[peteeng]

Excellent work Catahoula - Really interested in your machine stiffness when its done. What modulus are you using for the EG? I'd be interested in knowing the FE static stiffness with and without EG....Peter

[catahoula]

What modulus are you using for the EG?

I went to double check this and turns out I had mixed up the material properties from another model and was actually using the value for granite, so 55 GPa, which is definitely too much. I just re-ran it at 20 GPa and found 6.9 um deflection at 300N with the EG fill in the gantry, and 7.8 um /300N without. So 43 N/um with and 38 N/um without, a difference but not massive. That's just for the gantry tube, I was still using EG fill in the base frame and gantry supports. So maybe another 1 um deflection with those unfilled too.

The reason these values are lower than what I quoted above is that I've messed about with the model and didn't have the Z-axis box in place, this was just with the spindle mounted on a 35mm thick aluminum plate which has a bit of cantilever to it. So with the Z boxed up it'll be back up to 50-60 N/um in FEA. It'll be a few months before I can attempt to compare this to real measurements.

All in all it shows how difficult it is to get a gantry machine much beyond 50-60 N/um. The best results I've been able to get utilized a custom machined spindle mount that also served as the spindle mounting plate, so it was basically a solid block of aluminum 125mm x 125mm x 350mm bored down the middle to accept the spindle. Inspired by the Datron Neo spindle housing- they must be doing something similar to that. That made a big difference in managing the cantilever and stiffness issues with shimming tramming plates, but that's quite the machining project so I'll only tackle that it if I really need to. Of course, this difficultly is mostly because I'm set on having close to 200mm Z work envelope. If you drop the Z height to 125mm things get massively easier, because most of the cantilever can be taken up by the spindle itself. For my application I could actually be OK with that, but I still want the workholding flexibility of being able to mount a standard 75-100mm vise when necessary, and still have a bit of travel to work with.

Edit to add: I found this example of modulus (https://www.basetek.com/zanite-plus-polymer-composite), so de-rated a bit from 32 to 20 GPa for good measure. Just an approximate guess tho