Hi,
my first mini-mill was of similar dimensions, but of a more normal bed mill type, made out of cast iron. I got some great big cast iron elevator weights (1000 x 200 x 100) from the scrap yard and milled the
axis beds and other parts out of them. Worked pretty well. I used it for 61/2 years before building my new bigger and more powerful mill.
While the machine was rigid enough for metals, the majority of my work is for PCB's. I too have a Mechatron 800W spindle, and I've used it, and used it, and used it.....for
eight years. The only time it EVER goes below 24000 rpm is if I'm cutting steel. For just about all materials with tools 1mm and under you will be rpm limited, run it a s fast as you can.
I used 20mm Koruda 5mm pitch C5 ballscrews, because I could get them second hand. Sourcing good quality ballscrews is about the most challenging part of the whole build.
You are right to choose ground screws, I've used C5's and had great results, C3 would be better but really only for skiting, C5's will be as accurate as you could possibly require.
If you use thin screws then they will tend to 'whip' at speed, and so you'll need coarse pitch screw to avoid having to spin them that fast. The minimum I would consider is 16mm diameter,
then at the lengths you are talking you can spin as fast as you want, say 3000rpm without whipping.
Fine pitch, say 5mm or less, allows great thrust from modest servos but slower rapids. Higher pitch, say 10mm or more, requires more torque from your axis motors but blinding fast
rapids. With such a small machine fast rapids are not really required, so I'd recommend 5mm pitch.
My new mill has 32mm C5 5mm pitch screws and I can still get 25m/min by spinning them at 5000rpm (Delta 750w servos), but I find to too scary fast and have detuned them to 15m/min,
and when making PCBs I reduce it further still. My recommendation is do not get too hung up on fast rapids, 5m/min or thereabouts is ample. In a production environment 50m/min is
more likely....but yours is not a production machine.
Acceleration is overall more important than speed, the higher the acceleration potential of your machine the better the machine can follow a toolpath. Acceleration is best with
small (5mm<) pitch screws.
I bought my ballscrews and linear rails from this outfit:
https://www.ebay.com/str/industrialp....m47492.l74602
I was able to get absolutely the top brands THK, NSK etc, new-old-stock for a small fraction of new price.
Are you sure you want granite? Its about 1/4 as stiff a steel, 1/2 as stiff as cast iron, costs way WAY more and is a PITA to drill and attach anything to.
Just as a matter of interest price a piece of steel say 32mm thick, 400x250 from a profile cutter, and find out how much it would cost to take it to a tool maker and have him throw it in the surface grinder.
I'll lay you odds its way cheaper and so SO much stiffer than granite.
Craig