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Offline Tweakie.CNC

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Re: Lost Steps
« Reply #10 on: July 04, 2011, 04:48:51 AM »
Thanks. I just hate having to ask tons of questions without offering some form of suggestion.

So, Your Z is physically short on dimension when coming back to zero and Mach is OK. I would guess at two possibilities;

1) Missed steps and the cause being that the motor does not have enough torque to raise the axis under all conditions. This could be stiffness in the axis (which you have already eliminated) or the motor drive electronics (drop in voltage from the PSU, insufficient current setting, etc).

2) Something physically loose (motor coupling, leadscrew nut, etc.)

Don't know if this helps.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline SMA

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Re: Lost Steps
« Reply #11 on: July 04, 2011, 05:48:40 PM »
Hey Tweakie,

This machine is belt driven.  Had some new belts come today and replaced them on z and X.  Checked the movement of Z and it is free and clear.  Slides easily fully up and down.  I am pretty sure it is not mechanical within the motor since i swapped z and y with no change.  Exactly 3mm short of zero.

I am not sure how to test PSU.  I did try changing the Dir Pulse to 5 from 2 but it did not make a difference.  Kinda at a loss at this point.  Very strange how it is always 3mm short of z=0. 

Thanks for the help though !

S

Offline RICH

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Re: Lost Steps
« Reply #12 on: July 05, 2011, 08:33:38 PM »
I can appreciate your problem and the difficulty your having.

I had a heck of a time trying to use my mill to do precise 3D machining and could not figure where those steps went or where and when it was skipping.
Like your code there were small increments of XYZ moves and changing from exact stop to CV or any other settings just wouldn't help.
Drove me nuts since  the machine otherwise worked well for all the other stuff i have ever done with it.

So here is what i did:
Rebuilt the X,Y,Z axis ....new motors of greater torque, new thrust bearings, and meticulous adjustment of the ball screw nut and thrust bearings.
Now no  backlash to deal with and feel very confident on the mechanics and controller side of things.
Now after 10000 lines or more  of code at much higher feed rates and rapids, the out of position is .001" at the end of the program.

I think the major cure on my end  was the new stepper motors which could deliver the acceleration and velocities required by the program.
Of course the other mods and adjustments provide for repeatable accuracy.

Even though i have the same motors,same gear ratio, same type / make of ball screws, same type belts and timing gears each axis has a different steps per unit
value ( maybe somewhat small but still different ) and I don't find that unusual.

Not saying you need to do the above, but, 3d maching places a higher demand on the system.
RICH

Offline SMA

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Re: Lost Steps
« Reply #13 on: July 05, 2011, 09:08:14 PM »
Hey Rich,

Thanks for the input. 

My machine is brand new.  Has less than 4 hours of machining on it and the builder has advised that the motors or over kill for what we do with the machine. 

I feel like it is a setting in Mach.  Some sort of offset that I don't know about.  Lost steps should change with motor speed setting adjustments but even speeding the machine up, both speed and rate of acceleration, leads to the same result.  I think I am missing something in settings?

S

Offline Tweakie.CNC

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Re: Lost Steps
« Reply #14 on: July 06, 2011, 12:57:19 AM »
I understand your frustration S but your machine should follow the instructions it is given by Mach and be in exactly the position the Mach DRO says it is. When there is a discrepancy, as in your case, then for some reason steps have been lost (or gained). Gained steps are often caused by electrical noise pickup but steps gained in one direction can appear as steps lost when the axis travels in the opposite direction. Lost steps are often caused by insufficient drive torque or mechanical problems.

As your machine is belt driven, could the belt be slipping or jumping a tooth on the pulley ?.
Have you checked the tightness of all the grub screws associated with the drive train to the Z Axis ?.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline SMA

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Re: Lost Steps
« Reply #15 on: July 06, 2011, 04:15:14 AM »
I am new to this but I agree that a machine does nothing without instruction and then only what it is told.  Learned that pretty quickly and how true it is.  It is strange how it does not achieve Z0 when crossing over but what gets me is that not matter then lines of code it loses the same amount of mm's (I am not sure how many steps this would equal) everytime.  You would think more lines of code means bigger discrepancy in result.  Coupled with motor setting changes you would see some significant changes.  This is just not the case.  Always 3mm off at home.

I just replaced the belt yesterday, just to be sure and continue to eliminate all possibility of mechanical issue.  Swapped Y and Z and in that process made sure the motor was well secured to the Z axis and as well that the Gear was well aligned to the idlers.  I loosened the belt a bit to reduce binding at idlers and conversely I tightened a bit more to see if slippage was the issue.  Idlers spin freely and true and belts are snug.

It is kinda like an odyssey without really going anywhere :)
« Last Edit: July 06, 2011, 04:19:35 AM by SMA »

Offline Tweakie.CNC

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Re: Lost Steps
« Reply #16 on: July 06, 2011, 05:00:26 AM »
Quote
It is kinda like an odyssey without really going anywhere

You are right there.

What are the steps per mm that you have set in motor tuning for the Z axis ?

Tweakie.

Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Tweakie.CNC

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Re: Lost Steps
« Reply #17 on: July 06, 2011, 05:24:46 AM »
Just answered my own question  ;D

OK just looked at your .xml and the steps per mm for your Z Axis are only 83.333. Your velocity setting for this axis is probably way too high for this ratio thereby reducing the available motor torque considerably.
By comparison, for my Z Axis I use a velocity of 1500 but my steps per mm are over 500 and that represents a reduction gear ratio of around 6:1 over your 83.333.
I think you should reduce your Z Axis velocity to around 1000 or less and try again to see if the error still persists.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline SMA

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Re: Lost Steps
« Reply #18 on: July 06, 2011, 07:58:42 PM »
Hey Tweakie,

Thanks for posting again.  I'll try anything to get his figured.

I ran the file through and same issue.  I slowed z down to 1000.  No change.  Still 3mm over the file and z still does not make it back to 0 on the crossover.

Weird and weirder. 

I am reading like crazy about MACH to see if there is a bad setting since I keep getting the same result.  3mm.

Pulled all belts off one more time to be sure there are no binds and things are all good.

S

Offline SMA

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Re: Lost Steps
« Reply #19 on: July 06, 2011, 08:01:07 PM »
And yes I did save the settings and even opened up setting tab to be sure they changed.  You don't need to reboot Mach for Motor Setting Changes do you?