# Machining Teflon



## gunboatbay (Oct 5, 2008)

I don't know if this is the right place for this post, but I need some help. I am making one of Ridder's coffee cup stirling engines and his plans call for a graphite or steel piston. I don't have any source for graphite in my area, but before Boeing Surplus closed I bought a 1" thick sheet of teflon. I am thinking, because of the slickness of the material, it would make a dandy piston. Does anyone have any experience they could share about machining this material???


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## Tin Falcon (Oct 5, 2008)

GB:
 I have machined it with no problems. Most plastic machine fairly easily. a couple of tips off hand do not machine it too fast or the tool will get hot and you end up melting it instead of cutting. The other thing that comes to mind is for your final cuts control the dust breathing Teflon powder is not good If it gets in you lungs it may stay there. 
When you do the final cuts set you lathe compound slide top slide to just under 6 degrees. That way a one thou feed on the compound will give you a one ten thousandth feed. 
As far as graphite rod MSC, Mcmaster Carr and Enco all should carry it. What is "Your Area"
Tin


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## gunboatbay (Oct 5, 2008)

Thanks for the info/help. Yes, I know some of the suppliers you mentioned carry the graphite, but my home is in a relatively undeveloped area of British Columbia and shipments to Canada can be a PITA. By suggesting that I use these suppliers, were you implying that I probably shouldn't use the teflon for a piston??


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## Tin Falcon (Oct 5, 2008)

Graphite may be better for your application but Teflon should work. My first attempt a a low delta T Stirling had not run yet so not qualified as an expert in this area. My piston is Teflon. 
Sometime the material at hand is better that the difficult to find.
Tin


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## Mainer (Oct 18, 2008)

For graphite, look at EDM electrodes. I think MSC www.mscdirect.com may sell them.


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## raggle (Oct 18, 2008)

This page may be of interest

http://www.btinternet.com/~sylvestris/phoefiles/phoelix.htm

I think the engine was covered in Model Engineer in the 80s or a little earlier with warnings about the material

Ray


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## ScottM (Nov 27, 2008)

I've machined Teflon, speed does not seem to be an issue as in other plastics - it does not melt easily if at all, think frying pan.

Sharp HHS tooling works fine.

post pics when you're done please.

I think you are proposing to used this in on of the low temperature Stirling engines which should be fine as far as I know never having built one :-} but please read this quote.

Safety
While PTFE itself is chemically inert and non-toxic, it begins to deteriorate after the temperature of cookware reaches about 500 °F (260 °C), and decompose above 660 °F (350 °C).[12] These degradation products can be lethal to birds, and can cause flu-like symptoms in humans.
By comparison, cooking fats, oils, and butter will begin to scorch and smoke at about 392 °F (200 °C), and meat is usually fried between 400450 °F (200230 °C), but empty cookware can exceed this temperature if left unattended on a hot burner.
A 1959 study, (conducted before the U.S. Food and Drug Administration approved the material for use in food processing equipment) showed that the toxicity of fumes given off by the coated pan on dry heating was less than that of fumes given off by ordinary cooking oils.

http://uk.answers.yahoo.com/question/index?qid=20080706100329AAIzM68

- Scott


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## raym 11 (Nov 27, 2008)

Would the weight or the pston enter into the mix?

I saw a guy take a deep whiff of gassed off Teflon. Not a pretty sight.

Ray M


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## Noggin (Mar 31, 2009)

Its a late reply but 

http://www.ensinger.ltd.uk/

Have a data sheet for machining many plastics


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## David Morrow (Mar 31, 2009)

GB, I live in Vancouver and my girlfriend lives in Bellingham, Wa. I'm going to order some graphite rod from McMaster. Getting it across the border is easy as she can bring me a "gift" each visit - something like $50 or so. I"m not in a rush but can order it sooner rather than later if you like. Let me know what size & part number you want. I'll order it and we can settle the bill after it arrives. 

If you don't live at the end of a long gravel road, I do a lot of long distance riding by motorcycle and you never know where I may go for little tasks like delivering graphite rods when the snow is gone and the opportunity exits to go for a ride.


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## Stan (Mar 31, 2009)

gunboatbay: I assume you have mail service. If you tell me what you need, I will mail you a piece of graphite from Edmonton. Just send me an email.


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## hitnmiss (Apr 1, 2009)

My only concern with Teflon is dimensional changes with humidity...

You want a perfect fit in a LTD stirling. 

I made my piston out of graphite, cylinder out of 303SS. Ordered the graphite from MSC I think. 

Mine's ran almost two years everyday on top of the coffee pot for an hour a day or so. There is no graphite worn and fallen on the displacer. Pretty amazing really.


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