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Author Topic: Backlash strangeness  (Read 4618 times)

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Offline Ansen

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Backlash strangeness
« on: December 05, 2012, 10:25:50 PM »
I am pecking a fairly demanding pattern in plexiglas and I was hoping that backlash compensation would fix a subtle, but noticable banding pattern in my dots.

Attached is a photo showing (top) backlash OFF and (bottom) backlash ON. There are some big gaps (0.25 in) in the pattern as well as some completely missing holes. I filled the holes with ink to make them show up better.

Can anyone explain this? My backlash settings are X=.009, Y=0.0065, Z=0, shuttle speed=0.005.

Also, the drilling pattern starts on the left and moves to the right in 1 inch vertical strips. Notice how the patttern is about 1/2 inch longer with backlash compensation turned on due to the skipping.

Thanks
« Last Edit: December 05, 2012, 10:36:24 PM by Ansen »

Online Tweakie.CNC

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Re: Backlash strangeness
« Reply #1 on: December 06, 2012, 02:07:08 AM »
Hi Ansen,

I am most interested in this but cannot quite see exactly what you mean by 'banding'.

I notice variations in 'shade' in the top example but it appears that not all your dots have been filled with ink.

Presumably you are producing the pattern bi-directionally and that is why you think backlash comp may help but I would like to hear more details of the problem you are seeing without backlash comp being used.

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

Offline RICH

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Re: Backlash strangeness
« Reply #2 on: December 06, 2012, 06:40:04 AM »
The backlash settings may be wrong or actualy varying some. By varying i am meaning that the force to remove the backlash
varies. More likely to be indicative of play in the thrust bearing.

RICH

Offline Ansen

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Re: Backlash strangeness
« Reply #3 on: December 06, 2012, 09:59:37 AM »
Here is the real problem that I was trying to address using backlash compensation. The small "rivulets" correspond to the algorithm used in the drilling order, but they do not appear in the original design. They only show up when small holes are relatively close together. I know, pretty minor, but they do show up disturbingly well at a distance.

My theory was that the X axis was doing a lot of back and forth as it travels up the Y to complete a block of holes. Backlash would "short" the distance to the next hole, creating a predictatable gap between the blocks of holes.

Online Tweakie.CNC

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Re: Backlash strangeness
« Reply #4 on: December 06, 2012, 10:39:33 AM »
I understand what you are seeing now.

How is the image created ??  ie. from a horizontal axis travel across the whole picture, one line at a time or some other method.

Tweakie.

Addition;

Just read your first post again and assume that you are doing it horizontally in 1" wide vertical stripes. Although it is a very small amount backlash could well be the issue here.
« Last Edit: December 06, 2012, 10:46:59 AM by Tweakie.CNC »
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Ansen

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Re: Backlash strangeness
« Reply #5 on: December 06, 2012, 11:17:02 AM »
Imagine the whole image broken up into 1 in. squares. The holes are then drilled starting from bottom to top in each square. Hoes are sorted by Y, so the X does a lot of traveling back and forth, but only within that 1 in. box. Then it moves to the next box.

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Re: Backlash strangeness
« Reply #6 on: December 06, 2012, 11:36:30 AM »
OK, I understand how it was done.

If the bottom left hand dot for each box was always approached in the same direction then any backlash would have been taken up in the same direction so would it really be the cause of the problem - I am not sure.

I know it does not solve this issue and I am not sure if it will be of any interest to you but there are alternative methods of producing images from dots or drill holes which produce the image one complete horizontal line at a time which would not be affected by a small amount of backlash which occurs with bi-directional working.

Although I produced this image using a laser it is essentially just a series of small holes or dots which could just as easily have been produced with a drill (although it may take considerably longer to complete due the Z axis movements).

Tweakie.

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

Offline Ansen

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Re: Backlash strangeness
« Reply #7 on: December 06, 2012, 01:51:56 PM »
That image looks great.

I'm really more interested in the sculptural quality of the image. I create four different layers in acrylic and paint each layers' holes in a different color--typically cyan, magenta, yellow and black. These are the four colors used in almost all color printing.

Regardless of my original problem, so far the cure has been worse than the disease! The gaps in the original test image are completly software generated. All I did was turn backlash comp on. This pattern of holes is totally repeatable.

I obviously have some kind of hardware issue with 0.009 of backlash, but that doesn't explain why I would get giant 1/4" gaps in the output.

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Re: Backlash strangeness
« Reply #8 on: December 07, 2012, 03:30:51 AM »
Basically, as I see it, you have 2 separate issues here ‘Banding’ and ‘Backlash comp’.

I have never used backlash comp so that issue is outside my comfort zone.

If you would care to Zip and email me a GCode sample file (preferably with approx. equal sized dots) which exhibits the banding issue I would be happy to run it on my machine and post the result. This should (with luck) determine if the banding is machine or software related.

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

Offline RICH

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Re: Backlash strangeness
« Reply #9 on: December 07, 2012, 06:13:59 AM »
Ansen,
Post the file first so it can be checked. Don't think the 1/4" gaps are due to backlash.
RICH