New Machine Build- 55"x70", leadscrew advise needed

[Gads]

hi all I could use some input on a new machine build i'm trying to work out.
my plan is a 55x70 or 60x60 moving gantry router machine. all riding on roller bearings. my gantry is 8020 (3060) with porter cable variable speed router, weight without my slide stuff at this point is about 50lbs and estimate total finished weight between 50-100lbs. my steppers are 640oz 4 wire motors and gecko's i have running my mill right now. my thought is to use hi-lead 3/4-.500 multistart acme with plastic nuts on x and y. the x i plan on using 2 screws coupled with belt/pulley and no gear reduction on any axis. I have tried to calculate screw torque requirements, critical speed and desired ipm etc.. but am no engineer. my thoughts are based on 20-150ipm feed and 200+ rapids keeping my motor rpms below maybe 700rpm. any experience or advise would be welcomed as i need to make a decision before my toy goes any further.

Thanks all

[Jesse B]

I think that you will get excessive whipping of the leadscrews only going with 0.75" over such a distance. I'd suggest upping the diameter.

[CarveOne]

Originally Posted by Gads hi all I could use some input on a new machine build i'm trying to work out. my plan is a 55x70 or 60x60 moving gantry router machine. all riding on roller bearings. my gantry is 8020 (3060) with porter cable variable speed router, weight without my slide stuff at this point is about 50lbs and estimate total finished weight between 50-100lbs. my steppers are 640oz 4 wire motors and gecko's i have running my mill right now. my thought is to use hi-lead 3/4-.500 multistart acme with plastic nuts on x and y. the x i plan on using 2 screws coupled with belt/pulley and no gear reduction on any axis. I have tried to calculate screw torque requirements, critical speed and desired ipm etc.. but am no engineer. my thoughts are based on 20-150ipm feed and 200+ rapids keeping my motor rpms below maybe 700rpm. any experience or advise would be welcomed as i need to make a decision before my toy goes any further. Thanks all

For considerably less than the amount of expense that you will incur by using 3/4" multi-start lead screws, bearings, end supports, anti-backlash nuts, rails, and other parts, you could use cncrouterparts.com rack and pinion drive assemblies and carriages. You will then have none of the whipping issues and easily get 1400+ ipm jog speeds. Everything about them is easier to figure out and to install. Anyone designing a machine with over 3-1/2 feet of travel should seriously consider using them.


CarveOne

[Aryantes]

I'll agree with the previous posts.

With a machine that large, I would seriously look into getting a Rack and Pinion set up.

It won't be much more expensive. It might be cheaper actually.

You will avoid the issues that screws have, which is normally whipping and resonance, when they have to be extremely long.

Unless you are going to invest in screws that are more than 1" in diameter... maybe 2"... but that'll break the bank unnecessarily.

[johnmac]

Take a look at commercial built machines of this size. They use rack and pinion, no screws.

John

[ratpack]

I used rods on a 60" gantry but they whiped to much not very good at all.

[Benonymous]

All the large scale machines I have seen use rack and pinion. Screws that long will really slow your rapids down due to whip. It may not sound like a big deal now but watching your machine crawl along between cuts will drive you nuts. R and P all the way!

[Grunblau]

Turn the nut instead of the screw. My table is 98" long with a 5/8" lead screw. By fixing the screw, this allows you to tension the screw to eliminate sag and you don't have to worry about any whip.

BTW... All commercial machines (100"X50" laser cutter and 100" X 50" Techno-Isel CNC) I have used are lead screw based but are fixed. The laser cutter has a 3/4" screw and the CNC has a 1" ball-screw.