Machining Teflon, need some help on this VooDoo material!

[MattBSME]

I am trying to machine a taper on some virgin teflon.

I'm using a CG5 insert from Circle Tool, with a .007" nose radius. Talking with Circle's engineering group, this is the correct type of insert.

I'm making a 6* taper, with 1 pass with flood coolant. 1400 RPMs @ .0015 IPR

I'm getting a great finish on the part, but when I assemble the part into the mating part, it will not seal. Some parts seal, others don't.

I've spent the past week changing feeds and speeds, adjusting the angle slightly and nothing is giving me the correct result.

I'm starting to wonder if temperature is coming into play, causing the Teflon to warp to different sizes? Perhaps after machining, it is changing after sitting out?

I'm making a light pass, a max of .060" cut at the start of the taper, coolant should be at room temperature.

Any advice?

 

[Compositepro]

Why do you expect that it should seal? What are you doing to try to create a seal? Teflon tapers usually require a silicone grease to complete the seal. These are often used with laboratory glassware.

 

[MattBSME]

Without going into to much detail, the mating part is also a 6* taper (not teflon).

Basically everything is a ball valve, where the teflon is the rotating part to open/close water flow. I am getting leaks at the top and bottom of the taper. I've been able to elminate a leak at the bottom by bumping the OD larger a couple of ten thousanths, but left the top alone.

My frustration is coming from that I am using a new machine, where my old machine was worn out and very slow. Now that I have my new fancy CNC, I can't get it....D'OH!

 

[Compositepro]

That kind of valve is called a stop cock and does need to be lubricated. A couple ten thousands is meaningless with Teflon. It is plastic and will easily distort or cold flow that much.

 

[MattBSME]

Correct.

What I can't figure out, is that if I run a group, some leak, some dont. The machine is fine, so my guess is the material is distorting from heat on the process right before it.

But with flood coolant on, I would think the part would be okay. Unless the coolant is chilling the part, then later when the part warms up, we are getting a larger part than intended? I can't seem to find anything that relates temperature vs thermal expansion of teflon.

 

[CH5OH]

try using a slight different angle on the cone, so it rests in its mating part, first on the side it needs to seal.
for example:cone=30deg,bore=30.2deg

 

[MattBSME]

I've done that, it works at a certain point, then later the part must relax and no longer seals.

I thought it might be thermal expansion, but I don't think it is now. I think I'm relieving the stresses, and it's changing shape...but almost like it's shrinking? I would think by relieving the internal stresses, it would expand some.

 

[williedawg]

Perhaps the part-off op is deforming the part.

A HS parting tool with high back rake and plenty of relief might make a difference.

Something else to try is making a partial cut where the parting op will be; then do the taper turning, then complete the part-off op.
Teflon is probably the worst material for squirming.

 

[metman]

You might try cutting with very sharp (lots of rake and side relief) high speed steel. Typical carbide inserts have very little relief because they are strong in compressive strength but not so much in shear strength. So you are possibly correct in assuming heat genneration during the cutting and/or working the material which could be inducing residual stress.

Sharp HSS tool bit will cut rather than rub the material off.

 

[EdDanzer]

Your problem could be tool nose radius compensation. Without proper lead in and lead out steps and the proper G41 and G42 you probably will not get matched tapers.

Use High Spot Blue and see if the tapers match at both ends.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[jbel]

If the material is exhibiting a cold creep condition it may be changing shape after the machining process. Talk to a materials specialist and they may be able to describe this condition to you.

 

[btrueblood]

In order to seal, you have to put some compressive stress on the teflon. As soon as you get above about 200 psi, in our experience, the teflon will deform slowly to relieve the stress. The material "creeps" under load, if you want a term to google.

The only way to use teflon is in compressive stress loading, with a spring or other device to maintain the stress at a relatively constant level, and some means of containing/restraining the teflon so that its inevitable cold flow will not interfere with the part's operation.