motor gearing (cog) help please

Attachment 9207Attachment 9208Attachment 9209Attachment 9210Bought 2 new motors to replace the old original motors on the machine, nema34's so same frame size to make it easier plus same depth as there is not a lot of space where fitted. New motors arrived today so went to fit, they are 3/8" single spindle the same as the original motors so thought this would be straight forward but having taken the gear cog off the original spindle there is no flat on the spindle so was just relying on an the tight fit.
Probably because of the flat the gear fits on the new spindle without problem so is not tight enough.
Looking for suggestions on the best way to try & sort this so the gear doesn't' spin on the spindle. Any help would be much appreciated.

Hi Martin, [I] think you will have to drill and tap the cog in the root and put in a (two?) grub screw. G.

Yes Geoffrey that was my initial thought but posted on here just in case someone knew of another way it could be done.

Loctite 638 or 641 (if disassembly in future needed)

Wouldn't use these gears on CNC machine without some form of mechanical fastening.
Would Either buy gears with key ways or have some machined into existing gears. Both Drilling/Taping and keyway cutting could be difficult has they may well be hardened gears.? try a file on them to check before attempting either.

If not then I'd simply drill/tap and use set screw along with Loctite.

I have a feeling they are hardened although not actually checked yet, any idea where I might be able to buy something off the shelf or is it going to be a case of having to get some made? They are probably imperial as the rest of the machine seems to be, guess that is because Gerber are an American Company.