How to machine this?

With nothing better to do while waiting for some material to arrive, I cobbled together this sequence of drilling/milling procedures to make something approaching what appears in the OP's photos.

Drill the holes then turn the piece on end to mill the large slot and "keyway".

With a milling cutter of suitable diameter, remove the area shown (left) and make a couple of blocks, using bolts to align, solder them to the main body.

That's how I would approach it, anyway, but sorry, that's taken up quite a bit of real estate in the thread. Mods feel free to delete it if you consider it wasteful of space.

John

Edited to try to get the photos side-by-side

Edited By John Hinkley on 21/07/2023 15:15:19

As someone who earns a living from making sawdust it is worth pointing out that your typical Ovolo form as found on sash window glazing bars is not actually a 1/4 circle as it is longer than it is wide. Sometimes as shown below other times a 1/4 of an ellipse

Hence why I suggested making form cutters. John's and possibly Howards method assum it is an exact 1/4 circle.

Edited By JasonB on 21/07/2023 16:19:31

That’s basically correct. The one I’ve chosen to be standard for my home-made tools is based on this sash ovolo plane:

Which has dimensions:

and the circle is 1” OD :

Which I’ve made #8 hollows and rounds to cut (amongst many others)

problem is, the Ovolo dimensions above don’t match the sash template I have, or various other related tools. Hence the need to make lots of tools.

When you consider that as Jason says, it’s an arc of a circle and not 1/4 of a circle, machining it all get more tricky.