New CNC build gantry questions

[Capsuna]

Hello all,

I am beginning to plan my CNC router build and have a few questions about my gantry design.

First all, I came across a large lot of 60x60, 60x120, 30x90, and 30x60 extruded aluminum that I will be building the machine frame from. I also sourced a 1" 6061 T6 aluminum plate which I will be machining for supports, plates, etc. I plan on building a machine with a cutting area of 1850mm x 1250mm.

I did some math for the gantry, with the 60x120 extrusions, 68kg load in the center, and the span being about 1250mm, it will sag about .14mm. So I think that will be unacceptable and I want to stiffen it some without buying additional extrusions. So I was wondering how do I calculate the sag if I attach pieces to the tops and bottom of the 60x120 and make it a C shape (attached image)? So I will place a 30x90 on the top and bottom of the 60x120.

Also, are there cons of placing linear rails on the top and bottom faces of the gantry? Or should I stick with the side the z axis will be on? I am just thinking about how I will fasten these 30x90 extrusions to the 60x120s.

I look forward to this build and sharing my progress with everyone :)

Thanks for the feedback!