Hm, since you mention it…
The tip wouldn’t even need to be carbide, just to etch plastic (my default choice of medium would be CD jewel cases, made from styrene). I could probably do something like a length of clothes-hangar wire, work-hardened (i.e., bent back and forth until it breaks) and then filed to a point, and mounted in a pen case.
The space’s policy of “feel free to bring in your own tools” might or might not extend as far as a DIY hack like that, but use of the machine requires a safety training class, anyway, so I could ask about it then. And I wouldn’t mind getting that training badge, anyway: Who knows when I might have some other project that uses a CNC in a more conventional way?
A systematic XY offset wouldn’t be a problem at all (heck, I’m positioning the design on the surface by eye, anyway), and even a varying offset of a few millimeters wouldn’t be too bad, as long as it doesn’t vary during a segment.
This just might work…
I mentioned an HSS or carbide tool only because it’s likely that the makerspace already has a box of scrap/dull/broken tools and it may be possible to repurpose one. Otherwise, yeah, anything that will scratch the material you’re interested in should be fine, at least for a while. You do want something sharp as opposed to just coming to what appears to be a point, since really you’re making a shallow cut in the plastic, not just forming a crude trench.
Of course, this is all stuff you can experiment with by hand outside of the shop. When you get something that cuts the way you want, the machine should be able to work with that.
I was looking around on the Shopbot site and see that their Desktop model has much better (“engraving level”, they say) precision than their large gantry models. I had been thinking of the latter since that’s what our makerspace had. If yours has a desktop model, rotating engraving might be an option.
If that’s the case, I can mail you one or two of these. I have a box that I haven’t used yet.
The makerspace has pointed engraving bits. I’m just not sure if an engraver would actually work for my purposes. Although, come to think of it, I do have a rotary tool at home I could test that with.
And in case anyone hasn’t figured it out, the project is abrasion holography. I wasn’t satisfied with the results using a compass, so I calculated how to do it better. And it’s driving me crazy that I still haven’t been able to test my calculations.
The thought crossed my mind, but I wasn’t sure.
My own experience is that you don’t want engraving at all. A scratch with a compass gave worthless results, because the interior surface finish wasn’t specular. Instead, it had a diffuse surface which was highly visible as a scratch but didn’t have the reflective properties you need for holography.
Instead, what worked reasonably well was to use a dull point that just gets pushed into the plastic. Instead of a two-point compass, I used one with a single point and a lead holder (but without lead). The lead holder I could just kinda push into the plastic and it made a nice smooth-walled scratch.
I’m not entirely sure how you’d achieve this with a Shopbot but it does seem like there should be a way. Something like the Cricut, which can rotate the head, combined with a dull roller head, would probably be ideal.
Maybe a ball-point pen again, but this time actually using the ball to roll across the surface? A fine-tip ball should have the effect you want.