Machining teflon

btm · May 2, 2008

I have several parts that need to be machined from Teflon. In my past experiences with Teflon, I've found that it is not very stable.
I've read somewhere that it can be stress relieved, but I can't find any details on this.

Has anyone ever tried to stress relieve teflon? How well did it work?

Thanks
btm

Kyle Smith · May 2, 2008

I machine virgin electric grade Teflon on a regular basis and have never had stability issues.

Hidalgo · May 2, 2008

teflon compresses easily. If the part has any size the best way I have found is on 3M two sided tape. Just be sure the side on the tape os flat. I lap it on 800 grit to ruff it up a little
then clean it with acetone.

patindahat · May 2, 2008

Positive rake, and a real keen edge on a tool seems to make a big difference.... Inducing less stress maybe. Not sure but I was shown this to get a better finish, and I found hitting sizes easier too.

Pat

Limy Sami · May 2, 2008

IME and like most plastics, heats your biggest enemy as the material can't take it away very easily, Teflon has a really big coefficient of expansion so really sharp tools and keeping it cool will help to avoid stability problems.

Oh yeah, ( just in case you don't know) DON'T smoke while machining it.

Mike Ortega · May 3, 2008

Onsrud

Single flute cutters. Razor sharp edge, deep gullet. Get a nice finish with few burrs. Man I love those things

doug8094 · May 3, 2008

Teflon "Cold Flows"

FWIW, when working in chemistry labs we had Teflon stoppers and stems get "stuck" in glassware from thermal expansion or simply deforming under pressure at room temperature (depending on the situation). The solution was to run it under cold water until it shrank. Just gives an idea of how easy it is to "distort" Teflon.

jim rozen · May 3, 2008

Yes teflon cold flows under pressure. Not exactly distortion. And yes as a polymer
it does have a much larger thermal expansion co-efficient than metals. The
reason not to smoke while machining it, or to use good ventilation when heating
to around soldering temperature, is a thing called "polymer flu."

http://en.wikipedia.org/wiki/Polymer_fume_fever

I avoid torching anything with teflon in or around it.

Jim

jim fuchs · May 3, 2008

teflon

at work we machined some high temp teflon to be used in a 280 deg mold . it swells bad so bad it would lock up the mold so we did some heating in a oven and checked it before heating and after . it got a bunch bigger when heated and then when it cooled down it didnot go back close to its cold size .so now we pre heat it then put it in the molds works great hold size good . before it would grow and try and break bolt etc. jim

Madpickinskills · Mar 5, 2013

What about being able to hold a tight tolerance like +/-.001?

oxford · Mar 5, 2013

We machine some Teflon rollers. The Teflon is tubing pressed onto a steel roller. Approx 4" dia by 48" long. I don't know what the interference is but it is a pretty tight press, nothing else is used to keep the Teflon on the roller. The rollers come out with less runout after sitting on the shelf for a while if they are rough turned first, let sit for a couple of days and then turned to final size. I guess this is somewhat stress relieving it.

dazz · Mar 6, 2013

Hi

Teflon molecules do not cross-link. Cross-links tie molecules together. Without these cross-links Teflon molecules are free to move around. Under stress, Teflon will creep.

The high coefficient of expansion and stress creep properties often makes the goal of tight machining tolerances a futile exercise. The items need to be designed to allow for the properties of the material.

WizardOfBoz · Mar 6, 2013

Not to quibble, but I think you guys are all talking about polytetrafluoroethylene, or PTFE. Teflon® is a Dupont trademark for it's brand of PTFE. Teflon is always capitalized, and the first instance should contain the "Registered" symbol, ®, (alt 0174). The generic question, I take it, is about PTFE.

PTFE was discovered by Roy Plunkett in 1938 when he inspected a small cylinder of tetrafluoroethylene and found no pressure. He was trying to develop fluorinated refrigerants for automobiles and other uses. He had the cylinder cut open and found a white powder which was characterized and found to be polytetrafluoroethylene. Roy was working for a DuPont subsidiary at the time. As a former DuPonter, I find it very interesting that the key act of discovery (cutting the cylinder open) would probably not be permitted by DuPont's safety rules of today. Something they kind of glossed over in safety orientation!

Raul McCai · Mar 6, 2013

Too much rake in a fly cutter can rip the work from the vice. I've never had a stress issue.

Teflon is like Xerox or Kleenex. It's become generic. Dupont hasn't lost it as property, but in common usage it's like pig tracks on the farm. No need to use trademark symbols unless the communications is commercial in nature.

jamie76x · Mar 6, 2013

As some one said, Positive rake.
Most "machinist" these days only know carbide insert negative rake tooling. You know, the "machinists" that can not sharpen a drill.

HSS tool bits, positive rake, coolant and the right feed/speed. If you go to slow the surface is kind of "runny" and if you go to fast it tears.... You have to push it just enough to get a good chip. Read the chips, they will guide you.

vettedude · Mar 6, 2013

Madpickinskills said:
What about being able to hold a tight tolerance like +/-.001?

unlikely if you are turning anything larger than 5" and even lots under 5 if it is very thin

RUFUS1 · Dec 17, 2021

vettedude said:
unlikely if you are turning anything larger than 5" and even lots under 5 if it is very thin

i have hade success holding .001 tol. And stability for teflon "ptfe" by stablizing the material as it is machined.
First teflon tends to expand uncontrolibly (unpredictably) when used in and oven at temperatures above 125c.
However this can be brought to a level of use if you process the machining of the material.

The first step is the rough machine the teflon with machining stock on the dimensions
place it in an oven for appro. 1 hour at 145c let cool to room temp.

Semi-finish and place back in oven at 140c for another hour.
Let cool to room temp.

Finish to size.
As long as you do not exceed the 140c temp. In use the teflon remains stable when placed in an oven.
We use teflon in potting molds curing at 125c and below

kenton · Dec 17, 2021

Madpickinskills said:
What about being able to hold a tight tolerance like +/-.001?

Machine parts, then adjust thermostat to bring into tolerance.

Chris999 · Dec 18, 2021

I've had a 4" dia Teflon piece grow 0.045" after stress relieving at 350F. If the part is not at an elevated temp in service you might be able to get away without stress relief but I'd do it anyway, especially if you want to hold tolerances.
If it's service temp is much above ambient you must stress relieve it or it'll definitely change shape. Some guides recommend 525F but I've used 350F with and the part remained stable after but the service temp was less than 200F.

Annealing Guidelines for Plastic Stock Shapes | Boedeker

boslab · Dec 18, 2021

I have some rod and bar stock, I avoid sheet no prob with the round and square but the sheets another story, mill and it curves, very movable though I’m told it’s extruded not cast.
I use single flute router bits and HSS tools, honed, light cuts
Not tried inserts but the Ali ones look ok
Mark

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