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Question about R1 design: why is the Y-axis belt off-center?

Discussion in 'General Questions' started by Europa, Nov 16, 2020.

  1. Europa

    Europa New Member

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    I'm on a kick to decrease lash/increase rigidity of my R1+, and I'm curious - from a design/engineering standpoint - why the Y-axis belt is attached off-center of the Y end brackets. There are no packaging/space constraints with the Y belt components, so I'm wondering if I can move the stepper off-center, and relocate the idlers to the centerline. Was the off-center belt mounting on the brackets chosen specifically because the Y brackets are made of a fairly flexible plastic (mounting the belt toward the rails does reduce deflection somewhat), or is it important that the stepper itself be on-center? And, for that matter, on the X-carriage, why is the belt attached off-center? Is it because of the way the hot-end lock rotates?

    Rigidity project: My original Y-axis brackets were warped after many years of printing, heat cycles, and belt tensioning experiments, leading to a lot of skew and sloppiness with the bed. I replaced the Y-brackets with extruded aluminum 2020 rails, and mounted the rods using aluminum flange couplers. Linear rods were replaced with new units (dead straight!), and I replaced the linear bearings with Igus Drylin solid linear bushes. I built a secondary carbon baseplate that I attached to the existing Robo case, and bonded the original acrylic baseplate to the carbon fi ber* plate. Z-axis rods will be mounted with aluminum flange couplers, along with secondary reinforcement at the top of the case. I've also reinforced the X-carriage, with carbon fi ber on the extruder mount and lower X-carriage, and 6.5mm aluminum hot-end locking plate. Along with the solid bushes, the X now has nearly zero lash. This leaves the Z-brackets as the last weak link, which I might solve using a combination of reinforcement material, flange couplers, and sensorless homing on new stepper drivers (eliminating Z-endstops, and freezing the Z-nuts in the brackets).

    As part of this, I want to move the Y-belt idlers to the centerline, since the new bed carriage is rigid, and the Igus bushes can be finicky with anything other than perfectly linear motion (having the belt off-center necessarily pulls the bed in a skewed fashion, which is less of a problem with LM8UU bearings, but can be problematic with solid Igus bushes).

    I've also reinforced the X-carriage, with carbon fi ber on the extruder mount and lower X-carriage, and 6.5mm aluminum hot-end locking plate. Along with the solid bushes, the X now has nearly zero lash. This leaves the Z-brackets as the last weak link, which I might solve using a combination of reinforcement material, flange couplers, and sensorless homing on new stepper drivers (eliminating Z-endstops, and freezing the Z-nuts in the brackets).

    The goal with all of this is to 1) reduce noise; 2) improve print quality through having everything perfectly trammed/leveled, while also having reduced resonance; and 3) faster print speeds.

    Am also replacing the Arduino+RAMPS with to a 32-bit control board so I can really take advantage of the more robust mechanical components. I might even try adding a spindle for light milling/CNC stuff, although I don't think the Z components will be able to keep up.

    *Apologies for the spaces in "carbon fi ber;" the forum software thinks this phrase is spam when I remove the space in the second word, and it won't allow me to post.
     
    #1 Europa, Nov 16, 2020
    Last edited: Nov 16, 2020
  2. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Just to be clear X = left-to right as you look at the printer from the front and Y is the along the bed

    The rails center the movement for the Y (and the pulleys are roughly centerline so that helps) the X is not far off center.

    These are fair standard reprap style printers... not quite the same as the more modern CoreXY and the others.

    Certainly have fun with it :) I have rebuilt one with a much larger aluminum bed (and a different arrangements for the belt) and a number of folks have extended the Z axis as well. The replacement of the electronics is more about preference than performance. If you front the machine with an OctoPrint instance you get most of what you get with same a 32-bit control board in terms of function and you really are not going to get a lot more print speed unless you actually do serious work with the X and Y axis. Post a project thread and keep us informed.
     
