficult to cross thread.

If the thread is 1.125" OD, use an Imperial pitch ( Why produce a "bastard" connection, for the sake of it? )

I know that from time to time we do, often for covenience rather than absolute need, but not an ideal choice.

To my mind, with an 8 tpi Leadscrew, I would feel tempted to go for 10 or 12 tpi. (8 tpi is an easy choice of changewheels ) with the others not far behind. 16 tpi might be getting a bit too fine to have a decent thread depth, only 0.040"in Whit form.

8 tpi will be 0.080" deep while 12 tpi is 0.0534", and 10 tpi will be 0.064".

Howard