A new hope: Epoxy granite and steel.

[JAZZCNC]

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?

I understand what your saying and I'm not arguing that you are wrong, my point is that it doesn't matter if the loads are never going to be high enough to deflect the tubes.
The application of the machine shown is cutting plastics with minimal cutting forces, so even at the top of travel, there will be no significant cutting forces that will come close to deflecting those tubes.
End of the day it's about the application and if I was chasing strength then I would take measures to cancel out the effects of cutting slots in tubes, but in this application, which is the only application this machine will do all it's working life then it allows me more scope to full fill other design requirements. Like, compact a design, etc.

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.

Again your correct and no argument from me. But there is a big difference in the direction of the cutting forces and like you say proportions/aspect ratios working together. So in this direction and at this width then the racking will be much higher unless the bearing spacing was proportional and this isn't practical for a Z-axis as it would need to be huge so increasing height or lowering travel, take your pick. Using two ball-screws takes all this away and reduces any risk of racking.

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.



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

Erm.!! tunning and mapping are IMO two completely different things, but if you are using Linear scales then screw mapping isn't required so no chasing is being done. This is why all high-end machines use linear scales ( and Very expensive ballscrews)

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.

Again we mostly agree, but we will have to agree to disagree about the relevance of the slot because while you may see weakness from your Armchair in the VIRTUAL example you provided I can tell you the REAL WORLD example I provided doesn't deflect to any significant degree and is a magnitude stiffer than is required to do what it as been designed to do.
My test of every router/mill I build is to cut the Aztec calendar in 10mm Aluminum because of the very fine detail in it. Then straight after I cut it again over the top to make sure there are no errors. I do this because if there were any errors or significant deflection then the second pass would destroy the fine details cut in the first. (See the poor pic)

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.

Well first I haven't compromised anything, Also I can't be responsible for others' assumptions and if like you they assume I've just cut slots in the sides without knowing or caring about the effects of doing that then that's on them not me.!
Anyone who is building should design based on their own application and research or ask questions based on those needs. If they just copy from a single picture or video without asking or getting in touch with the designer then more fool them.