corexy build build plate questions

Javajoeuk

Hi I have some questions about altering the design of my printer in regards to the build plate which I am just about to build. At present it has 4 linear rods and two z motors lifting the bed frame which I have working fine but I am thinking about adding a third z axis motor so I can have true bed leveling as I have a spare driver on my duet 3 board.
So the bits I am not sure about are, will I be able to keep the 4 rods that are holding the bed plate frame at the moment move my two existing z motors forward of there original postion and add a third motor at the back to give me my triangle?
Any advice would be great

Thanks

deckingman

@Javajoeuk Consider this. Assuming that you've managed to align all 4 rods so that the build plate moves up and down without binding (difficult to achieve but you say you've managed it). So you can say that plate is flat and level but with respect to what? It might not be "tram" with the X and Y rails. That is to say, the bed could could be sloping from (say) front to back with respect to the X and/or Y axes. The rods must be square with respect to the plate (otherwise it would bind). So if the plate is sloping front to back, then the rods must be sloping as well. That is to say, they are truly at right angles to the bed, but not at right angles to the X and Y rails (because everything is sloping from to back).
Now you want to level the bed with respect to the X and Y rails. This means you'll have to correct that front to back slope. But if you try to do that, then it will bind on the 4 rods because they were square with the bed when it had a slope, but now you want to raise the front of the bed to correct that slope. So they only way you can adjust the bed so that it is tram with respect to the X and Y rails, is to re-align the 4 rods.
It doesn't matter how many lead screws you fit, or how you position them. It will always bind if you try to level the bed with respect to the rails (make it tram) because it is over-constrained by the 4 rods.

DIY-O-Sphere

@deckingman said in corexy build build plate questions:

because it is over-constrained by the 4 rods

However, this has nothing to do with the number of guides, but with the type of connection. For true leveling, the connection between bed and rails must allow a displacement.
Much easier to realize with 3

engikeneer

@Javajoeuk to simply answer your question, yes it does work, but may not be the best arrangement as the others have said...

I have exactly this arrangement on my printer (4 10mm rods with linear bearings in the corners and three lead screws that form a triangle for independent bed levelling). It works okay, but only because there is some slop/flex in the system that means it's not quite over constrained. The issue I have is that the flex/slop in the system then means that my bed is not always very stable and I get some z banding on my prints. For this very reason, I am in the middle of redesigning my z axis to sort out the over constraining and fix these problems.

Javajoeuk

Thanks for confirming the problem. That is exaclty my setup with the 4 rods.
I think I will redisgn it and move th 3 rods next to the screws.
How much deflection do you think I need to allow for ?
2-3 mm movement or more?
My plan is to home z at start with zmin then when they are homed move them up to my z probe and re do the bed leveling with a bl touch which should then be able to do the compensation and adjust the screws. Or have I misunderstood what we can do moving to 3 z axis?

deckingman

@DIY-O-Sphere said in corexy build build plate questions:

@deckingman said in corexy build build plate questions:

because it is over-constrained by the 4 rods

However, this has nothing to do with the number of guides, but with the type of connection. For true leveling, the connection between bed and rails must allow a displacement.
Much easier to realize with 3

Actually it's a combination of both. But 4 rods is one more than the optimum

DIY-O-Sphere

@Javajoeuk said in corexy build build plate questions:

2-3 mm movement or more?

As an inspiration, check the jubilee project...
https://www.youtube.com/watch?v=a9irK9rOUHY&feature=emb_logo

Javajoeuk

Thats what I was thinking about just need to use a rod instead of the rails.