I'd contend that you can't buy from a spec table. The ACTUAL ACHIEVEABLE accuracy of a machine is a function of many things. Fixturing, cutters, coolants and speed of operation are but a few things that will determine how accurate you can get one to run.
Example: we're running cam masters on the neighbor's VMC - the brand is in your list. The masters are all run with the same code structure but the profiles/shapes differ from piece to piece.
Yet, each and every time we run them, there are small subtle differences that show up in fits and finishes. We're getting better with reach pass but still, simple tool wear (we're cutting steel) and temp changes in the machine will show up in part sizing (we're shooting for 0.0001"s in tolerances).
Since thermal growth in a machine is not considtant nor necessarily predictable, I'd contend that a "temp comp" factor that a machine may have is NOT something you can stake your reputation upon - life is NOT that simple.
Theoretcialy, the machine we're using can NOT support the sort of tolearnces we're trying for - their literature clearly says so. Yet, we do it.
How? Jigs, fixtures and iterative tweaks in the tool path/cutter feeds/speeds to find what the machine "likes" to run at. Thankfully, even though we have CNC machines which can automate things for the machinist, there still is a segment of the profession that requires art and skill to achieve exemplary results in a finish machined part.
The FIRST thing I'd look for is service. Then I'd look for an agent who's sold some machines AND who has customers you can go see them run. THE CUSTOMERS will tell you if the machines REALLY perform as advertised. They'll also tell you if the dealer is worth a damn or not.
The last thing you want to buy is a machine is one from an agent/broker who is merely shuffling the papers and not servicing/supporting the machine. There are any number of horror stories from guys who bought "vaporware" machines and then they didn't work as promised or all of a sudden you couldn't get them serviced if the OEM vacated the market - lots of "pacific rim" suppliers did that at one time or another.
HINT: if you want a good simple test for some of the issues you're looking at, have them cut a perfectly round circular disk AS FAST AS POSSIBLE in both steel and aluminum - do it for several sizes. Also do several for ID holes of several sizes.
This is a good fast check of repreatablity and for follower error. It will show you if they have axis tuning and motion control figured out under constantly varying speeds and feed.
If the circle isn't round (should be so with 0.0003" or better on DIA), they can't reliably repeat or hold tolearance, no matter what their spec sheet claims.  |