Jon Elson wrote:

On 11/27/2010 08:31 PM, Ignoramus18541 wrote:


See a specs page from my manual.

http://igor.chudov.com/projects/Brid...Mill/specs.jpg

``Positioning accuracy +/- 0.0005".''
This is the old thing about resolution does NOT equal accuracy.
Bridgeports are hand-scraped for accurate alignment of the ways, and ARE
pretty accurate. But, the kind of stuff you need to maintain .0005"
accuracy over a work envelope of 12 x 18 x 18" is pretty tough.
Just rest your hand on the table for a minute and the table will warp
enough that you will have several times that error at the extremes of
travel for 20 minutes or so. Run the X axis back and forth rapidly for
a couple minutes and the screws will expand and cause a dimensional
error for a few minutes. getting .0005" accuracy on a 2" cube isn't
that tough, if you can account for tool and workpiece deflection.
getting .0005" accuracy on an engine block-sized piece that you machine
over a 2-day period will drive the experts NUTS! High-end machines have
the spindle running in a coolant bath, and run coolant through hollow
ballscrews, too.

Jon

Yep, and that coolant is circulated through a special refrigeration unit
to maintain a very consistent temperature (not actually cold), it's not
to be confused with the coolant / cutting fluid which is an entirely
separate thing. Those high end machines have specific warm up procedures
you have to run them through at the start of the production day to get
them stabilized before you start your precision work. Even stuff like
night time setback thermostats on building HVAC can cause you problems
when you need real precision on big machines.