How close can I space THK rails?

[bgolash]

Hi Group

I am building my gantry with 4 x 4 structural steel tubing for the Y axis. One THK rail will be mounted on the face of the tube. The second will be mounted on top. I wonder if the spacing of the THK rail assemblies is too close together. I`m wondering if anyone has design input on this mechanical detail. I`ve checked out some commercially built machines and I`ve seen the use of three blocks used on one rail. I`m wondering if this would make for a more solid axis considering how close the rails will be.

Regards barry

[ger21]

The closer the rails are to each other, the easier it will be to twist the gantry. How tall will your Z-axis be?

[bgolash]

Hi

I haven`t designed it but I believe the working distance will be
7.5 inches. The overall height of the Z might be close to 18
inches.
Thanks for your help...regards Barry

The closer the rails are to each other, the easier it will be to twist the gantry. How tall will your Z-axis be?

[Torsten]

A common design problem with bridge mills.
Because most of the work is probably done profiling thin sheets of material most
cutting will be done with Z-axses fully extended the worst scenario.
Extending the unsupported point of force much further then the width of rail spacing
will deteriorate rigidity very quickly.
How about designing your gantry height with several mounting positions?

[ViperTX]

I'd look at the specs on the rails and see what the torsional spec is, you won't get any worse then that. I'd use 2 trucks per rail and space the rails about 6 inches apart on the gantry.

Design for the spindle you'll use and the type of material (thickness, hardness, etc.) also throw in those little details like accuracy.

[ESjaavik]

The type of rail also matters. Some rails (SHS, HSR) will take the same lifting force that they do in downward force. This is the type you want, since when ther is a tilting force like that, one rail will be forced downward, the other will be lifted. All forces here as seen in relation to the top of the rail as shown in the data sheet. Another type of rails are designed to primarily take a downward force, like when carrying a heavy object.

I may not have understood how you plan to place the rails, just make sure they are in the same plane, facing the same way. Not one facing sideways and the other up. That would make it impossible to adjust them to parallel.

[JavaDog]

Why doesn't anyone move the Gantry up/down instead of the router?

[ViperTX]

Why doesn't anyone move the Gantry up/down instead of the router?

Think about it....if you move the gantry up and down then each gantry leg must have a rail or guide that is vertical....where would you place the leadscrew / ballscrew.......well it makes sense to center it between the legs...how will you support the ends....need a crossmember between the legs for the bottom bearing the upper bearing would be on the gantry's outer non-moveable legs....that's basically why.