Machining virgin Teflon

Hi:
I'm experiencing some difficulties in machining the seal as shown in the attachment.
-I started by chucking a 6 inch length of dia. 4.062 round stock in the 3 jaw.I faced one end and flipped the stock.
-The ID was bored (using a DCGT 32.51 Al cutter)and measured using a 3 point bore micrometer.
-The OD major diameter was turned using the same cutter geometry
-The OD step was then turned.
The surface finish was excellent, with no chatter marks, but here's the problem, the part grew uniforly by approx .020 in diameter :(
Does anyone have some suggestions to solve this problem. I'm considering underturning all diameters by .020, but I doubt that I'll be able to hit the numbers.

thanks

How much stock overhang from the chuck? Is holding compression part of the problem.
Do a rough-out run of the whole part and then do finishing.
Can't say as I have ever noticed this but my teflon work is usually blocks not thin little gaskets.

Thanks:

The stock overhangs by about 4 inches. I figured that the compression would not appreciably effect the ring 3 inches away from the jaws.
Everything measured OK prior to parting, and post parting. The problem reared it's head 24 hours later...
I'll try to undercut all OD's (using cutter wear comp) and let it sit for a day or so. Hopefully I'll be able to correlate the amount of undercut to the growth with a few tries.

thanks

It is possible you are doing everything right and the Teflon is creeping after you machine it due to residual internal stresses. I've made some pieces from Teflon (and similar materials like Delrin and Nylon) and had to go to great lengths to get a "plate" to be flat. However, after some period of time, the block will not be flat anymore, it starts to curl. I recall making some fixtures for a customer that were 8" x 6" x 1". Visiting the customer months later, these plates had bowed about 1/8".

So it is probable you are making a perfect part when it comes off the machine, but it may not stay that way.

Thanks for the feedback, I'll let you know if I succeed.

regards

Try annealing it before machining. You could use an ordinary cooking oven, take it slowly up to around 400F and then slowly down. To allow it to heat and cool slowly I would wrap it in several layers of aluminum foil and to check the temperature of the teflon put a meat thermometer in it in a small hole that will not interfere with the machining.

I had the unfortunate task of working virgin teflon early in my career. And the most important thing I, and my supervisor, learned was to use HSS tooling, and have as sharp a cutting edge as possible. Razor like if possible. The material has a propensity to tear if the tooling isn't very sharp. Though in my case the rod was Ø.300 and making rings for a clients.

Hope this helps some.

I worked it out:
As mentioned before, I undersized all diameters by .025. Within an hour of parting it gained .010" on diameter @ room temp. By mid afternoon it fit snugly (just a bit tight) onto a mandrel that replicates the shaft diameter that the part was intended for. I measured the OD, and it was within the required dimensions. The material obviously has a tendency to "relax", and I'm confident that it's creep characteristics will stretch it to the correct size. The seals are for a PVD coating machine, manufactured in the Ukraine, so getting parts would have been slightly probematic.

thanks again for the input

I did similar pieces about double the size and I had a heck of a time getting them to size.
Wish I knew what works best, what I did was turned O.D. I.D. and left about 1/8 /side.
then made a steel sleeve what went snug into the hole and clamped it like this...use very sharp tooling, I ground my own
finished O.D, I.D left about .015 /side, part off, not thru, stay away about 1/16 then finish size and part off.
Measure and compensate for next pieces.

Konrad