Some thoughts on cnc machine design principles. . .

[routercnc]

Ok, so maybe that was too much of a mind dump!

In summary, for the machine being considered:
There seems to be some benefits to the lean back style of gantry where the C of G ends up in the centre of the side rail bearings. This reduces the bearing moment and reduces the risk of binding.
There is a formula for the ideal Z location for the X axis ballscrew which drives the gantry, which ends up being near the Z location of the tool
During constant velocity cutting the ballscrew forces are dominated by the cutting forces
During acceleration, the inertia of the gantry is only a small part of the total force, which is again dominated by the cutting forces
With well chosen geometry (ballscrew/rail/tool), the side rail bearing moment is small, meaning the bearings can be placed close together give more overall gantry movement along the X axis.


Finally, as mentioned in another thread, here is a further update to a spreadsheet calculating deflections and twisting of a gantry. Now added a further calculation for the gantry bending when cutting in the Y direction. So the gantry vertical deflection due to spindle weight, twisting due to cutting in X, and bending/lozenging due to cutting in Y direction can now all be approximated. This gives a rough minimum tool deflection. Comments and errors gratefully received . . .