Quote Originally Posted by Ross77 View Post
0.025mm doesn't sound that unachievable.
Set your micrometer to .001", look at it and decide how you're going to divide this tolerance up between the slides, the screws, the spindle runout, the surface finish and flex in the system.

On a good day with an obliging material I can cut better than .001" in the Y, but the X cuts around +0.0015" or worse. All bearings and ballnuts are preloaded, the Gibbs have fine adjustments and pukka slide oil, I even have a pneumatic quill lock to de-slop the Z, BUT, it's a round column mill and even wound right down that 6" diameter, cast iron tube at the back is more willing to twist than it is to bend. It's a kind of torsion bar suspension.

You might think that I could just go around again and whisk off an errant .0015" but it doesn't work like that. Once you get inside the flex parameter it prefers to rub rather than cut. Wind it in until you pass the flex point to get a cut and whammo, instant undersize.

You might think that I could cut the X undersize on the finishing path, but it's erratic. I think I've reached the limit of the machines' capability.

As soon as you skimp on the iron you also get to worry about sympathetic vibration. There's a reason why the troops break step when they cross an iron bridge even if it can carry a much heavier load than them.

Perhaps milling to fine tolerance with less than 2 tons of cast iron to back up the tool is a black art. I could try hanging around midnight crossroads in case Old Nick turns up so I can do a deal for some fern seed but I may just try filling the column with ferro concrete first :whistling:

Robin