Need Help!- maxium width of single leadscrew drive...

[brucer]

i'm sure this has been covered, but i couldnt find anything.. if someone could point me to a thread that they might be familiar with i would appreciate it..

what i'm wanting to ask is:
i'm planning on building a machine loosely based off of cncrouter parts design, i like the fla100, but want x axis to be around 52" wide so i can fit a 4ft wide sheet through the machine, and Y axis i'm planning on around 40"... actually laying the machine out like a cnc mill would be so the axis are actually flipped to what most router kits are that i see.. on all the cnc mills i've ever ran the x axis is always the widest axis so thats how i'm going to set up my plasma/router..

would it be possible to run the 52" wide axis off a single 1/2-10 5 start leadscrew?

mainly just trying to keep costs down not going with dual racks or dual leadscrews.. i have a stack of 80/20 so thats not a problem, i want to do it as nice as i can as cheap as i can.. i'll be doing alot of machine work myself, actually all of it..

my main focus is going to be plasma cutting, but wouldnt mind being able to do light to medium routing, for plaques,signs, misc engraving and such..

any help would be great
thanks.

[DonFrambach]

Originally Posted by brucer would it be possible to run the 52" wide axis off a single 1/2-10 5 start leadscrew?

I don't have any personal experience so take this with a grain of salt.

I think the problem you'll run into with a 52" leadscrew will be screw whip. To limit that you'll need to run your machine slowly. You'll definately want a multistart screw. A larger screw diameter would help but that will add greatly to your costs. If I needed to make a 52" axis, I'm pretty sure I'd go rack and pinion.

If you do go with a 52" leadscrew, please post your experience. Actually, please post whatever you decide to do.

[acondit]

You can get a 5tpi x 3/4" x 6' length of ACME threaded rod from MSC for $60.58 (#03199387). When I bought 4 of them a few years ago I paid $30/stick.

I believe that it is the Nook website that has a calculator that shows deflection(whip) for a given length at various rpm for a given diameter screw.

Alan

[ger21]

You're gantry probably needs to be closer to 58" to allow the tool to reach both sides of a 49" wide (MDF width) sheet.

Whether or not it racks depends on how you build it. If you run a 12-15" wide plate across the bottom and mount the nut to it, it's unlikely it'll rack, but it also depends on bearing spacing, and how rigidly the gantry and vertical sides are attached.

As for whipping, If the screw is close to 60" long, it'll probably whip at around 400-500rpm. Depends on how straight the screw is.

[johnmac]

If I were going to build a machine in your size range, I would use rack & pinion. That said, if you stay with screws, I would say a 3/4" multi start screw with as few turns per inch as possible. Stepper motors produce best torque at slower speeds, so "gearing it down for more power" isn't needed. The multi start screw will get the travel speed without spinning the screw so fast. For a direct drive setup, 2-3 turns per inch of travel is a good combination of speed and resolution, IMO.

John