Cutting a 1 1/8 fine pitch thread into PTFE!

hi, i have been asked by a home chemist to turn a fitting for him so he can distill solvents from a paint an (dont worry i know the guy hes not a back yard meth chef hes a genuine home chemist inthusiast haha).

anyway one side of the fitting needs a 1 1/8 fine pitch thread put on it and ill turn a nut while im at it.

im fairly new to cutting threads on a lathe, i have a myford ML10, oud someone advice me on selecting what change gears i will need? currently i the only gears i have are a 65T, a 70T and a 75T lol.

any advice much appreciated

cheers,

josh.

All depends on what you or your mate consider a fine thread to be, how many TPI. I would strongly recommend purchasing a book on screwcutting, you would then be able to calculate your own gear train and know what gears to purchase. “The gearing of lathes for screw-cutting” by Brian Wood being one such book.

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.

PTFE is a fairly rigid plastic which I think might be able to support a thread of 1.25 mm pitch. Gears needed for the gear train are 50, 55 and 70 set out as follows:-

Ist stud Driven 50, linked to Driver 55

2nd stud will need a suitable size idler to link the chain to the leadscrew---maybe 40 would be about right

Leadscrew 70

Result will be 1.247 mm pitch. very close to 1.25 as a target.

ML 10 lathes are unusual in that they have a permanent fixed 25T gear on the spindle, which in this case will need to mesh with the 50T wheel

Standard Myford change wheels are readily available

Regards Brian

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

Good advice Andrew. I will amend my thread pitch gearing accordingly and come back with something else. My sample of PTFE may not be typical.

Regards Brian

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

Why are all of the suggestions for a metric pitch? Presumably the ML10 has an 8tpi leadscrew, or am I mistaken? If it is imperial, then I'd suggest 16tpi. Simple gearing, exact, and the half-nuts can be engaged anywhere.

After all, the OP is using imperial sized stock.

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

Simpler still would be to aim at 8 tpi, bearing in mind what Andrew Johnson has to say about the material. The gearing then is 25 on the leadscrew directly coupled to the spindle gear of 25T with idlers linking the chain

Brian

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

Maybe you could use a standard SS union coupling, Get one oversized and sleeve the the inside with flanged Teflon inserts so no chemical touches the SS coupling bores?

**LINK**

Edited By John McNamara on 10/08/2023 04:17:27

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

If you use an insert thread tool you can get a full form shaped insert which will form the crest of the thread as well as the root of the thread as long as you use the correct insert for the thread that you choose. this will trim any miss formed thread when you come to the corect depth of thread you are cutting. Have a look at APT tool web site.

have a look at cutting a thread on my lathe recently although it is in bronze but 8 TPI https://youtu.be/7IgjID3RHkQ drop me a message if you want any further info.

David