1. I'm building a cnc for wood and aluminium.

I have a 1000mm y axis ballscrew, 1605
Also, 700mm x axis 1605. (Haven't purchased yet, 2005 maybe?)
Z axis will probably be 250mm ballscrew.

I'm looking at my z axis. It has the 2.2kw water cooled spindle.

I was automatically going to get a 1605 for z axis but, smaller would be easier on the build and am I right in thinking the 4mm pitch would be more accurate, but slightly slower?

Any real disadvantage of using 12mm ballscrew on Z axis? Cost isn't a big factor, just wouldn't mind a slightly smaller assembly.

Am I better with 4mm pitch over 5mm? Accuracy preferred over speed.

Thanks.

2. For a Z axis it's more about holding force over speed. The shallower pitch will almost certainly deliver as much Z speed as you need whilst requiring less holding torque.

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4. Ok. So ideally, I'll see if I can find a 2004, or I'll use a 1604.
Thanks again.

5. Generally, use the smallest diameter screw that will do the job. In this case, doing the kind of back of envelope sanity check I like to do:

Target max Z speed = 2000mm/min. You probably don't need this speed for Z but let's start here. 4mm lead screw needs 500rpm at that speed. What is max critical speed of 250mm screw, 12mm diameter? I plugged these numbers into a critical speed calculator (use Google to find - there are plenty online) using 8mm as ballscrew dia to allow for thread depth. It came out with something like 4K rpm, so this screw is well within its max speed. So I would seriously consider 1204, and keep weight down. It will be plenty strong enough.

In case you haven't met it, critical speed is max rotational speed of shaft before it starts to whip and become unusable.

500 rpm is well under typical corner speed (I.e. roughly max usable speed of stepper before its torque falls away) so 1-1 drive would be fine.

That's the kind of thinking I do to get a starting point - you can refine things from there based on trade-offs of the various parameters.
Last edited by Neale; 22-01-2018 at 09:57 AM.

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7. ThatS excellent info. Thanks guys. 1204 is what I'll go for.
Thanks again.

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