Not even *very* short cylinders?

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    So, I have an interesting problem. 

    I need to engrave the outside a thin aluminium ring (diameter between 45 and 60mm). The characters would only ever have to be about 3mm high.
    (I build custom camera lens mounts and things like a distance scale and serial number would be nice to have).
    Now, I understand that the eggbot is intended for spheres, so, I guess my question is: would the minimal height required for the characters really be a problem. Thinking of a 100mm sphere, the curvature on the top really wouldn’t be that big, would it really be an issue to draw 2-3mm high characters on a cylinder? (assuming the characters are perfectly centered under the pen).
    Holding the ring wouldn’t be an issue. I tend to build jigs for myself for this kind of thing anyway. 
    The engraving itself could be done with a very small engraving pen, since the aluminium (or nominally, brass) would be coated/anodized black, so, the actual engraving can be very shallow.
    If an eggbot wouldn’t work, any suggestions on what would? This is a hobby for me, so a commercial cylinder engraver isn’t really an option ($$$). I work as a software engineer, so, while building the physical engraving machine would be fun, too much software to write for it would make it feel like work, not a hobby (hence my reluctance to go with a DIY solution).
    Any help/advice would be welcome.
    Windell Oskay

    A very thin cylindrical ring is a lot like the center slice through a sphere, so the Eggbot can probably work okay on the surface, but I’m really not sure about the engraving part.  The Eggbot is meant for drawing, and does not exert the kind of downforce that you would need to make a good mark with a handheld scribe-type engraving tool, and cannot support the weight of heavier, traditional engraving tools.   

    Two things that *might* work well, depending on budget and requirements:  Our own diamond-point engraving tool– normally meant for hard surfaces like glass and stone –could actually be used to mark anodized aluminum, on a low vibration setting, with a slow travel speed.  However, the marks will not be much finer than those that you get with an extra-fine point Sharpie marker.  High-speed, lightweight engraving tools, such as the Turbo Carver could be used to good effect and make very fine lines, but cost a fair bit, and will take some work to mount to the Eggbot. 

    Thanks for the quick response. 

    The turbo carver is an option, but at the moment, it’s probably a little out of my price range. Mounting it wouldn’t be an issue, I have a mill and lathe, so custom parts aren’t really a problem.

    The type of engraving tool I was thinking of was (I’m not sure about this forum’s ebay link posting policy) the type of thing you find if you search ebay for ‘engraving pen’. I’d probably replace the diamond tip with a sharper carbide one.
    Another option that may or may not be a smart one would be to coat the entire ring in something like polystyrene and have a pen that writes in (precisely controlled quantities of) acetone, thus dissolving the polystyrene at that point. The ring could then just be etched. Or the reverse: the pen could write with some contaminant and the ring could be anodized afterwards, leaving that region clear. I’m not sure about the accuracy of something like that though.

    Extra fine sharpie type result is probably ok (ish).

    The other option would be to buy the engraving kit for my Proxxon MF70 mill and turn the horizontal motion into rotation. I’m not sure how much accuracy that would give…
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