Need Help!- 4th Axis Slave Stepper or Not?

[Dusty Mills]

I need a recommendation. I’m putting together a 36” x 48” router, utilizing ½ -10 Acme lead screws. I’ll be using one of the stepper motor packages from Keling Technology with a Gecko G540. I plan on running one of the water cooled Chinese spindles on an aluminum hybrid gantry.

The question is, on such a setup, does anyone have experience or a suggestion on whether it would be beneficial to use, or not, an additional 4th stepper as a slave on the X - axis?

(Mods - I may have originally placed this question in the wrong forum "FAQ")

[ger21]

1) If it's a moving gantry, than driving it from both sides with a slaved motor will eliminate racking that can occur when using a single screw.

2) 1/2-10 acme is a terrible choice. It'll whip like crazy, and severely limit your speeds. 1/2-8 2 start can get you around 200ipm, but the 48" screws still may whip a bit at higher speeds. 1/2-10 5 start is the preferred screw, for maximum speeds.

[Aryantes]

I agree with ger21, definitely use a screw with a higher lead.

1/2 10 - 5 start at the minimum in my opinion.

With screws that long, you want to make sure your RPMs are not that high on the screws or else you will get whip and unwanted results.

I had two 1/2 10 single start screws for my 60" x axis, 2 motors and couldn't do more than 50 ipm or so before I started getting resonance and whipping issues. That translates to 500 rpm on the screw.

If I had 5 start screws, my stepper motors would have been giving more torque at that RPM AND at 50 ipm gantry speed, the screws would have only been turning at a stable 100 rpm.


On another note, I think that driving both sides gives you a lot more functionality, like auto-squaring by homing, no racking, and more torque.

[aslatte2]

I am building a similar machine except the x axis is going to be 72". Will "3/8 X .500 Torqspline .5 in./rev. 4 start" be a good choice? Also when adding distance to the in./rev. do I need stronger stepper motors? I am currently plaining on using 160 oz. steppers.

Thanks,

Alex

[Aryantes]

With a machine that long, I would definitely recommend against using leadscrews.

In fact, maybe even 48" is borderline ( or maybe I don't like leadscrews because of my recent experience. slightly bent, wobbling, whipping. ugh)

Look into roller chain drive or rack and pinion.

If you do want leadscrews, go bigger than 1/2", definitely not smaller.

Stepper motor torque drops off as they run faster. using a 2 Turns Per Inch screw like you are planning is a good choice.

[debetto]

Have you looked at Ahren's R&P drive at cncrouterparts.com? This is a good way to go for a machine the size you intend to build.

[Dusty Mills]

Originally Posted by debetto Have you looked at Ahren's R&P drive at cncrouterparts.com? This is a good way to go for a machine the size you intend to build.

Thanks for the info and help guys!

I considered going with a rack and pinion system, however my professional experience is that racks, unless you have a precision double loaded pinion, leads to some undesirable backlash problems. While this may not be of much concern for some of the uses on shop built machines here, I'm more inclined to go with a larger dia. ACME screw or best choice rolled Ball Screw.

My dilemma is I want to build a machine, within a budget, for proof of concept, where the parts carry an equal value to performance characteristic.

Speed on such short distances is not as big a factor as you may believe it to be.

[ger21]

Speed on such short distances is not as big a factor as you may believe it to be.

A lot of people will tell you that it's a far bigger factor than you believe.

[Dusty Mills]

Originally Posted by ger21 A lot of people will tell you that it's a far bigger factor than you believe.

I have personally run tests on 5' x 10' tables with both systems working complex patterns utilizing most of the bed cut dimensions. The differences are seconds and can be eaten up in material handling time. Since I will be doing mostly 3D woodwork, a maximum of 150/ipm is all I expect with the cutters and material.

Where speed is an issue, is when you are placing multiple parts on the bed and running long distances between them, or between stops on patterns such as with point-to-point machines.

[Aryantes]

Originally Posted by Dusty Mills Since I will be doing mostly 3D woodwork, a maximum of 150/ipm is all I expect with the cutters and material.

I don't believe you'll be able to get this out of single start screws before getting screw whip and resonance.

And, your stepper motors will have lost a lot of their torque going at 1500 rpm. At the same ipm on a 5 start screw, you'd have 300 rpm, and be in a position where your motors are performing much better.

This is a pretty helpful resource for all leadscrew information.
http://www.nookindustries.com/acme/AcmeCalculators.cfm

Speed is only one of the benefits of a multi start screw.

[Dusty Mills]

Originally Posted by Aryantes I don't believe you'll be able to get this out of single start screws before getting screw whip and resonance. And, your stepper motors will have lost a lot of their torque going at 1500 rpm. At the same ipm on a 5 start screw, you'd have 300 rpm, and be in a position where your motors are performing much better. This is a pretty helpful resource for all leadscrew information. http://www.nookindustries.com/acme/AcmeCalculators.cfm Speed is only one of the benefits of a multi start screw.

So probabaly a 5/8-5 would work best for 48" at those speeds.