Finegrain
Diamond
- Joined
- Sep 6, 2007
- Location
- Seattle, Washington
Hi guys,
I'm machining a few 100 pieces of 3" OD x 1/8" wall 6061 tube. Pieces are ~8" long and I just need to do some simple end work, +/-.005".
The tube came in at average 3.02" OD, but it is generally ovaled between 3.01" to 3.03".
The challenge is how to hold onto it without distorting it. I'm doing it in the mill, where I can control the clamping pressure better.
I made an exquisite fixture, which gave a loving 95% contact conformal grip ... for 3.00" tube. OK, the tube is oversize, so redefine the conformal surface to 3.02". Oops, the tube is too big in one orientation and jams in the fixture. When rotated 90* so it goes in the exquisite fixture, I end up with 2-point contact and ~.01" distortion which caused the part to be out of spec when taken out of the fixture and the material springs back. However, the amount of distortion is very unpredictable due to the 2-point contact.
So I tweaked the conformal fixture surfaces a bit to relieve the center and outside areas, and now I have 4-point contact which is at least predictable, such that I was able to write the distortion into the program and am getting finished parts within a couple thousandths of where they are supposed to be.
There's got to be a better way though. Some way of holding onto asymmetric thin-wall pipe without distorting it, but accurate enough to locate within a few thousandths so the machined features are reasonably coaxial with the tube.
Ideas?
Thanks, and regards.
Mike
I'm machining a few 100 pieces of 3" OD x 1/8" wall 6061 tube. Pieces are ~8" long and I just need to do some simple end work, +/-.005".
The tube came in at average 3.02" OD, but it is generally ovaled between 3.01" to 3.03".
The challenge is how to hold onto it without distorting it. I'm doing it in the mill, where I can control the clamping pressure better.
I made an exquisite fixture, which gave a loving 95% contact conformal grip ... for 3.00" tube. OK, the tube is oversize, so redefine the conformal surface to 3.02". Oops, the tube is too big in one orientation and jams in the fixture. When rotated 90* so it goes in the exquisite fixture, I end up with 2-point contact and ~.01" distortion which caused the part to be out of spec when taken out of the fixture and the material springs back. However, the amount of distortion is very unpredictable due to the 2-point contact.
So I tweaked the conformal fixture surfaces a bit to relieve the center and outside areas, and now I have 4-point contact which is at least predictable, such that I was able to write the distortion into the program and am getting finished parts within a couple thousandths of where they are supposed to be.
There's got to be a better way though. Some way of holding onto asymmetric thin-wall pipe without distorting it, but accurate enough to locate within a few thousandths so the machined features are reasonably coaxial with the tube.
Ideas?
Thanks, and regards.
Mike