PTFE machining properties

[eski1]

hi
we have got to machine lots of various sized PTFE seal rings biggest being about 220mm outside diameter , 200mm inside diameter and they all are about 10mm thick . Obviously they have to be fully machined then parted off due to there flexible nature .But what we have been told buy the customer is that the material will change diamensions after a period of time , so they recommend roughing out the seals then after about a week finish turning them . This seemed a strange request and not practical , what i would like to know is if the ptfe does change can you work on a percentage to allow for this . We would be machining the seals from a 9" billet 300mm long and would be using coolant .

Cheers
Chris

 

[rnd2]

Machining PTFE is a black art and those that know rarely divulge their hard won knowlege for free. We sometimes make seals from PTFE but generally avoid using the stuff. It has a low modulus, low resilience, practically no compressive nor tensile strength and because it has high thermal exapansion moves all over the place while it is being machined. Although chemically unaffected by moisture, PTFE is slightly porous, therefore will be affected by moisture imbalance between it and the prevailing outside environment. I supect in addition to all of the above, your customer is aware from past experience the dimensions change significantly after machining. They are giving you a heads up. Ignore that at your peril!

Fortune favours the bold and avoids the unprepared.

 

[plastx]

Is the material virgin PTFE? As mentioned above virgin PTFE is fairly soft and hard to machine accurately. There are filled PTFE for seals that help some of these problems but also change the properties in other ways. PTFE, especially virgin, does shift over time after it is produced but that should have happened long before you received your billet. You could cut a few and let them sit a few days and see if you do get changes. That's the tech side.

The business side is that if the customer requests a specific procedure, deviating from that procedure can lead to misfortune. I'm sure they compensated you the extra set-up and machining time ;-) .

Regards,
Mike

 

[eski1]

hi
Thanks for advice
material is virgin ptfe
What i forgot to mention is we have to machine the mating surrounding piece for these as well from billet brass same sort of dimensions as above . So we will machine these first as the brass hopfully won't move to much , then at least we can put the ptfe seals in them to hold them round enough to get a decent measurement .

chris

 

[ozchevy]

I've machined seals as described, maybe not quite as big in diameter. You should cut your stock PTFE bar to a managable width to be held in a four jaw chuck, dial it up, bore your internal dimension. Part off pieces at say 13mm widths
Now with that done - machine up a mandrel to be held in the four jaw chuck, dial it in as above and machine a step 8 or 9mm in width and 1/10th larger in diameter than the internal diameter of the seals.
You will now be able hold the seals on the mandrel to machine the OD and face them off to 10mm dimension.
Your cutting tool should be HSS not TCT, you will need a positive rake cutting tool with no chip breaker. speed of spindle around 1500 rpm.

 

[eski1]

hi
Thanks for the replies
Just an update job was done
Mat'l used virgin PTFE , customer happy with 8 out of 10 seal sizes but had problems with 2 which did change dimensionally . After a few phone calls we have now been told the seals should have been made from glass filled PTFE instead ( not specified before ) anyway job now on hold , the seals are for a plastic extruding machine i would imagine there might be a bit of heat involved ( can only guess at this stage as need to be informed ) but what advantage does glass filled have over PTFE

Cheers
Chris

 

[patprimmer]

Tensile strength, Flex mod and HDT

Regards

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