Dual rails redundant?

[PsyKotyk]

I scored some THK HSR35 rails and blocks off eBay, and proceeded to design my machine using 1 rail and 1 block for the X axis (as pictured).

I've looked at a lot of machines, and see that most people opt for two rails, and sometimes with two blocks per rail. My question is why? The dynamic load spec for the HSR35 is in the thousands of pounds with no play in all 4 directions. What's the reason for two rails or multiple blocks? Is it to somehow improve acuracy, or just over engineered?

[drawbar]

If I were you, I would try a real world test of the "no play" expectation. Bolt down one of your rails rigidly. Then bolt something long and rigid (thick bar, angle, whatever) to the slide block perpendicular the the rail motion. See how much play really exists when you rock it back and forth. A lot of people that build machines worry about even 1 or 2 thousandths backlash in their screw/nut/bearing combination of the drive system. Any "play" that you get from the rocking slide block will be considered backlash and will be added to whatever (if any) backlash you have in the perpendicular axis' drive components.

Brian

[Climatepro]

The other concern is bending moment loading. One block will be loaded on only two of it's opposite corners to handle a twisting load (for example a load on the side of the cutter in your gantry drawing). If you have two blocks, they would each carry half of the moment equally.

It's kinda like trying to hold onto a 4 ft long broomstick with one hand right in the middle, and then sweeping. Your hand and arm have a very high load capacity in the push/pull direction, but a 2ft moment arm creates a huge bending moment. If you use two hands, then the moment is transferred into two opposite direction axial forces.

[jeffs555]

You need to look at the torque(moment) specs for the bearing. All of the cutting force will be applied through the moment arm of the Z axis, and if you calculate the torque, I am sure you will be exceeding one or more of the torque specs.

[jeffs555]

I just looked up the moment spec for the HSR35 and it is higher than I thought. It is different in different directions, but for a single carriage, the lowest is around .65 kN-m. This is around 6000 in-lbs and if the end of the cutting tool is 10 inches from the carriage, this equates to a cutting force of about 600 lbs. I would still question the amount of play you would get with a single bearing. The advantage to using multiple rails with multiple carriages is the torque or twisting force is converted mainly to forces perpendicular to the bearing surface.

[High Seas]

PsyKotyk (great handle BTW)

I wonder, if you want to use only one rail and "truck" on the X Axis; why you don't put the rail and truck UNDER the table?

It would be clear of most swarf and chips generated during milling.
And you'd sorta "split the difference" on the twist you might generate on the Y axis.
I'm sure everyone is waiting to see if/how the approach pans out if you pursue it!
Jim

[mxpro32]

with a router design, I guarantee you are going to have more flex in your frame than you will in the hsr35 block. A 35mm block is extremely rigid. using one block will make alignment easier as well. Your design should work fine.

[spalm]

On a side note about your drawing, I like how you added vertical stiffeners to the sides of the gantry walls. I did this to a wood machine with amazing results, and no real downside. It added no practical width to the machine since the motor and bearing stick out anyway, and is cheap and easy. Maybe this is a wood machine, I donno. If it is, glue the stiffeners; don’t just depend on those screws. Stiffeners could also be added to the back of the beam and the bottom of the gantry connect plate/leadscrew nut holder.

IMHO that single rail across the top of the beam just does not look right to me, but hey, give it a try. You can always add a second rail later.

Steve