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I've been wondering why most (hobby) CNC machines have two actuated linear rails like this:

enter image description here

Besides the extra force for moving the spindle, is there a need for both rails to be actuated? Could the rails not be designed this way:

enter image description here where the red cylinders are bearings and the yellow is an actuator. Will this setup lock up?

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What happens in the arrangement is that you are applying a force away from the center of gravity.

Imagine you have a pencil on a flat surface and you try to push. If you don't push it around the center of gravity you will cause it to rotate.

Although using linear bearings (especially longer ones) will mitigate the problem, you will be creating more normal forces, therefore more friction, and the motor will draw more Amps in order to provide the required torque. And that would be bad design.

Surprisingly, even if the actuator is in the middle you might run into problems depending on the tolerance of the bearings. That was an experience I had when designing a (budget) linear translation stage (that was almost 10 years ago)

enter image description here

10 or so were build from these, (with different lengths - so only the rods had different lengths). Generally they worked well, however the initial prototype was troublesome with respect to getting the alignment of the bars correct, and using linear bearings of proper tolerances (too tight tolerances, were not good when there were misalignments in the rods, while too loose were making the stage move like a drunk sailor in an alley).

Generally, the most important thing is: not to overconstrain the assembly. And that I'm afraid is something you can't read, you have to make mistakes and learn.

And just to be clear: what I said in the previous lines, does not mean that the power screw in the middle is worst that two power screws either way. The dual motor design if not properly implemented (e.g. if you don't make sure that both motors are sychronised), can end up is similar or worse self locking problems.

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  • $\begingroup$ Right. What if the actuator was right in the centre? Would that eliminate the said problem? But the load force could still be off centre. $\endgroup$
    – John M.
    Oct 3 '20 at 19:30
  • $\begingroup$ A single actuator in the center would be just as bad (well, only half as bad, but still too bad) when you were cutting at the edge of the work area. $\endgroup$
    – alephzero
    Oct 4 '20 at 13:01
  • $\begingroup$ Read Alexander H. Slocum's excellent resource: pergatory.mit.edu/2.007/resources/FUNdaMENTALS.html $\endgroup$
    – D Duck
    Oct 9 '20 at 19:07
  • $\begingroup$ There's nothing inherently wrong with using more power than the minimum necessary, but the twisting is a problem. $\endgroup$
    – user253751
    Nov 3 '20 at 18:55

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