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Author Topic: Missing steps! Upgraded my screw drive to R&P  (Read 9171 times)

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Missing steps! Upgraded my screw drive to R&P
« on: December 17, 2011, 12:07:16 AM »
I just upgraded my router to a 5x10' R&P drive and now I seem to be having accuracy issues. I've been playing with the settings in Mach, slowing velocity & acceleration and also bumping up step & direction pulse width but I can't seem to get my accuracy back. I think I'm missing steps but I have no idea where. I'm going to rule out the possibility of losing steps going from the computer to the breakout board to the drivers because it worked fine when I was running my smaller screw drive system.

I'm basically cutting 4 parallel lines that are 4 inches apart. I'm getting inconsistency whether I cut them in X or in Y. I've attached a quick diagram of what is happening.

In the attached diagram, The blue lines are what it's supposed to look like and the red lines are what I'm getting. You can see how it's almost random where the errors lie. 2 or 3 lines can be right on and then one may be slightly off. Any ideas about what could be going wrong? Everything worked great on my smaller, screw drive router and accuracy was fine.

Also what is happening is when I cut out a profile, the position where the bit is on the exit cut is slightly off from the position of the entry or start of the cut. This is telling me that steps are being lost in the process of cutting the profile and not ending up where it started.

Now could there be an issue of noise or interference in the cables? I did just replace my Y & Z motor [/img]cables with larger 18ga 4 conductor shielded wire but have not done so yet on my X & A motors, which still have thinner 24ga unshielded wires. Could this make a difference? Also, could it make a difference if the cable lengths are different? I'm looking for all possibilities here.

Any thoughts would be greatly appreciated. Thanks,
Jon

Offline RICH

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Re: Missing steps! Upgraded my screw drive to R&P
« Reply #1 on: December 17, 2011, 07:54:24 AM »
Would recommend that you test and confirm your steps per unit. Don't have the  router on when you do the testing.
Five and 10 feet are long distances and the distance testing is only as good as the measuring device you are using.
Test each axis and one axis at a time. Set the steps based on the longest distance, then check for linearity of the axis,
ie; use 5'  / set the steps per / then check at say every 6" of movement and see if you are over or under and note what your findings.
Now move the same distances but with the router on as that would show if noise is involved. Do the above at a reduced velocity and at
your max.

RICH
Re: Missing steps! Upgraded my screw drive to R&P
« Reply #2 on: December 17, 2011, 11:34:27 AM »
Hey Rich, I don't think it is my steps per unit. I've already played with that and determined that the deviation is more random and not compounded over distance. I actually cut a 24" x 48" profile and the measurements were right on.  I put my vel to 100 and accel to 2 and cut 13 lines, two inches apart along Y.  Measured between them and there was a .02 variation at most. I put the ruler on the first line, the second & third were slightly off from the first but exact in relation to each other, the the next 2 or 3 were slightly off the the next was on and the next 2 were off, etc. very random in the variation of distance between all the lines. I will check with the router off but I'm keeping the router cable away from the motor cables and even have it in a separate power outlet. I may try to put it on a separate circuit just to be sure. The thing is, when I ran the exact same electronics & motors on my smaller 2" x 3" table, I didn't have any issues so everything is still the same except now I'm connected to an R&P and the axis' are triple in size. It sounds like noise to me because of the very subtle errors in accuracy (.02) I just don't know id its coming from the router. Any other thoughts?  Thanks,
Jon

Offline ger21

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Re: Missing steps! Upgraded my screw drive to R&P
« Reply #3 on: December 17, 2011, 11:43:45 AM »
IF it seems to lose position, then gain it back, my guess would be something mechanical.
Gerry

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Offline RICH

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Re: Missing steps! Upgraded my screw drive to R&P
« Reply #4 on: December 17, 2011, 12:48:22 PM »
Quote
the deviation is more random and not compounded over distance

The axis can gain, lose, or deviate but it is just a matter of degree relative to the total  ( a perfect linear plot of travel) and will be consistant.
If you have / know that  info to begin with, then when the router is used, and if it is no longer consistant, it is being caused by noise or even something else. Just want to completely eliminate any mechnical issues to begin with.

RICH

 


« Last Edit: December 17, 2011, 12:50:58 PM by RICH »
Re: Missing steps! Upgraded my screw drive to R&P
« Reply #5 on: December 17, 2011, 03:07:53 PM »
Ok, I will stick a sharpie in the collet and draw some lines with the router off and see if there is any difference. What other mechanical issues could there be? Keep in mind that I have checked & tightened all the motor shaft pulleys, tightened the belts, even tried to move each axis by hand to see if there was any play or back lash issues in the rack & pinion and all of these checked out ok. What other mechanical things should I look at?  My motors are not whining or stalling either. 

Could the difference in wiring be a factor? Having the Y & Z motors using 18ga shielded wire and the X using 22 or 24ga un-shielded wire?  I'm going to switch the X today over to the heavier shielded wire just in case.

Thanks,
Jon

Offline BR549

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Re: Missing steps! Upgraded my screw drive to R&P
« Reply #6 on: December 17, 2011, 05:06:35 PM »
What you need to understand with your conversion IS with the R&P it normally requires MORE motor torque than you needed with a Screw. This is because with the R&P you cannot get the reduction you had with a Screw. Yes you get more speed BUT that comes at a price of needing more torque to drive it.

The R&P i have seen either use belt reduction(which can limit top speed) or LARGE motors to drive them.

Just a thought, (;-) TP
Re: Missing steps! Upgraded my screw drive to R&P
« Reply #7 on: December 17, 2011, 05:28:55 PM »
Yes TP, I have the CNC Router Parts R&P system, which has a 3:1 belt reduction. I'm also using 495oz NEMA 23 motors which should be sufficient. Again, I haven't seen any stalling or heard anything funny yet with the motors. I'm getting pretty good results keeping my velocity at 100ipm and accel at 2.  I ran my 2" line test again and it seems to be getting much better. I actually have my power supplies for the motors & drivers plugged into their own circuit, separate from the router, computer and dust system.

Offline BR549

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Re: Missing steps! Upgraded my screw drive to R&P
« Reply #8 on: December 17, 2011, 05:42:01 PM »
How do the steps/ per unit  compare from the old setup to the new setup?  Close? not so close?

(;-) TP
Re: Missing steps! Upgraded my screw drive to R&P
« Reply #9 on: December 17, 2011, 05:53:35 PM »
Steps per unit on the R&P is ~1388 and the steps for the old screw drive was 8000.  My Z is still a screw drive and it's at 8000. They were all ACME 1/2" 2 start screws.

My drivers are currently set a 1/8 microstep. Do you think I can change them to higher or lower microstepping to get better results? Will that help?
Thanks,
Jon