Questions about milling machines with linear scales

I'm having a hard time making round circles with my milling machine.  When cutting an OD path 1.25" in diameter I'm seeing 0.006" diameter variation.  If you have a machine with linear scales I have a couple of questions:

1) What kind of accuracy are you getting when contouring a circle?

2) What does the PID error look like in the scope?


Chris
 

Also if the mechanics of the machine have backlash, you will struggle with imperfections at the points of direction reversals, even with linear scales. And that backlash will likely cause instability and preventing you from tuning the PID loop stiff enough to get good results.

I have AC servos that accept stepper input at a configured resolution of 100,000 steps per inch.

When I use the HAL scope to trace the X and Y error signals I see a following error of 1 to 2 tenths except during the direction changes where I see 5 tenths on Y and 1 thou on X. I don't think these numbers account for the 6 thou error I see. I'm left with two possible causes; backlash in the linear scales and slop in the axis ways. I've adjusted the gibs multiple times trying to resolve this.

I'd like to know what other people get when they cut an OD so I can know what reasonable results look like.

Slop in the ways or gibs can easily cause very large imperfections at direction reversals, due to the table rocking on the slop. The biggest problem is when the ways are worn unevenly, and they are too tight at the ends, but to loose where most worn.

Get a good indicator and check for mechanical backlash on the machine, .0001" per division.
There should be no backlash on the scales. Not sure sure what you mean. Reader should follow the machine. 
Is this a ball screw driven machine? AC bearings on the screw? Quill on the Z? Everything that moves has some play and will add up especially under load. I usually write down all play values for each axis/components and have that as a reference for wear down the road....

Glass scales intended for machine tools have a carriage inside the read head that rides directly on the scale with little ball bearings. The read head is isolated from the carriage by a ball joint on the end of a piece of spring wire. This keeps the gap between the scale and reticle constant even when the motion of the slide is not perfectly straight and flat. The presence of the ball joint introduces backlash into the assembly.

I'm using tenths gauges to measure everything. The machine itself is old and worn. The spindle is surprisingly stiff, I can get a couple of tenths if I pull on it hard enough. The X axis is also pretty good, but the Y has a big dip in the center. I'm planning on moving the limit switches in so I can tighten the gibs further without risk of wedging.

Slop from the table rocking on loose ways, will be magnified depending on the position of actual milling vs the pivot point (which would normally be centered on the drive screw nut.) The further away from the pivot point the spindle is, the greater the change seen, and may not even be visible to the linear scale depending on how/where the scale is mounted to the axis and the read heads distance from that pivot point. (Conversely, if the scale is mounted so that it sees more movement from the rocking than the spindle, the control would compensate and over correct from the perspective of the spindle.)

The table (X axis) has dovetail ways and a ball screw down the center. The saddle (Y axis) has box ways and a ball screw on the side. The Y axis scale is mounted on the same side as the ball screw. The spindle is a about 6 inches away from the scale and ball screw. So it seems likely the Y axis is the issue. I'll see how much I can tighten the gib and try again.

Tightening the gibs has reduced (but not eliminated) the error. Pulling on the table to prevent the rocking improved it a little more but resulted in the 45 degrees being different.