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I am designing my own optical rotary encoder with a non-standard number of encoder positions for a specialty application. I have gotten a 0.35mm acrylic with the pattern printed on it but now I need to punch out the center hole.

A smaller hole could be much more easily punched precisely by hand or machine.

If a punch could be designed with a small rod in the center, it would align just right with the center hole.

This method could also be used to punch out the whole disk. First a big punch with rod could be used to punch the disk, and then a smaller one to punch out the center hole.

The biggest problem that arises is where/how to get a bit that has an alignment rod in the center. Perhaps they already exist, but I just don't know how to search. I don't trust my lathe to drill out the center of a punch correctly, as the punch will likely be made of the same material as my drill bits. How can I precisely punch these holes in thin acrylic sheet?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Wasabi
    Commented May 18, 2021 at 1:21

2 Answers 2

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The problem is you are missing a reference edge or hole to indicate anything off of, and you can't trust the edges of your rectangle plate.

Maybe cut out the acrylic in an oversized circle (for backplate mounting holes), mount the acrylic on an aluminum back disc then rotate it in a lathe (even just turning the chuck by hand should work) chuck to see if the edges wobble. Once you fiddle with it so they don't then press a divot into the disc with a center which you can then use with the punch. Or if, beforehand, you drill a slightly oversized relief hole in the back plate (so it is always under the center mark even with fiddling, but not so large there is too little support against the punch) you could hand punch the pilot hole on the lathe right then and there. I'd be wary of actually drilling it on there lest you tear the thin acrylic off the back plate, though if you think you can then you don't want the hole in the backplate beforehand.

EDIT: Or get an optical center punch. This is what I would probably do. Sounds like a good investment and you always have the above method to fall back on.

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  • $\begingroup$ SFAIK they don;t make indicators that follow ink marks, but perhaops you could put a microscope on slide and use that to zero the part $\endgroup$
    – Jasen
    Commented May 15, 2021 at 20:27
  • $\begingroup$ @Jasen Yeah that's a good idea. You could use a fiber microscope if you can get it into the lathe to observe the edge wobble as you rotate it on the lathe rather than with the naked eye. $\endgroup$
    – DKNguyen
    Commented May 15, 2021 at 20:30
  • $\begingroup$ @Jasen You know, there probably is a center punch tool out there with a sight on it. $\endgroup$
    – DKNguyen
    Commented May 15, 2021 at 20:49
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    $\begingroup$ @Jasen Yep. Optical center punch. After the toolmaker at work keeps showing me all his toys, one thing I realized is if there's a stupid problem that seems like it could comes up more than once, someone has made a tool specifically to deal with it because you can't go around wasting time jury-rigging things when you're making money off of it. $\endgroup$
    – DKNguyen
    Commented May 15, 2021 at 20:59
  • $\begingroup$ @DKNguyen I have looked into optical center punches and this looks like the best fit for my application. That is exactly what I was looking for! $\endgroup$
    – Hackstaar
    Commented May 17, 2021 at 21:23
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The fact that it isn’t Gray code is a reason to start over. When you do, add a tooling hole for the center. With careful use of a punch properly sized for the tooling hole you should get good enough alignment for this large of a pattern.

PCB manufacture uses tooling holes and fiducial marks to get good alignment between layers. If you want to make more of these that’s a good way to make high precision encoder wheels.

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  • $\begingroup$ it is a gray code. just phase shifted a bit, thius it's nothing that can't be corrected by sensor placement. $\endgroup$
    – Jasen
    Commented May 15, 2021 at 23:06
  • $\begingroup$ in pcb manufacture the holes don't perfectly align with the plating or the silkscreen $\endgroup$
    – Jasen
    Commented May 15, 2021 at 23:10
  • $\begingroup$ They align within a mil or so. Hole tolerance is very tight. And no way that is Gray code - more than one region changes state at a time. $\endgroup$ Commented May 16, 2021 at 0:44
  • $\begingroup$ The timing of the changes depends on the sensor placement, offset the sensors by half a stripe angle in each lane and and add an extra at the original position in the MSB lane and you get a gray code with N+1 bits. $\endgroup$
    – Jasen
    Commented May 17, 2021 at 5:00

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