Choosing a drive screw and motor for a long axis?

[CnoEvil]

Hi Guys,
I'm working out my plans for building a cnc router unit and I have a couple of questions. Although I know I probably should start smaller since this is my first machine, for the type of work that I want to do with it I will need a large one. I want to be able to cut full sheets of plywood and mdf so I've determined that something in the 5' x 9' size will be necessary. Due to space constraints and since the x axis is such a long travel, I've decided to go with a moving gantry and a 4 axis setup with dual drive screws.

I've been looking at the xylotex 4 axis kit with 425 oz./in. steppers. My question is, will dual 425 oz/in steppers be sufficient to drive such a long x axis or should I get them to upgrade my motors to the 497 oz/in ones? If not the 425 oz/in steppers, would the 497 oz/in ones be enough?
Once this riddle is solved, here again I am confronted with a question of drive screws for a 9 to 10 foot travel. I was planning on using acme precision threaded rod. At such a long span, I'm wondering what my most effective choice would be to minimize sag and flex in the rod and still keep the costs down. Initially I was thinking of going with a 1/2"/10, but maybe I should go to a 5/8" rod or even a 3/4"? What is the general consensus on that?


BTW, this is a great site and I've enjoyed reading and learning about cnc here. I hope some of the wiser and more experienced builders can help me with these questions.
Thanks,
Mark

[jcc3inc]

Sir,

With a 9' long leadscrew, you will have high speed "whip" of the screw. If you look at a web site of some leadscrew mfg there is a chart for determining critical speed. You MUST consider this in your design. From experience, I would use rack and pinion as all manufacturers of commercial machines do.

Regards,
Jack C.

[ger21]

The Xylotex does not have enough power to move a machine that size at reasonable speeds.

And as for screw size, I'd say at least 1-1/4in or bigger.

[CnoEvil]

Wow... that's a really big screw, ger21. I hadn't anticipated it having to be that big. The cost of a screw that size makes it easy to take the plunge and go rack and pinion. I was thinking acme rod because typically rack and pinion is more expensive, but with a screw that big I'm not so sure.

Would the size of the motors needed change if I went to rack and pinion?

As for the amount of power I would need, I've looked at the charts that folks use to determine that but it looks like brain surgery to me. Does anyone have a more simple "rule of thumb" for determining these kinds of things?

What would be the largest table I could make using the xylotex steppers? Does anyone else know where to get stronger ones at a reasonable price? Would I have to go to Gecko drives to power a motor strong enough to use on a big table?


I may not be able to afford a large table afterall.

Mark

[wcarrothers1]

I'm glad I went rack.. Actually if it didn't eliminate weight off the gantry where ever Z was located I'd say having both x and y rack would be nice since with rack vs screw you can move the gantry with out the motors needing to be activated..

My X rack became practical cost wise on my machine after I found a guy willing to sell me a 10:1 thompson planetary trans. With out that (which only cost 75 bucks on ebay) X would not have probably worked as good as it does as I'd have to do multiple belts to decrease the gearing and such.

Still for long runs I think it better then a screw especially if your machines tollerances arn't perfect enough to deal with a center only screw..

And rack moves faster. (least in my case)



b.

[Dylwad]

We're running 640 oz steppers with Geckos on a 6 foot 1/2-8 2 start acme with big angular contact bearings and dumptstercnc anti-backlash nuts. All steel frame with probably 500 Lbs of tension on the 6 foot acme to get 300 IPM rapids with no whip. Running Deskcnc. I would call this the limit for our components, but i have to say the machine has been running flawlessly for 14+ months now.

9-10 feet is big for a first machine, and may be too big for xylotex if you want reasonable rapid speeds. Remember steppers make the most torque when they are stopped, and torque drops off with RPM.

A 5 foot wide gantry will have some mass as well, which should be a consideration, imagine the power it would take you to accelerate a 300LB weight from stopped to 300 IPM in less than a second, with accuracy, now add drag from a router bit cutting 3/4" mdf, lots of power if you ask me.

The best information i have found is Geckos stepper motor basics http://www.geckodrive.com/support.cfm should be a big help with some of the design process. Another good source is nook industries http://www.nookindustries.com/

steppers and Geckos from http://www.homeshopcnc.com/
anti-backlash nuts from http://www.dumpstercnc.com/
angular contact bearings from http://www.vxb.com/page/bearings/CTGY/ANGULAR_CONTACT
acme rod from http://www1.mscdirect.com/CGI/N2DRVSH

I built our own power supply usiing the info from Gecko, out of an old microwave transformer re-wound for the correct voltage.
www.partsexpress.com has decent prices on toroidal transformers.

Good luck, its an amazing feeling watching your machine move for the first time.

Dylan

[thkoutsidthebox]

Are the racks for the rack & pinions machined at the ends so that they can simply be placed end to end to have longer lengths, or do you have to buy a rack the full length?

I havent seen them over 8ft yet.

[Dylwad]

You can butt pieces together, using a short scrap piece of rack to align the teeth before bolting them down.


Dylan

[walter]

Going small first is a good advice. You could build a cheap mini router using Xylotex, learn the dynamics and have fun.

If you go 5' x 9' (especially with Xylotex)- you'll be greatly disappointed.

Check out this site, Zone member used 9 ft 1.75" ballscrew with triple stack bipolar 1100oz stepper and Gecko drive. That didn't work- he had to go servo route...

http://oneoceankayaks.com/madvac/x_axis.htm

[macona]

Another way to eliminate whip is to rigidly attact he ends to the frams. Using a couple thrust bearings and a pulley you spin the nut with the nut being coupled to the y axis gantry through the bearings. Not only no whip but you get faster acceleration through not spinning a big chunk of steel.

[ger21]

Originally Posted by walter Check out this site, Zone member used 9 ft 1.75" ballscrew with triple stack bipolar 1100oz stepper and Gecko drive. That didn't work- he had to go servo route... http://oneoceankayaks.com/madvac/x_axis.htm

Where on that linked page does it say it didn't work?? I didn't see any mention of servos.

[CnoEvil]

OK, another thought / question / shot-in-the-dark.


If I were to stick to keeping my gantry at 100 lbs. or less and go with dual rack and pinion drives - one on each side - would I be able to get away with using two 497 oz/in steppers on the x axis? I don't intend to use a big 3 hp router or anything like that, just your typical standard sized router. I doesn't need to be super fast...just not mind numbingly slow.


I know I would probably be much better off building a smaller unit to start with, but doing the work that I do, a smaller unit would be of limited usefulness. Since my funds are pretty limited as well, who knows how much longer it would take me to scratch up the loot to build a big unit after I spend my money on a decent smaller one.


It's not that I'm dead set on xylotex either, it's just that they sell a whole package in a price range that I can afford.


Thanks,
Mark