Centroid Community CNC Support Forum

From the looks of the setup, I would guess it's running through the original miter gears and the stock Acme screw. If you really want this to work well, replace the acme screw with a ballscrew and drive it directly and not through the miter gear set.

cncsnw wrote: ↑Wed Apr 28, 2021 1:31 pm

eng199 wrote:The Ki term normally drives out the last remaining error, which I would think could get the mechanicals to their "happy place" where current is low.

I respecfully disagree. Ki will indeed drive out the remaining error (provided the "Limit" term is high enough), but when the motor gets there, there may still be significant tension in the drive train. Nothing in the PID loop will necessarily relieve that.

With a heavy load, excessive friction, and perhaps some springiness in the drive train, there only way to relieve tension would be to go back the other direction a short distance.

Think about what you would feel in the hand crank if you manually moved the knee up to a certain position, slowly creeping in until you reached the exact target position, and then kept holding the crank handle in that exact position.

I was thinking of a stick-slip situation, where you might be able to get past the "stick" part. However, I will concede the point since you have more experience in the area.

tblough wrote: ↑Wed Apr 28, 2021 4:12 pm From the looks of the setup, I would guess it's running through the original miter gears and the stock Acme screw. If you really want this to work well, replace the acme screw with a ballscrew and drive it directly and not through the miter gear set.

I agree with Tom, I've used machines that are lifting the knee as an axis, it works if there is a 5 turns per inch ballscrew in there and nice 2:1 belt reduction to give you an overall turns ratio of 10. ( Brigdeport themselves installed both a ballscrew and a Air counter balance system to achieve good performance when they built a CNC axis using the knee)

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I just wanted to thank everyone for their input.

Converting to a ballscrew is not a reasonable option especially in light of the fact that this is not a cutting axis. It is going to be used for TLO's. I have a GT05 (.005mm) scale on the knee from before it was driven and I set it up as a manual axis just to display the knee position on a DRO. After tweaking backlash and reduction ratio, the knee tracks extremely well with the scale. In a 6" up and down move, stopping at 1" increments, the maximum difference between scale DRO and W servo DRO was .0006". Some pretty good mechanicals from Bridgeport.

I have the servo tuned the best I can and it will be fine as it has been for the last 5 years or so that I have used it.
I will probably put a M93/W in my M6 file to release power from motor at the end of tool change.

Just venting my frustration....

Yesterday I powered up machine and homed just fine. I then entered a MDI command for the Z axis and got a 411: (Z) full power without motion error. Grrrr... Machine had been running well for several weeks after my full cabinet gut and "rebuild", which I implemented a bunch of new functions like Auto/manual for the spindle speed and control, etc.

I swapped the Y and Z axis encoder and power leads and the error stayed with the Z axis. I measured the H-bridge output to the motor, issuing a single click on the MDI and the Fluke meter on the output stayed dead on zero in both directions while the power output bar in CNC12 went to red and the control then faulted. So that eliminated the H-bridge MOSFET's from being the problem, which would have been an easy fix.

So, probably some of the logic driving the bridge or even the FPGA is bad...$375 minimum and $30 postage each way...I am looking at a minimum of $435 for this repair on a control that has at most 300 hours on it, which is brand new in the machine tool world. It is an expense that I don't need at this point. My luck.