Teflon shrinking and expanding?

[AndrewJP]

The company I work for received 2 similar jobs both to be made from Teflon. I received one of these two jobs and the other one went to another programmer. Both of our parts passed our inspection and were sent out to the customer(coolant was used on both parts). My parts went through and the other programmers parts were rejected. The part was .045 smaller on the length. We rechecked the program and it was fine. Anyone know how this could have happened? My boss asked me to Google Teflon to see if I can find any info on this.

[ckelloug]

Hi,

Most of what I'm saying below is speculation based on the material properties as I haven't ever machined teflon to high tolerances.

My first question would be if both of your pieces from the same lot of Teflon? Different lots may have slightly different internal stresses from the sintering process.

Teflon has around 10 times the thermal expansion of steel so if there was a large temperature difference, this would cause tolerance problems.

Based on the large variation that you mention, my instinct would be to guess too high a clamping pressure perpendicular to the face where material was being removed was used. Teflon has a much lower modulus and a much higher Poisson's ratio than steel.

In essence, a part that was heavily clamped would have expanded perpendicular to the clamping force during machining and slightly contacted in the plane of the clamping force. A part squeezed in this manner would be longer on the machine and shorter when the clamps were removed.

Regards,

Cameron

[AndrewJP]

Ya that's the same conclusion we came up with. Plus the the operator admitted to tightening them up really hard.

[RomanLini]

Originally Posted by AndrewJP Ya that's the same conclusion we came up with. Plus the the operator admitted to tightening them up really hard.

That made me grin...

And they let that guy machine expensive plastics?