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Machining a large radius, on a large part, on a manual mill

With such tolerances if you happen to have a 2.25" annular cutter then just clamp a scrap piece of 1x3 to the edge and drill through on the correct centre point. Very fast and easy.

(Gustafson it is just a 1" deep bore)

That is a 2.25" radius. Got any 4.5" annular cutters laying around?
Consider doing two at once, (face to face, not stacked) perhaps with some sacrificial material in between, to lessen interrupted cut loads if they're an issue. Plus you can mess up with half the effort.
I regularly cut a radius with a 3.5" diameter shell end mill ( also known as a side and face cutter )which has a R8 collar on a Bridgeport mill. I would assume that a similar 4.5" diameter end mill would be available and I believe the bigger diameter would not be a problem. It is a quick operation and only requires one cut. Hope this helps.
We need to machine a 2.25" radius 'half moon' into the side of a 1"x3" piece of CRS 1" deep into the flat side and all the way through the short side.

View attachment 275463
Hi Shaun, just did that the other day on 1/2"x 6"X 12" long CRS with a small R8 manual mill using an old(40 years) 5" hole saw & locking the quill up using the Z axis control to cut a little deeper slowly, a little at a time. It came out with a very clean acceptable finish. It would have cut quicker & cleaner with a new quality bi-metal saw on a bigger more robust machine, but it got the job done with just a little deburring. Customer very happy.
Cheers Peter
4.5" diameter Insert face/ shell mill would do that easy.

If you had a horizontal attachment for the mill a 4.5" diameter cutter would do it in one pass
Well we ended up getting the job and I ran it this weekend. I had ordered a hole saw from McMaster (ended up being a Morse brand) and gave it a try. Seemed like I couldn't get my mill to run slow enough so it chattered like crazy. Ended up drill along the scribe line with a 5/16 drill bit and knocking the big chunk out with a hammer/chisel, cleaned it up more plunge-milling with a 5/8 EM, and finally finished it up with the boring head.
Thank you all for the help.