Anyone got a good strategies for repeatable indexable fixtures? Asking for a friend…
Coin is 40mm across, for context.
This was done using a 1/8 endmill to make a pocket the blank was a tight fit in, changed to an 3o degree v bit and somehow lost alignment. I know the step over is wrong. But I like the effect anyway.
This side was a scratched circle with the 30 degree bit and then double sided tape - but I could see it moving as it engraved.
It’s not great for a circular coin, but I use 3d printing for workholding pretty religiously.
For instance, I will create a model that perfectly holds my part, and anchors to the spoilboard at 4 corners. Then before I even setup my job, I will setup my new coordinate system, and use a quick macro to drill 4 small holes at the four anchor points. This guarantees that my workholding is aligned with the XY plane of the tool, positionally and rotationally (though it can’t accommodate skew). Using G10 L20 commands will fix the coordinate system and keeps it active even through a power cycle. It’s then all down to the accuracy/repeatability of your endstops, which I’ve found to be sufficient, though I’m not usually carving something as small as you have here.
It’s more challenging with a coin. You could maybe print your workholding with dots or notches that align with key points in your engraving, so that once you have started engraving, it’s easy to visually align it.
Hmm, maybe I could 3D print something like a flat topped cloche with a hole the diameter of the tool bit and the base a slip fit for the coin. Then I could pick up a reference for the center.
What you’re describing could work too - registration pins and maybe a grub screw to hold the coin.
It was for circular wooden blank “coins” but I 3D printed a press fit fixture that held the coin securely and was screwed to my table. Kind of a custom vise of sorts. I think I put double sided tape on the coin to be extra secure. It was tight enough. If I did it again I’d add a slot to pry the coin out, or similar.
This is pretty cool. You could add screws on the top and bottom and make it two parts that you can clamp together. But that might be too much as well. 
What about leaving tabs in the square stock, removing them later? Or are these pre-made blanks?
They’re pre-made blanks
A chunk of aluminum with a close fitting pocket to the coin size…and superglue. heat the coin up to release the glue. And the aluminum will work as a heat sink so cutting heat doesnt melt the glue prematurely. Using this method for watch dials and it is working great.
