How to machine a circle when it isn't

Here is how I do it if I can't be bothered writing G code.
Punchout.JPG
Use an end mill with a diameter equal to the distance across the flats cutting from left to right and then a small amount of cleanup to fit with a file.
A smaller end mill can be used with two passes rather than one.
 
I like a drill, then nibbler followed by a file. These are small holes, only 4 of them. You can be done in the time to read this thread. Betcha.
 
Thanks for all the responses. I'll have to figure out what's the best method for me. I can't recall the exact dimensions.
 
Okay, here's the dimensions. So, it looks like a 11/16" or 5/8" end mill would work. Of course, I have neither! I only have 1/2" and 3/4" in that range.


AvConnCutout.jpg
 
If this is sheet metal, consider using a Step drill or Unibit. In my experience, they leave a cleaner hole and are much less likely to grab and deform the material. A 4 flute end mill or a hole saw might also work well.
 
devils4ever said:
I'm working on making a panel to mount Aviation-style connectors for my CNC mill conversion.

The ideal cutout would be a circle with two flats on it on opposite sides.
Click to expand...
That's a double-D in metal punch jargon.

In aluminum, scribe, drill in the middle, nibbler and finish with a file.
In harder stuff, just make the circular hole and when the gage goes in, the bezel covers the gaps.
If you really need that shape done with non-CNC machine tools, it's custom-punch time. Greenlee makes a line
of 'em, and I've done the crank-with-wrench type often enough to enjoy falling back to nibbler-and-file.
You could also do a rectangular backing plate to a circular-hole panel.
 
All: This is being cut in 1/8" Aluminum.

RJ: I think I'm going to try your method. Thanks for the drawing/dimensions. I'll just have to watch out for backlash since I don't have a DRO.
 
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