Smooth rods dimensions for coreXY 300x300mm? (22 replies)

I am planning to build a machine based on coreXY that has a printable area of about 300x300mm with one extruder. I would like to be able to switch between direct feed and Bowden with one extruder, as well as to mount more than one extruder (up to four, with a Bowden setup of course) and still get at least 200x200mm as a printable area. I am still writing a tentative 3D model of the printer, but I guess the smooth rods of the X and Y axes will be about 500mm.
I don't know the weight of a typical direct feed extruder: given that 4 hotends should not weigh more than 400g, or alternatively a nema17 motor should not weigh more than 400g, I guess that the central carriage bearing the hotend(s) should weigh not more than 1kg including the nema motor with one extruder, or the 4 hotends.

So what would be the right diameter for the X smooth rods?

I tried to do some deflection calculations (for example here and here) to get some worst case numbers, although many variables cannot be taken into account (for example rods are fixed, but the frame is not perfectly rigid, etc). However, with a carriage weight of 1kg and 2 steel smooth rods with free ends, the deflection should be:

• 0.15mm for rods with diameter 10mm
• 0.07mm for rods with diameter 12mm
• 0.04mm for rods with diameter 14mm

Given that this is really the worst case scenario for deflection (rods are not free but fixed, the deflection should be calculated over the length of the printable area and averaged because of the length of the carriage that is not zero, etc.) my guess is that 12mm diameter should be more than enough.

On the other hand, the Y axis (the outer one) should support a heavier load because of the X smooth rods and their respective supports. I don't know exactly how heavy they may be because I cannot find weight information for all the parts I am planning to use, but I guess it may reach 3 kg (the 2 X smooth rods would weigh 1 kg on their own, plus the central carriage that may reach 1 kg, plus the other supports). In this case the deflection should be:

• 0.12mm for rods with diameter 14mm
• 0.07mm for rods with diameter 16mm
• 0.04mm for rods with diameter 18mm

So, with a diameter of 16mm for (unsupported) smooth rods, there would be a theoretical total (X + Y) vertical deflection of 0.14mm in the center with respect to the borders (worst case), which should be still acceptable for the first layer if the bed is flat enough, the rods are straight (for example 50 microns / 300mm, although I would like to know how to compute the correct value for 500mm), and bearings quality is decent. Alternatively, I may use supported rails.

What is your experience?