teflon turning

[azzrock]

hi Ive recently been turning some teflon bar to help a friend out.
I found the result disappointing compared to other Teflon parts ive seen.
i have not played around much with grinding cutting tools yet. i used a fairly pointy
carbide turning tool.
does any one know what sort of cutting tool gives a good result with teflon?
Is emery used in finishing teflon parts to improve the finish.
thanks aaron


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[Michael G]

I can't remember all the details when I was in a place that turned telfon, but we didn't use emery (I wouldn't want to as the dust is likely to be nasty). Looking a the tool though I think that it needs to be sharper. When I turn other plastics like PE a sharp HSS tool seems to work much better than anything else (even a blunt HSS tool :roll:)

Michael

[Steamwhisperer]

Hi AAron,
I use a very sharp piece of HSS with a decent rake on it and definitely not emery. From memory I think a decent nose radius helps too.

Phil

[azzrock]

thanks guys. im going to give it a try. It was fairly lazy of me to use that cutting tool.

[BobL]

I've turned a bit of teflon and it definitely doesn't like emery. It also gives under pressure so if it needs to be finished to a very specific size it has to be well supported and slightly over cut. As others have already said, a very sharp cutter helps.

There is also teflon and teflon, in fact there are dozens or hundreds of different forms. The solid white stuff is called PTFE and is relatively easy to deal with. If you want a challenge try a turning a form called PFA which is softer and has a more rubbery texture.

[Metalman]

One place I worked at in the 70's had a large sign on the wall of the fitters workshop which said 'Do not turn PTFE (Teflon) without first getting approval from the foreman' The reason for the sign is that if PTFE is overheated it gives off very toxic gases. I am not very knowledgeable in this area but I think it would be worthwhile for everyone to check on the dangers for their own safety.
Mm.

[BobL]

From Wikipedia;


The pyrolysis of PTFE is detectable at 200 °C (392 °F), and it evolves several fluorocarbon gases and a sublimate. An animal study conducted in 1955 concluded that it is unlikely that these products would be generated in amounts significant to health at temperatures below 250 °C (482 °F). More recently, however, a study documented birds having been killed by these decomposition products at 202 °C (396 °F), with anecdotal reports of bird deaths as a result of non-stick cookware heated to as little as 163 °C (325 °F).[18][20]

While PTFE is stable and nontoxic, it begins to deteriorate after the temperature of cookware reaches about 260 °C (500 °F), and decomposes above 350 °C (662 °F).[21] These degradation by-products can be lethal to birds, and can cause flu-like symptoms in humans.[21] In May, 2003, the environmental research and advocacy organization Environmental Working Group filed a 14-page brief with the U.S. Consumer Product Safety Commission petitioning for a rule requiring that cookware and heated appliances bearing non-stick coatings carry a label warning of hazards to people and to birds.[22]

Meat is usually fried between 200 and 230 °C (392 and 446 °F), and most oils will start to smoke before a temperature of 260 °C is reached, but there are at least two cooking oils (refined safflower oil and avocado oil) that have a higher smoke point than 260 °C. Empty cookware can also exceed this temperature upon heating.

So keep your cutting tools below 260º and you should be safe :D

In 1988 I was cleaning up about $1000 worth of solid PTFE labware by boiling it in 50% strength nitric acid when I received a phone call that my car had been stolen. The upshot was that by the time I got back to the lab 8 hours later the container had boiled dry and the PTFE decomposed into a browny grey powdery blob. Fortunately everything was in a fume hood. Needless to say the boss was not very happy.

[azzrock]

hi every one. thanks for the good advise.
bob your right it doesn't take much pressure for it to start to deflect away.
the jobs hollow and over 6" long. I made a steel support that runs up the center of the job
and fits into a live center with changeable tips.

basically it appears keep it cool with a nice sharp tool. Pretty basic really.
I think the safety advise is well founded. from what i understand a lot of the chemist's that worked for dupont developing teflon died of cancer.

[.RC.]

I have machined plastic products with a dremel in the lathe...

[BobL]

In the early 1980's I was doing some experiments with very very small amounts of radioactive waste (e.g. for example no special OHS precautions were required). We would dissolve up the waste using concentrated acids in PTFE beakers made out of solid teflon rod in our in-house workshop. The beakers were 50 mm high x 25 mm wide with a 6 mm wall and base. Because the teflon was then very expensive we decided to refuse the beakers and cleaned them by boiling them in concentrated acids. However on doing a cross contamination check I discovered that this method of cleaning was insufficient to clean them. It appeared the teflon was porous and absorbing small amounts of the waste.

At that point I decide to conduct an experiment to see how far the contamination extended into the teflon and took one beaker down to the workshop and had the machinist shave off layers as thin as he could cut from the base of the beaker which I stored in clean plastic bags. I then analysed these shavings to see what was in them. As the bottom of the beaker thinned it got harder and harder to cut. The machinist fitted a steel plug inside the beaker to provide support but when the beaker bottom reached a thickness of 1/2 mm he could not take off any more shavings as the pressure required would puncture the teflon and tear the base of the beaker.

Anyway I analysed the shavings and found the radioactive waste did not penetrate beyond the inner 1/2 mm. We then examined the inner surface of the beaker under a microscope (not an easy task) and found it was not as smooth as it looked and machining marks were clearly visible and it was possible that stuff was being trapped in these grooves.

I then altered the cleaning procedure and we found success using a chemical shock method which involves repeated boiling them in concentrated acids and then is ultra pure water and this was successful. A few years later she lower cost moulded PFA beakers became available we found these could be reclined even better than PTFE. I still have some of those PTFE beakers, they were beautifully made and I can't bear to throw them away.

[azzrock]

bob good story. teflon beakers sound handy. if you drop them they wont break.

[Ueee]

bob good story. teflon beakers sound handy if you drop them they wont break.

They would be slippery little suckers too......:D

[BobL]

They would be slippery little suckers too......:D

Actually they are not too bad (about the same as wet glass) especially as we have to handle them with double plastic gloves. The worst thing is you cannot write on the suckers so we have to engrave codes numbers onto them and the pray that one does not confuse the codes with what is inside them.

[azzrock]

thanks guys. after playing with a few diff cutting tools i got the finish i was looking for. .
thanks again.
this cutting tool looks rough but shore did a great job.
aaron

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