Unimat Problem

[Melkal]

Hello, I have been learning how to use a metal lathe with an inherited Unimat.
I have stumbled trying to shape a 2” steel round into the flywheel. Making the face cuts are producing a lot of chatter.
Side cuts are ok. I can face aluminum, brass etc.and get a nice finish. Steel seems to be the problem.
I’ve ground new bits, tried brand new carbide tools. I’ve tightened up everything I can, the cross slide, spindle, endplay, etc.
Am I missing something, have I exceeded the capacity of the machine? The steel came as part of a kit from Little Machine
shop for a mini steam engine, so I don’t know what it is, but some hardware store steel works
The same. Any thoughts would be appreciated. I should point out that I can cut the steel but I can’t stop the chattering leaving a lousy finish.
Thanks in advance.