Author Topic: Slightly odd behavior when cutting triangles  (Read 1734 times)

ggallant571

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Slightly odd behavior when cutting triangles
« on: September 07, 2016, 09:09:20 PM »
Attached is a small gcode file that cuts two triangles with x & y set at 20mm. The difference is the order in which the cuts are taken. On my machine they are different. There appears to be a 1mm error.

Test was performed with GrblController and Grbl 09j firmware. Firmware was modified to support 100% pwd at S100 so the cuts are at 20% power (if power is linear).

I would appreciate it if someone else could either duplicate or assert that it is just me.

Thanks
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Re: Slightly odd behavior when cutting triangles
« Reply #1 on: September 07, 2016, 09:13:45 PM »
What software were you using?

Never mind...I see it now...Grbl Controller.

Did you write this gcode?
« Last Edit: September 07, 2016, 09:16:39 PM by Administrator »
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Re: Slightly odd behavior when cutting triangles
« Reply #2 on: September 07, 2016, 09:20:53 PM »
I don't have to actually run this gcode to know what it will do. I can read it and I know exactly what it will do.

The triangles are not the same...by that I mean they will not cut in the same pattern, if that is what you are expecting.

And they certainly will not cut on top of each other, but I'm positive you know that since you wrote the gcode.

Do I understand your question correctly? You are asking why these two triangles do not cut in the same pattern? Or am I mis-understanding your question?

Code: [Select]
M05 S0
G90
G21
G01 F500
G01  X0 Y0
M03 S20
G01 F500
G01 X20 Y0
G01 X0 Y20
G01 X0 Y0
M05 S0
G00 X25 Y0
M03 S20
G01 X25 Y20
G01 X45 Y0
G01 X25 Y0
M05 S0
G1 F500
G1 X0 Y0
« Last Edit: September 07, 2016, 09:24:47 PM by Administrator »
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ggallant571

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Re: Slightly odd behavior when cutting triangles
« Reply #3 on: September 08, 2016, 08:13:30 AM »
They are offset by 25mm to high-lite the problem. Each triangle should have a X & Y that are 20 mm. The diagonals should be the same. The diagonals are different depending on where they start. In the first case, the diagonal ends at 19,1 and has a vertical cut to 19,0 before doing horizontal back to 0,0.

In the second the diagonal is good, it stops the final burn at 1,0.

I have tried this with various speeds, lengths, laser power, wood, paper, and starting position on bed. The finer the cut, the more pronounced the effect. The non-randomness is eerie. Today I will download a prebuilt grbl hex file and try again.



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Re: Slightly odd behavior when cutting triangles
« Reply #4 on: September 08, 2016, 09:23:16 AM »
Did you calibrate/check your $100, $101, $102 Grbl settings? This shortage (in this case) sounds like you didn't calibrate the distance your laser head moves against a known measurement.

Our 'default' values should be 80.00 but may need some small tweaking to get the laser head to move the exact correct distance.

To calibrate simply do the following:

1. Burn a line 20mm long.
2. Measure it.
3. If it is not exactly 20mm long, then you need to adjust your $100, $101 and $102 Grbl values. You have to use trial and error and retest, etc.

(You can use inches but be sure you set your units in gcode).

Adjustments needed will be very small, something like from 80.00 to 80.10 to start with and adjust from there. I can't remember which correction to make for which distance error, you'll just have to test some changes.

Since you know how to write gcode, just write a simple test where you burn a line, come back to X 0, jog down Y-5 to get ready for the next line burn test.

Alternatively, BCL can do this quite easily for you if you wish to use it. See attachments.

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ggallant571

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Re: Slightly odd behavior when cutting triangles
« Reply #5 on: September 08, 2016, 09:35:39 AM »
I understand the calibration issues. I don't understand why the order of the cut matters. It almost as if the firmware is rounding wrong. I await BCL for Linux!!!!
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Re: Slightly odd behavior when cutting triangles
« Reply #6 on: September 08, 2016, 09:40:49 AM »
>I don't understand why the order of the cut matters.

I guess I don't understand what that statement means?

Can you re-state it and dumb it down for me  :)
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ggallant571

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Re: Slightly odd behavior when cutting triangles
« Reply #7 on: September 08, 2016, 09:45:19 AM »
Going around the triangle clockwise (0,0), (0,20), (20,0), (0,0) cuts different from counterclockwise (0,0), (20,0), (0,20), (0,0). From the gcode the points are the same. The plot inside GrblController show the same.
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Re: Slightly odd behavior when cutting triangles
« Reply #8 on: September 08, 2016, 09:55:59 AM »
>Going around the triangle clockwise (0,0), (0,20), (20,0), (0,0)
That is clockwise gcode but its not what you posted originally. Are you just using the above as an example in this specific reply?

>cuts different from counterclockwise (0,0), (20,0), (0,20), (0,0)
This code was in your original posting.

The code at the top and this code immediately above, will cut on top of each other, but going in opposite directions. However, that is different from the code you originally posted. Are you aware of that?
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ggallant571

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Re: Slightly odd behavior when cutting triangles
« Reply #9 on: September 08, 2016, 10:03:04 AM »
That is the problem. They are not exact. My original post offset the two triangles by 25mm for a better picture of the problem.
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Re: Slightly odd behavior when cutting triangles
« Reply #10 on: September 08, 2016, 10:06:56 AM »
Post your entire test gcode so I can see it. I just want to verify it over lays correctly.
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ggallant571

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Re: Slightly odd behavior when cutting triangles
« Reply #11 on: September 08, 2016, 10:24:31 AM »
Did you try the triangle_20.gcode file? With a light burn (chrisp lines) the difference on my machine is very visible.
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ggallant571

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Re: Slightly odd behavior when cutting triangles
« Reply #12 on: September 08, 2016, 10:53:44 AM »
Attached is a version with the 2 overlaid. Some of the differences/problems get blurred by the burning.
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Re: Slightly odd behavior when cutting triangles
« Reply #13 on: September 08, 2016, 11:26:55 AM »
Attached is what I burned and I measured with my digital micrometer.

The bottom burns is obviously the side by side file and the top one is the over lay file.

Every line I measured exactly matched the other one! I measured 3 times, each line.

The overlay was like watching a mirror...2nd lines laying down right inside the first lines.
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ggallant571

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Re: Slightly odd behavior when cutting triangles
« Reply #14 on: September 08, 2016, 12:32:52 PM »
Thanks. I can assure you that is not what I get. Looking for camera but it is among the missing.
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