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Author Topic: Pocket widths different on x and y axis  (Read 1156 times)

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Pocket widths different on x and y axis
« on: August 01, 2017, 02:07:48 PM »
Hi all,

I have a strange one for you. Personally, I'm at my wits end. Let me preface this by saying I have a home built Greenbull Clone (it's similar to the Greenbull by BuildYourCNC.com) and it cuts 4' x 8' sheets of MDF on a regular basis. It runs on roller chain on both the x and y-axis, and with a screw on the z-axis.

Here's an image that hopefully helps to illustrate my problem:



In this image, the square outside dimensions are 15" x 15". Each of the pockets inside have a width of 0.66", to allow me to fit 15mm MDF snugly. Now, here's where I have an issue. When I cut the one on the right (horizontally aligned in the image), it cuts fine. The gantry travels along the x-axis for the length, and the width of the pocket is enough to support the snug insertion of the MDF. When I cut the one on the left, however (vertically aligned in the image), the width of the pocket is different. It is so that the MDF will not fit into the pocket, and the edge of the wood actually 'bumps' into the edge of the pocket. I physically have to sand the piece to be inserted, or else, risk damaging the pocket or the wood.

Now, here's where I am having trouble understanding what may be happening. I thought, "oh, well, the x-axis may not be properly configured, and that's why all of my pockets seem to be tighter", and that's when I modified the motor stepping settings. I cut and recut the same piece using different settings, until such time as the tight pockets were the same width of the proper ones. Here's the problem. When I finally measured everything out, the 15" x 15" square was now 15 1/8" on the x-axis side. Not good.

So, my question to the experts here is, what could this be? Could this be backlash, due to the longer chain drive on the x-axis? If so, can backlash affect internal (pocket) cuts and external cuts differently? The chain drive on both has very little play in it, so I think I've firmed it up as much as it can be, so if I assign settings in the Mach3 backlash settings, may this also affect the dimensions of other cuts, outside of the pockets?

I hope I've explained this properly. As I mentioned, I'm at my wits end. I really don't want to go into every design I've created, and manually modify the pockets on the x-axis to be 'wider' than the y-axis cuts. (Especially considering I regularly rotate parts on a sheet, to fit other cuts, and this would cause significant issue.)

I really, really hope someone has seen this before, or has some advice on how I may rectify this issue. I appreciate any input!

Thank you!

Rick
Re: Pocket widths different on x and y axis
« Reply #1 on: August 01, 2017, 02:40:04 PM »
It sounds kind of like a backlash issue, have you checked it? If so have you set backlash comp? If you cut a circle is it round?

I'd avoid compensating in your drafting, it's sets a long precedent of adjusting models instead of addressing the real issue up front.
Re: Pocket widths different on x and y axis
« Reply #2 on: August 01, 2017, 02:53:22 PM »
It sounds kind of like a backlash issue, have you checked it? If so have you set backlash comp? If you cut a circle is it round?

I regularly cut 1 1/8" circles through, however, I don't recall having much issue with ovals. (Usually, when this happens, I notice because the bit needs to be replaced as MDF likes to eat bits somewhat.) I have not yet set any backlash, but did find an article (here: http://www.cnccookbook.com/CCBacklash1.htm) which I'm reading through, as well as this video (here: https://www.youtube.com/watch?v=A27nOkTFYDY) which also seems helpful.

I am interested in the best way to test, and then, to assign the settings in Mach3, if warranted.

I'd avoid compensating in your drafting, it's sets a long precedent of adjusting models instead of addressing the real issue up front.

Agreed. I've opened and closed these files so many times already, I'm happy to just leave them be. LOL.

Offline RICH

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Re: Pocket widths different on x and y axis
« Reply #3 on: August 02, 2017, 07:47:13 AM »
Suggest the following:

1. Calibrate each axis so the steps per unit are correct. The calibration moves are always with backlash removed.
    Consider the following:
    - based on the longest travel
    - check calibrated steps in different sections of the bed. Idealy doing a profile check of each axis  ( both directions )
       will show the variations of the axis. Profiling gives you data for how accurate YOUR system's positional  accuracy is.
2. Check backlash for each axis. Again it can vary, so check along different spots of the axis. Idealy backlash should be minimised.
    You can live with it, use backlash compensation, or make modifications to get rid of it.

Spend the time doing the above first, even though it takes time.

3. Find max velocity and acceleration settings for the machine, reduce max to provide headroom for actual machining.
 
Now go on to step 4.

4.( No use rambling on if you don't do 1,2,3 first !)

RICH