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Backlash control on Y axis is bad
« on: February 28, 2007, 02:37:38 PM »
Hi,

The backlash on the Y axis does not work properly.
When the backlash option is turned on (only 20% of Max) the Y axis step will output a high rate of pulses,(only in certain parts of the G-code) causing the motor to loose steps. I hooked a logic analyzer to the direction and step pins to verify the it was not noise. I found the the signals were clean, and captured the waveform when the problem happens. When the problem happens, the Y steps pulses are 38.6 us appart. This is about 25.7 KHz, which is my pulse frequency-- Hmmm.. This causes the motor to miss steps.

Note: This does not happen in Jog mode. The backlash function works perfect in the Jog mode. It only happens when running G-code.
Turning the backlash function off fixes the problem, but now I have backlash issues. I tried 5% and 100% of max values, but it makes no difference.

I noticed that someone else posted this exacted problem. This maybe the reason for a lot of posts of "ugly circles" posts.
I could send you the captured waveforms from my logic analyzer if you need them.

Thanks,
Robert


Hood

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Re: Backlash control on Y axis is bad
« Reply #1 on: February 28, 2007, 03:06:18 PM »
Try running in exact stop mode, with most stepper systems CV and Backlash comp dont like each other.
Hood

Offline Chip

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Re: Backlash control on Y axis is bad
« Reply #2 on: February 28, 2007, 03:13:07 PM »
Hi, Robert

Art stated on the Yahoo site that he had been working on some issues on backlash.

Hears what he sed,

Hi All:

I had been playing with backlash a bit in the last couepl versions, this one
goes back to "normal".
I "think" I may now know how to handle backlash correctly, its starting to piece
together. So, we'll
deal with that at some point in future.

There is a new lazycam in this version as well. It is somewhat better at
chining, has a few structural changes,
and has been prepped to be able to do pocketing and offset Gcode. It has a text
block that will be used to
provide status messages while its doing a job, so you can tell when its done..

This required a few alterations.. but its coming along nicely.


Thanks,
Art
www.artofcnc.ca

Thanks, Chip
« Last Edit: February 28, 2007, 03:15:02 PM by afn09556 »

Offline Chaoticone

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Re: Backlash control on Y axis is bad
« Reply #3 on: February 28, 2007, 03:13:32 PM »
I can't run backlash comp. on my mill. The reason is, my motors don't have enough power to move that much that fast. The best I understand it, when an axis has to make an adjustment for backlash, it has to do it very fast. If not, the path would be wrong. An example. Lets say you are cutting a circle in CV mode. At the 3:00 position, one of the axis slows to almost nothing and then stars in the other direction at almost nothing and gradually gets faster. Now if you have backlash on, when you get to the point it changes directions, instead of just having to chage directions it now has to move the amount you have in your backlash comp. + the amount it would with no backlash in the same amount of time for the path to be true. In jog mode, it doesn't matter because it has not done the math in advance to keep the path on track because there is no path to figure. I hope this makes sense.

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: Backlash control on Y axis is bad
« Reply #4 on: March 01, 2007, 08:39:53 AM »
Hood,

I tried your advice by running in exact step mode, but no luck. I have backlash comp on both X and Y axis, but only the Y axis goes crazy.

For right now, I am going to try to tighten up my machine. I can not afford ball screws right now unless you know where to get retrofit kit for a benchtop mill cheap.

What is an acceptable backlash tolerance could I get from lead screws? I have 0.025 inch of backlash in my lead screws and was wondering how good could I get them. If I could tighten up my machine, it may hold me over until I could get ball screws and just run Mach3 without backlash comp.

Thanks,
Robert

Offline Chaoticone

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Re: Backlash control on Y axis is bad
« Reply #5 on: March 01, 2007, 08:57:25 AM »
Hey Robert,
    The amount of backlash that is acceptable for you all depends on you and how close the tol. is on the parts you want to run. A high precision lead screw can get you arund .009. They make self adjusting, preload nuts for lead screws, I just can't remember who right now, they would help. These folks could give you a quote for ballscrews.

http://www.rockfordballscrew.com/

I hope this helps,
Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!
Re: Backlash control on Y axis is bad
« Reply #6 on: March 01, 2007, 10:14:12 AM »
Hi,

I just downloaded the latested verison hot off the press, 2.050, and the backlash comp. works great! I only cut one part, so I have not fully tested it yet. It makes my sloppy machine look great. I am a happy camper.

Thanks,

Robert

Offline Chaoticone

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Re: Backlash control on Y axis is bad
« Reply #7 on: March 01, 2007, 10:19:00 AM »
Robert,
     That is the best news I heard since the time I found out my girlfriend wasn't pregnant. ;D


Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!

Offline ardor

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Re: Backlash control on Y axis is bad
« Reply #8 on: March 01, 2007, 01:48:23 PM »
Robert,
     That is the best news I heard since the time I found out my girlfriend wasn't pregnant. ;D


Brett

So, are you the trucker that broke his arm when he pulled out to avoid a kid and fell off the couch?

 ;D
There is no approved dental use for WD40

Offline Chaoticone

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Re: Backlash control on Y axis is bad
« Reply #9 on: March 01, 2007, 03:17:28 PM »
Nope, I'm the mechanic that broke his arm when he pulled out to avoid a kid and fell off the couch?   ;D

 ;D

Brett
;D If you could see the things I have in my head, you would be laughing too. ;D

My guard dog is not what you need to worry about!