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Sheet Metal Brake and 3d Printer.
S. Heslop:
Been putting this off because I just can't settle on a design.
Went back to CoreXY since it fit into what i'd already come up with the easiest. It's a little over-built though using the 40mm extrusions. I can't help but overbuild the wrong parts. It's often said that any idiot can make anything work, but it takes a real engineer to make something that barely works. I might use 30mm extrusions in the end.
I really like the look of your machine WeldingRod, and it's clearly a superior design to what I can come up with. But i'm not sure how you'd enclose it without making its footprint huge. The area I want to fit the machine is about 600x600, and id like the build volume to be at least 350x350x500.
So assuming I continue with this design then... where would be the best place to put the Z axis screws?
In the middle seems most obvious, but I can see it tipping as the gantry moves past the center point. But at the ends seems like there'd be alot of leverage to flex stuff...
WeldingRod:
Fully enclosed, my printer is 690x690x660 deep for a 380mm build cube. Yeah, it's pretty big!
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WeldingRod:
I would make choices to minimize and balance loads. Your 3d model with the screws has good locations.
Btw, speaking from having two screws... I highly recommend coupling them with a belt (if posdible), even if you have steppers for each. When the power is off they can get desynchronized and tilt your z axis. Also, watch out for backdriving the z when the power is off. I use multiple constant force springs to counterbalance my z carriage and make it so it doesn't backdrive when it's off. Belting helps this too, as you have to turn multiple motors to get in trouble vs only one.
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S. Heslop:
Coupling them at the bottom would be fairly simple I think, under the table. I figure it'd be a good idea to put the pulleys between bearings so it doesn't flex the screw.
Thinking about it some more, i'm going to be using cold rolled steel for the verticals. Ground rods are too expensive and at that point i'd probably do better with Chinese MGN guides. I sorta wonder how the bearings sticking would affect stuff. Maybe i'm blowing the problem out of proportion but i'm just imagining if I put the screws in the middle then the whole thing is going to be see-sawing about. I guess if that happens then I could add some struts under the table to add some more. 8 bearings! Overconstrained isn't a word i'm familiar with.
A bit ago I tried using autodesk inventor since it has alot of neat seeming features. Especially the kinematics and the deflection modelling stuff. Turns out the kinematics don't work too well for complicated mechanisms like straight-line linkages. And I find the method of modelling stuff really hard to get used to. Maybe it's a learning thing but I feel with sketchup it's really easy to move stuff about to brute force my away into a design, when with Real cad then you need a good idea of where youre going to begin with. But then I can't actually simulate deflections.
Maybe I should learn some maths.
WeldingRod:
Exactly constrained means that a moving part on a machine has parts that locate it in X, Y, Z, Theta, Rho, and Phi. Over constrained means that one or more of the directions and rotations have two or more parts fighting to control them.
A common example of this is an axis running on two linear bearings. To make this simple, I'll talk through it in terms of two ground rods with a total of four tubular linear bearings running on them and a lead screw. Lets say X is the direction parallel to the rods. The X direction is controlled by the lead screw; easy. The Y and Z directions are controlled by the linear bearings. Rotation around the X axis is controlled by the linear bearings, as are rotations around the Y and Z axes.
There are two big over-constraints. The first is that the pair of linear bearings on a ground rod MUST be co-axial to be able to move. This is typically pretty easy to do, as you can often stick them on the rod and then screw them in place. A lot of linear bearings include some facility to provide a small rotation to self-align.
The tough one is the two ground rods. If they are not parallel, then the motion will jam. Getting them onto a common mounting plane helps alignment a lot, but you still have to get the two parallel (within the flex/slop of the system) to get motion to work. We often solve this by using a flat surface to mount the ground rod ends and then doing the trick where you go to one end, tighten the mounts some, go to the other end and tighten a bit, rinse and repeat...
As for the rotations, lets think about X first. If the table rotates around the lead screw, nothing happens (other than a little motion). Thus, the lead screw doesn't control this rotation. If we consider the right hand ground rod to be the master one, we can imagine rotating the table around it. This rotation is prevented by the TWO bearings on the other ground rod; this means that its over-constrained, and that we have to do some sort of special alignment to get it to work. Y and Z have an even higher degree of over-constraint; they each have 2 or 3 extra bearings fighting for control plus the lead screw is going to resist rotation around Y and Z (and bind up).
If you go to a total of three linear bushings, things get quite a bit happier. Think of "three points define a plane". Using a fork/slot bearing as the singleton is the best choice, or putting the singleton linear bearing IN a tight slot. One or both bearings on a common shaft need a spherical housing (ideally something you can clamp after installation). In a perfect system, the coupling between the lead screw and the table would have a couple of flexures so that the nut can seek its own happy spot. Set up this way, X is only controlled by the lead screw. Rotation about X is controlled ONLY by the fork bearing.
Y is controlled by the bearing pair (remember the fork is pointed in the Y direction). Rotation around Y is also controlled by the bearing pair; the fork won't contribute any significant rotation resistance as its short relative to the pair. Finally, Z is set by the three table bearings. Rotation around Z is just like Y; the bearing pair.
After I funded the Cobblebot printer, I found out the hard way that they TOTALLY didn't understand this stuff. They had a quadruple bearing group running in grooves on EACH of the four posts. Thus, the darn posts had to be totally parallel for the Z to move at all! And, the two screws had to parallel also. Further, the motors were on the bottom, so the most critical position (starting) was the one most likely to bind up! The Y axis had two bearing quads. Again, requiring two linear rails to be perfectly parallel to each other. X was the only one that would run reliably.
Sorry for the long blather! Its a topic dear to my heart, and one that I spent a LOT of time on to get my printer working right.
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