I would probably have had more luck finding the answer if I knew the term for the configuration of the MPCNC. It is not a gantry. What is it called when the z-axis is mounted at the moving intersection or two axises.
The question is. What are the advantages of the design. Is it one of the reasons that so many table top CNCs can’t cut metal?
There is a thing called a parallel robot where multiple drives each constrain the position of the end effector but not in a mechanically serial fashion. A delta printer is an example, or a six axis flight sim using a Stewart platform.
This is in contrast to a serial robot where each axis is carried by all the previous ones. A robot arm is a typical example of a serial robot and Xcarve for example has the Y axis carry the X axis which in turn carries the Z axis.
MPCNC has X and Y in a parallel configuration, neither one carries the other, and they both jointly carry Z. I don’t know a more specific name for the MPCNC arrangement.
A serial robot puts larger and larger stresses on the axes that are closest to the base. A flight simulator on a robot arm would have the stresses multiply and the base would be under tremendous load.
Does this make the key difference? I dont know. It all comes down to engineering tradeoffs of cost for stiffness. Serial robots can work fine. Can they work at $400? Maybe. If linear bearings were cheap and strong against torque then the machime might be shaped differently. Once the design gets concrete, all the rules get tossed out the window (mostly) and you have to follow what works.
I also don’t know the name for the type of configuration that an MPCNC has.
One advantage of using two tubes at 90 degree angles is that they form a plane. The center assembly then has something to support it (and keep it from tilting) in both the X and the Y direction. A traditional gantry that only rides in one direction is prone to rotating about the axis perpendicular to its movement (or twisting if it’s not very rigid). A traditional gantry is also heavier, because it needs to carry the hardware for an additional axis.
The Lowrider shows that a low cost “gantry style” can also work though.
It’s also distributing the load across four rails instead of just two. This helps us get away with cheap conduit and plastic instead of more traditional/expensive components. We can also now get away with smaller motors, which can be run with less expensive controllers, power supply, etc.