[blitz355]

Hello,

I'm building a machine with an external footprint of about 1500mm x1000mm (see picture). I was reading up about ballscrews and the factors that should go into making a decision. It seemed to me that the biggest factor was ballscrew whip.
My plans for the machine are mainly based around aluminum machining and engraving and thus chose 5mm lead as it is reputed to give better resolution/torque.
Next I started considering ballscrew diameter and after looking at a few charts found on the web came up with a critical speed of about 1100-1300rpm when considering a 20 or 25 mm ballscrew.

I'm not totally decided on my drive system yet, but will most likely run a g540 w/ some larger steppers (recommended specs anyone?)

What are the disadvantages/advantages of going with 20 vs 25mm ballscrews?

Thanks in advance!

[ger21]

A large diameter screw 1500mm long has a lot of inertia, and probably needs more power than a G540 can deliver.

[blitz355]

A large diameter screw 1500mm long has a lot of inertia, and probably needs more power than a G540 can deliver.

Really? I was under the impression that alot of larger machines used the g540. Although I haven't seen a ton of ball screw driven routers that are larger.
The long axis will be dual driven if that makes a difference.

[blitz355]

A large diameter screw 1500mm long has a lot of inertia, and probably needs more power than a G540 can deliver.

Hi Ger21,
I'm following up with math (scary). Found a good site that showed how to do some calculations for stepper sizing (calculating inertia etc..) I started doing all the calcs by hand then saw nearly the whole way through someone had worked up a spreadsheet with them built in
I input an estimation based on what I know about the weight of my gantry, and the assumptions made in the article I found (see the attached spreadsheet screenshot)
I pulled some values for motor specs from cnc router parts motors paired specifically for the g540 (detent torque and rotor inertia unknown from specs)

I assumed more cutting load than assumed in article for alloys (20N, I used 25N)
According to the spread sheet as long as I stay under about 70 ipm when cutting aluminum at 25N of force, I'm ok.

Now, knowing that you are the man around here (not joking, I see you everywhere, and value your input greatly), do you think my calculations are something I can stand by?

I'm also assuming(you know what they say about that), that having two motors slaved to the long axis, can only help with overcoming the inertia needed to move the beast.

Thanks!

[ger21]

You mentioned a critical speed of 1100-1300, but 70ipm is only ~350 rpm.

What kind of rapid speeds are you looking at?

[blitz355]

You mentioned a critical speed of 1100-1300, but 70ipm is only ~350 rpm.

What kind of rapid speeds are you looking at?

To be honest, I haven't really set a 'goal' for rapids. As long as I can do around 150ipm I'd be happy (I don't do many cuts over long distances). So that's roughly 762rpm. When I mention 'critical speed' I'm referring to the speed at which the ballscrew will start to whip.
I'm pretty sure the spread sheet in the above attachment is referring to the same thing (ball screw whip).

Am I overlooking anything, or do you think I'm ok moving forward?

Thanks Ger,
Dan