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Z Issue After Tool Touch Off <resolved>

Posted: Sat May 08, 2021 9:59 am
by ridekp
Hi Folks,

I am having an issue with my tool touch off macro. I think this really a problem with G30/G28/G53 moves but I will explain in detail with some background info to see if anyone has some insight. My machine can Z home in a repeatable manner and typically does so well under .0005". I have my Acorn configured for a tool probe and tool touch off device and have been using Probe App V2 for my probing functions. My tool touch off device is one of the common knockoffs similar to the link below and it performs very well.



When manually touching off tools using the WMPG jog wheel the TT repeats within .0003" from the readout in CNC12 so I am very confident that my TT works as it is supposed to. I have cut programs using the manual touch off method and the tool offsets sync up to the the locations probed by my TP perfectly.

My issue is that when doing an automatic tool touch off cycle, after the tool is probed, the spindle head does not return to its true Z home. Each iteration of the probing cycle it returns about .14" shy of the true home location. If I do multiple offset measurements with the same tool in a row I can watch each offset drop roughly that amount each time i.e. 7.3", then 7.16", then 7.02" etc. I can also see it stop well before touching its homing switch. The first tool offset probed after homing z is always exactly as it should be. If I home z between each tool touch off the values come out as they should. At first, I thought this might be a mechanical problem but I have checked that all connections between the ball screw and the stepper are tight and free of play. The z axis is moving as it should (tested with an indicator) and the part I just cut came out to the correct dimensions. I have also run a test program multiple times to rapid up and down in z then settle to an indicated location. This test rules out slippage from the ball screw to the spindle shaft as each time it returns to +/- .0002" of the original location as measured with an indicator. I am using Stepper Online closed loop steppers with the error output monitored by acorn so I so I am confident I am not loosing steps. I have also been checking the error logs and do not see anything that would indicate that I am having signal interference issues.

I think this has something to do with the G30/G28/G53 commands since I am assuming that is the last command in the probing cycle macro. I am also curious where the macro for this is as I could not readily identify it when browsing the cncm folder. When issuing G30 and G28 commands via MDI this problem also presents itself. I can't imagine why any of those commands would cause the Z to stop short of its accurate location. When jogging with the handwheel the z axis returns to the correct home each time (as long as I do not issue a G28/G30/G53 command).

I have attached my report and recorded a video of the issue at the link below:



If anyone has some time I would love to hear any ideas on how to possible fix this.

Thanks!

Re: Z Issue After Tool Touch Off

Posted: Sat May 08, 2021 11:03 am
by Sword
I'm not sure if this is your problem or not, but I see that your G30 return coordinate for the Z axis is -0.1000. Perhaps Swissi will chime in with the answer for ya, as it sounds like something within the probing app settings.

Code: Select all

CNC12 v. 4.64 coordinates file
Axis     Minus     Plus    Return     Return      Return     Return
         Limit     Limit   #1(G28)     #2(G30)      #3         #4
 X      0.0000   13.0000    0.00000   12.78050    0.00000    0.00000 
 Y     -7.3000    0.0000    0.00000   -1.64960    0.00000    0.00000 
 Z    -10.9000    0.0000    0.00000   -0.10000    0.00000    0.00000 

Re: Z Issue After Tool Touch Off

Posted: Sat May 08, 2021 11:47 am
by ridekp
I did change that in my trouble shooting thinking that if I did not go all the way to zero maybe it would resolve the problem. But it still happens with the z-.1 location as well.

Re: Z Issue After Tool Touch Off

Posted: Sat May 08, 2021 12:01 pm
by Sword
It didn't look like you were using a G52 Z offset in the report either. I use that by way of MDI from time to time and forget to clear it when finished.

Re: Z Issue After Tool Touch Off

Posted: Sat May 08, 2021 4:40 pm
by cncsnw
When you do a G28 or a G53Z0, and Z stops ca. 0.140" below where home should be, you should:
1) Press Alt-D to switch the DRO to machine coordinates
2) Read the Z value there.

If the Machine-coordinates DRO says Z is at zero, but the head is not in fact all the way back up, then either you are losing counts, or the drive train is slipping, or (extremely unlikely) one of the macros you are running includes an M26 code to reset machine zero on the Z axis.

Yes, I know you said that mechanical slippage and dropped counts were not a possibility. If I had a dollar for everyone who has said that about their Acorn installations, I could retire.

Since you are not getting any alarms from your servos, the counts are probably being lost on the command side: not all of the pulses clocked out by the Acorn are getting to your drive.

Re: Z Issue After Tool Touch Off

Posted: Sat May 08, 2021 5:19 pm
by Gary Campbell
Yes, I know you said that mechanical slippage and dropped counts were not a possibility. If I had a dollar for everyone who has said that about their (insert brand here) installations, I could retire.
Let me add a "times ten" to Marc's statement. We are all guilty of assuming that the operations we performed are correct and the problem lies elsewhere.

A good diagnostician will assume nothing is right, and verify all conditions. And, don't forget Occam's razor.

Re: Z Issue After Tool Touch Off

Posted: Sat May 08, 2021 5:53 pm
by ShawnM
Where do I send my dollar Marc? :mrgreen:

Re: Z Issue After Tool Touch Off

Posted: Sun May 09, 2021 12:06 am
by swissi
ridekp wrote: Sat May 08, 2021 9:59 am .
I think this has something to do with the G30/G28/G53 commands since I am assuming that is the last command in the probing cycle macro. I am also curious where the macro for this is as I could not readily identify it when browsing the cncm folder. When issuing G30 and G28 commands via MDI this problem also presents itself. I can't imagine why any of those commands would cause the Z to stop short of its accurate location. When jogging with the handwheel the z axis returns to the correct home each time (as long as I do not issue a G28/G30/G53 command).
.
The ProbeApp uses the return address that's been chosen in the configuration settings and the options are G28, G30, G30 P3, G30 P4 or a G53 address. The macro that the ProbeApp generates and executes is always c:\cncm\ncfiles\probing_cycle.cnc.

The fact that this problem also exist when you execute one of these return addresses from the MDI is a pretty clear indication that this problem is unrelated to the ProbeApp.

-swissi

Re: Z Issue After Tool Touch Off

Posted: Sun May 09, 2021 7:35 am
by ridekp
Well I believe you all are right! I need to do some more vetting but I believe I found the issue and it lies with my closed loop stepper drive. The drive has two correction methods to simulate a servo, which are position and tracking mode. When the any z rapid to machine zero is initiated, the motor is lagging where acorn thinks it should be by some small amount. Over almost the entire z travel this accumulates to .14". The drive is set to position compensation by default so it should continue to move after acorn stops sending pulses, the problem being that once acorn hits it's soft limit at g53/g28 Z0.0 it stops all movement before the drive can compensate its position. I'm surprised that the drive didn't alarm out at .14" of error but that may be an accumulated amount that over 11" of travel does not trip the default alarm threshold.

I changed my z max rapid to 150ipm and what do you know, perfectly repeatable tool touch off values. In my head this makes sense as to why my test programs were still working because I was rapiding z around away from the soft limit so it has plenty of room for it's compensation to take place. I also loaded a much longer tool and noticed that the lag from the stepper dropped to .08" when issuing a g53/g28 Z0 from closer to the home position.

I will change the drives to tracking mode and maybe set my G30 location 1" away from z home to make sure the soft limit doesn't shut my drive off too soon.

Thanks for all the input! If my hypothesis is incorrect and I am still having problems I will let the brain trust here know.

Re: Z Issue After Tool Touch Off

Posted: Sun May 09, 2021 7:45 am
by tblough
Be sure to send your $1 to CNCSW :D