Scratching my head on this one trying to figure out what's going on and hoping one of you might be able to provide some insight.
I've got lathe job with a thin walled aluminum part that I am taking a finish pass on the OD that I am noticing 0.005" out of round afterwards but only on the outside on the last inch and half of the part on the chuck side. ID is still round.
The part: 4.5" OD x 11" 6061 Aluminum 0.100" wall thickness
The setup: ID pie jaws grabbing 3.5" deep on an 8" 3 jaw chuck on a Mori NL2000.
The process: We rough turn the OD leaving 0.020" finish stock, then we OD grip the part with 4" pie jaws to drill and bore the ID to size. Last op is ID gripping the part with pie jaws to finish the OD.
More details on the problem: I check roundness after finishing the ID and everything looks good <0.0004" all around. I've got my clamp pressure dialed as low as it will go and if I clamp, roundness still looks good on the machine. I take the part out before machining, you got it, roundness still looks good. However, after I take a 0.005" skim pass on the part is when all hell breaks loose. Once I take the part out and check it on the CMM, it has the classic trilobal shape you'd expect from jaw distortion but here's the kicker, its only on the OD. ID is still perfectly round. The distortion is only present on the last inch or two of the part on the chuck side, everything else is round. I used dykem to verify the part is properly seated on the jaws and making contact across its entire length & width.
Questions I can't find an answer to:
1)If its jaw distortion, why can't I see it from just clamping and unclamping the part? (only shows up after I take a skim cut)
2)The ID pie jaws probably grip close to 90% of the ID with minimal pressure, how can they be distorting the part?
3)Why is it only affecting the OD and not ID?
4)If its something else, say tool deflection, why it is then that if I spin an indicator on the part after cutting while still clamped, I get only a few tenths of runout? This tells me the machine is cutting round.
Any help is greatly appreciated.
I've got lathe job with a thin walled aluminum part that I am taking a finish pass on the OD that I am noticing 0.005" out of round afterwards but only on the outside on the last inch and half of the part on the chuck side. ID is still round.
The part: 4.5" OD x 11" 6061 Aluminum 0.100" wall thickness
The setup: ID pie jaws grabbing 3.5" deep on an 8" 3 jaw chuck on a Mori NL2000.
The process: We rough turn the OD leaving 0.020" finish stock, then we OD grip the part with 4" pie jaws to drill and bore the ID to size. Last op is ID gripping the part with pie jaws to finish the OD.
More details on the problem: I check roundness after finishing the ID and everything looks good <0.0004" all around. I've got my clamp pressure dialed as low as it will go and if I clamp, roundness still looks good on the machine. I take the part out before machining, you got it, roundness still looks good. However, after I take a 0.005" skim pass on the part is when all hell breaks loose. Once I take the part out and check it on the CMM, it has the classic trilobal shape you'd expect from jaw distortion but here's the kicker, its only on the OD. ID is still perfectly round. The distortion is only present on the last inch or two of the part on the chuck side, everything else is round. I used dykem to verify the part is properly seated on the jaws and making contact across its entire length & width.
Questions I can't find an answer to:
1)If its jaw distortion, why can't I see it from just clamping and unclamping the part? (only shows up after I take a skim cut)
2)The ID pie jaws probably grip close to 90% of the ID with minimal pressure, how can they be distorting the part?
3)Why is it only affecting the OD and not ID?
4)If its something else, say tool deflection, why it is then that if I spin an indicator on the part after cutting while still clamped, I get only a few tenths of runout? This tells me the machine is cutting round.
Any help is greatly appreciated.