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strange step loss.
« on: January 23, 2010, 11:12:37 PM »
I have constructed a new mill and have come across a strange problem. My z drive has been losing steps. It was losing them in the negative way and the bit just gets deeper. I tried slowing the drive down. I am running at 300mm per min with an acc of 6.8. still the same. I swapped a motor from my y to the z. Still the same. Now if I swap the wiring on the z, so it is set up to run the opposite direction then set motor outputs to be "dir low active" unchecked. I then loose steps on the up stroke. So I wrote some code, g01 z0, g01 z-30, g01 z-35, g01 z-30, g01 z-35 etc till I had 100 up and 100 down movements. Then went back to zero and was within .002mm so no lost steps. I cant see it being a motor problem as I changed it. I cant see it being a mechanical problem as when its wired in reverse then sent fwd with software the step loss follows the software. What am I missing. I am using a dell laptop d610 that is exclusively for the mill. The motor tests are great. I am using univelop 4/5 axis drivers. The particular code that is giving me problems is a converted jpeg, so there is tons of minute z movement.There's a total of 30,000 lines of code. About 50% of the lines have a z directive in them. After the program runs the z drive is out by about 6mm or some 3700 steps. Some of this movement is smaller than my resolution. I presume that if the command is less than the resolution then there is just no movement. I also presume that mach keeps tally of the numbers though and still calculates them into the next line. Is that correct? What should I try next?
Many thanks Bazza.

Offline Mattw

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Re: strange step loss.
« Reply #1 on: January 24, 2010, 12:09:55 AM »
hmmmmm, I'm probably one of the least qualified folks here, but I think you may be onto something with your diagnosis.  Is your program making relative movements or absolute?  If you are making relative movements in increments smaller than your resolution, I can see how you'd accumulate errors.  Can you set up your code to make  moves to absolute positions?  Just a thought.
Re: strange step loss.
« Reply #2 on: January 24, 2010, 12:19:18 AM »
I have tried both, one is better than the other. Can someone explain the difference between the 2 and its accumulated effect
Thanks Bazza

Offline Hood

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Re: strange step loss.
« Reply #3 on: January 24, 2010, 03:35:36 AM »
What do you have the Pulse width and Dir Prechange set to? Try 5 for each if you have it lower or even give Sherline mode a try.
Hood
Re: strange step loss.
« Reply #4 on: January 24, 2010, 03:46:33 AM »
I'm no expert but I remember some people have problems with laptops not giving out a consistent 5V, that may be something to check.

Also what do you mean converted jpg? I use CAD/CAM to generate G-code but never heard of jpg, as that's a picture file? Please explain, always looking to learn something new.
Re: strange step loss.
« Reply #5 on: January 24, 2010, 07:11:55 AM »
Bazza is referring to a raster to vector conversion....

Bazza, I have a question... If you remove the motor drive from Z, does the spindle drop or is it sprung upward to neutralize its weight?

And are you using stepper motors?

Offline ger21

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Re: strange step loss.
« Reply #6 on: January 24, 2010, 08:53:27 AM »
Set your accel to 2 or 3 and see if it still loses steps.
Gerry

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Re: strange step loss.
« Reply #7 on: January 24, 2010, 09:41:31 AM »
Hello bazza, sounds like the same trouble as I am having, see engraving at an angle. Await third control board setup.
Re: strange step loss.
« Reply #8 on: January 24, 2010, 01:30:07 PM »
Ok to answer a few questions. Yes I am using stepper. Without the motor attached to the z it does not drop.It is however easier to turn the screw down than up. I tried with a heap of rubber bands on it to support it with no effect. I then discounted that as being a problem as when I reversed the motor wiring and software direction, the step losses followed the software and lost its steps on the upstroke. I will reduce the acc and try that. I think I did already try it at 1 with no effect but will try again.I am running pulses at 5, I will try sherline mode. Whats the difference with sherline mode?
Bazza
« Last Edit: January 24, 2010, 01:33:45 PM by bazza »

Offline Hood

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Re: strange step loss.
« Reply #9 on: January 24, 2010, 01:38:40 PM »
Whats the difference with sherline mode?
Bazza

Some drives require a very wide pulse, Sherline mode give you that, 40us if I remember correctly.

Hood