What motors should I run? 5' x 10' CNC Router

[digidem]

I'm currently in the process of building a 5' x 10' CNC router with the help of my father, and we are unsure what motors we will be using. I was looking into some of the 3 axis kits out there with 400oz/in or greater stepper motors, drivers, breakout board, etc, but am concerned these small motors are not enough for such a big machine. We also have a connection with a local shop, that has a lot of extra stepper, servo motors / drivers lying around. I will most likely head down there this week and check out what they have, but hoping to get some suggestions before hand, so I know what I'm looking for.

Thanks for any advice you may have.

[ger21]

How much weight are you moving?
How fast do you want to go?
How are you moving it? Ballscrews? Rack and pinion?

[lgalla]

You need a 4 axis kit.2 for the X to prevent racking.
Larry

[digidem]

The gantry will probably be around 50 lbs. We planned on using a single 3/4" diameter precision acme lead screw (multistart?), centered in the frame with an anti backlash nut, because of the cost of a ball screw. Our design allows us to upgrade to a ball screw in the future. The two side plates of the gantry are connected at the bottom with a plate that will prevent racking of the gantry.
We are using 1" thompson linear rails and bearings for the x axis and 3/4" for y/z axis.

As far as speed goes, I'm unsure of what would be best. What should I be looking for (200ipm, 300ipm, ???).

[generalsocial]

I'm pretty new to all this. But here's some food for thought.

Is a 4x8 piece of wood harder than a 2x2?

Larger machines are just cutting bigger pieces of material so in theory don't need bigger motors.

Okay, so the gantry gets heavier which would require some extra force. But unless you're wanting to cut really fast, the inertia will not be large compared to the force of cutting.

Just a thought...

[digidem]

generalsocial, thanks for the response. I have actually thought about this, and while browsing google images have seen many 5' x 10' commercial and homemade machines that appeared to have small nema stepper motors, like those sold in some of 3 / 4 axis kits out there. I assume those I've seen were definitely 400 oz/in or more based on their size.

The random motors I have access to from the local shop, are all off heavy duty CNC milling machines. So I have a feeling they will be overkill, but if the price is right in comparison to new/weaker stepper motors, then I assume it will be the better option.

I apologize for not being more knowledgeable on the subject. Began diving into the CNC world less then a month ago, and trying to learn as fast as possible. My father has been working with CNCs and building his own machines for over 25 years, so I'm kind of leaning on him at the moment, to get a solid machine together fast. He is taking care of the frame/drive system, and I'm figuring out the electronics setup.

[LeeWay]

A 10' long screw is going to be tough on anything to spin up to speed. If I were going to build one this size and had to use screws, I would make the gantry 10' long so you only move the Z axis with the long screw rather than the whole gantry.

Personally I think I would really consider R&P on such a big machine. It will require 4 motors, but will be a better machine and easier to repair if needed than a screw driven machine. I'm not sure what type ways you are using, but CNC router parts has a nice and simple relatively inexpensive system that I would consider for anything that size. I built similar carriages on my 2x4 build and they work great. Just a suggestion so you don't lock into a design that will have some issues from a design point.

[ger21]

Originally Posted by generalsocial I'm pretty new to all this. But here's some food for thought. Is a 4x8 piece of wood harder than a 2x2? Larger machines are just cutting bigger pieces of material so in theory don't need bigger motors. Okay, so the gantry gets heavier which would require some extra force. But unless you're wanting to cut really fast, the inertia will not be large compared to the force of cutting. Just a thought...

Unless you're running a 7-10HP spindle at very high speeds, cutting forces will be negligible, probably 30lbs or less.


I think you're estimate of 50 lbs for the gantry is 2-3x too light.

As for the 10ft screw, I'd recommend a 1" pitch, and 600-800oz stepper. A servo with belt reduction would be preferable. That's how big heavy duty routers do it, if they don't use rack and pinion. You can probably get away with 400oz on the Y, but it really depends on the design.

And for a machine that size, I'd want at least 500ipm rapids. It would take 30+ seconds to go from end to end at 200ipm.

[digidem]

Originally Posted by ger21 Unless you're running a 7-10HP spindle at very high speeds, cutting forces will be negligible, probably 30lbs or less. I think you're estimate of 50 lbs for the gantry is 2-3x too light. As for the 10ft screw, I'd recommend a 1" pitch, and 600-800oz stepper. A servo with belt reduction would be preferable. That's how big heavy duty routers do it, if they don't use rack and pinion. You can probably get away with 400oz on the Y, but it really depends on the design. And for a machine that size, I'd want at least 500ipm rapids. It would take 30+ seconds to go from end to end at 200ipm.

I will be using a Porter Cable 1 3/4 HP, 23,500 RPM, that I already have.

Do you think 750oz or higher Nema 34s, would be suitable? Would a single one centered on x frame, be able to drive the 10ft acme lead properly?

I should know within the next few days what is available to me from the shop. I may be able to get servo motors for the price of steppers.

The machine will be used for large scale art and sign making primarily, possibly furniture later on. It will be built with 18" or more of Z travel. After I've got a good feel for 3 axis using Mach 3, I will be using a 9" or bigger rotary table w/ motor for 4th axis. I plan to use the 4th axis for creating various complex objects.

[Phife]

5x10 w/ 18" Z! You will have issues with the long Z and the single X screw. a 5ft wide gantry driven from the center will rack when cutting near the edges, unless its seriously beefy.

Also the entire gantry will have to be pretty beefy to support such a long Z. Waay more than 50lbs, For such a large machine with a 18"Z you'll be looking at 200+lbs for the gantry.

Dont forget you'll need to machine the surface the rails ride on..

Its a possible build but will require some $$ and a good design.

When you get some drawings done post them up, lots of people who can help ya out.

Good Luck!

[digidem]

I mentioned all these posts to my father. He is still certain that racking won't be an issue, and that the gantry will be under 100 lbs. He has designed and built various manufacturing / packaging machines his entire life, so I'm trying to just trust his design, and inform him about other peoples opinions.

He wants to help me get the machine up and going very fast, so I'm not sure we will even get to creating digital drawings of many parts.

[LeeWay]

I think there are ways that you can get around the racking issue, but I will be interested to see how you achieve rigidity at that size for the gantry.
I will keep watching. Good luck with it as well.