operation cnc

[JoeHarris]

I am working on a similar sized machine and have been through the same questions on ball screws - seemed to me there was a lot of different opinions floating around. I too decided that spinning ballnuts was a bit out of my league! I went for 2005 in the end, the larger dia will hopefully help with whipping and I am going to gear it 2:1 so as to get faster feed rates, with the option of gearing 1:1 for high res work...

I am coming round to the idea of using steel (ill have to learn to weld tho!) for the frame and x axis but Ali extrusion does seem the best option for the gantry. The L shape arrangement used in a few builds allows you to keep the gantry squat and rigid. Why does your gantry need to be so tall and thin...?

If you need any help with sketchup just ask I have been using it for years and have just about got through the pain in the ass stage !

[Jonathan]

I went for 2005 in the end, the larger dia will hopefully help with whipping and I am going to gear it 2:1 so as to get faster feed rates

Not really, you'll end up worse than with 1610 regardless of the drive. Critical speed is proportional to the diameter, so going from 16 to 20 means the ballscrew can rotate 25% faster. However to get the same feedrate with 5mm pitch (2005), you need to spin the screw 100% faster, but can only go 25% faster... so you end up with 62.5% of the feedrate attainable with 1610.

If the L-shape gantry is reinforced by putting some box section on the back at 45° to make triangles it will be very strong.

[Jonathan]

I'm not keen on how far the Z-axis overhangs (parallel to X). Presumably you've done that to position the Y-axis ballscrew more optimally than having it on the back, a long way from the cutter? That's sensible, but I would at the very least try and add pockets to compact the design, or add a second piece of extrusion above the existing one such that the ballscrew can run between the two, i.e. directly between the rails which is optimal.