Anyone a fan of awesome kinematic designs? I came up with this design a little while back. It’s similar (though mathematically identical) to the kinematics of the Markforged 3d printers, but this design is capable of 2 individual X axis without a single motor (unless you use a direct drive extruder) on the xy gantry. I have built both an H-not and Core-xy and I have never been a fan of how finicky the belt tensioning is. Both those designs require a very rigid gantry. I want my next printer to be very lightweight (carbon fiber frame, etc) so I wanted a gantry that didn’t have to deal with the racking forces of those designs. Also I just want to do something original. For now though the printer will just live in my head, but I figured I would share the kinematics for anyone else.
https://hackaday.io/project/19543-haq-xy
Using two belts for Y should eliminate the need for a racking-safe and super stiff crossbeam for X. Markforged uses only a single driving belt for what is the X-axis in their machines.
Moving either X axis motor will induce a racking force on the gantry.
I proposed running the X axis the same way a while ago. In that case, the idea was to locate everything on one side so that the X beam could be a cantilever (on a small, bowden-driven printer). I wasn’t thinking about using it for independent X carriages, but that should work, too.
I would not do this with a non-bowden extruder. You want to balance the mass on the axes as much as possible, and the Y axis is already going to be carrying significantly more mass than (each) X axis.
@Wylie_Hilliard Not much of one. At least compared to the racking forces you get on a core-xy or H-bot printer. In an ideal world with “frictionless” bearings there technically shouldn’t be any racking force. We don’t live in that “ideal” world unfortunately which is why I made the Y axis the way that it is compared to how Markforged does it.
This design will WANT to rack, but unlike in an H-Bot, the Y axis here will prevent any meaningful gantry twisting. (That’s good.) But why deal with the added kinematic complexity of having Y motion affect X motion? It seems like you’ve basically invented a harder-to-control version of the gantry used in the Sli3dr or the Cartesio.
CoreXY simply does not rack, all the drive forces are intrinsically balanced. I also find it very easy to tension up the belts. (I use slotted motor brackets.) Not sure why you’re having issues with it.
ryan, the racking force is the same as in an H-bot arrangement. The only advantage here is that the belts are shorter.
@Wylie_Hilliard racking is caused by opposing forces on opposite ends. This has self-negating forces on each end. The independent Y axis will shut down any uneven forces that fail to be negated due to loose bearings.
@Wylie_Hilliard In an H-bot, the only thing keeping the gantry bridge square is the stiffness of the linear hardware and frame. In this design, X-axis acceleration DOES produce an unbalanced Y-axis force on one end of the bridge, but the two Y-belts and torsion rod mechanically couple the two sides of the bridge so it can’t skew out of square.
BTW, adding torsion rod synced belts like these should solve the reading problem with an H-bot, even if they’re completely passive, as long as the backlash is thoroughly eliminated. I’ve thought about using a passive cable loop for this purpose, too.
@Whosa_whatsis This arrangement does create unbalanced racking forces. As B1 accelerates counterclockwise, it pulls up on the top left pulley on the gantry while reducing the tension on the lower left pulley. This creates a moment about the Z axis, trying to twist the gantry.
It resists movement, but it doesn’t try to change the angle of the axis the way an H-bot does… Yes, if it does manage to pull the axis out of position, it will do so on one side and the whole axis will be a moment arm, but that’s a secondary effect, and probably negligible if everything is properly tensioned. The unbalanced forces of an H-bot actively try to pull things out of square, as a primary effect. That’s the difference, and that would only happen with this configuration if both X carriages are moving together.
@Ryan_Carlyle While the racking forces of each belt in Core xy cancel each other out, unless you have a very rigid gantry, over tightening one belt over the other can skew the gantry to where it isn’t square anymore. This can be a bigger issue with larger gantrys. I actually don’t have any problems with my personal core xy printer. It prints great, and reliably. Overall I am just not a fan of the Core xy design.
As for the design of my Haq xy if you look at the actual kinematics the only racking force would be if the bearings on the x axis ends have enough resistance, which they will because there is no such thing as a perfect bearing, but it is still much less than an h-bot would produce, and the y axis will solve whatever racking forces there are.
There are some good advantages to this design imo. The key thing being the dual x axis with a very lightweight gantry. That along with there not being any belts that pass in the front of the printer, although that is really only an advantage for my specific printer design. I like the open concept, similar to the Sigma or the Type A.
@Ryan_Branch In addition to frictional forces from all the x axis bearings on the gantry, there will also be racking forces during acceleration of the carriages.