I just engraved the full circumference of a 2.5" diameter glass tumbler.
Nice!
How many times did you have to stop and rotate it?
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I didn’t stop to rotate it. It rolled along the floor dragged by the fixture. I shared the design for the fixture here: Rotary Engraving a Yoyo on my Glowforge
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Interesting.
Is there an sensitive balance of tape to give traction to roll and not to much to grab the rotation tray?
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Well done! Your hard work pays off.
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Wow. Almost anything is possible with a GF and an imagination. Fabulous!
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And I remember people saying that rotary could never be done on a Glowforge. Never say never. I tip my hat to you, sir.
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sneaky trick hitting the shoulder of the glass. Any thoughts on theoretical maximum usable diameter? 4" would have to be absolute maximum (after that the “shoulder” of the glass would be perpendicular to the laser beam, so no go. And that is ignoring the geometry issues of the head colliding with such a large cylinder)
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Hmm there is also a theoretical maximum mass that will interfere with the stepper motor’s y axis movement. I bet it would take like a 18" long 3" cylinder of solid metal or stone to get there though.
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Yes! I addressed this in the yoyo post, but essentially what seems to work best for me is two strips of masking tape, each 3 layers thick and sticky side down, placed just beyond each end of the object you are engraving. Between those two strips, I place 1 or more single layer strips of masking tape face up, secured at each end with a small piece of tape. The purpose of the three layers face down is to prevent the fixture from sticking to the single layers face up. The sticky layers help the object roll uniformly without sliding or rocking back and forth in the gap.
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2.5" is about the maximum diameter that will allow a focused beam to strike the surface. Larger is possible with a less focused beam. The yoyo in my other post was engraved with a slightly defocused beam.
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So I did some theoretical limit diagrams. The green bar is the laser focusable 1/2" height, the two cylinders are resting on the machine bottom, no tray.
The diagram on the left shows a 2.11 inch diameter, and results in a 45 degree focused laser strike achieved by offsetting the center of the object by 0.75" in the Y direction from the laser path.
On the right, we get a 2.73" diameter offset by 1.22 inches. While it hits focused, the incident angle is right about 160 degrees, which is very oblique. I’m not sure how well that would work, it might be fine, it might not work very well at all.
You could get slightly larger than that, but the 2.5" max seems like a reasonable limit to adhere to, but that limit will depend on the Y-axis offset.
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Did you experiment with other materials? I’m thinking possibly 1mm silicone rubber mat. It would be a little grippy but not sticky grippy.
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This is looking to me like a Collins glass, which typically might have a 2.5in diameter, might work 
I’ve got a lemon tree… suppose I ought to pick up some more Gin, and then get this project started.
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Yes I did try a silicone mat. The problem that I had with it was that it was too much friction for the fixture, causing it to grab and jerk at times. The silicone did improve the tendency of the object to slip rather than roll, but because it wasn’t tacky, it would still bounce back and forth between guide edges producing gaps in the engraving. This may be different for different formulations of silicone mats.
The masking tape has been perfect for my needs because it is thin, giving me maximum space for my large objects, and the tackiness provides just the right amount of tack and clean release to both ensure the object rolls, and stays held against one edge of the guide.
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Nice illustration!
Here is the calculation I made for the glass. It’s 1.863 inches from the bottom surface, with tray removed, to the top of the 1/2" focusable zone. You may notice, in the photos I posted, that I made a new guide tab to offset the fixture 1.089" from the laser beam centered position.
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Innovative and awesome! Thanks for the explanation.
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Wow! Amazing work!
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Here is the file for the 1.08" offset tabs:
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Sorry for the late to the conversation question-but what settings did you use to engrave on glass?