Gavztheouch's Mk2 Laser

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Re: Gavztheouch's Mk2 Laser

Postby iGull » Wed Jul 18, 2012 7:08 pm

Hi Gav

Just noticed your new setup - looking really good ! Loved the anti- whip support for the ballscrew end machining - looks like my old music stand :D
50um, I wish that was what I had :D

Regarding the snake data to show up backlash, I swapped to 'waves' of half arcs - the backlash showed up instantly - it's amazing how accurate your eyes are.

Anyway, regarding the dsp and 'backlash' (although this isnt realyl backlash). On my system, I had to set the Y axis steps/um different from the X (X was as calculated using the pitch circle diameter of the pulleys - bang on) - to date, I haven't found out why (I did actually swap drive pulleys and drives to see if that was a cause). Both axes now use exactly the same steppers and belts - the belts are even from the same reel.
I did the settings over 500mm on both axes - checked the diagonals were equal too. I also measured multiple square test pieces. All dead nuts on, but the axis steps are different. I began to think like you ,that there was some issue in the pulses from the dsp. I no longer have access to any decent test equipment - a pulse counter would be a good thing to hang on the step outputs during a 45deg diagonal move with the same steps/um on each axis. I also tried a change in step polarity

I don't remember having this issue using the lasersaur board, but I had a lot of backlash at that time caused by those split couplers which may have masked it :D I no longer have the board, I mailed it to a friend for his laser.
Hopefully, you'll get to the bottom of it using mach - look forwrd to yourr results.
Cheers
Neil
EMOs are a sign of weakness ...
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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Wed Jul 18, 2012 7:55 pm

So, are you saying that what you were observing and reporting as backlash may have been uncoordinated motion by the DSP controller?

Stuff like that is why I was recommending a static measurement of backlash rather than inferring backlash from print quality or some other indirect method. True backlash should be relatively easy to measure. If something can be measured directly, that is by far my preference. No point introducing complexity and uncertainty about what you're measuring or observing. That can lead to replacing belts with ball screws when the belts were actually OK. ;)

Of course, hind sight is always 20/20.

My current CNC retrofit project uses LinuxCNC and I've been bumbling along on the edge of effectiveness for too long. I'm finally starting to see some progress. If I knew what I was doing when I started I could have saved at least 90% of the effort so far. That knowledge comes at a high price. Welcome to the bleeding edge of project build craziness. On the plus side, my next two (or more) LinuxCNC projects should be much easier, but you probably aren't planning on building another laser engraver.

Maybe I can learn from you and others on BuildLog.net, so my future laser build goes a bit more smoothly. It'll probably be a LinuxCNC project!
Apparently, I didn't build that! :-)
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Re: Gavztheouch's Mk2 Laser

Postby iGull » Fri Jul 20, 2012 3:31 pm

Liberty4Ever wrote:So, are you saying that what you were observing and reporting as backlash may have been uncoordinated motion by the DSP controller?
Stuff like that is why I was recommending a static measurement of backlash rather than inferring backlash from print quality or some other indirect method. True backlash should be relatively easy to measure. If something can be measured directly, that is by far my preference. No point introducing complexity and uncertainty about what you're measuring or observing. That can lead to replacing belts with ball screws when the belts were actually OK. ;)


This is a slim possibility, but I have no real way of proving it - unless I can get my hands on a pulse counter - I've attached a screendump of my system settings. I'm not entirely sure it's actually the case. I have also cut a 500mm L shape where the line is cut contiguously - effectively nulling any backlash as that would have been taken up in it's initial positioning (I purposely set the directions then nailed the datum) and there have been no reverse directional changes made - the lines were orthogonal and of equal length.
Doing a quick calculation, over 500mm, the step count difference between the two axes is 22328 steps. At the X step size, that equates to just over 1.9mm - which seems quite a lot over 500mm. I did some further testing last night, and my machine cuts dead square and size accurate to within 0.05mm 'ish - over the 150mm my vernier works. Over 150 to 500, it's difficult to gauge accurately - flat ended steel rules aren't really that accurate (dimensionally), but are fine for consistency - my sizes are consistently the same in X & Y. Different materials give differing results of course as expected. I do see an amount of backlash on wee circles (3mm or less) - it's visible, hardly measureable accurately - but I now cheat and set those circles to be cut at a much slower rate which improves matters.
I use an offset of 0.1mm which is fine for any work that I need an accurate fit on (mainly balsa/liteply/acrylic) - this is with a 63.5mm lens.
To be honest, it can all get a bit anal can't it :D At the end of the day, I'm only cutting bits of wood and what I get from the system is perfectly OK. My real feeling is that this is all mechanical imperfection, but I'd like to just count the pulses to give me a nice psychological warm feeling anyway :D :D :D

I apologise if this is hijacking Gav's thread, I'm not intending to - I just don't want to see his ballscrews go to waste LOL :D :D It may all be a factor.

