[FuriousGeorge]

I've noticed that most of the time when I'm squaring ~6" long stock I get a sort of reverse bowing where my cuts are about 1 thou thicker in the center. I squared a .5"x 1.5"x 6" piece of Delrin with a face mill today, and ended up with h=.5" of material for the first .25" or so of length, .501" for the next 5.5" of stock and then back to .5" at the other end. It's an easy problem to work around, but I'm curious as to why it is happening. Any thoughts?

[SCzEngrgGroup]

How are you holding it? Soft plastics, like Delrin, are difficult to hold without compressing it, which will cause "squeeze out" somewhere else. Unless you're holding it with screws from underneath, or something similar that does not compress it, I would think holding better than 0.001" will be difficult.

Regards,
Ray L.

[FuriousGeorge]

Ray,

I'm holding the block in my vice, so I think your 100% spot on with the answer. I've noticed that I can crush plastic in the vice if I try, so what your saying makes a lot of sense. Thanks!

[Wog]

Just an idea...
If you have enough depth of the Delrin clamped in the vise, you could place a piece of alum or steel on both sides before clamping. Cut them just a little under the Delrin size to act like a "stop" for your vise. You'll need to play with it to find just the right size.

[nitewatchman]

Soft plastics, like Delrin, are difficult to hold without compressing it, which will cause "squeeze out" somewhere else.

It would seem that if it were squeezing out or bulging up in the center due to the jaws squeezing the material that the part would actually be thinner. When the jaws are released and the bulge retracts the cut surface would go back down. Am I looking at this wrong.

I do agree that holding closer than 0.001" in Delrin may be more a matter of luck than skill.

nitewatchman

[kevinro]

It would seem that if it were squeezing out or bulging up in the center due to the jaws squeezing the material that the part would actually be thinner. When the jaws are released and the bulge retracts the cut surface would go back down. Am I looking at this wrong.

I do agree that holding closer than 0.001" in Delrin may be more a matter of luck than skill.

nitewatchman

Inverse is also true. If the center of the vise was off, then the two ends would squish up, cutting more than the center leaving it high.

I think the correct test would be to check the part without removing it from the vise, then check again after.

Sent from my GT-P5113 using Tapatalk HD

[FuriousGeorge]

Inverse is also true. If the center of the vise was off, then the two ends would squish up, cutting more than the center leaving it high.

I think the correct test would be to check the part without removing it from the vise, then check again after.

Sent from my GT-P5113 using Tapatalk HD

I actually did check the part with a micrometer before taking it out of the vice and the bowing was still present. I checked after a roughing pass and the height varied by about .002." The pass was a rapid with a my brand new Glacern FM45-250 (http://www.glacern.com/fm45). It was was roughly .05" depth of cut, 4000 RPM and rapid feed rate. I took a .005" finishing pass (same speeds and feeds) and measured again and got a tolerance just over .001."

The stock was sitting pretty high on a set of parallels with only about 1/8" being held in the vice.

[Gerry Sweetland]

I had a friend that was a rep for a carbide tool suppler that told me once about one of his customers was a shop that specialized in precision machining of various plastics including Delrin.
He said that they would freeze the material to keep it stable while machining. I don't have any details, don't know if they put the stock in the break room fridge/ freezer or some other method such as dry ice.
Gerry

[Steve Seebold]

Delrin is wierd stuff. If you don't hold it exactly right, or you clamp it too tight, or not tight enough, it will move around like it's on wheels.

[Geof]

I actually did check the part with a micrometer before taking it out of the vice and the bowing was still present.....

Something you could check is any play in the Y axis. On some machines I have used when the X axis was to the left of center the small amount of play in the Y would let the table tilt very slightly left, then as the X moved the table to the right it would level up and then tilt to the right. This gave a result just the same as you describe.

Take the table all the way to the left and bring a dial gauge mounted to the spindle down to the workpiece. Try lifting and pushing down on the left end of the table and you might see the gauge deflect which will indicate some play. It is possible you will not be able to do anything to correct it but really 0.001 variation in Delrin over the distance you have is not that bad.

[C*H*U*D]

This may not be related to the problem you are seeing, but another thing to consider when machining Delrin is stress relieving. If you take too much from one side it will warp. You really want to start with a size as close to the finished size as possible. And I agree with Steve...Delrin is weird stuff.

[takewhatyoulike]

Delrin and PTFE filled Delrin AKA Turcite http://www.boedeker.com/turcax_p.htm are really not very dimensionally stable in terms of CTE also are somewhat hydroscopic. That’s why I don’t use large diameter or long lengths of the stuff. For large diameter bearings using Turcite, I embed multiple small diameter plugs of Turcite in an aluminum bore that has the same CTE as the aluminum bearing housing then machine the assembled Turcite/aluminum bore as a unit. In this way dimensional changes of the Turcite stay small.

Don Clement