Hi Guys. I had initially planned on using two screws to drive the X part on my CNC build. I am wondering if this is necessary though. My rails may be 40" long and hopefully yeild about a 32" travel of cut on the x. The y axis might yeild a 20" cut. It will be a traveling gantry type. I am using 5/8" acme rod. I'm using 7/8" hex stainless for these rails. 270 oz. steppers. Using some iteration of six skate bearing trucks on the hex. Three bearings on front and rear of each truck.
Pros and cons?
Thanks guys.

If you are using a traditional gantry design (sides of the gantry extend below the cutting table and are tied together underneath the cutting table), then it makes more sense to use a single, centered drive screw.

ACME screws have an energy transmission efficiency of only between 35% and 48% -- so the extra screw is going to eat up a lot of your motor's torque.

The extra cost (don't forget belts, pulleys, end bearings, etc.) make it even more unattractive.

If your gantry is some other design (e.g. an overhead gantry riding on elevated rails), then that might force you to go to the two-screw method.

-Chris

Thanks Chris.
I originally had considered not joining the gantry at the bottom. With this concept, the logical soloution to the twisting of the gantry was the two screw method.
I do think that the idea of raising the whole machine up some and hard framing the whole gantry is a much more efficient way to go. A singlle screw should easily push the 10" to 12" on either side without wanting to rack. The trucks should be solid and have a good span (6" to 8") on the rails.
Thanks much for the input.
The parts are starting to arrive and I am going to get a final plan drawn up to share. Nothing like the caliber of plans that I have seen on here. Just something to show which end is up. :)
I will also start a build log when I get to framing the thing.

LeeWay,

You would have to buy matched screws to prevent the twisting, the drive motors would have to be perfectly synchronized...2 screws is a "big headache!"

Lee, 90% of the people who hang out here (including me) haven't made it past the planning stage (yet) -- so if you are moving forward on building, the rest of us are going to be watching, and cheering you on...best of luck to you! :cheers:

I have to move forward on this. I can barely keep up with orders for the product that I am making. I love the way my metal cutting bandsaw works. Set it up, turn it on and it does the work. Its not even CNC. :)
I am cutting out, then hot glueing parts to a template to rout out by hand, then welding the parts together having to insure squareness now. With the CNC approach, I have a design where I can cut the part all out of one small sheet and then fold and weld. Should be a temendous frustration saver and possibly time saver as well.
Its going forward. :)

The material is polycarbonate that I'm welding. Sounded like I was talking about steel when I reread my post.