fine tuning - x-axis resolution - HELP!!

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25 Oct 2011 10:53 #14203 by Rick G
If you have not already perhaps a simple test will help.

Move the gantry with emc to some location and leave the power applied.

Set up your dial indicator.

Try to push / pull the gantry by hand with a reasonable force and look for flex/play/movement.

Try in a few locations.

Then

With power off double check for binding.

Rick G

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25 Oct 2011 11:06 #14205 by whampoo
Thanks for the information I will try that tonight.
Where would I find the images of the plasma table?
I may have to gear down the x-axis and a few images may help with the new setup.

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25 Oct 2011 11:45 #14208 by BigJohnT
A couple of photos are here

gnipsel.com/shop/plasma/plasma-gallery.xhtml

and some more are here

s47.photobucket.com/albums/f163/johnplctech/Plasma%20Cutter/

darn I do need to finish the photo gallery of the plasma on my web site...

John

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25 Oct 2011 14:08 #14219 by andypugh
If I was modifying your axis to gear it down (and I think that would be better than going to smaller sprockets, as you have very few engaged tooth with a 14 tooth, so low stiffness/high wear.) then I think I would move the motor to one side, and mount it off the rail.
Move the plummer-blocks to the outboard flange, to reduce the bending moment on the shafts.
Mount a large-ish sprocket on the shaft inboard of the plummer block. Then mount the motor so that a smaller sprocket on the motor shaft can drive that sprocket with a short belt.
This does require you to make a new cross-shaft. I think I would use tube of somewhat larger diameter than you have.
No, who am I kidding, I would use whatever is propped up in the corner of the garage, and is long enough :-)

However, this isn't getting to the root of the problem. Something very odd is happening if large moves are accurate, but small moves are not.

If large moves are accurate then that indicates that the machine is capable of being accurate. The question then is about what is different between long moves and short moves.

Bear in mind that 0.001" move at a scale of 666 is not a full step, so the motor won't move. However 0.01 should give you 6 microsteps. it is actually possible that 7 microsteps might not actually move a stiff/heavy axis. You need to get up to a full step to be positive of a movement.

I still want to hear what the result is of slowing things down a lot. Your machine really _ought_ to work.

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25 Oct 2011 14:18 #14226 by BigJohnT
Speaking of tubes I had to replace my solid shaft between the Y belts on the plasma with a tube due to torsional flexing on the rod causing the far side to lag behind the side with the motor. And we are talking a very light gantry made from 80-20 extrusion. The tube was considerably lighter than the solid shaft and very rigid.

John

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26 Oct 2011 05:58 - 26 Oct 2011 06:01 #14268 by whampoo
Hi all,
I haven't had a chance to mess with the ini file yet I got sidetracked making salsa tonight. Any chili pepper heads out there ?? I grew some ghost chilies and Ecuadorian devils breath they make great salsa, won a contest last week at work and now they want more. The boss says its the last batch tho she didn't like the burning eyes, nose and throat. :laugh: I am attaching another pic to show the reason for the timing gears being so far from the pillow blocks. Its a design flaw... I might be able to put the timing belts on the inside of the gantry legs .... possibly? This is an older pic I had the motor on the rails at first but there was too much movement in the motor mount so I cut the shaft in half and put the motor in the middle. and the table has been leveled since this pic. It looks nice tho. :)
Does anyone have a good source for timing gears ? I am looking to make the 5:1 ration suggested, as it makes sense that the more steps I can get out of the motor before there is movement in the gantry the better the resolution.

Thanks again I will keep you posted on the ini file changes and let you know if that works.
Attachments:
Last edit: 26 Oct 2011 06:01 by whampoo. Reason: pic didn't upload

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26 Oct 2011 08:08 #14270 by Rick G
You might try here...
www.mcmaster.com/
Good catalog, good info and fast shipping.

It is curious that large moves are consistently good and small moves are poor. This seems more mechanical than computer. However as Andy said it is a good idea try reducing the acceleration and velocity to see what is going on. Try different moves at different speeds. Look for flex, play etc. both unloaded and loaded.

The machine looks good, lots of work to get this far, now for some fine tunning.

Rick G

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26 Oct 2011 08:18 #14272 by Rick G
The gantry does appear to have a lot of mass and will probably benefit from a long acceleration ramp up to speed especially at the scale you have.

Rick G

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26 Oct 2011 11:56 #14284 by BigJohnT
Just looking at the first photo again I would add a short piece of angle to the outside and mount the pillow block bearings out there.

John

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27 Oct 2011 06:00 - 27 Oct 2011 06:01 #14317 by whampoo
I tried slowing down the x-axis by a multiple of 10 and man I have seen the postal workers move faster than that and that is slow .... hope none of you work for the usps .... sorry if you do ... :laugh: ... I have ordered some new timing sprockets on 10 teeth X1/4" for the stepper and one 48 teeth with a 1/2" bore for the shaft... found a robot website that had the formula to figure the timing belt length so I am hoping I didn't mess that up too much. I am having them shipped out 2nd day ups and hopefully they will be here before Saturday. The company is only a few states away so I might get lucky and receive them on Friday. I suppose I will be spending much of the day doing math figuring the new inches per step since there are now 3 different size sprockets. but that is the least of my worries. With the new setup I should be able to get a much finer resolution. I am sure I will be picking your brains again shortly.

Thanks again for all the help and input. With all the information and help you have provided you have made my choice to use EMC2 as my controlling program a fantastic experience. When I first started working on the software part of the CNC machine it was a bit overwhelming, I am much more familiar with some of it, but still have a long way to go, this is all new to me ... all I wanted to do was make a router tracing machine... hmmm ... I got carried away.... but it's a blast.

Even tho I have been having issues with the small movements I was still able to kick out a few projects that didn't have tight tolerances ... the fonts were not as round as I thought when I designed this but I still think it came out just fine, no one will know the difference.
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Last edit: 27 Oct 2011 06:01 by whampoo. Reason: quick fix....

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