I have a chance to buy a 6" suburban chuck with a 5 c mounts for a great price. My question is how useful are they in a shop. I'm not sure on the holding power,to be able to mount to lathe and take cuts. Are they made for cutting forces or just light grinding? Thoughts appreciated thanks.
What Plastik said, mostly.
My ~ 6" Hardinge-branded, Walker-made allegedly grips at over 700 lbs. Allegedly.
You understand magnetism and air-gaps or imperfect surfaces?
Least gap, the force is still trying, the friction GRIP it makes use of has GONE. Darwin Awards can't pay your bills.
In practice, it will only ever be utilized for far less than 100% of the many alloys of Iron. Useless for the Bronzes and many Stainless alloys. I avoid Aluminium like a bad rash. Plenty of better men already cover any need for the working of that.
Even then, only for on-lathe
low-load grinding or polishing. And I do NOT like to grind on a lathe if it can be avoided. That said, it requires less floorspace than even a tiny surface grinder. Provides for inverted Blanchard'ish possibilities, writ small, if you will.
Special use item. Your call if you even have such uses. I do not, really. Just one more tool-whore.