How to machine a circle when it isn't

If this is thin sheet metal and you need to have nice clean holes I would recommend that you glue your sheetmetal down to some plywood (or similar material) backing before any drilling or cutting happens....Just be sure to use a glue that can be dissolved easily with something like mineral spirits so you do not ruin the part.
 
FWIW, I always "make a sandwich" when doing thin material. That is have sacrificial material both above and below your part. Then clamp it all together. leaves a perfect clean cut if you do this.
 
Drill it round and solder in a separate piece with correct width rectangle underneath to create the flats.
Oops, already sugested by whitmore.
 
I put some of those in my control box, just drilled the hole and popped them in. I didn't worry about the flats since they hold just fine with the nut that threads onto the backside. I would humbly suggest that it's probably not worth the effort to make a hole with flats on it since the chances of the connector spinning and causing any type of problem are pretty low.

As has been suggested above, you can always make something temporary and replace it with a nice custom part once you have the CNC conversion working. That would also allow you to engrave labels into the new part for a professional finish if that's what you're going after.

Having been through enough of this process to get my machine up and running I'm sure you will have plenty of other issues to deal with that will actually make the difference between having a running system or not. Did you get your steppers yet? I posted to your other thread about stepconf probably three times but the forum issues wiped out my responses, It should be up there now.

Let me know how those settings work for you.

Cheers,

1ohn
 
I realize I could just mill a circle with my boring head, but part of this journey is for me to learn how to manually do things properly and accurately.

I have all my other cutouts done. These are my last ones. I'm slowly learning how to do things!

I do have everything now: stepper motors and motor controllers.
 
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