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Steps. see drawing. but the principle is good for any size keyway to be cut.
Turn the outer holder shank to suit.
In this case, drill and ream 4 mm dia.
Open up the end to 5 mm dia, finish with a 5 mm end mill to create flat bottom.depth important.
Turn a up a spud, note dimensions.
Slot with a 4 mm wide slitting saw to 4.2 mm deep. as you need a bit of daylight between the end of the brazed spud and the cutting bit,
this ensures the grub screw?holding the bit comes down fully.
Push the spud into the slot of the holder, and braze.
This creates the square (rect)? for the bit. 4mm x 4.2 mm.
Final op, back onto the lathe and drill and tap the end 4 mm x .75.

Tool bit made from 8 mm dia drill rod, or Silver steel.using a 5 or 7 degree end mill.
Hold in a J Stevenson 4 sided block.
Mill all sides to 4 mm square. IE important remove 2 mm per side.using the base of the endmill.
Turn 90 degree in the JS block. Using the side of the endmill mill the side rakes. to leave a witness.
Mill the front rake using the side of the mill.
Rotate the JS block 180 degrees and tilt using a angular parallel say 5 degrees and with the end of the mill, finish the top rake again to a witness.
Harden and temper the bit and slice off with a Dremel. to suit the final length required.
I prefer to leave the drill rod longer during making the bit, you need to allow extra length when holding in a collet, and something to grip
on when dunking in oil during harden and temper process, put the bit into the holder after HT and Dremel off.Sparks will also
tell you the HT is correct.
Naturally the bit cannot be longer than the bore of the keyway to be cut, neither can the diameter of the holder bit end.
Example, 10 mm dia gear bore, 4 mm wide keyway. holder entering the bore, will be 7.8 mm. or slightly less.
The cutting end of the bit can be exposed say 1.5 mm.
This means you can cut 1.5 mm deep of key, move back to start position, move the bit out 1.5 mm and repeat to final depth.
Normally keyway depths be it in the spindle (slot) or gear are 1/2 the size of the key.The idea of square bits is that you can slide
in and out of the holder without fear of rotating it and losing side position.
Sounds confusing but doing it you will get the idea.
Very quick to make, and reliable.
Hope of interest.

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John

I forgot to add attachment.

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John

You can use square HSS blanks, already hardened, but you would need a surface grinder for accuracy to produce the 4 rake angles.
Using a off hand bench grinder possibly, but certainly a Surface grinder is a good machine to have, but with my workshop space limitations,
I don?t think having a SG in the dining room would keep peace and harmony with SWMBO.?
A good book to read is Ivan Law?s Gear cutting,. it fully explains how to produce involute fly cutting tools, similar method to making keyway cutters.

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John