CoreXZ Thoughts and Questions

CoreXZ Thoughts and Questions

Postby pjoyce42 » Mon Mar 16, 2015 6:19 pm

I am working on modifying the current CoreXZ design to use bent sheetmetal for the frame rather than lasercut MDF, and I have some questions about other design changes I am contemplating.

How sensitive is the XZ pulley system to weight? I am thinking of using U-bolts to secure the linear bearings because my current experience with zip-tied linear bearings is that they will eventually loosen and introduce slop/vibration into your motion system. I think that a properly sized u-bolt will be far more secure even if they are more costly. My main concern is with the weight sensitivity of the gantry when there is no power to the motors. Would using a bowden extruder be enough to offset the extra weight from a few small u-bolts?

Does the size or depth of the groove on the pulley bearings matter? I found some v-groove bearings on amazon with a much deeper groove than the 625VV bearings in the model. I can't see any reason why this would matter as long as the pulleys are all coplanar, but is there something else I'm missing that would be affected by the groove depth?

Lastly, I am thinking of making the design more compact. Is there anything about the pulley spacing that I shouldn't change, or pulleys that need to stay proportional distances to each other?

Thanks
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Re: CoreXZ Thoughts and Questions

Postby Nicholas Seward » Tue Mar 17, 2015 2:28 am

Thoughts on bearings: No it shouldn't matter. I picked 625VVs for a few reason. 1) They are thinner. 5mm vs 6mm. This is very important for my work on Simpsons. I also don't like the idea of the string force being applied to the cantilevered smaller bolt with a smaller spacer. 2) I also can use standard M5 washer for spacers. A standard M4 washer won't work for the 624VV. 3) Bend radius is related to string life. 4) I standardize on M3, M5, and M8 hardware. 5) I also use 625 bearings for other things. 625 and 625VV have the same footprint and can be interchanged for most things. 624VV and 624 have very different footprints.

As you can see, most of these issues are personal choice or close to negligible.

Zip-ties: I am sure you are right that they eventually loosen up but I have been going for over a year with my machine and everything is still tight. Of course, I tightened them with a needle nose pliers. It would take a very short amount of time to retighten the 16-24 ties.

That said, it sounds like you want a more finished look. The ubolts could give you this but it would be way too easy to over tighten them and cause the housing of the linear bearing to deform (finding a ubolt with the perfect ID will be hard but even if you do it will cause a pinch spot) which will cause the balls to not circulate. So how do you get a clean professional look? Pressing them into a machined hole is the ideal way. For a clean RepRap machine, print a plastic strap/block. A thin enough spring steel strap would also be pretty safe. Bottomline: even force is very important.

Bowden: I matched the inertial of the X with the Y so switching to a bowden won't be an improvement. In fact, I think you will have more problems. To your real question: is weight an issue? In short, no. The dual steppers used to move the gantry in the X direction will be more than enough for whatever you want to put on the gantry. The 3:1 reduction in the Z should keep the gantry from falling when the power is off.

Compactness: All the horizontal and vertical runs need to stay that way. The strings leaving the spools need to have a constant length run of more than 200mm to keep the triangulation error very small. (This means you can't route straight from the spool to the gantry.) Most of the bulk of my design is from support structures on the side. The way I designed it force me to have a certain margin on each side of the z rods. I only imposed that constraint to keep the number of parts down and to keep the design simple. Metal offers more possibilities that should alleviate this constraint.
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Re: CoreXZ Thoughts and Questions

Postby pjoyce42 » Thu Mar 26, 2015 2:11 am

Thank you for all the detail! That simplifies a lot of the work that I was going to do. It's nice working on a design that was made by a mechanical engineer :)

Regarding the u-bolts vs zipties, I think I will have the shop at work make me some machined pillow blocks (or I will just print them), that are in 2 pieces so that I can install the bearings using regular bolts.

When you say you matched the inertia of the XZ and Y gantries, did you simply match the weight or was it more complicated than that? I was hoping to replace the MDF in both of those parts with sheet metal as well (keeping all the other dimensions the same), but if they need to be the same weight, then it might complicate things a little.

Also, one of the upgrades I will be making is to find space for a raspberry pi running octoprint, and wire that directly into the on board power supply, so that the unit is fully wireless. I will also be adding a power switch next to the power inlet switch.
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Re: CoreXZ Thoughts and Questions

Postby Nicholas Seward » Thu Mar 26, 2015 7:57 am

The effector is about 1/2 the weight of the bed but the bed has 2:1 reduction. This allowed me to use aymmetric acceleration, jerk, and velocity values. It isnt a perfect symmetry. Even if they weren't close I would set my values for both axes to the lowest common denominator.
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Re: CoreXZ Thoughts and Questions

Postby pjoyce42 » Sun May 31, 2015 9:09 pm

Would you be able to share the math you used (it doesn't have to be pretty) in determining jerk, acceleration, and velocity values. I have replaced all of the wood parts in my model with metal, and I am working on getting the mass of the moving parts to be the correct ratios. I expect that my parts will be heavier than yours, so I want to adjust my firmware settings accordingly. Also, can you PM me a copy of your modified Marlin firmware, so that I can look through it and see what I need to adjust to get it working on a Printrboard.

Lastly, how did you route the wires for the extruder motor? I am considering either getting some cable carrier chain, or running the cable bundle through some nylon tubing.
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