I have a mechanical background, but very limited exposure to CNC mills/routers. I have cut MDF using a micro mill, but accuracy was not a major concern for the end-use. I am familiar with 3D printers kinematics and structural considerations, but the forces acting on these machines are quite light compared to the forces generated from milling aluminum. Aluminum extrusion profiles are readily available (in only a few sizes) for me, and I’ve found the bolt-together design to be quite convenient. I posted this on another forum, but CNCZone seems to have many experienced machinists and DIY CNC designers who can offer their expertise. I took a look a DonFrambach’s design thread and CatalystGilles design as well, but I don’t have easy access to those sizes of extrusion, or a welder.
I’m planning on building an aluminum extrusion frame CNC mill from scratch that is capable of (slowly) milling 6061 aluminum (ideally faster than a reinforced 3018 engraver, at least as an initial goal). All extrusion pieces are 20mm x40mm and will be secured using aluminum inside corners as well as aluminum corner plates wherever possible. My goal for accuracy is +/-0.1mm. The planned build volume is ~12″ x 12″ in X & Y, with ~2″ of Z travel.
The two plates at the bottom of the Z-axis supports are planned to be 0.25″ aluminum plate with custom drilled holes, very similar to T joining plate that can be purchased. The idea behind this feature is to maximize the stiffness about the Y-axis by maximizing contact area. The piece of extrusion below the X-axis gantry that appears to intersect several of the Y-axis profiles will actually be three separate pieces (one piece in the middle and one piece on the outside of the linear rails’ extrusion) to add rigidity to the Y-axis extrusions.
Linear motion would be controlled by a GRBL controller, driving Nema23 motors, connected to 12mm ballscrews (for all three axis’). Each axis will have two parallel linear rails (15mm Hiwin style seems too flimsy due to the M3 mounting screws, so 20mm rails are currently the top option on my list). The kinematics will be similar to a 3018 CNC where the bed moves in the Y-axis, while the toolhead moves in X and Z
I have a few questions:
1) Do you see any mechanical problems with the frame for slowly milling aluminum to within 0.1mm? (such as a lack of stiffness, etc.) I’m open to suggestions.
2) What is everybody’s experience with Chinese linear rails? I’ve heard mixed reviews, particularly in terms of problems with binding (poorly machined blocks) or lots of play (due to undersized ball bearing sizes)
3) This design’s current cost is below a (Chinese) 3040 or 3020, but do you have any specific recommendations for an alternate below $1000 CAD?
Thanks in advance.