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Author Topic: Skipping steps, X-Axis, plotting (cutting) curves  (Read 5225 times)

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Skipping steps, X-Axis, plotting (cutting) curves
« on: July 18, 2009, 08:09:57 AM »
I have a "Home-Brew" lathe fitted with SloSyn SS2000-DP4 drive and a Probotix PBK-RF isolator.  The Mach-3 software is licensed. I also have an XY plotter at home, I use for debugging profiles.  I have been running profiles on the lathe that consist entirely of straight lines and the lathe will return to its starting point, precisely and exactly... ready to start another piece without any compensation, over and over...

I am running Mach-3 Turn, in the Diameter mode.

I am having trouble running profiles that contain arcs.  At the end of a run, the X-Axis returns short of its starting point by two pulses, repeatedly. Motors are 200 pulses/revolution... 50 pulses/mm. In Diameter mode, the X-Axis in incrementing in 0.04mm steps; the Z-Axis (of course) increments at 0.02mm/step. So my "Return to Zero" is "dead on" for the Z-Axis, but 0.08mm shy for the X-Axis.  If I remove the "curves", thus skipping over them, the cutting point returns to zero, every time.

My question is...  Do you think the intermediate calculations for the curves seek points beyond the resolution of my stepper motors? Could this be the source of my problem?  I can control the end points of each line in the G-Code, to make sure the value for X divides evenly by 0.04mm, but I have no control over internally-calculated values for X when the curves are plotted.

I DO have "Steps Per" set to 50 in the X-Axis motor Tuning.  Could there be something else I am missing?


Offline Hood

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Re: Skipping steps, X-Axis, plotting (cutting) curves
« Reply #1 on: July 18, 2009, 03:02:52 PM »
Mach will move to the nearest it can get to a programmed position with your minimum step, it will not however end up two steps short. When you say it is 0.08 short is this by measuring or is it shown in the DRO?
Hood
Re: Skipping steps, X-Axis, plotting (cutting) curves
« Reply #2 on: July 18, 2009, 06:27:18 PM »
I have a mark on the handwheel that I line up when I begin the process. If I plot the entire profile including the curves, the marks do not line up at the end of the run. It takes two single-step clicks to get the marks to line up on the X-Axis.  The DRO is showing the correct number at the end of the run. After I correct to get the marks to line up, the DRO is off by two 0.04mm... or 0.08mm repeatedly. I have to re-set the DRO. Once I line the marks up again, re-running the profile does the same thing.

If I'm doing this with the pen plotter, the second (and third) retrace sits exactly atop the previous provided I adjust "X" before re-running.

I've played around with motor tuning and acceleraton to no avail...

If I skip the curves, the plot returns exactly as it is supposed to... over & over.

I have other profiles that do not have curves. and they also work just fine.

If the problen has to do with electrical noise, it seems strange to me that the tathe and the plotter would act the same. They are in different electrical environments.

I must tell you, that the stepper-motor wiring is not shielded. The cables are relatively short but they pass through a pair of DPDT 5toggle switches.  The switches allow another computer to connect to the systems that runs DOS (and profiles written in Microsoft Basic).   This was intended to be an intrim hookup that would allow the older PC ro turn parts until we get the new system debugged.

Is that a mistake to even try?
« Last Edit: July 18, 2009, 06:32:50 PM by TunerTom »
Re: Skipping steps, X-Axis, plotting (cutting) curves
« Reply #3 on: July 19, 2009, 08:02:08 AM »
I think I have found the problem.
I have turned "Tool Path" off, in Debug, and I reduced the Accel rate for the "X" motor from 100 to 75.

I have run the complete profile four times on the plotter and the pen path has superimposed itself exactly on top of the previous drawings and both the "X" & "Z" handwheels have returned to their starting posiitions every time.

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Re: Skipping steps, X-Axis, plotting (cutting) curves
« Reply #4 on: July 19, 2009, 08:20:07 AM »
I would think it is the accel that was the problem, turning the toolpath off will help with low spec computers but as your DROs were showing correctly it would seem to me the correct pulses were put out to the drives.
Hood
Re: Skipping steps, X-Axis, plotting (cutting) curves
« Reply #5 on: July 19, 2009, 08:40:54 AM »
Thanks for your help.

One question...  Since this all seems to have to do with timing issues. Are there advantages to increasing thw kernel speed  (currently at 25000).
That would mean recalibrating the motor speeds again (sigh) maybe, I should simply leave well enough alone.

Thank God, it doesn't appear I need to rewire the stepper motors...

Tom

Offline Hood

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Re: Skipping steps, X-Axis, plotting (cutting) curves
« Reply #6 on: July 19, 2009, 08:50:01 AM »
Dont think  increasing the kernel will have any  benefit unless your steps per unit require that you need to increase the kernel to get the  rapids you require, this is not the case in your setup so I would leave well alone.
 
 Increasing higher than you need  may also have a detrimental effect if your computer or parallel port  is borderline.
Hood
Re: Skipping steps, X-Axis, plotting (cutting) curves
« Reply #7 on: July 19, 2009, 10:54:45 AM »
Thanks...