Need Help to Improve Surface Finish

I ran the circle test again with the VFD on and spindle running at 2200 rpm. DTI needle bounce was the same as the previous test. Looks like the vfd has no effect, which is good. While running the test I monitored the Z axis step and dir pin lights on the BoB. The step pin light (pin 5) stays on throughout the run. The Dir pin light (pin 4) turns on only at the end during Z retract.

Looking over my driver and BoB wiring, again, and had a "Doh" moment. I have the drain wire connected to ground on both ends. Easy enough to fix but still can't believe I did it. I'll disconnect one end and run the test again and see how the DTI needle responds. If no change then reconnect and disconnect the other end. Going to run the test as jbolt suggested too.
 
jbolt said:
I see in your photos that the enable wires are not connected to the BOB, are they still hooked up to the drives? If so disconnect them at the drives. Also make sure the bare ends are not touching the metal back panel. On the BOB enclosure is the 110vac earth ground attached to anything else besides the back plate?

What we learned from the school ground issues was that terminated wires or improperly terminated wires acted like antennas, the grounding system was undersized and it was easy to create ground loops.

Where the motor drain entered the metal enclosure we had not terminated them at the enclosure wall but had instead ganged the drains inside the enclosure to a single ground lug. By not terminating the drains at the enclosure wall the drain wires inside the enclosure acted as antennas picking up interference from the inside.

Another issue was all the components of the system were grounded but the ground was not robust enough and we had daisy chained the grounds. The best strategy is to do a star ground system where each enclosure or device like the VDF and the machine are grounded to a single point separately and to not daisy chain the grounds. We moved to a 3/8" wide braided ground strap that was grounded to the machine structure, electronic enclosure case and VFD base as separate runs being careful not to inadvertently create loops. The VFD on this machine is mounted outside of the electronics enclosure. The electronics enclosure also contained the main power disconnect so it was also terminated to earth ground. The electronics enclosure was divided inside by full depth metal baffles to separate the high voltage from the low voltage and again for the signal wiring. Basically separate Faraday cages in one enclosure. In the course of troubleshooting the original build we tried separating out the the encoder termination boards in a separate metal enclosure but in the process mistakenly created a ground loop which made things worse.

On my PM-932 CNC conversion I removed any unnecessary wires (stepper enable) and any unused wires from the system. All the motor wires, limit switch wire, and spindle control wire drains terminate at the metal enclosure wall. There are no shielded wires or drain wires inside the electronics enclosure(s).
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All very good points. Thanks.

If I understand correctly you are saying to run a ground from each enclosure to the machine. Then terminate/star ground all incoming cable grounds at the enclosure wall. And yes, the 110vac ground is connected to the back plate only. Probably should ground the back plate too.
 
Tom,

Yes - star ground, not daisy chain. Also, PMDX advised me to run a drain from the 126 mounting holes to the star ground point inside the enclosure and not rely on the board to backplane mount screws. This may not be an issue for you, but if you're in there fiddling around and doing some wiring it's easy enough to do.

Good to hear there's no difference in with the VFD on or off. And I'm interested to hear how the DTI responds when you're indicating the head to column.
 
spumco said:
Tom,

Yes - star ground, not daisy chain. Also, PMDX advised me to run a drain from the 126 mounting holes to the star ground point inside the enclosure and not rely on the board to backplane mount screws. This may not be an issue for you, but if you're in there fiddling around and doing some wiring it's easy enough to do.

Good to hear there's no difference in with the VFD on or off. And I'm interested to hear how the DTI responds when you're indicating the head to column.
Click to expand...

Jbolt's post got me thinking about my wiring. I've gone through 3 upgrades since the initial installation and the wiring is beginning to be a bit untidy. Before doing any more testing I'm going to clean things up. Finding the driver/BoB ground loop may be significant. We'll see. And I will run a drain wire from the BoB to the star ground lug.
 
TomS said:
All very good points. Thanks.

If I understand correctly you are saying to run a ground from each enclosure to the machine. Then terminate/star ground all incoming cable grounds at the enclosure wall. And yes, the 110vac ground is connected to the back plate only. Probably should ground the back plate too.
Click to expand...

For the grounding system I use the location of where earth ground enters the system as the starting location for each arm of the ground runs.

On the VFD you have earth ground from the 220vac so nothing else needs to be done there other than the drain wire from the spindle control board to the VFD base ground.

For the BOB enclosure, if you ground the BOB to the ground system, don't run a separate ground to the back plate. I would keep the BOB isolated from the back plate to reduce the chance of the back plate picking up interference.

With your VFD and BOB enclosures being plastic you don't get the added protection of the Faraday cage affect of a full metal enclosure. Even with a full metal enclosure the openings in the enclosure need to be kept as small as possible for it to work properly.
 
TomS said:
Jbolt's post got me thinking about my wiring. I've gone through 3 upgrades since the initial installation and the wiring is beginning to be a bit untidy. Before doing any more testing I'm going to clean things up. Finding the driver/BoB ground loop may be significant. We'll see. And I will run a drain wire from the BoB to the star ground lug.
Click to expand...

Your comment about untidy wiring got my attention.

Cable dress can be important when it comes to reducing magnetic interference. You have a situation where you have conductors carrying high current to your steppers, and conductors that are relatively high impedance signal lines. Magnetic coupling between them can cause problems (ask me how I know). Shielding in the form of copper braid really doesn't address this kind of interference. Good cable dressing, and keeping high current lines separated as far as practically possible from signal lines, is the way to do it. The cable dresing should minimize the area between your drain lines and signal lines. See my illustrations below

Example 1, bad lead dress (lines far apart)
signal line ---------------------------------------------------------------------------



Drain line ---------------------------------------------------------------------------

Example 2, better lead dress
Signal -----------------------------------------------------------------------------
Drain-----------------------------------------------------------------------------

Example 3, Even better lead dress (this is hard to draw so I won't): make a twisted pair using the signal and drain lines. Induced voltages tend to cancel. I've even seen this approach used in high frequency integrated circuits.

Reason: the larger the spacing between the signal and drain lines, the more magnetic flux can pass through the loop that is formed, producing more induced voltage.

Ideally you'd also route your cables so the high current lines are at right angles to the signal lines but that could be difficult to do.
 
jbolt said:
For the grounding system I use the location of where earth ground enters the system as the starting location for each arm of the ground runs.

On the VFD you have earth ground from the 220vac so nothing else needs to be done there other than the drain wire from the spindle control board to the VFD base ground.

For the BOB enclosure, if you ground the BOB to the ground system, don't run a separate ground to the back plate. I would keep the BOB isolated from the back plate to reduce the chance of the back plate picking up interference.

With your VFD and BOB enclosures being plastic you don't get the added protection of the Faraday cage affect of a full metal enclosure. Even with a full metal enclosure the openings in the enclosure need to be kept as small as possible for it to work properly.
Click to expand...

Thanks for the clarifications. We're on the same page.
 
My plan for today was to have test data to share. Unfortunately not. I stripped out the unused X and Y Enable - wire and rerouted all of the drain wires. Also added a ground wire from the BoB to the star ground. Ran individual ground wires from the BoB/ESS enclosure and the motor driver enclosure then terminated them at the VFD ground lug. Powered up the system and the Y axis driver fault light is on. Checked all the wiring making sure the termination points were tight then double checked again. Made sure I hadn't crossed a wire and all is good there. Gave up for the day out of utter frustration. Tomorrow I'll swap out the Y driver with the X driver. If the green light comes on then it's the driver. If no green light then it's back to figuring out what changed. Sure would like to be making parts.
 
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