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MikeI think you misunderstand; I am suggesting you mill the slots i.e. the shaft is in the x axis thus no abnormal loads on your machine!Mike
Assuming a rotary table is available, I'd use a carbide endmill to rough. each slot. Either tilt the rotary table or the mill head the angle needed and feed in the X axis. After the forces on the broach will be much smaller.The Z axis of most mills are much stouter than the X and Y if you choose to use the head as a press.
We know someone with a slotting head
Looks like Mike has two possible ways of doing his splines and they don't involve the excitement of setting up and trying to use my slotting head Jo