# Wait... HOW is Proofgrade measured?!

In all the years I’ve owned a Glowforge laser cutter, I’ve never needed to measure my Proofgrade… until now. Building a project with my Daughter, and knowing Proofgrade materials are 20" x 12" we planned something around that. But, it turns out, the material isn’t 20" x 12"! In fact, it seems to me, it’s not even measured in inches at all! As far as I can tell, it’s exactly 52cm x 30cm (20.47" x 11.81"). Now, the fact that I’m crazy isn’t in question here… but I don’t think I’m wrong. So is this the case? Are the sheets measured in cm? Am I the only one who didn’t know until now?

And then I have to wonder… Does the software actually think in cm natively? Would I be better off, in some way, creating my artwork in cm? I mean, when it comes to precision, and I want 1/8th of an inch, does the software actually calculate 0.3175cm? Or does it round off to 0.318cm? Or, worse, 0.32cm?

Yes… These are among the things that keep me up at night…

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I’ve known they weren’t exactly 20x12" ever since I started using my first stack of cut-down Baltic birch…but I never really thought much about it, I guess since lumber is almost never really the exact dimensions it says it is.

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Seriously… let’s talk about 2 by 4s. The tendency to round up (sometimes way up) in woodworking is maddening.

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In b4 someone explains the “logic” behind dimensional lumber.

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Oh, and in answer to this question, yes. At least in the early days when the forum was mostly people waiting for their machines and the handful who had gotten them picking apart the software.

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It probably thinks in steps, or fractional steps. Stepper motors rotate in discrete amounts, and that is turned into translational distance by virtue of gearing ratios (or in this case pulley ratios since we use belts).

My guess was that they started with desired travel per step and backed into motor specs and pulley size, with certain maximum torque values that could not be exceeded as dictated by head mass, belt strength and elasticity, etc. Once they knew the motor size and fan power requirements, they probably backed into the power supply requirements, and so on.

A lot of this stuff was probably worked out well before Glowforge got there, there should have been no reason to do much research into motor size and whatnot, it probably came down to equations to balance the system. I’ve always thought the hardware portion was probably simple compared to the software.

So yeah. Glowforge software sees a 1” square and at some point has to translate it into steps. It’s always going to be an approximation to the nearest step. Given that they advertise accuracy to 0.001” you might assume that the designed travel per step is 1/1000” but only they could say for sure.

As for designed vs actual travel per step that’s tougher because it comes down to tolerances of the parts. It would make sense that one part of calibration is to actually measure the true travel per step for that machine and then setup a constant to be applied when deriving the actual cut instructions.

TLDR I doubt it “thinks“ in any real world measurement.

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Just as SVG is not defined in cm or inches… might as well be nano-snoots.

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All the internal state and communication between the UI and cloud service is (was?) in metric and it was only translated to inches on screen. This was true, ironically, when there was no metric option in the UI.

Obviously at the machine level it’s all in motor steps.

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Yes, you would. That doesn’t only apply to creating artwork, but to every situation you’re likely to encounter

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Oh, Europeans

Now if you’ll excuse me, I am going to have 99/50000ths of a hogshead of water and cut some materials to 1 and 3/50ths cubits wide.

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Interesting. I pointed that out back in the pre-release days. No one seemed to care then.

Proofgrade does have it advantages, but as of late lack of a availability, inconsistent sizes and in some cases colors (transparent acrylics) has caused me to move on to other vendors. I buy a lot of it, get them oversized and cut everything down myself.

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Mostly, still don’t, because the precise size of the sheet doesn’t matter. As long as it’s a bit bigger than the cut area and fits in the tray, who cares? Those few square inches of material that you “lose” aren’t worth chasing down, economically speaking.

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