Project: Turning Teflon parts

[Jerry]

Actually got paid to make something today. A friend needed to build a device similar to a UV sterilizer. A quartz tube inside of a pyrex one. A UV-C lamp in the center starts a polymer reaction in the fluid in the unit. I made the end pieces out of teflon. Not virgin. Also made some threaded parts for some glass cold traps and an adapter for the arm. Threads were 1.375-8. The teflon machined very nice. Used generic carbide for the most part. Tried HSS but the finish wasnt any better. It cut to dimension very well. I dialed 1.375 on the DRO and it was right there. Not springy like some plastics.

And I also got a real job again. At least for about 8 weeks or so. I will be a "Camera Tech" for Laika on their new stop motion film.

A little vid of my 10EE cutting teflon. Kinda shaky since I was holding the iphone with one hand and running the lathe with the other. Click on the video twice to see it in HD on the youtube site.



Teflon parts by macona, on Flickr

End cap by macona, on Flickr

[Doug Coulter]

Teflon is fun to work with when it's big like that, and short. Cuts like a dream here, anyway, decent finish if the tool is sharp. You don't want to get mixed up with trying to machine a long skinny rod, though! Even a steady rest and a tool follower don't seem to help that much. Did you do the threads on the lathe too?

I like the way your lathe just stops right now. Mine has enough inertia to render the emergency stop switch kind of a joke.

[chrismb]

I like working teflon, too. It's my dielectric of choice for little in-vacuum bits. One thing about it is that it has some strange 'liquid' properties. After a while, and under load, it appears to creep and change shape. On a longer time-scale, I get the impression a block of teflon would just sag into a puddle, given enough time! So I use it only as separator- or low-load-parts. For mechanical loads, I try to use peek, cost permitting.

I only have very elementary hand tools. I have noticed that with both peek and teflon if I want to thread a, for example, 12mm rod with a thread then I have to use a die that usually goes with a 1/2" metal core. If I try to thread 12mm with a die that cuts a 12mm metal core, what happens is that the die doesn't self-centre and the thread digs so deep in that it ends up getting pushed over to one side, so to speak, and you get a deep thread cut on one side of the rod and a very shallow cut on the other. That's especially true for peek, whereas with teflon I tend to get to a point where I am not able to hold the piece at all, as the die cuts so deep that the piece slips around! So I use undersized rod relative to the quoted core size that goes with the die.

I don't know if this is 'known' in the machine-shop world, but it is the reality I have to live with in my under-budget hand-tool world! I think the two materials actually do this for two different reasons; the teflon is too 'soft' for normal dies and the die just bites in too deeply, whereas for the peek there seems to be lines of differential resistance to cutting, according to how the plastic was flow-formed. Not sure if that is correct, but it's my best guess.

...and nice looking work, Jerry! Maybe I should get you to make my teflon bits in future!!...

[Jerry]

There is no "seems like it flows" with Teflon. It does flow. The stuff is known to change dimension on you as soon as you cut it. If you want precision parts you rough machine to shape and then late it set for a few day and then finish them.

Teflon flowing is a problem outside if structural issues. It was found to flow around wires when used as insulation. If there is too much pressure against a metal pin or something it will eventually flow out between the wire and pin and short. Another problem with teflon is it slowly sublimates in a vacuum. There have been some issues in satellites because of this.

Your problem with the die is nothing new. Many materials exhibit this behavior when the wrong tool geometeries exist. Brass and acrylic have similar issues.

[Doug Coulter]

Yes, I'm fighting (mentally for now) with teflon's flow properties around my new HV rig, which I plan to insulate with teflon all the way to inside the tank. Turns out about .4" of teflon, or in other words, a bored out 1" piece with a wire in it, is good for my projected 180kv or so. But if you expose the end of that in vacuum, it's not going to work. So I will probably turn it down to fit into quartz or pyrex tubing with a vacuum seal on the end with a conductor passing through, and use teflon as the filler inside that tubing, which will be the mechanically strong part and seal through the tubing coupler. It can be thicker outside the tank so the connection can be buried inches deep in that end so as not to have arcs outside the tank.

Fusor HV feedthroughs are a world of their own, as the hot D ions reduce things like quartz to silicon and they become conductive....fail. So you have to shield the business end with pure Al2O3 which reduces to a lower oxide only, which evaporates...I have found that an outermost pyrex shield is conductive enough so that it stays close to ground at the "action end" and doesn't attract the hot D+ as badly,,, but can't do that if it's sealed around the conductor! And you can't just leave the conductor floating in the gas in there, or you get lengthwise arcs from Paschen's law effects -- this is truly a hard one. (I'll discuss this more on a more topical thread for it) Sadly, even the FT guys at CERN and ITER don't really have a solution to this weird environment at this point, though they have made some helpful suggestions. Most of those involve deliberately warping the E field so as not to have the ions hit the thing, as at these energies they will reduce almost anything in the chemical sense.