stiffness measurements cnc mk3

[Neale]

At a quick glance, it looks as if a fair bit of the loss of stiffness as you move out from the gantry towards the tool is actually coming from play or flexibility or otherwise in the Y and Z rails, plus a component from gantry twisting. Your first figure is presumably bending of the gantry, primarily, and then the other effects become larger as you move out from the gantry. Maybe the Z bearing spacing could be increased? Reduce thickness of Z moving plate to reduce leverage? Deeper gantry? I'm playing with ideas for my own design and just finished my first pass (on paper) at the whole Y/Z assembly so very interested to hear your thoughts. I think that by reducing clearances and cutting clearance grooves in moving and fixed Z plates you can just about fit a ballscrew nut in between while still using 15mm rails but it's all a trade-off, of course, as you are weakening the plates a little.
Thanks for giving the data - it's good to see real numbers to start to get a feel for what is happening in the structure.

[routercnc]

Hi Neale,

Glad it is of interest. I'm hoping that the method and setup is simple enough for others to try and compare to. I know Jonathan has measured something similar so perhaps others have too.

Yes the first figure (10,000N/mm) is the basic gantry stiffness and everything else drops away from this due to various factors you have mentioned. What you cannot see in these photos is that the Y axis ballnut screws to a wooden panel on the Y axis (!), so this probably accounts for some of the 10,000N/mm gantry side dropping to 5000N/mm on the Y plate. An upgrade to aluminium should help here. It's a carry-over part from an old design when I had less machining capability.

I've always felt I could have done a better job, especially on the Z axis, along the lines you have mentioned above plus other ideas. This data shows there is still room for improvement although I have been accused recently of only using the machine to make a better machine, and not actually making anything with it. Easy to get drawn in . . . .