Hi,
To make the M3 Castellated Nuts. Start with some hexagonal brass rod 4mm across the flats. First face off the hexagonal rod to get a good surface to work from, use a centre drill to mark the centre and drill 1.6mm (1/16"), through for about 10mm. Use a carriage stop on the lathe saddle and ensure that the lathe bit is touching the hexagonal face (I skim the face to get a good reference). Set the compound slide DRO/graduated scale to zero. Then using the compound slide, without the lathe bit touching, move the lathe bit in 1.2mm towards the chuck.
Then move the cross slide in until it is just touching the hexagonal bar and set the cross slide DRO/graduated scale to zero. Use the hand wheel on the saddle to move the lathe bit off the hexagonal bar. Adjust the cross slide in ,by 0.05mm, then using the hand wheel on the saddle move the saddle towards the chuck until the saddle touches the carriage stop. Move it back off the work and again adjust the cross slide in by 0.05mm. Again move the saddle towards the chuck until the saddle touches the carriage stop. (This method of using the carriage stop is good as it gives you a consistent point to cut to) Repeat this method until all the hexagonal surface has been taken away, for me that was a total of 0.3mm cut into the hexagonal bar. This should give you the same as in the photo below:
Then using a parting tool cut off a 3mm section
which leaves the Castellated Nut blank, as shown below.
I made a jig to help locate the cuts to form the castellations. This was made from a small piece of tubing, which had a dot punch tapped into it to flare it out than a section of hexagonal bar (4mm AF) tapped into it to form a hex shaped hole inside. Then a slit was carefully cut to act as a guide when cutting in to the top of the castellated nuts. A smaller piece of tube was used to push the castellated nut out afterwards.
The castellated nuts were then tapped M2 and all the sharp corners removed.
You will be able to make two castellated nuts before having to re drill the centre hole. If the centre hole is made any longer, then the drill bit can start to wander and there is more chance of snapping the drill bit in the work.
Hope this is useful
Sarah