How would you complete this part? Another beam upfront? Another milled part?
I would add a second bearing on each rod, spaced about 1/3rd the distance the COG of the cantilever protrudes from the rods. What you’re doing here is putting a giant moment load on the bearings and there’s a real good chance they’ll bind up or have lots of flex if they’re not high quality and perfectly preloaded.
How about pulleys and cables just to be different.
@Ryan_Carlyle how about adding a third rod to do that? I don´t want to decrease the possible build volume.
Where would a third rod help? A second screw on the opposite side would help.
The problem is that all cantilevered Z stages act like levers. The bed weight and inertia put a lot of wrenching force on the Z bearings. It’s pretty typical to put two bearings 3+ inches apart on each rod or to use one really long bearing to minimize that effect. Check out an Ultimaker or Replicator Z stage to see what I mean.
Usually you can put the extra bearing length above the build plate and it will occupy the same Z height within the frame taken up by the extruder and gantry.
So I could do the same with two longer bearings like lm10l?
I am wondering because the z stage of an CraftBot has also only two lm8uu bearings and a long lever.
Yep long bearings are fine. A LMxxL series bearing is literally two LM bearings in one housing.
Design a sliding double-bearing piece that, when it will hit top or bottom, will slide in relation to the bed while still providing the necessary distance to lower the loading moment.
I’d ditch the unsupported smooth rod entirely and go with MGN9/MGN12 depending on your particular design. I’ve seen cantilever beds being an limiting factor too often, on unsupported rods they like to resonate and wobble around when you hit the right frequency.
They are going to be supported when the printer is done.
@Daniel_Stauffer Thomas means “end-supported” rather than “unsupported.” Common vernacular. End-supported round rods are pretty flimsy. I don’t recommend using them with length more than 25 times diameter.
@Ryan_Carlyle Okay, got it. Sounds funny though. What would a supported rod be then?
@Ryan_Carlyle correct, I was referring to the rods only having contact to a rigid structure on their very ends. This is disadvantageous with a bed that has no other constraint for rotating about the Z- and X-axis (like another rods at the opposite side of the bed).
Go with 1x2 extrusion for the bed support rails. Rigidity in the z axis system is critical on a cantilevered setup. May also want to mount the z motor on top, that way the motor bearings and lead screw are under tension rather than compression.