[jackie_burton@m]

I am having a problem with milling a slot in thin wall (.014) telescoping brass tubing. There is a .003 difference between the outside diameter of the inner tube and the inside diameter of the outer tube. They telescope perfectly before I cut the slots but stick afterwards. There is no problem with burrs so I can only assume that the tubing is being forced out of round by the milling process. Any thoughts on how to true the tubing afterwards or how to prevent this from happening?

[ZipSnipe]

instead of milling the slot try grinding the slot. If you lucky and have a surface grinder or access to one this would be easy. Or maybe take a wooden dowel turn it the inner diameter of your tube and put it inside the tube so it reinforces the wall as your milling your slot. And on that note you might be able to make a dowel and press it inside the tube to force it back round again. Good Luck!

[jackie_burton@m]

Thanks for the reply. No access to a surface grinder. I will see what I can do with a wood dowel.

[HuFlungDung]

Is the tubing bent lengthwise afterwards or is the cross section elliptical after the cut?

Brass work hardens very easily if the tools are dull. Indeed, the tubing is probably work hardened in manufacturing. The degree of work hardening does not make it all that much more difficult to cut, but the bad thing is that the piece is no longer homogenous in strain qualities. So if you remove a portion of the surface, the piece warps.

So it might be necessary to figure out a way to anneal the tubing before working on it.

Use very sharp tools when milling. Low rake angle or zero rake angle (straight flute) tools would be preferable if you can find them. If I have to work in thin brass, I will often touch up the drill flute to convert the rake angle to zero, or take an old endmill and grind a zero rake face at the end of the tool.

Try to avoid plunging through the surface in a drilling action, instead, cut through tangentially with the sharp corner of the tool.

[Geof]

If your slot is longer than the diameter of the tube there may not be much you can do. All tube unless it has been normalized has stresses from the manufacturing process and when a tube is cut lengthwise these stresses cause it to distort drastically.

[jackie_burton@m]

The tube is elliptical after the slot is milled. I am using a brand new end mill so I do not think sharpness is the problem. The slot I am cutting is 4.5" long and 1/4" wide on an 8 1/2" tube. O.D. is 9/16" (.562) I.D. is .534. O.D. of the interior tube is 17/32 (.531).

I will have to think about the tangential cut and annealing the tube. Thanks for the advise.

[jackie_burton@m]

BTW - Tried wood dowel. No help there.

[HuFlungDung]

How are you supporting this part for machining?

[jackie_burton@m]

Supported by 5" long aluminum v block tube drilling jig and cushioned by router table pad end cut. Clamped by bar clamps at overhangs

[lerman]

Try supporting it by putting a.530 (or so) steel rod inside of it. Notch the rod to match the notch you are milling. You might want to lube the rod prior to inserting is so as to have a better chance of getting it out.

Ken

[HuFlungDung]

If the tube is going elliptical rather than arching into a bow, I think maybe it is being crushed. Have you tried filling the tube with the wooden dowel and then milling it?

I would prefer to use a 5C collet to hold something that is delicate like this. A collet does not deform the circle. It is also possible to back up the tube by inserting a mandrel inside the tube before chucking in the collet.

If you don't have a 5C collet setup, I'd recommend that you look for a 5C collet block set. This consists of a square block and a hex block, both bored to accept 5C collets and come with nuts and a closer. You can find an inexpensive set at Shars Tool, page 77 of their catalogue.

You can grab one block in your milling vise. Support the far end of the mandrel on a dead center or perhaps rig up the other collet block to hold the free end of the tube. You would need some method of identifying the orientation of the mandrel, because it is going to get milled a little bit. You can do this by making it overlength, and milling a flat area (or drilling a cross hole) that you can set up with a level or a square on subsequent parts.

Edit: Oops, lerman beat me to the punch

[jackie_burton@m]

OK, good ideas that are worth trying. I ordered the collet block set from 5c Tooling. They were $5 cheaper than Shars.

I did try the wood dowel inside with no real success. Next I will see what I can do about the mandrel. Thanks a lot to all of you for taking the time to think about this. I will let you know how it works out.