cnc knee mill gearing

[mechhamo]

hi everyone , i have a question if someone could help

i have a acra knee mill i want to convert to cnc ,i have posted before about this when i was looking into doing it about a year ago but only now i have the time to do it
i have done my research in relation to ballscrew gearing ,critical speed of the ballscrew what servo motors and other things as well , i have decided to go with a 5mm lead with a two to one gearing and 750 watt 3000 rpm servo motors and achieve about 7500 which to me is fairly fast for a knee mill and i most likely won't be able to rapid at that speed due to being a knee mill so i thought that combination would be ok and at 3,000 rpm not to reach the ballscrew critical speed and still have fair speed
my question is if i was to use a 10mm lead with 3.1 gearing with the same servo motor speed, instead would that have an affect on the resolution of the machine considering the encoder works over 360 degrees
meaning will it affect the accuracy to achieve a rapid speed of 10000 and be able to lower the ballscrew speed to 1000 way under the 2000 critical speed for a 25mm ballscrew

thanks in advance
kind regards wayne

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[Jim Dawson]

Every BP type CNC knee mill that I have seen either uses a 0.200'' or 5mm ballscrew. I can think of no reason to use a 10mm ballscrew. I'm not sure what your value of 7500 is, mm/min?

My machine has 750W, 3000 RPM servos, 2:1 reduction, and can hit 300 IPM rapids, but at 200 IPM it is physically unstable and frightening, and I limit the rapids to 100 IPM on the XY, and 30 IPM on the quill. I could bolt the machine to the floor and use higher rapid speeds, but I haven't found a good reason to do that. With a 32x14 work envelope, it doesn't take very long to rapid the full travel at 100 IPM and I'm not in that big of a hurry. For most work, the rapid moves are short anyway. If I'm in a hurry, I use my Haas with 400 IPM rapids. The BP clone is not a production machine, the Haas is.

[toyoman]

I second what Jim is saying, 300 IPM is way too fast for table rapids on a knee mill. 120 IPM is what my Acorn is set to and its plenty fast. Accuracy of axis movement is not just affected by the servo encoder but also the quality of the ballscrew/nut assembly, thrust bearing preload and arrangement at the ends of the ballscrew, the controller backlash compensation, and the overall geometric accuracy of the mill itself. A 5mm lead ballscrew and 2:1 gearing will work just fine.

[mechhamo]

thank you very much ,toyoman and jim your advice is very appreciated ,your advice was just like i thought

thank you i will post videos as i am converting it

thank you