I'm milling some thin metal lock caps. I'm not really sure what speed to set the milling at. When I mill metal, it seems to give me more problems then brass.

There is more vibration and what I notice is if I mill a straight line and then go back in that same direction, it will actually mill more metal off. I don't know why it is doing that considering I have the table locked on the other axis and went really slow. I don't think my object is moving in the vise either.

What speed should I use for steel with a 4 flut 1/8 mill bit?

What speed should I use for milling brass?

I've been using like around 500.

Thanks for the help, this is a real newbie question which I'm sure has been asked a million times.