Take nothing for granted, used machines!
Posted by
Jon Anderson
on 2000-08-20 00:47:02 UTC
A few months ago, I bought a '76 vintage Millport CNC mill, sans the
fridge-sized control. Being a home based business, this was the huge
step up I needed, and I knew that for the $2500 price, there would be
some issues.
I was right.
The X axis has always been noisy, and it would bind and stall the motor
well before reaching the end of travel at X+. I attributed the noise to
lead screw thrust bearings and the binding to wear. Couple days ago I
pulled the left end bearing housing. Seller told me he'd upgraded to
angular contact bearings and specially adjusted them for minimum
backlash. I found standard bearings, and no retainer for the outer race,
the bearings are located by press only. Luckily, that seems to be
enough, though I did draw up a retainer ring and will make and install
it when I replace the bearings.
But it was still noisy.
Tonight I got brave and took the motor side apart. Soon as I popped the
bearing support loose from it's two roll pins, it dropped over a quarter
inch and moved forward about the same. Hmm, seems to be a problem here,
says I.
I'm almost about to panic here, seeing what could be a real problem, and
just about to start a $30k job! The other bearing support had also moved
a bit but not enough to catch my attention.
All turned out well however, I reached in to the ball nut flange and
loosened the screws, replaced the bearing supports at both ends, and
retightened the nut flange bolts. Took the right side bearing support
loose again, and it sat right where it should.
Needless to say, leadscrew support bearings at both ends are shot, but
the screw appears to be OK.
Table now travels the full length in rapid with no stalling.
Word of advise to anyone buying older retrofit machines, if something
doesn't sound right, it probably isn't right. Don't assume that just
because the seller was using it, everything is fine. Don't assume that
the friendly old-world tool maker seller really did the bang up tune up
he thinks he did either... (I don't blame him for the screw
misalignment, that was probably from the factory)
Jon
fridge-sized control. Being a home based business, this was the huge
step up I needed, and I knew that for the $2500 price, there would be
some issues.
I was right.
The X axis has always been noisy, and it would bind and stall the motor
well before reaching the end of travel at X+. I attributed the noise to
lead screw thrust bearings and the binding to wear. Couple days ago I
pulled the left end bearing housing. Seller told me he'd upgraded to
angular contact bearings and specially adjusted them for minimum
backlash. I found standard bearings, and no retainer for the outer race,
the bearings are located by press only. Luckily, that seems to be
enough, though I did draw up a retainer ring and will make and install
it when I replace the bearings.
But it was still noisy.
Tonight I got brave and took the motor side apart. Soon as I popped the
bearing support loose from it's two roll pins, it dropped over a quarter
inch and moved forward about the same. Hmm, seems to be a problem here,
says I.
I'm almost about to panic here, seeing what could be a real problem, and
just about to start a $30k job! The other bearing support had also moved
a bit but not enough to catch my attention.
All turned out well however, I reached in to the ball nut flange and
loosened the screws, replaced the bearing supports at both ends, and
retightened the nut flange bolts. Took the right side bearing support
loose again, and it sat right where it should.
Needless to say, leadscrew support bearings at both ends are shot, but
the screw appears to be OK.
Table now travels the full length in rapid with no stalling.
Word of advise to anyone buying older retrofit machines, if something
doesn't sound right, it probably isn't right. Don't assume that just
because the seller was using it, everything is fine. Don't assume that
the friendly old-world tool maker seller really did the bang up tune up
he thinks he did either... (I don't blame him for the screw
misalignment, that was probably from the factory)
Jon