You can buy a piece of 1" X 12" Thomson shaft from McMaster for about $16, for example. It's guaranteed to be straight within .001"/ft, but I think it's likely to be better in this large a diameter.
Ned Simmons
Even my stiff as an anvil Hardinge HLV-H will, depending on the material and diameter, take a spring pass.
A good way to check, is to make your cut normally, right to left, then reverse your feed and go back to the right without changing the cutting depth. You will be surprised on how much and how close to the headstock it will shave off on the return.
Gunner
"At the core of liberalism is the spoiled child - miserable, as all spoiled children are, unsatisfied, demanding, ill-disciplined, despotic and useless. Liberalism is a philosphy of sniveling brats." -- P.J. O'Rourke
Reminds me of Dave Gingery giving boring advice: Advance the cutter say .025" and take a pass. You might take off .017 the first, and (some more, etc....) and left with a thou or half a thou shy of the actual tool position after a few passes.
I still don't know how he managed to build all those machines with that 4" long 1/4" boring bar.
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @
As your experience "grows", you'll find .002 inch on the dia. is more in the realm of grinding.
With lathe work, more consistent results are obtained with a heavier cut. Suggest you use .010 on dia. as a finishing pass.
Why? You are putting more of a force on the whole setup, which takes up the slack in bearings, etc.
It also puts the actual "point" of the cutting tool below the surface of the material as opposed to just skating across the existing surface.
Spring passes are fine when necessary, but in most cases should try to avoid as a common operation.
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.