PTFE machinging properties 1

[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

 

[Philrock]

It is common for massive pieces of plastic to creep after machining. It's unlikely that you can compensate for this with a fixed percentsge correction. For some plastics, the problem can be reduced by thermal annealing. The manufacturer of the compound may have information on this.

 

[jbel]

This seemed a strange request and not practical

This may seem strange and impractical until you machine them to size in the first shot and then a week later you have a great big pile of scrap parts because of the cold creep.

PTFE is strange stuff if you have never machined it before. It is weird even if you have some experience with other plastics.

Take their advice about the wait time or you could find out by the school of hard knocks.

As for the coolant, it is not necessary. Just a good strong air blast. You will find that chip control will be your limiting factor in machining parameters. On a lathe you will get one long stringy chip that is almost impossible to break. Keep it away from the chuck with the air blast or you wont like the results.

For tooling use high positive rake polished inserts designed for aluminum. Try

http://www.rouse-arno.com/products/prod_index.htm

Again, chip control will be your limiting machining parameter, not speeds and feeds.

 

[Kenneth]

http://www.boedeker.com/anneal.htm

http://www.boedeker.com/fabtip.htm

 

[PLCRookie]

I have a friend in the seal business. He told me once that he uses a shopvac attachment at the tool post to suck up the chip(s). I have also seen production machines designed to machine seals specifically. They have a Vac with a type fo shredder blade attached that actually chops up and pulls the chips away. Chip control will definitely be an issue.

 

[ozchevy]

Allow 3% to the outside diameter for cold creep in PTFE.
Temperature changes also effect the amount of creep in this product even a 2-3 degree centigrade change in temperature will greatly effect the material, especially on the dimensions you are machining to.
Take the clients advice and wait the interval time on finished sizing. Obviously this will make things very differcult to set back up in a lathe chuck, so you will need to be very precise in your method.