Milling Table Unevenness

C

Chewy

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I got a PM-30MV mill and trammed the table. I set it up with a dial indicator on just the table surface and got pretty good, a couple of of tenths around the center of the table. I found out the left side X is .003 higher then the right at around 27+ inches. OK. Got a disk brake rotor around 8"diameter and started checking the table out. First, disclaimer! The rotor is flat. Readings are repeatable no matter how it is turned. It is bolted down finger tight and I don't see where it is off more then 1-2 tenths. The tram is a 1/2" round rod in an end mill holder. The dial indicator is hard bolted to the rod. Used several different indicators and holders and the difference between them is negligible. The table unevenness stays consistent.

To the left of center, the table is flat, maybe .0001 off X-Y front to back, side to side. Go 4" to right and Y is off -.0006 at column. Go 4" more and Y is +.001 at column. I didn't include the X but waves up it and down every so many inches. The table has about .002 Between max high and low. It just rocks back and forth. The X-Y readings are at 7" diameter taken from the spindle. If I leave the vise at the good spot and put rotary table around 7-8" away. the work will be off about .001 over 6-7 inches. And that's BEFORE I screw anything up!

Am I making a mountain out of a mole hill? I had hoped to be able to do two setups some times. Two vises or a vise and rotary table. But I am going to have to map out the table to get two spots with the same X-Y readings. A couple of tenths don't bother me but a .001 swing up, down and around is annoying.

I am curious to find out how other people deal with it or just ignore it. Is it noticeable in the work when you work with bigger pieces? Thank Charles
 
Precision in nice, but blood pressure is supposed to stay within limits. If chasing tenths is going to cause blood pressure rises, take up bird watching. If not, ignore the very minor fluctuations in your table. This is a hobby, meant to be enjoyable, IF you don't enjoy it don't do it.
 
just curious but if you set the indicator directly under the spindle and move the table back and forth, do you see the unevenness that you are describing?

BTW, I would be a very happy camper any day that I can get the finished part +- .001 (or.002 for that matter)
 
When the table is at its extreme left or right position, the overhanging weight can cause the table to lift. In indicator in the spindle would see that as a non-flat condition. This problem is inescapable with dovetailed ways as there must be some clearance in the ways for lubrication and freedom of movement. Tightening the gibs will improve this. Both the x and y axis ways will contribute to this

There are several way to check if this is the cause. If you lock your table, this should remove the free play and will reduce the lift. Another would be to offset your indicator so that the measurement position is left or right of the the spindle axis. A third would be to place weights on the table to counterbalance the overhanging weight and repeat the measurements.
 
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