Tripod bed mount angle?
o_lampe last edited by
I asked this in my hashPrinter thread, but got no feedback. So here's the question again:
I want to design a tripod bed mount where the bed rests on three ballstuds sliding on dowel-pins to allow temp-expansion.
I"ve seen this on other printers, but I'm unsure about the angles of the dowel pins?
Q: Should they aim to the bed-center, or should I place them at equal angles?
Here's a sketch of the angles in question
cosmowave last edited by
@o_lampe I would prefer the solution with "90 + 2x150deg". Because i think, it's better adjustable. But i have no experience with this kind of bed mount...
mrehorstdmd last edited by mrehorstdmd
You're essentially asking about the difference between a Kelvin and a Maxwell kinematic coupling.
The "rails" have to run parallel to the direction that the plate expands, relative to the chosen reference point. The reference point location relative to the adjustments is what makes it a Kelvin or Maxwell coupling.
If you choose the reference point to be the center of the bed, the "rails" should be aligned to point at the center of the bed at all three support points because the bed expands outward in all directions from there. That's the classic Maxwell coupling.
OTOH, if you choose one of the screws to be the reference point, say one of the two along the bottom edge, that point won't need rails- just a "hole" to sit in. The rails at the other point along the bottom edge will simply be parallel to the bottom edge (in your drawing) of the plate, and the third will just support the flat bottom of the plate and is allowed to slide in X and Y. You'll adjust the bottom edge point first to set the edge of the plate parallel to printer's X axis, and then set roll using the third point to set the plate parallel to printer's Y axis. This is a Kelvin mount.
The Kelvin mount's square angles are easier to set up accurately - most machine tools are square- than the odd angles that will result if you choose the reference to be any other point (as in a Maxwell mount), and you only need "rails" at one point, not all three.
If the rails are set square to each other (Kelvin style) and the lines they define are parallel to the X and Y axes of the printer, leveling the bed is super easy because a) you'll only need to adjust two leveling points and b) when you make the adjustments all tilting will be in the directions of the X and Y axes of the printer.
I used a Kelvin mount in my printer, with the reference being a point on one of the "ears" on the plate:
In the Maxwell mount, every leveling adjustment, tilts the bed in X and Y, so tramming isn't quite as simple. Also, every tramming adjustment moves the reference point vertically, so you have to adjust the Z=0 position once the bed is level. It's only a Maxwell mount if you accurately aim the rails at the reference point. Any error will cause the bed to tilt and move up and down a bit as it heats up.
With the Kelvin mount, the reference point doesn't move vertically when you tram the bed, and each adjustment (provided you adjust pitch first, then roll) only tilts the bed along one axis. That means that when you adjust roll, it doesn't affect the pitch adjustment.
Of course, if you use auto tramming and zeroing, difficulties in adjustment shouldn't matter...
o_lampe last edited by
Thanks for the elaborate and exquisite answer. I'd give you two thumbs up, if I could.
So, with three Z-motors and a Maxwell mount, I'd need to level in several passes.
I think, I've seen a meta-script that does that already? IIRC it was posted from @dc42 ?
The Kelvin mount has advantages for manual leveling, but also needs three different designs for the mount.
mrehorstdmd last edited by
@o_lampe Actually, with the Maxwell mount, as long as two of the leveling points lie along one of the printer's axes (as you have drawn), leveling should be as easy as it is with the Kelvin mount.