I'm not sure I understand what a 'static backlash' measurement is 'though ? By inference, 'backlash' has to include movement?

Cheers

Neil
Attachments
Screen shot 2012-07-20 at 15.42.48.png
System settings
EMOs are a sign of weakness ...
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Re: Gavztheouch's Mk2 Laser

Postby jv4779 » Fri Jul 20, 2012 9:28 pm

iGull wrote:I'm not sure I understand what a 'static backlash' measurement is 'though ? By inference, 'backlash' has to include movement?


http://www.cncexpo.com/MeasuringBacklash.aspx shows the procedure. In summary, move into a dial indicator, zero the indicator, then move off. The difference between what the computer thinks it moved and the reading on the dial is the backlash.

This is obviously easier to do on a cnc machine that has jog or mdi support.
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Sat Jul 21, 2012 11:33 am

I heard someone using the term dynamic backlash to included things like belt stretch and other variables related to the acceleration/de-acceleration of the gantry. So I assumed "static" backlash could be used to described just the mechanical backlash, ie only measuring backlash when the gantry is not moving, but there are multiple movements in-between. Not quite sure if that makes sence, I think I just confused myself. :? :D
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Re: Gavztheouch's Mk2 Laser

Postby Greolt » Sat Jul 21, 2012 9:41 pm

Have you tried your test while setting Step Edge to "Falling"?

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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Sun Jul 22, 2012 4:49 am

I made up the term "static backlash". There is probably a real static backlash, and my made up version is probably different from the real version. ;)

It's true that backlash inherently requires motion, so in that sense, I don't see how there could be static (no motion) backlash.

What I meant by "static backlash" is real, mechanical backlash, the measurement of which was described in the post by jv4779 a couple of posts earlier in this thread... as opposed to what was being described as backlash but seemed to be inferred from laser cutting errors. Some forms of control errors or electrical problems could mimic backlash, and if they were assumed to be backlash when they weren't, a lot of time and money could be wasted trying to fix nonexistent backlash.

I plan on using LinuxCNC when I eventually build a laser, so I'll be able to enter MDI commands or manually jog the axes, so measuring backlash (and entering backlash compensation) should be very easy. Of course, engraving with LinuxCNC requires a multi-step image-to-Gcode conversion. I do a lot of engraving, but it tends to be the same engraving over and over again, as opposed to the custom work that a sign shop, trophy shop, or electrical machinery label maker would do where every job is different. The tedious but infrequent conversion for engraving won't bother me too much and I can just run the same G code each time I make new parts.
Apparently, I didn't build that! :-)
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opinions needed on backlash : Youtube Video

Postby gavztheouch » Wed Jul 25, 2012 6:31 pm

Hi, today I made a video of myself testing the backlash in my y-axis.

The first few movements are 1mm, then I move on to increments of 0.1mm then 0.05mm and finally 0.01mm

For each different increment value I first make multiple movements in one direction then reverse the direction for a similer number of movements.

I reckon there is about 0.03mm backlash, what do you guys think.

Im using a 10mm travel dial test with 0.01mm increments on the dial

Cheers

Gav

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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Wed Jul 25, 2012 8:50 pm

I may have found the answer to my "dynamic backlash issue", I think it is down to the servo motor tuning. I have them way to spongy.

I didn't relise this was possible but you can hook the panasonic servo drives up to a pc via a RS232 cable while the dsp is running and plot a grapth of the postion error on the screen. It shows the motors losing postion at the change of direction, I think this is due to the motor not being tuned so they are stiff enough to resist the momentum of the gantry.
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Re: Gavztheouch's Mk2 Laser

Postby Techgraphix » Wed Jul 25, 2012 8:58 pm

So all the ballscrew-effort wasn't neccesary at all???? :oops:

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