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roundness problem on long shaft


Aug 14, 2023
Hi guys, im new here, but as a machinist i've been 10 years into it

it recently bought myself a CNC lathe 5 axis that can machine up to 120'' long.
usually on part that goes up to 100'' long i've had no issue on my cylindricity nor my roundness , i've machine with tolerances that goes to TIR and cylindricity of 0.0003'' from the 2 ends of shaft ( those are journals for bearings)

i've put a longer shaft this week ( 3.937''dia x 115'' long) the centerhole as been turned on another lathe to ensure it was centered perfectly. after doing the roughing operation ( 0.170'' over) im not able to to turn a spot on the shaft for the steadyrest that dont have run out ... it machine like a 3 leaf clover ... about 0.001'' at peak on the dial even stranger , when i dial on the top of the shaft (x axis) the dial indicate close to 0.0003'' run out but then when i move the dial to the side ( Y axis) its gives me the 0.001'' when im getting close to the tailstock though it turn closer to 0.0001''

what can cause such a thing ... i should be reading the same 0.001'' on top and side ? could it be my chuck is at problem? i've looked the high seems to be where the jaws are clamping, but thats not a perfect fit im chucked on 0.5'' long and after the rough i always unclamp the piece to remove any stress made during the process

the tailstock pressure is very low to make sur it dont bow due to pressure i've tried to change it without any sucess.

the shaft is in steel1018 annealed

anyone ever experienced this kind of problems? i dont know if it will work, i've tried to attached a video but it too large for the website

You're shaft is flexing. Tool pressure and depth of cut combined with material inconsistencies alone will cause this. Tailstock pressure will change if the part heats up at all. You didn't state the tolerance required, however you may be trying to turn what should be a grinding job. If you do have to turn it try sharp tooling with lots of coolant but I doubt it can be done without support.

Just checked 115" is almost 3 meters long, the workpiece will droop at that length. Are you familiar with the term "shovelling shit up hill" no ways you can do this unsupported.
the tolerance on cylindricity is 0.0003''(0.007mm) and TIR tolerance is 0.001''(0.025mm) usually i dont need to send these safts to grinding its pretty easy to achieve that(2-3'' dia x 100'' long in banana or 50.8mm-76mm x 2.5m in the standard system ) i take 0.005''(0.127mm) doc with vcmt inserts

i've figured some way to achieve that on this part, i've put a steady rest at the tail stock than turned 1 other spot in the middle placed the steady there and did another spot at the 1/3 of the shaft and was able to finish machine at 0.0003''(0.007mm) TIR and 0.0001''(0.0025mm) cylindricity...

i was scared that my chuck had something due to the reading on the dials.
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