Small Mill very early design stage

[Ross77]

So has there been some confusion over the level of accuracy then??? :whistling:

0.025mm doesn't sound that unachievable. when I was looking for the xy table for my mill I'm sure I saw some precision tables in that travel range for around the £300-£400. bolt it to tee slot table you've all ready got, add a solid fixed gantry for the z and go from there on a trial and error basis.

I'm sure most of the accuracy you require will be achievable by finding the machines best running conditions for each material type and maybe establish a warm up routine so its not used when really cold......then it wouldnt matter if everthing was tight and binding when cold.

[Robin Hewitt]

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

[John S]

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

Robin has nicely quoted an example of what happens in the real world regardless of endless calculations.