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C-Beam Z axis?

Discussion in 'CNC Mills/Routers' started by John Meikrantz, May 1, 2015.

  1. John Meikrantz

    John Meikrantz Veteran
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    Would using C-Beam (one of the linear actuator bundles) make a more rigid Z axis than the current Ox design? Seems like there would be less potential for flex than the offset wheels/spacer design, and if mounted directly to the existing Z axis plate, it wouldn't stick out much farther.

    Wonder how it would compare to a commercial Z slide.

    -- John
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    The main weakness in the current Z-axis system is the lack of appreciable vertical distance between the wheels. The spacing has already been increased once to improve the stiffness which helped but separating the wheels even further would help even more. The wider the vertical grip, the stiffer the system gets. The next level of weakness is torsional rotation of the X-axis beam which is why many people are bonding the two pieces of V-slot together. But increasing the stiffness of the Z-axis itself using the C-beam system would not likely show any discernible improvements. You would get some side to side stiffness improvements if you used outset wheels as a grip based on the 80mm width would obviously be better than the current 60mm grip but as far as dipping caused by the Z-axis flexing about X-axis, there would be no noticeable difference if the vertical wheel spacing was the same in both scenarios. There might be a perceptible difference using both inset and outset support wheels but that would take a bit of testing to verify.

    As far as using commercial Z-slides, it would depend on the slide. While these inherently offer greater rigidity due to considerably tighter tolerances, the width of the grip on both directions still plays a factor.
     
  3. John Meikrantz

    John Meikrantz Veteran
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    I'm not sure I completely agree. I see two areas of front/back flex in the Z axis. The side wheels, even if tight still seem to have some flex forward/backward, as opposed to compression in the direction of the V Rail itself. Also, the way the Z axis rod is supported, it allows some movement forward back as well, i.e. the rod flexes with the acme nut as the pivot point. I don't see a lot torsional rotation around the X beam.

    I would think that the C-Beam would prevent some of those things.
     
  4. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    And this is exactly the problem. And the best way to reduce this flexure is to reduce the force on the wheels. And the best way to reduce the force on the wheels is to increase the spacing between them. This is simple leverage.

    Other ways to reduce the flex on the wheels would be increasing diameter of the bolt, reducing the length of the bolt (i.e. reducing the distance between the wheel and the plate which is why the double standoff in the extended plate was not a good idea) and reducing the diameter of the wheel. Of these potential solutions, the first is not an option due to the lack of an eccentric spacer for a larger diameter bolt. The other two are an option but for either to be properly implemented would require a rework of the existing plate layouts. Chris Laidlaw has overcome the issue of the double spacer by extending the spacer blocks such that all 4 wheels each side only have one 1/4" spacer between the wheel and the support. Personally, I'd like to see him take those blocks up another 40-50mm and provide holes for two wheels at the top both sides as this would reduce flexure even more. The final option of using smaller wheels is workable but the wheels still have to be at the outer edges of the rail section. While concentrating them at the center as the C-beam system does would provide same resistance in the vertical flexure as ones on the edges, it greatly reduces the side-to-side stiffness of the assembly which is why it is not a good idea.

    As far as the Z-axis rod goes, it is not there to stiffen the system, it is merely along for the ride. If you can see it moving, the system is failing it not the other way around.

    Truly if you want to give the C-beam system a try, go ahead but I honestly don't believe it will solve the problem. Ultimately the Ox while being an excellent system is still a hobby grade system and has its limitations. It will never match pro grade systems costing 10(+) times as much but it should not be expected to.
     
  5. James Gao

    James Gao Well-Known
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    That's an interesting perspective I haven't thought of... The mini v wheels might actually work better on the z axis. I'll have to incorporate that into my double z rail design!

    Also, why does the ox move the rail, rather than move a plate like most other designs? Based on my experience, the rail itself functions as a large lever, applying significant forward/backwards force on the wheels. Attaching the z rail directly to the x carriage transfers torque on the z axis into axial force on the x rollers, which is what they were designed for. Any front-back force on the wheels is centered so there shouldn't be any torsion on any of the wheels.
     
  6. John Meikrantz

    John Meikrantz Veteran
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    That was kind of my thought with using C-Beam for the z axis. Seems like it would greatly reduce the torsion on the wheels, combined with the small diameter wheels having even less leverage against their axle.
     
  7. James Gao

    James Gao Well-Known
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    If you have any capability to mill your own plates, you can try my idea here: http://www.openbuilds.com/threads/double-z-rails.1673/

    That also moves the z-axis closer to the gantry. With a stock OX plate size, there's probably enough room to use a pair of 20x40's, and you can pinch from both sides with mini-v wheels. That would be pretty strong.
     
    John Meikrantz and sigrx73 like this.

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