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Discussion Starter · #5 · (Edited)
What is that you're cutting, Richard? Foam? Where did you set your XY zero on the work piece (how did you locate that point)? It looks like the actual size of the pocket is correct, though.

David
It's Ytong, ran out of mdf and have to go downtown for it.
Dust is nasty and all over. :bad:
I set my X0Y0 on ytong bottom left corner. Yeah, size of the pocket is 99.9% correct.

I think this is because i updated grbl to v1.1h and flashed back to stock version (0.9j.20180122).
Honestly? I have no idea what i was doing. :nerd:
 

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David
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It's Ytong, ran out of mdf and have to go downtown for it.
Dust is nasty and all over. :bad:
I set my X0Y0 on ytong bottom left corner. Yeah, size of the pocket is 99.9% correct.

I think this is because i updated grbl to v1.1h and flashed back to stock version (0.9j.20180122).
Honestly? I have no idea what i was doing. :nerd:
Have you checked your drives for backlash, loose couplings, etc.? Although, with the pocket sizes being accurate I doubt this is the issue.

If you home your machine to the XY zero you set and issue a manual command to move 1" or 5" (any distance, really), for instance, does it move that prescribed distance on both axes?

David
 

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Also check your spindle settings. I had a case where the x was off about 1/4 inch to the right. My usual spindle speed was 18000 and when I saved the toolpaths I left the 1 off the speed. Redid all the toolpath settings since I wanted to be sure they were all right and everything was fine.
 

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Does the machine software have a feature to run a calibrations test on your axis's, ie. "Steps per Unit"? Not being calibrated properly can effect the size of the finished cut.
 
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Mike
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Pockets look good but that is just 10mm so might be slightly off.

Where is the CNC homed to, Did you zero on the corner of the foam or did you home the machine and then run the toolpath, that may be where the difference in start point comes in.

You could also set a point at your start point in your design file to make a shallow plunge at that home point, then start in a good solid part of your material, run the file and see if the measurements from the center of the plunge to your pocket is correct or still off. That way you are not depending on trying to zero on a less than desirable spot like the corner of the sponge.

Also if you are trying to calibrate your machine you will want to use the longest cuts possible so you have greater accuracy.
 

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You need to calibrate the steps per mm for each axis. ($100-102). GRBL updates don't normally wipe out previous settings but it's possible.

First thing to do is get a good ruler. Then jog the machine in X a fairly far distance (200mm is good) and measure how far it actually went. The use this formula to change the GRBL steps per mm setting for the X axis:
new_steps_mm = old_step_mm * (actual_distance/jog_distance)​
set the new number in GRBL and verify it actually is update. Do that for Y and Z.

Then repeat the process until you get no changes. You can use calipers for a bit more accuracy but use the longest possible distance they can measure. 150mm is about the smallest I would use. It is important to use long distances to reduce the effect of measurement errors.
 

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Discussion Starter · #13 ·
Images are the results of running G91 G21 X100/X200/Y100/Y150

Workspace = 300mm x 180mm

$$
$$ < $0=10 (step pulse, usec)
$1=25 (step idle delay, msec)
$2=0 (step port invert mask:00000000)
$3=0 (dir port invert mask:00000000)
$4=0 (step enable invert, bool)
$5=0 (limit pins invert, bool)
$6=0 (probe pin invert, bool)
$10=3 (status report mask:00000011)
$11=0.010 (junction deviation, mm)
$12=0.002 (arc tolerance, mm)
$13=0 (report inches, bool)
$20=0 (soft limits, bool)
$21=0 (hard limits, bool)
$22=1 (homing cycle, bool)
$23=0 (homing dir invert mask:00000000)
$24=25.000 (homing feed, mm/min)
$25=500.000 (homing seek, mm/min)
$26=250 (homing debounce, msec)
$27=1.000 (homing pull-off, mm)
$100=400.000 (x, step/mm)
$101=400.000 (y, step/mm)
$102=400.000 (z, step/mm)
$110=1500.000 (x max rate, mm/min)
$111=1500.000 (y max rate, mm/min)
$112=1500.000 (z max rate, mm/min)
$120=10.000 (x accel, mm/sec^2)
$121=10.000 (y accel, mm/sec^2)
$122=10.000 (z accel, mm/sec^2)
$130=200.000 (x max travel, mm)
$131=200.000 (y max travel, mm)
$132=200.000 (z max travel, mm)
 

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Discussion Starter · #15 ·
Pockets look good but that is just 10mm so might be slightly off.

Where is the CNC homed to, Did you zero on the corner of the foam or did you home the machine and then run the toolpath, that may be where the difference in start point comes in.

You could also set a point at your start point in your design file to make a shallow plunge at that home point, then start in a good solid part of your material, run the file and see if the measurements from the center of the plunge to your pocket is correct or still off. That way you are not depending on trying to zero on a less than desirable spot like the corner of the sponge.

Also if you are trying to calibrate your machine you will want to use the longest cuts possible so you have greater accuracy.
Hey Mike, I did zero on the corner of my work piece. I never use home and default setting is off.
 

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Discussion Starter · #16 ·
Have you checked your drives for backlash, loose couplings, etc.? Although, with the pocket sizes being accurate I doubt this is the issue.

If you home your machine to the XY zero you set and issue a manual command to move 1" or 5" (any distance, really), for instance, does it move that prescribed distance on both axes?

David
Hey David, i checked them and they're good to go.

I never use home and It's off by default.
 

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OK, it's properly calibrated. Though, to be honest, I'm surprised that it's exactly 400 stps/mm. Those little genmitsus and other chinese machines are notorious for being inexact.

OK, wrapping my head around this one a little better. I wonder what you are calling 0,0. Are you taking into account size of the cutter, aka tool offset? 0,0 is where the center axis of the cutter is when you reset x and y. If you are going to mill into a precut piece, to position your holes precisely, you will need to have a zeroing process that takes tool offsets into account. Or, you could simply use an oversized blank and cut the outline too.
 

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Mike
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I think Phil hit the nail on the head.

Looks like you are zeroing the side of the bit on the inside edges of the material at the corner. You should be zeroing the center of the bit on the corner.
 

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By the way, I spoke a bit too soon. It looks like your calibration is a bit off. Your 200 mm jog looks to actually be around 199.5. Your 150mm jog looks to be slightly worse, around 149.5. So 401 steps/mm and 401.3 steps/mm respectively.

While these aren't horrendous errors, they can make a difference in getting a good fit with parts not made on your router.
 

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David
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And if you really want to be certain you're setting zero off the center of the bit then put a V-bit in and use the point. Then you can change to the bit you'll cut with and know that you're as close as it's going to get for XY zero.

David
 
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