Thanks all for your replies.

As I mentioned, I currently have a pneumatic drawbar in use with 500# bellevilles and a 3 inch cylinder. It works fine, but I have been experiencing some instances of tool slippage. I have read the Tormach white paper, and there are probably things I could do, such as make sure the bore of the Tormach collet is clean and grease/oil-free, that would minimize this; but I came away thinking that my 500 # holding force was likely a little low.

I don't usually do heavy cuts (typically 1/2" endmill at .030 DOC), but I noticed anytime there was any vibration I could usually find the endmill slipping somewhat. So the natural assumption (especially reading Ray's many posts on the subject - thanks for those, Ray) was that I needed to up the spring force and upgrade my cylinder. I have been attempting to stay away from mechanical leverage just to simplify things; so the question was if 500# was too low, what was likely high enough?

I think I might opt for an interim approach and try upping the bellevilles to 1000# by doubling the number and stacking them together like <<>><<>>.
Then I could use a standard 4" bore at 100 PSI. There is bound to be an improvement; and maybe for my mill that would be enough.

As far as constructing my own cylinder; I'm not dead set on doing it; but I already have all the materials and the construction seems easy enough. It would be single-acting with a light spring return; and I could get exactly the travel I need (no more than .25") and have the lowest possible profile. I am also trying to remedy the problem of my current system, which simply pressed down on the spindle and therefore subjects the bearing to the force. I have a design that solves that problem; so I am ready to start. I just don't want to do anything unsafe.

I went back and looked at the specs for drawn 6061-T6 tubing in .065 wall thickness, and saw specs for Ultimate KSI = 42, Ultimate Shear strength KSI = 27, and Yield KSI = 37. If I am correct that KSI equates to 1000 PSI, that seems to be a safe margin vs 100 PSI even though I'm not sure which spec is most applicable. And my Bimba cylinder wall thickness seems quite thin (I didn't measure it when I had it apart) but I have no idea what material it is made from.

I guess I'll go check ebay for any good deals on a 4" bore cylinder, and if I find any maybe that's the safest approach. But I thought it was worth posing the question here before I went off and made myself a bomb.

Thanks again, everyone, for the helpful responses.

Dave