how to accurately mill a rectangle with rounded corners (no cnc)

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picclock:
@ Jason B

That's a very impressive picture of the hit and miss engine plate, and pretty much spot on. I assume that Robs hit & miss part was made on a CNC machine.

Yeah, I screwed up on the radius dimension :scratch:. I was thinking about a method to do this with reasonable accuracy.

The work is 128mm x231 x 15mm. The slot dimensions are 3mm wide x 1.35mm deep. The corners are radiused 10mm (centre of slot) and the overall size of the rounded rectangle is 120x224 (centre of slot). I know these are correct cos I took them from the drawing ::)

But seriously, I assume that Robs hit & miss part was made on a CNC machine.

The best I can come up with is do the 4 corner quadrants, getting the dimension to the quadrant edges using an edge finder and my DRO. Then once off the rotary table use the same edge finder to align the part and join the straight bits.

For stuff like this CNC rules :bow:

Many Thanks

picclock

David Jupp:
You could perhaps adapt the principle of a copy lathe - make up a suitably sized template from sheet steel (file the corners) - mount this on the milling table, offset from the stock and a short distance above the table. Arrange an (adjustable) follower rigidly mounted to mill structure or bench. Manually drive the table so that the follower stays in contact with the template all the way round whilst cutting the groove.

Will still take quite a bit of care, but I think it should work.

picclock:
@ David
That's a neat way to transfer complex shapes. It may come to that if I can't work anything else out.

My current thinking is that the problem is aligning the mill centre with the slot centre. If I can do that accurately then I can ignore the errors that occur when I measure to the edge and set from the edge. So really I need to be able to centre on the slot. My Starret edge finder would work well except that its a 6mm tip.

So going down this route, a mod to the Starret edge finder, drill a hole in the end and superglue in a piece of 2mm silver steel, or obtain another edge finder with a smaller tip. Not sure how the Starret would behave with a smaller diameter driving it, but at least it's a non destructive mod.

picclock

Jasonb:
Why do you need to locate from the centre of the slot? Locate the straight slots from one edge (4mm one way, 3.5mm the other)and the pivot point of the 10mm corners can be located from two adjacent edges (9mm and 8.5mm)

picclock:
@ JasonB

I figured that if I located both edges then I can use the DRO to get the centre of the slot, and any inaccuracies in the measuring technique or tool will be cancelled out.

picclock

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