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