A new hope: Epoxy granite and steel.

[pippin88]

Like I said the first time, it depends on the application if it's worth it or not. This machine has a specific usage (which I can't mention because of customer confidentiality) that doesn't require high strength so deflection won't be any concern even if it deflected 10x.
This is not my first rodeo, I wouldn't build something that wasn't more than up to the task it's been designed to do by a large factor.

If I was building a machine for cutting more aggressively and I will be very soon, then you'll see it's designed very different with much more bracing and thicker wall tubes, but it will still have the slots because they very much protect the screws. Also, you are making a big assumption that there is nothing inside those tubes.!!

I'd also be interested in the forces you applied to those tubes.? Are they realistic or would we need 30Kw servos with 2mm pitch 30mm ball-screws to repeat...

The absolute value of the deflection (and the absolute load) is not the point. The point is the comparison between slot and no slot under the exact same conditions.



You can double the thickness of the tube and you will still be behind. Better just to get rid of the slot.
(If you put the slot in the opposite side of the tube, away from the rails / load, deflection is roughly in the middle between 'no slot' and 'slot')

If you want to cover the screw, put it within another tube attached to the side, but don't compromise the stiffness of your main structure.

What is inside the tube that negates the loss of the major load path?

Racking can't NOT happen IMO when it's plunging into hard materials for operations like drilling, it's just by how much.?

Any axis where the cutting force is not directly centered and you are only using one screw is subject to some degree of racking. Every cartesian design has one axis where the cutting force is not centered. Your machine above has the Y axis (table axis) with one single screw. Are you concerned about that racking?

The risk of racking is about the proportions / aspect ratio between the spacing between the rails, and the bearing spacing along the rail.

The extra expense of making it more massive, etc to counteract this far outweighs buying another ball-screw as it doesn't stop at just the width, spacing, etc. As it gets heavier, wider, etc, it requires larger and more costly everything.
Doubling up means you share the loads so can use a smaller size and less expensive components which cancels out any cost offsets but gives a much better-balanced machine.!. . . Which looks like it was designed by someone who knows what there doing and not a tight-arse who's trying to save a few ££.

Adding a second servo etc becomes a non-trivial expense. Tuning two servos with two different screw errors (unless you are paying $$$ for very high precision screws) is non-trivial.

For my router I have used dual drive and would never go for a single drive on a wide gantry. For a wood router driven by steppers and cheap ball screws I totally agree with dual drive and strongly encourage everyone to drive both sides of the gantry.

Well first I wouldn't use belts on something any wider than that little machine I did and I wouldn't use belts if I was chasing accuracy to the degree of mapping ball-screws as it defeats the point and like you say makes it very difficult.

Regards Screw mapping then very few do that at the DIY level and if using linear encoders rather than rotary encoders then you shouldn't need to map the screw. Your only concern then is keeping the two sides in sync and I'd assume if using linear encoders the control system will be fully closed-loop so will handle that side.!

Knowing the error and compensating before it happens (screw mapping) is generally better than chasing it after it happens.

(Obviously the machine I have in mind with AC servos, linear encoders, 45mm roller rails is more than the average hobby build).




Machine design is always a series of compromises. I am not saying your machine design is wrong, or that mine is right. I am merely pointing out possibilities and weak points. For me, the slot in a tube to hide a ball screw is not worth it at all. There are other ways to protect ball screws. Less experienced people than you may not understand the design choices / compromises you have accepted and copy that feature without understanding the substantial impact it has.