Question about gantry construction

[nokode]

Hello all,
i decided on going for 2 motors on Y to move the gantry, to eliminate possible racking, and to leave clearance under the table to add a 4th axis in the future (4th axis with center at table level).
But now i start thinking in potential issues, and need your comments on this (regardless on using 1 or 2 motors on Y):
Connecting the gantry under the table will give more stiffness to that part, making it more rigid and resistant to lateral movements, right?
If i don't connect the gantry under the table (to alow the 4th axis), is it possible to keep the gantry solid anyway? Of course, assuming that the top and laterals os gantry are very solid and well connected, and also using 1 linear rails + 2 cars each side (from Hiwin, 20 / 25mm).

For those who have an "open" gantry under table, what are your thoughts on that?

Thank you once more for your precious help.
Rui

[RomanLini]

You should be able to make it plenty strong without connecting the bottom of the gantry under the machine.

If I read you right you are asking about trapezoidal flex, which is where a square pushes sideways to look like a diamond? The beam under the bottom of the gantry won't make much difference in that way.

One model is like a square, the other is like the square with the bottom line removed. Both have the same issues with trapezoidal flex. If the top 3 members are stiff enough as a group then removing the bottom member makes little difference.

[CarveOne]

Dual motor drives for the gantry work very well if the gantry and gantry supports are stiff and well connected together. Mach3 and EMC2 both can slave the two motors together in software. It is steadily becoming the more popular way to drive a moving gantry machine. I'm using it on both a dual rack and pinion drive machine and a machine using dual ACME screw drive.

You have twice the torque pushing or pulling the gantry. Cutting near one side of the machine doesn't cause the racking problems that one motor driving from the center of the table has had.

CarveOne

[Walky]

That's right, a bottom plate wouldn't really make a difference; the gantry itself (if well planned) should prevent the side plate "tilting".

I've recently been building a machine using double X screws with no bottom plate (to keep the work table low), it is very sturdy (no side plate tilting) and the key to this is the distance between the rails (or gantry structure in your case) and how well are they mounted at the ends. A bottom plate would help prevent side play (in a single screw design), but since that's the job of the screws on each side I see no point in using them.

[louieatienza]

Originally Posted by nokode Hello all, i decided on going for 2 motors on Y to move the gantry, to eliminate possible racking, and to leave clearance under the table to add a 4th axis in the future (4th axis with center at table level). But now i start thinking in potential issues, and need your comments on this (regardless on using 1 or 2 motors on Y): Connecting the gantry under the table will give more stiffness to that part, making it more rigid and resistant to lateral movements, right? If i don't connect the gantry under the table (to alow the 4th axis), is it possible to keep the gantry solid anyway? Of course, assuming that the top and laterals os gantry are very solid and well connected, and also using 1 linear rails + 2 cars each side (from Hiwin, 20 / 25mm). For those who have an "open" gantry under table, what are your thoughts on that? Thank you once more for your precious help. Rui

I also have a dual leadscrew setup with no lower gantry beam. A lot of the designs that DO have the lower beam are driven with a single leadscrew, of which the nut is attached to said beam.

That sais, I've had no issues with racking, since the dual screws help prevent this. You still want to have a solid gantry; I've seen many a design with very solid bridge, but flimsy uprights...

[nokode]

Hello,
thank you all for your comments, i will go as i intended in the first place, with no connection under the table.
But now i have another question, about the correct possition of the profile guides (from hiwin probably).
Can those guides be used in any position? i mean, for example to suport the gantry, can the guide be fixed to the table either in vertical or horizontal position?
I can assume, by looking at several machines, that yes it can be used in any position, but at long term, regarding the life of rail+car, is there a recomended position?

Thank you
Rui

[jscavitto]

hiwins like the lg and ag series with grease lubrication can be used in any direction. hiwins with self-lubrication need certain orientations.