Have you got a milling machine & rotary table?

Tony

Looking at your photo of the castellations, would it not be possible to use a fine saw and cut straight across the end (e.g from 10 o'clock to 4 o'clock hope that makes sense), and similarly with the other flanks of the 'teeth', then use a small drill to remove the bulk of the metal from between the sawcuts, and finish off to size with a needle file?

As Jason and others say it is simple enough with a rotary table to cut the parallel paths then rotate the table and offset to cut each side of each castellation to the taper
If you don't have a rotary table then you need to make a fixture with a central spigot to mount the part. The part will need a witness mark with calculated witness marks around a circle scribed on the fixture: clamp, cut, rotate, clamp etc.
One of the easiest ways of creating those fixture marks is to draw it in CAD and print, cut out and stick to the fixture

The primary cross-cuts are made on their centre lines but for the tapers the mill is moved 1/2 the cutter width off the centre line and only run circumference towards centre for 4 teeth then swapped to 1/2 cutter width the other side of centre line and repeat.

pgk

Would a thin (say 0,75mm or 1/32"  ) slitting saw pass below the southern edge of the eastern merlon and above the northern edge of the western one in the above pic?

If so, you could use the sawcuts as flank markers for end milling out the crenels. If you have a swivel vice you could presumably calculate and set the angles. You might have to finish with a Swiss file, Dremel or suchlike.

Edited By Mick B1 on 11/08/2021 14:50:19

Not having a rotary table is a bit constricting, but not insurmountable. In your position I would either make it from square stock and use the sides to do the indexing, against a vice stop and turn it down afterwards or make it out of round stock with a stub on the "back". Drill and/or bore a central hole in the aforementioned square stock, securing it with a grub screw or just superglue. Mount the square stock vertically in the mill vice and proceed to mill as described in previous posts.

A rotary table will be a useful addition to your workshop and I would suggest a 4"/100 mm one, preferably with division plates.

John

You don't really need a rotary table for that, infact you could do it by just rotating the part once, either way a simple printed out paper disc and pointer would do.

First thing is to look at the part and asses how it can be machined, simply by rotating slightly you will see that the radial faces can be lined up with the X and Y planes.

A bit of CAD to draw out teh shape, I've assumed a 5mm hole and shown your 1.6mm gap. Which a 1.5mm dia cutter will just fit through so offset by half cutter dia and machine one face, probably go down in 0.5mm steps

This will leave your part looking like this

You can either leave it in the same position and make another cut in X coming in from the right and offset 0.75mm below the x axis then two cuts from top and bottom either side of teh Y axis. Or if tooling permits rotate 90deg and make a cut repeat twice. You will end up with 4 slots forming 4 of teh dog faces

Now rotate the work by the complimentry angle of the 37.05deg, offset below X axis and make a cut. 

Which and then do it three more times by rotating 90deg or cutting in teh other three axis and part will then look like this

Edited By JasonB on 11/08/2021 16:18:40

Get a suitable size bit of square bar, machine spike on it to locate the bore of your castellated thing, then you can grip this in the machine vice and use it to index pretty accurately.

Or even easier make it from square bar, finally turn it round after you've done the teeth 

Edited By duncan webster on 11/08/2021 19:09:14

I used the rorary table to produce a similar type of drive, but with 7 segments, 51 degrees 26 minutes, approximately for a long handle for the knee of the Tom senior mill when there was a heavy weight on the bed.

1.6mm won't fit assuming you were measuring what will be the diagonal, you need a 1.5mm cutter