I was watching an advertisement for epilogue lasers the other day and they demonstrated engraving a cup made of glass, but the machine had an accessory that turned the cup so that the spot being engraved was always the same distance from the laser? How does GF handle this kind of idea. Here is the link for the video Im talking about https://www.youtube.com/watch?v=36kdUQXmaA0. Check around 3 minutes in.
The most direct answer to your post is that the Glowforge can not engrave directly onto a typical drinking glass.
The reason is, the maximum material thickness inside a Glowforge is somewhere in the neighborhood of 2 inches. Since most drinking glasses are larger than 2" in all directions, they simply will not fit inside the machine.
There are a few ways a Glowforge could be instrumental to getting an image onto a cup or glass though. The basic idea is that you would create a stencil with your Glowforge, place the stencil on the cup, and apply paint/etchant/whatever over the whole thing. After you peel the stencil your design will be left on the cup. Cups with straight walls (like pint glasses, coffee cups, etc.) will work most easily, but curved cups (eg. wine glasses) are also possible.
There has been a fair amount of talk on the forum about designing a Glowforge-compatible rotary attachment for round objects with diameters larger than ~2 inches. Unfortunately, I cannot recall anyone posting an actual design or prototype yet. Once more people get their hands on Glowforges (or, once people who have Glowforges are allowed to speak more freely) maybe a practical design will surface.
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@karaelena designed a nice little manual rotary indexer that works for an item small enough to fit. (Like you said - under two inches.) 
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Shot glasses anyone?
For normal drinking glasses I plan on going Glowforge my stencil, then sandblast.
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Shoot glasses definitely work but only if you have something to keep the item equidistant andself rotating. I think stencils are probably the only to go for this idea since glow forge has a short cutting depth.
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What material would you recommend in this instance for making a stencil to put on a glass cup? I know vinyl is not recommended to be cut with a Glowforge.
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That is really the big question. You want a stencil material that is easy to apply to the glass after lasering, and ideally does not require tedious weeding… Anything that has to be weeded will also limit your resolution. I’ve never seen a laserable etch masking material like this. Best I have seen is this, but it must be lasered AFTER you stick it to the glass, meaning it is limited flat objects for our GFs.
No reason it couldn’t be out there. There are already UV-sensitive masking products where some of the material becomes weak and is blasted away during sand blasting, while still holding together enough for you to stick the mask to the glass in one piece.
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While I do a fair amount of glass etching, I don’t expect I’ll every use the Glowforge to create stencils for this purpose. Vinyl stencil material is relatively inexpensive, especially when compared to lase-friendly stencil materials.
For me, the best bang for the buck has been a Silhouette Cameo to cut vinyl, apply the stencil to glass, sand blast and remove the stencil.
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Agreed, my Cameo with some decent vinyl from Amazon has been employed in making a few dozen mugs and wine glasses lately, as it’s our go-to gift for weddings, birthdays etc.
I’ll let the GF take care of cutting the super fine stuff and hard materials.
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varibale autofocus should allow for curved surfaces within a given range. Whether it can yet, and what that range is, I’m not sure.
I wish I could recommend something based on experience, but the only glass engraving I’ve done with my (non-Glowforge) laser cutter has been directly onto the glass.
I had seen this stuff…
http://www.johnsonplastics.com/engraving/popular-brands/laserbits/lasertape-12-x-100-laserable-stencil-tape-for-sandblasting
… but I don’t know if I noted the price until now. Pretty expensive! Maybe they have a 6" roll for half the price.
If you’re going to paint or etch the glass, I bet any number of tapes could be used. A quick Google search says that blue and green painters tape is fine for glass etchant.
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And there’s no law that says you can’t drink from a saucer!
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I’m really hoping that one of the eventual head attachments (replacements?) is a drag knife so that we can use vinyl in the Glowforge. I don’t own a Cameo, and I don’t really want another machine sitting around, but I’d really like to add the capability to use vinyl, even if it’s not quite to the extent of the Cameo.
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Really interesting information. Thanks for the reply! I have one of these machines coming later this summer, and I’m just trying to read all I can about it, so I can hit the ground running. I have zero laser experience. That Lazermask looks to be quite useful.
Vinyl cutting is very useful but lemme tell ya, you will rapidly tire of weeding! I am really looking forward to finding ways to laser my way to similar results with no weeding step. Lazermask should do that for flat glass, at least…
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When we are creating decals for our trucks, we cut a wax paper template of the same image. The thin waxed paper tends to weed itself when lifted off the vinyl cut and lightly flexed, but remains stuck just enough to the larger vinyl surfaces. Our guys will then lay duct tape over the whole thing and roll over it with a soft brayer. When the duct tape is removed, it weeds the vinyl out that is no longer protected by the wax paper. Tends to help things move faster when we charge by the hour. Granted, these are larger, generally not-so-complicated graphics for truck doors or ‘pinstriping’.
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I also hope for a drag knife. I already have two 3d printers and might get another soon. Too many machines and not enough space = pain in the neck.
But the difference is detail Ive done all my glass etching using the silhouette and vinyl as wells while the GF wouldn’t be able to do glasses,it could do plates and glass plaques. I’ve seen the rayzist mask do photo engraving using the laser masking on a plaque ( much finer/deeper detail than just lasering on the glass.
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