Width and Height difference

[andrewmchaffie]

I am fairly new to laser cutting [had my GF only a couple of months now]. Before I open a support ticket, I had a question about width vs height.

Is there usually a difference between the width and the height in a cut. What I mean is that I made a circle in Illustrator that is 1.5" by 1.5", saved as pdf, cut the file… and the resulting disk’s width [according to my digital callipers] varies from 1.5090" to 1.4965". OK… what’s a 1/64th [or less] between friends… but now that I am trying to have disks rotate in cut circles, the disks can not fully rotate because of the slightly ellipsoid shape.

I noticed this on even my first cuts, but wasn’t thinking that I’d need to worry about less than a 1/64th. I have checked that there is no noticeable dust stuff on the rails or belts. Before I contact support, is this difference normal? How close should the two measurements be?

Thanks
Andrew

[eflyguy]

There’s a good discussion here.

Small variation between x and y cuts, so how do I calculate kerf?

What is an acceptable cut variation between x and y coordinates? For example, I keep cutting squares, but they come out as rectangles. I spent the last hour or so reading forum posts with similar problems, but I just needed to know if my results are normal. Usually there’s a variation of 0.05mm / 0.002" between x and y axis cuts. A 20mm x 20mm square comes out as 19.87mm x 19.92mm (X x Y). I tried rotating the wood piece 90 degrees to change the direction of the grain, and the results were simil…

In-general, it’s accurate to about 1/1000" in both directions. 1/64" is not typical.

[gewubs]

I believe that the laser arm is out of square to the Y axis travel. Try to cut a square and see if the corners are 90 degrees. Out of square arm will distort the circle.

[rbtdanforth]

I would investigate your design software as a difference in the scaling at the last digit could make that difference.

[gewubs]

BTW, 1/64 th of an inch is a lot when you are rotating circles within circles. The clearance from loosest point to tightest point is going to be 1/32" In machinery that is so far out of round it would be a complete show stopper.

[andrewmchaffie]

@gewubs

I believe that the laser arm is out of square to the Y axis travel. Try to cut a square and see if the corners are 90 degrees. Out of square arm will distort the circle.

Thank you – and Aha. Indeed a square is not square. I also thought about rbtdanforth’s point about the design software idea… So I cut a GF Software created square of 1*1 – and the resulting cut must be trying to compensate for the kerf, but it is also not square – it is 1.0055" by 1.0010". I will sort through some of the Support, try their “Perfect Squares - Squaring the Laser Arm” as my next stages.

Andrew

[gewubs]

Could be it. Cast your mind back to high school math class and graphing. I am sorry if you hated it; maybe you loved it? Anyway, when your Glowforge cuts a circle it is graphing it by X & Y coordinates. Now, distort your graph paper so that Y xis is still vertical and move the X axis out of perpendicularity with Y. Now graph a circle. The points will be numerically correct, but not geometrically correct. They will fall in the wrong place, and the circle will not be round. Circles should be round within the limits of the machine’s resolution, which is very good. I would reasonably expect within .005" of true.

Bear in mind that the circle is actually a multi-sided polygon as produced by the Glowforge. It does mot actually interpolate a true circle, but it deviates from true only a very small amount with the faceting.

[andrewmchaffie]

Hi all
Thanks for your help – I managed to borrow one of the big squares shown in the support "Perfect Squares - Squaring the Laser Arm” page. And now the GF is cutting to within 0.005 to 0.007 off of totally square and circles within circles are rotating. Yeah! Thanks again.