Vertical vs moving table (bridge) cnc stifness

[vnuEndru]

Hi, the question is which geometry is more stiff and why? Lets say same working area (small), same machine bed material, same machine size. For metalworking. From my observations in diy machines the main difference between two is z axis stifness. Usually vertical design have more stiff z axis and space for installing low rpm spindle+motor. Bridge design usually mean less stiff z axis and high rpm woodworking spindle. Does that mean verticall design is better in terms of stiffness?



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[JohnDonlan]

Hi, the question is which geometry is more stiff and why? Lets say same working area (small), same machine bed material, same machine size. For metalworking. From my observations in diy machines the main difference between two is z axis stifness. Usually vertical design have more stiff z axis and space for installing low rpm spindle+motor. Bridge design usually mean less stiff z axis and high rpm woodworking spindle. Does that mean verticall design is better in terms of stiffness?

The vertical will give you much greater stiffness and greater resistance against hysteresis (twisting). The x and y axis will be controlled by the table and with proper way covers can be protected from contamination.

The bridge type design can work well with sufficient stiffness if the upright castings are beefed up enough in their design. The weakness in this design May allow for hysteresis (twisting) during cutting conditions, but if the castings are heavy and sturdy enough..... This problem may be eliminated, or reduce to such a degree that it will be negligible (maybe only .0002-.0003" maximum).

Also, a Gantry design in many cases allows for a larger work envelope, and the greater flexibility of adding on a tombstone or indexing head..... Simply because there's more room to do so.

[vnuEndru]

Yeah, I think the same way. Just thought maybe I miss something here.
Thanks for the reply, didn't expect anyone to answer

[pippin88]

The 'vertical' design is called a C Frame

The Bridge design / dual column are usually much stiffer

The ones you have posted are not good examples

[awerby]

The vertical design in the picture would more accurately be called a "L" frame. A "C" frame cantilevers the Z axis out on a massive casting, like the classic Bridgeport design. But aside from that, I'd side with Pippin; I think the bridge design shown would be more rigid than the vertical one in the picture if only because the z axis is closer to the slide - or appears to be, and the load is distributed more widely. Where are the FEA guys who can settle this issue?

[ardenum]

If you disregard the Z axis part, the double column is stiffer but a regular plate z axis on that is crapidy poopy do. Needs to be beefy to take any advantage of that. On the other hand the VMC frame all the axes are more or less balanced, Z axis is stronger but then the saddle has to be beefy otherwise it introduces a potential weak spot.

Are you thinking of buying one of these cast frames? What's the goal here?