I am machining an aluminum tube that is 4" in diameter and 60" long. The tube is .06 wall. I cannot take a cut on the o.d, not even for a steady rest spot.
Due to the material and its "egg" shape, the best I can indicate it in is within .015. I do have to machine bores in the tube holding 3.98 +-.002.
No matter what I try the bores are as out of round as the o.d. This doesn't make sense to me. I am using a steady rest at the end of the part (I cannot "throat" the part in the chuck) and the tool is rigid, why are my bores coming out of round?
Can anyone give me insight as to why this is happening? How about possible solutions?
the steady could be deforming the tube, causing lobing.
Or most likely, there is enough residual stress in the outer surface to warp the bore.
If you suspect it is residual stress, you may want to stress relieve the tube (at least 3x to 4x the length of your bores) Before you machine the tubes. Soak them in an oven at 350°F for 6 hours, then let it slowly air cool.
Another method is to rough bore the tube, to within, say, 0.020" of finished size - then leave it sit in the lathe for at least 12 hours to let the metal "relax"; then finish bore to size.
If you suspect lobing from the steady, and since you can take a cut on the OD, make a thick-wall ring that lightly clamp around the tube, that your steady can run on.
Another way to machine the job is to bore the tube using a Criterion or suitable boring head, held in the chuck. Dial in and block/lash up the tube to the carriage (support the far end with the steady if needed). Bore the tube without rotating it in the steady.
also, if you're chucking on the OD, use pie-jaws...or may a tight fitting plug, to keep from further distorting/stressing the tube on that end.
Most likely you're experiencing residual stress in the tube. Experiment stress relieving a few sample pieces.