Jigs / absolute positioning

@beerfaced, this isn’t my experience at all. I always do set height or focus and the extents are considerably off. Here’s an example where I haven’t lifted the lid. I’ve seen other people mention how inaccurate the de-warping is as well. I assumed everyone experienced and worked around the visual drift.

Are you really able to get the four corners of your design to show near-perfectly aligned with your cuts? Maybe there is a tolerance problem on my machine somewhere causing this. I have been caught by several mm drift in either placing new parts on old material or trying to re-hit existing parts.

@Jules - thanks for this, and yes, this technique works… but only if you have not removed the material from the machine or the previously cut design from the GFUI. I’m talking about doing a second round of parts, essentially with a “clean sheet”, or trying to accurately place artwork on an object or sheet that’s been previously cut with higher accuracy.

I think one thing I’m going to start doing is putting all of my artwork on 12x20 templates so that I at least have that as a reference point. The issue with that of course is that I routinely will move parts around to work around hold-down pins or an imperfection in the wood, or to leave added scrap on a design I didn’t gang very well.

@evansd2 - Thanks for the boot reference, I hadn’t come across that. Stabilizing the tray is a huge part of the equation here. I also decided to try using magnets rather than pins so that I can get the material and parts square using two edges of the CT as a reference.

Something you can try, but not ideal if you’ve taken your material up…i run a score of what i’m needing to add using a piece of paper-goes pretty fast. Then i place my material on top of that paper and align that way. It’s more accurate than guessing and hoping:-)


I think there may be a couple of different concepts being interchanged. The short answer is you can’t always trust what you see in the browser, but you can always trust a placement jig.

Focal height, set focus - the laser beam is an hourglass shape with the narrowest point being where the most power is concentrated. For engraves, how far above or below the surface of the material the focal point is set has a detrimental or beneficial effect depending on personal taste and some other technical factors. For cuts, it’s more about how wide the kerf will be and how much charring there will be. Changing the focal height, using set focus, also adjusts the dewarping of the lid camera and makes the view displayed in your browser change. It does not affect where the laser head travels.

Alignment - all about placement of your file on your material. Some machines are better than others, but the lid camera is probably never going to provide pixel perfect placement unless it’s coincidence. For that you need to jig. Using set focus or an accurate manual height entry will help, but perfect is unlikely.

For something you cutout you can always make a placement jig. If you have cut something out and then notice you forgot to engrave/score/cut something place a piece of cardboard in your machine. Cut the object from the cardboard. DON’T do any interior cutting/scoring/engraving - it’s a waste of time and cardboard is too flammable for a bunch of lasing. Remove the cutout piece of cardboard, without moving the larger piece of cardboard. Place the piece you forgot to do something on into the hole in the cardboard. Set focus on the piece and then do what you forgot to do. The image in your browser will show that the alignment is off - ignore it and use the force Luke. I have pretty good alignment on my machine for the initial file placement. After that, all subsequent placements are shifted a bit to the right. The placement on the material, however, is always perfect.

If you do not find this to be true one of three things is happening:

  1. I have not properly communicated how to do a placement jig.
  2. The artboard in your file is not 12"x20" (not sure if this still matters, but it did a couple years ago and it’s just habit for me.)
  3. There is something wrong with your machine.

Why will focal height not change if I add height to the tray? You’re saying that I enter .125" for material height whether i have thick double sided tape on the tray, no tape, or no tray? If the machine relies purely on autofocus to determine height then why am I entering material height for each job and each step?

It uses that entered material height to adjust the view from the lid camera. It needs to know how far away from the camera the material is to de-fish the image properly. It is FAR more accurate to use set focus instead of entering the material thickness manually. Use set focus and allow it to adjust the camera view then place your artwork. If you have performed the camera calibration and have your unit sitting flat or shimmed so that the lid closes properly and fully without touching on either side the placement of the artwork to where it actually cuts should within sub-millimeter.

I’d change that to read that it is potentially more accurate. It’s possible to measure your materials quite precisely.

What the set focus does better is ensures that the specific spot you focused on is as accurately represented by your lid camera as it can be.

Jigs will always be the most accurate way to position materials. The lid camera is at best a good approximation – sometimes it’s accurate enough for your needs, but if you need positioning accuracy to the sub-kerf level, jigs win hands down.

I think we’re talking about different things here. Based on what I’ve seen, the autofocus / broad material height setting is for the lid camera - it just sets the parameter for the de-warping algorithm to display your material and help you align your artwork. This in my experience, no matter what you do is insufficient when trying to deal with extents.

The material height in the per-part settings (cut/engrave/score) sets the laser focus for the operation.

I believe if I change the height of (with tape for example) or remove the tray, I will have to compensate for the focus height. This isn’t a huge ordeal, but I can’t believe that adding some beefy double-sided tape below the tray is not going to affect the laser focus.

Am I mistaken?

True. It also sets material height for cutting.

It does, but it works as an override. If you leave it blank it’ll follow the material height setting or the set focus value.

Yes you will, for both cuts and also for the camera algorithm.

Set focus would neatly sidestep this, as it will measure the height of whatever you put in there. But then so will using the boots from before with no changes to tray height.

Tape is a poor solution for a few reasons. Try the boots, your tray wiggle will be gone. Jigs will still be your best solution for extreme placement accuracy.

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A) you don’t need to.
B) entering a manual focus height different than the set focus is most commonly done to achieve a defocus effect. Smoothing out an acrylic engrave, getting a fatter score line or better cutting through a thick material with a second/third pass for example

I have found that the thickness of the crumb trays is not exactly the same from GF to GF. My local library has one and it is 1/16" less tall than mine. If I put material that is exactly 0.125 inches thick measured with digital calipers on my bed it will be 0.0625 inches closer to the lens that if I put the exact same piece of material on my library machine. That’s why I believe using set focus is a much better tool than using digital calipers.

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Each machine is calibrated at the factory so I suspect that that discrepancy is already accounted for. Also, 1/16" sounds pretty extreme, it actually sounds like they didn’t have the tray in the divots or that there was some other mistake in measuring.

Almost everyone’s tray height (tray, not lip) is right around 1.35.

How to properly measure tray height:

I’d be curious whether applying that technique to both machines – being very sure that the tray is properly seated – would yield 0.0625" difference in measured height.

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We measured the trays out of the machine sitting on a flat marble table with a machinists level from front/rear and using digital calipers from the machined surface on the level to the table.

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I have no idea what any of that means lol. You sound pretty detail oriented though so I am going to assume that was accurate. If I found my tray was a full 16th off from “normal” I would have asked support about it and pushed for a replacement. That sounds like a defect.

The thing is: Is mine the correct one and the library one faulty or is it the other way around? Or is that much variance normal among the different batches of trays that have been produced. To what specifications are they manufactured to. I have always though that there could be differences in heights of crumb trays and when I found that the local library had one I took it upon myself to measure them and did find a difference.

One way to be sure, ask support.

I would think for a machine that is accurate to the 0.001" range that tolerances on things like this would be far lower than 0.0625".

the boots are critical if you have a pro and you are moving material through the machine.

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having done QC in a machine shop, I can say that is a damn accurate way to make those measurements.

ah the good old days. I can smell the coolant now…

I used to work for these guys.

those cutters are amazing.

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