  3. Europa

    Europa New Member

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    Um... I, uh... I do know the axes. :)

    So, let me describe it another way: my entire Y axis has been upgraded to be more rigid, with new linear rods, solid bushes to replace the LM8UU bearings, extruded 2020 rails to replace the plastic Y end brackets, and aluminum flange couplers to join the rails to the rods. The pillow blocks are then mounted on the carbon fib er-reinforced base plate, making for a very rigid setup on the Y... but solid bushes are finicky and tend to bind if the linear motion isn't dead-straight (I knew this going into it, but I wanted to reduce lash and quiet the thing down, hence the extra work). The problem, then, is with the location of where the belts attach to the Y brackets. In the original design, the Y stepper is on-center, but the idler rollers are actually 44.5mm offset of center, which ends up being 55.5mm when you account for the diameter of the 608 bearings (where center is defined as the 224mm distance between the center axes of the Y rods). This works fine with linear bearings (less resistance; more play), but when using solid bushes, the off-center application of force means the Y sled is more prone to binding. The optimal alignment would be to attach the belt in the middle - 112mm from each rod's center axis. My question about design, then, was to ask why the decision was made to mount the belt to the Y brackets at 55.5mm off-center. Is it because it's important that the stepper motor be in the middle? Or, was it a compromise in order to use the plastic Y brackets (where you can reduce flex in the brackets by not applying force to the middle of the bracket)? I'd like to move the stepper and idlers so I can pull the 2020 rails from the true middle.

    As it relates to increasing speed, I'm certain I'll be able to get some benefit from the mechanical improvements, along with the board upgrade and Trinamic steppers. I'm aware that this design is inherently limited in terms of potential speed, but I submit that the improvements to the Y axis is, if not serious work, it's at least semi-serious. Work on the X assembly somewhat less so, but still meaningful: solid bushes, carbon fib er reinforcement to the X carriage and the extruder bracket, and an aluminum hotend lock. These mods add up to a far stiffer X assembly, but there's no getting around the fact that direct drive extrusion means there's a lot of mass attached to the nozzle. Here, again, the off-center belt mount - where it attaches to the X carriage on the rear left - actually makes some difference when using solid bushes. As I mentioned, I knew solid bushes would be challenging to implement perfectly, but I wanted to see what I could pull off. Without the reinforcements to the baseplate, print quality wasn't always good, as small misalignments and binding led to some resonance lines and skipped belts. But the thing is so much quieter without LM8UUs, and with Noctua fans all around.

    The Z brackets are now the weakest link, not so much because of Z-motion, but in how the X gantry is aligned and suspended. I have enough 2020 rail left over to do something with the Z, but I want to get sensorless homing dialed-in before messing with new Z-brackets.

    As I'm waiting on the board, I haven't run the machine with new electronics, but I'm really looking forward to how much quieter the machine will be, in addition to faster, better prints. I'm aware that the key advantages of the Arduino+RAMPS setup are the extensive documentation and cheap components, but I went with an SKR board, so it's not really that much more expensive (it's no Duet board).

    I'd love to do a core XY build, and I may do that eventually, but I'll probably build a CNC machine before that.

    I'll post pics when I reassemble everything.
     
  4. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    The Robo is modelled after the standard reprap design (specifically I think it was the Mendel).
    The Z brackets at the top are really not needed. I have three that run 24x7 and there is not even a top cover on the machine. (I have another that does still have the top cover on, but it gets less use).

    These often print models up to 8inches or more in height with no issues :) The brace that holds the smooth rods to the bed frame needs to be made thicker and there are those on Thingiverse (I attached one I use as well).
     

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    #4 mark tomlinson, Nov 17, 2020
    Last edited: Nov 17, 2020
  5. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Also the R1 has couplers on the threaded rods where the R1+ uses leadscrews (no couplers since the lead is attached to the stepper shaft directly). Make sure that your couplers are tight and straight -- that will solve any problems related to those.
     
  6. mark tomlinson

    mark tomlinson ༼ つ ◕_ ◕ ༽つ
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    Heh, I can't assume anything :)
    Helps a lot if you are already familiar with the process...
     

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