Well can't mill it either radially or axially as if you want a zero radius corner like your illustration you'll need a zero diameter tool.
I would mill an axial slot up the the 'head' and silver solder a rectangular key into it but if you want it to be square right up to the head you will need to file a radius on the end to the depth of your slot and the diameter of your cutter.
The zero radius is not really needed because when fitted into the shanks I have to make clearance for them for which I am using a dremel type tool with the 3mm diamond bit (its a bit pot luck because some shafts are hardened while others are not) basically as long as I can stop this turning when the user is adjusting the stop then its doing the job. The plan was to sell these as kits that people could fit themselves which is why I would prefer them to look better.
Thanks Philf I will look into getting a carbide cutter and see if thats any better, The CNC option may be something worth thinking about for the future but you have given me another idea which is to go maybe .2mm or so smaller on the small diameter so it wont touch at all
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