Servos on x/y - Stepper on z - will it work?

[WoodSnarfer]

I was thinking about this...it would be nice to get the direct-coupling of a stepper for the z axis, since that axis typically has the least torque requirement, and I'd like to keep the design of that axis simple. But for my x and y axes, I'd like to use servos.

Is it possible to mix servos and steppers this way? I'm thinking that I would just buy the appropriate driver module for each motor -- but what about the power supply? Can I get into trouble by attempting this?

Thanks,
Chris

[arvidb]

It should be ok, but as you say you will either have to find servos and steppers (and drives for them) that can run on a common power supply (i.e. same voltage), or you will have to use separate power supplies.

Arvid

[HuFlungDung]

I'd be cautious because the Z axis actually may have a high torque requirement when it has to raise the tool. Perfect counterbalance would help, but the inertia of what must be moved and stopped is still there. Plus, all servo axis is a good selling feature later on

[WoodSnarfer]

That's a good point...a smaller servo with the appropriate gear/belt might not be all that bad...

Related question: when designing a (for home use) cnc router, is it okay to 'ballpark' the weight and force requirements of the axes, and then start shopping for motors? In other words, if I take the z-axis and figure out how much the nut/plate/motor mount weighs, and add the weight of the router motor, figure the net torque factor transmitted at the nut (after both the servo gearing and the ball screw) add a coefficient of friction -- then add the required downward pressure for typical routing/drilling -- then add a buffer of about 50% -- is that the way to come up with an idea of my servo requirements?

I'd really like to buy/build all the electronics, and have something to play with on my bench (actually hooked up to a PC, with CAD/CAM running) -- before I start building the router itself.

Is that a good way to think about this?

Thanks,
Chris

[ger21]

Like Hu says, I sure read a lot of messages about people losing steps on their Z-axis, and always thinking it's some other problem, when it's really underpowered. Instead of 50%, I'd double what you think you need. The force required to push down isn't the problem, it's abruptly pulling it up while it's moving down.

[BigDaddyG]

All,
It may sound odd, but my "Old Man" (god bless his sole) told me years ago a formula that I still use to this day. Estimate conservatively and come up with a number - Double it and add 50%. This works for everything from Job pricing to force factors. I have spent fortunes over the years on all kinds of costing / estimating software, and I almost always fall back on this concept. Use your best guess based on your knowledge and what ever engineering formulas you have, double it and add 50%. You will be surprised to see how often it turns out to be correct and in concert with your best software / formula's that are out there.
Regards,
Glen

[arvidb]

Double it and add 50% - isn't that the same as triple it?

Or perhaps it's 50 % of the original estimation - in which case it's multiply by a factor 2.5?

Arvid

[BigDaddyG]

Yes, the math is easy and is what it is! The lesson and the premise - is that actual cost 2x your best guess and a profit factor of 50%, then there is the lessons learned from the past generation! As a young man, learning to turn handles in the garage with my old man and memories of better times as we worked on handguns (my dads hobby, my memories). There were plenty of other lessons learned over time, but a simple phrase of which has been with me for decades, and as expressed, works to this day. If he would have said to me at the time, multiply by 3 would I have remembered the lesson, I dont know. But the concept of doubling the best guess estamate and adding a factor was the lesson learned by a generation that went to the moon and back using slide rules. Something simple, something passed on and a respect to/of the past.

[HuFlungDung]

3 is dangerously close to Pi, which happens to be how long it really takes to complete a project, versus a ballpark time estimate