Titanium warping

[Hmathews]

Have any of you had any problems with titanium warping while it is being machined or ground. Temperatures did not exceed what could be held in your hand. The piece was 1/4 inch thick and was being tapered from 1/4 inch on one end to 1/8 inch on the other. If you have, is there any thing that can be done to prevent it?

 

[Fellhoelter]

Most metals will do that if you remove material unevenly...

Maybe that's it?

 

[STR]

You can see the titanium or thin sheets of stainless cold peen from bead blasting one side only too and they won't straighten back up until you do the other side.

STR

 

[Hmathews]

We tried it with a grinder and a surface grinder. The material taken off was very even. It bowed in the middle, not much but enough to not be flat.

 

[Nathan the Machinist]

This is probably not caused by heat. It can be caused by removing more material from one side than the other on material with uneven stress across its cross section such as cold rolled steel or extruded plastic, but probably not in this case.

The cause is probably a dull cutter or not feeding hard enough. Did it warp away from the cut? If your cutter smears the surface under the cut and stretches it out a bit it can make that surface larger, warping the plate in the opposite direction. Titanium can be bad about this because it is strong, but not stiff. So it is possible to accumulate a lot of stress from a cut, and the material warps easy because it is only half as stiff as steel.

Titanium must be milled at fairly low SFM and a fairly high chip load. This means a 4 flute 1/2" end mill may only be turning at 800 RPM, but it is still fed at 15-20 IPM. RPM goes up proportionately as cutter size goes down, feed rate stays the same because chip load would go down proportionately as RPM goes up. This will get you in a workable ball park.

Titanium is flexible and strong, so you need to take shallow cuts. It conducts heat poorly, so coolant can help extend cutter life.

So, sharp tooling (carbide and HSS both work), lower RPM than you would use for steel, shallow depth of cut and feed fairly hard.

 

[Nathan the Machinist]

I was thinking about it some more. It cuts better if you don't plow, meaning you should cut with only one side of the cutter engaged where you can. Also it responds well to climb milling (if your mill is up to it). And, counter intuitively, cutter geometry like you would use for aluminum can work better than steel cutting geometry even though this is a high strength material. You don't want something that rolls a tight chip.

 

[First Twin]

Thanks for the great info guys. Harry was asking the question while I was making a good knife out of a piece of warped mess. I was grinding the titanium and the side I was grinding remained surprisingly flat (I was kind of proud of myself for keeping the grind so flat and even). Then I noticed that the other side had bowed enough to create gaps where there should be none, when the folder was assembled. I did notice what appeared to be some surface smearing as I ground and changed the belt once to try and improve the cutting. I wasn't too concerned at that point because I was just trying to hog off some material. I thought keeping it cool would prevent warping.
Nathan, I'll make a hard copy to save on the milling suggestions. Great info for the future.