New Machine Build- 4'x8' table needs motors!

[HenryK]

Hello cnczone!

My name's Hank; long time lurker, first time poster. I just made my first post in the "new builds" thread here:

http://cnczone.com/forums/showthread...059#post810059

I'm feeling pretty comfortable with the structural design, and am confident in my mechanical abilities. Electricity, on the other hand, is magic, and I think I could use a bit of guidance.

A few basic stats:
- 120 lb gantry
- x axis: dual 380oz motors driving rack and pinion (cncrouterparts.com)
- y axis: single 380oz motor driving rack and pinion
- primary material being cut: plywood, up to 3/4 (which I hear requires more "oomph" than most any other material)
- router: 2.25HP Hitachi M12VC (although I may be pressured into a 3.5HP Milwaukee if push comes to shove

The simplest and most affordable option when it comes the electronic system is the 4-axis package Ahren is selling at cncrouterparts.com. It includes everything I need, and everything is correctly matched, and has been shown to work (quite effectively!) in built machines. My only concern is that given the size and weight of my gantry (about 120lbs) it may not provide the "oomph" I need.

I'm still learning about feed/jog rates, and I've found a couple of useful equations, specifically Ahren's math in post #6 of this thread:

http://www.cnczone.com/forums/showthread.php?t=103392

I applied that math to the 382 oz-in Keling motor, because I hoped the provided torque curve would be similar to the 380 oz-in motors in Ahren's kit. If this math actually works out, at a gantry speed of 1100ipm (the same speed from the linked example) The keling motor would *ideally* provide 60in/lb of torque per side, or a total of 120in/lbs of torque. The same math gives the Y axis a total of 60in/lbs of torque.

The problem is that I don't really know what this means, or if it's enough. I know some of the energy will be needed to push the bit through the material, some energy will be needed to accelerate/decelerate the gantry, and some energy will be flat out lost through mechanical means. The only estimate I could find for max force at the bit is 150lbs, which this setup would not be able to provide.

I've been looking at the alternatives, and I'm drooling at the powerful prospects, but don't want to double my electronics budget if it's not necessary. The gecko 203v's are the only driver alternative I would consider, and I would be leaning toward motors in the 640oz range (900oz seems excessive, no?). I would be willing to make this investment if I were convinced the 380oz/g540 setup would need replacement in the near future. There's just no need to waste $600.

I would love to get some input from those of you who have used similar setups, and if I'm convinced to upgrade to the larger motors, I'd appreciate any guidance in how to purchase parts that are well matched (volts, watts, amps... all that good stuff

Lastly, it seems I am a bit electrically challenged (bad luck in all my vehicles), but I just bought a fluke multimeter for a few upcoming projects, and I'm ready to roll my sleeves up and become just a bit more proficient! Just appreciate that it may take some dumbing down to get "electric magic" through this thick skull

Thanks again for sharing your expertise and experience!
Hank

[bobsch]

Many folks around here use the combos from here: http://www.kelinginc.net/CNCNEMA23G540Package.html

Check the third and fourth options on the page. A Gecko G540 and either three or four 381 oz/in steppers. The steppers are a perfect match for the G540.

[ger21]

120lbs is not a heavy gantry at all, especially for that size. I wouldn't be surprised if yours ends up at 200 lbs.

As far as 150 lbs at the tool, the Hitachi would stall well before it reached that point. As a guess, I'd say 50 lbs max, probably 1/2 that is more likely. The Hitachi will probably be the limiting factor as far as feedrates are concerned.

[HenryK]

One of the great things about discount steel's web site (onlinemetalstore.com) is that when you select a specific type and length of material, they tell you the weight as well. I'm a little relieved to hear that 120lbs is considered light, and will do what I can to stay under 200 pounds!

What I'm gathering from your reply is that even with the 380oz motors, you anticipate the router itself may be the limiting factor. If that's the case, I may just leave room for expansion to a Milwaukee 3.5HP router, so that in the future I have the option to upgrade if I feel limited.

Is anyone not convinced 380oz is enough? Or can I sleep soundly tonight on this mini consensus?

[ger21]

What I'm gathering from your reply is that even with the 380oz motors, you anticipate the router itself may be the limiting factor. If that's the case, I may just leave room for expansion to a Milwaukee 3.5HP router, so that in the future I have the option to upgrade if I feel limited.

Depending on what you want to do with it, the Milwaukee may not get you much more.

Is anyone not convinced 380oz is enough? Or can I sleep soundly tonight on this mini consensus?

The real question here, is, enough for what? I don't think even you can answer that question, so noone can really tell you if it'll be enough.

Well, actually, you may find plenty of people here that will tell you it's enough. But is it?

I didn't comment on the G540 / 380oz combo, because I've never used one, or even seen one in person. But here's my observation.

I've seen a lot of Joes 4x4 rack and pinion machines with that same combo, running at ±1000ipm. In every example I've seen, the acceleration is quite a bit less than I'd like to have, if I was running at that speed. I'd actually trade a few hundred ipm for more acceleration. Acceleration has a big affect on the machines performance.

I can tell you that if I were building a 4x8 machine, I'd be using bigger motors and drives. Hopefully in a year or so, I'll have some first hand info in this area.