Nice looking cat head there !!
A cat head is used when the material needs to by supported by the steady rest but the material is not round enough to set the steady rest right into the part.
The part may be hexagonal, a forging or any other non perfectly round shape.
The cat head will be inserted into the part and then the steady rest will have a round surface to do it's work.
Is more complicated than just that, since it needs to be indicated to run true.
If the cat head wobbles then this will be transfered to the machined surface.
I have used very large cat heads used for machining large ship shafts forgings during my apprenticeship and got my journeyman's medals doing that.
These shafts where anywhere from 7" to 13" in diameter and from 12 to 60 feet long.
Without a cat head is impossible to machine something not round to start and specially so long.
Those shafts weigh as much as 26,000 lbs.
The lathe was an American with a 58" chuck (would swing 58") and the bed was 65 feet long with three large steady rests.
Right after finishing machine tool classes a the Long Beach City College, the first night at work for Jorgensen Steel they put me on this lather and the two guys on night shift had a ball with me.
I had only run 10" South Bends at school.
Ohhh ! the good old days !!
After about five years I started to get skittish handling such large and heavy parts and moved on to better pastures.
I still would run that machine if I had to.
Now I seat on my behind mostly staring at the monitor, but spend about 50% of my time in my home-machine shop.
The only big part now days in my life is my behind
