how do I buy a used VMC?
Seller said I can run the machine, but not cut anything.
How would I run the machine to verify that it still has the accuracy.
I am a newbie, What should I watch out when buying an expensive 4 axis VMC machine?
Unless you have squares and such, not much you can do other than check backlash. If it is an expensive used purchase, I always recommend you find a local service guy to check the machine out. There are a few thing to check, not just accuracy.
Backlash, you must set comp to zero to see how much mechanical slop is in the axis. Dealers will sometimes try to hide it.
Squareness- 5 sided square is required
Drawbar tension-need a gauge
Spindle noise and heat
ATC condition, alignment
Electrical cabinet- make sure everything is where it's supposed to be, and not chopped up.
Ballbar test- good snapshot to see whats going on, however if the machine is just on a dealers floor, it may not be level enough to use this.-same with some squareness tests.
Cannot cut anything? I would tell him to pound sand. Like test driving a car, it will idle and rev up fine in the driveway, but could totally die going up a steep hill. Worn spindle bearings may sound and act fine until they encounter the stress of a cut. A tired spindle motor could get up to speed easily cutting air or bog down or overheat in a cut that should only use half it's horse power.