Aluminium FEA optimalized CNC router

[PotatoMill]

I few weeks ago, I decided to get a CNC router to cut foam, carbon fiber and maybe some aluminum. I had also decided not to build one myself, because I knew router building often end up as the new hobby instead of the hobby you originally built the router for. Then someone directed me to this forum and all my plans changed.

I am now planning to build an aluminum-framed router that will eventually have capability to cut different ferrous alloys. In addition, I hope I can do some high-speed tool-paths in aluminum. In the early stages of the design, I planned to use aluminum profiles for the build, without any need for welding, but profiles are expensive and did not have the stiffness I wanted. Then I went on to steel, but that got to heavy. So now I am going to use 60x60mm t=4mm aluminum square tubes welded together. I have to practice my aluminum welding some more before I start with the build. I have done FEA on most of the build, where I started with requirements of a frame stiffness of 10-20 N/μm (10'000 N/mm - 20'000 N/mm). Deflection is proportional with force so I have checked the stiffness with lower forces and then multiplied up to 10'000 N and 20'000 N. I have done many iterations on the frame and the FEA images under is only the most relevant for the current design.

Planned working area is 1000x500x250mm

Complete assembly



The plan is to use 20mm profile rails and 16mm ballscrews on all the axis. With dual 1000mm drive one the X-axis. I'm going to use nema23 stepper motors but i'm still uncertain on the torque requirements I have to do some more calculations there. Though it seems like most of the steppers have a very high corner speed and to utilize all the torque and speed of the motors they should be geared down about 3:1. Any thoughts on this?


Base assembly (500N loads)



Stiffness; 223 N/um. I tried some different setups for vertical loads on the base. 5 vertical supports on the sides gave 4.8 times more stiftenes on the worst case compared to 3 vertical supports.

Stiffness; 53 N/um. No need for cross bracing on load in the X axis it's more than stiff enough.

The weak link where the forces in the Y axsis, this was fixed with some side bracing

Stiffness; 51 N/um. The side bracing gave 4.7 times higher stiffness. Success! (I tried a lot of different potential setups before deciding on this one)



Gantry (100N loads)

The whole weight of the gantry including the Z-axis came to 32kg, i'm going to see if I eventually can cut the weight down. I had hoped for something in the range of 20kg.

Single aluminium tube thick gantry. Were within 10 N/μm but not 20.

Stiffness; 32 N/um. Double thickness improved the stiffness 2.7 times, and the weight gain was small compared to the gain in stiffness.



Z - axis (100N loads)


The Z -axis ended up on 12kg. I have looked for ways to reduce the weight but often the stiffness suffers.

I'm going to use a 2.2kW water-cooled spindle. However if it is going to be use for ferrous alloys I have to gear it down to reasonable speeds. The plan is to use a belt reduction, but it's probably going to be a later update.

Under is some pictures of the results on the Z-axis. This is clearly the weakest link on the whole machine. It's no point making the the rest of the machine stiff if this part has 1/10 of the rigidity of the rest og the machine. The two vertical bracings on the front improved the stiffness but it's still not inside my requirements. The best solution to this that I have seen is Routercnc's MK4 design. However I dont have a CNC machine to build a CNC machine and I don't want to over-complicate my first build. It would be great if someone could come with input and suggestions on the design before I start ordering stuff, or it could become very expensive.

Stiffness of the Z-axis in the X direction (worst load direction) 3.9 N/um.

Stiffness; 0.85 N/um