Gantry ACME Screw Placement

[evena4]

Okay, this is my first thread. I am building a 4' * 6' Gantry style table. I have just completed welding up the frame out of square tubing. I am planning to use the table to push a plasma cutter across a table of metal grid and then use it to do double duty with an MDF top and wood router due to my various projects. May want to put a mini-milling head on it later. But I suspect I will be converting my Harbor Freight mill for heavier work after thsi one is working.


This is also my first attempt at building one of these.

The gantry will be an aluminum frame with 1" linear bearings sliding on 1" shafting suspended about an inch over one axis of the frame. I have not cut the end brackets for the shafting yet since my original plan calls for one end bracket on one of the shafts to also house the stepper drive and bearings for the end of the 1/2" ACME screw. My question is whether it is better to install the ACME screw so that it pushes the gantry above the rail, below the rail or on the outside of the gantry parallel to the rail. Also, is it better to place the screw as close to the rail as possible or if the gantry has a height of about 12" should it be located above the rail at the midpoint.

This is probably a crazy question but I am concerned that thrust from the actuating assembly may cause the gantry to crab down the rails. The bearings I have chosen do not have ball bearings in them as this is a hobby project for now. If I need to go to better bearings in the future, they are standard sized and held in with snap rings.

Any thoughts appreciated.

-Manny

[paulC]

Try to position in such a way that the screws are protected from dust etc.
With a 4' gantry router you will need to drive both sides of the gantry.

Paul

[evena4]

Paul, thanks.
I have considered using two opposing motors one on each shaft or possibly a single larger motor mounted in the center of one end driving a belt that would go around both shaft cogs turning them the same direction from the same end.

I expect the gantry to weigh in at 20lbs with a router and considerably less holding a plasma cutter-torch assembly.

I have seen other DIY machines on this site and elsewhere and am trying to incorporate what others have done though each machine seems to be sized to particular applications. I am a hobbyist and expect to find many uses. If two screws/motors is a must for a gantry that size/weight that's okay, I'll consider cutting the brackets and put identical ones on each side. I don't have another matching screw so I'll try it with one and if it doesn't work, order another screw and use my spare motor.
After you mentioned dust control, I started thinking that I could put the screws outside under a U-Channel which would make the positioning moot. I probably should do some more thinking and see if any more suggestions arrive before I start in on it.

Thanks,
-Manny

[ger21]

Originally Posted by evena4 I expect the gantry to weigh in at 20lbs with a router .....

Care to share how you manage this? I'm guessing the Y axis rails and bearings will weigh more than 20lbs. And your probably not counting the Gantry (Xaxis) bearings, which are part of the gantry) Add 5-8 lbs for the router, an entire Z-axis, and I'd be surprised if you're gantry weighs less than 50-75lbs.

[evena4]

I guess I should put collect all the pieces, put them in a box and put them on a scale before I speak.

I was thinking of extruded aluminum for the gantry cross-supports, 1/2" shafting and some machined inverted T-shaped pieces for the sides about 12" tall, 8" long with the lower 2" being bored to accomodate the a linear bearing at each end (2 bearings per gantry side).

Time to either dig out the scale and ponder real life values or build it and see what happens, Hmmmm! I wonder which will happen first...

I had not accounted for all those extra rails/screws/motors that will be hanging on the gantry.
Thanks for the reality check.
-Manny

[ger21]

Mines wood, with conduit/skate bearings for the Y axis, and weighs about 75lbs. Thought it might weigh 40-50 before I built it.

[spalm]

Hey Manny, welcome to the Zone.

I must say, I just had a chuckle as my screen size had your first post reading “I have just completed welding up the frame out of square” and thought to myself this is going to be interesting.

Six foot length is ambitious for a first attempt. With a 4 foot gantry width, it seems dual screws are mandatory, no matter where you mount them, or how you drive them. There will be a lot of leverage for side to side racking.

Does “1" linear bearings sliding on 1" shafting suspended about an inch over one axis of the frame” imply that the shafting is unsupported? If so, we’ve got to talk.

Got any pics?
Steve