Aluminium FEA optimalized CNC router

[PotatoMill]

[QUOTEmagicniner;79528]Have you done an analysis of resonant frequencies with the gantry and z in a variety of positions?[/QUOTE]
Not yet but eventually I’ll get to it. I don’t know how accurate it will be because of all the simplifications i need to do on the analysis. Frames like this have a tendency to vibrate quite a lot.

I would advice you to make bed analysis as well. Consider the surface you will mount the hole machine. On floor or on table. Maybe you need adjustable legs. This will probably change all calculations you made so far.

I'm not completely satisfied with the base fixturing myself. I do not have the most ideal location for my machine so I have to move it twice a year. Through a door up some stairs. Hence, the plan is to put weight on it when it is in place. If I find a better location for the machine, I will change the design accordingly.

Although aluminium is 3 times lighter than steel it is also 3 times less stiff. Using steel in the base assembly, where the extra weight is of no consequence and is actually very beneficial, might negate the need to add the extra triangular bracing. Especially if you can go for high wall thickness.

I totally agree. Steel would be first choice if I find a good place for the machine. The Young's modulus for steel is certainly better than for aluminum.

Don't forget that the stiffness you are aiming for (10-20 N/um) is for the WHOLE machine, from tool tip to bedplate. If you design each sub-assembly to be 10-20 N/um you will get somewhat less stiffness in total because they will behave like a set of springs in series. In other words each sub-assembly needs to be significantly stiffer than 10-20 N/um so that when added together the total stiffness is in the 10-20 N/um range.

20 N/um is an optimistic goal. I had to do a lot of simplifications in the analysis. The worst case for the base is currently at 51 N/um and the gantry 33 N/um. As you pointed out, the twisting of the gantry, the deflection will increase with the length of the Z-axis. The base will not be as stiff either due to the fact that it's fixed to the ground in the analysis. I forgot to write it in the original post, but 500N loads where used for the base and 100N for the gantry. Adding it now. Deflection is about proportional with force, so I just multiply it up to find the stiffness in N/um.

The Z axis is always a challenge with this style of machine. Try 20-30mm thick aluminium plate and see what that does. I suspect it will not give what you want especially at 450mm long. So another option is to use a piece of RHS (steel) with say width similar to the current plate width, depth 80-100mm, and length 450mm. Then house the spindle inside with cut outs or access holes to the spindle as required.

I will explore this setup, and some other potential setups and post what I find out.