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Ox Z Axis thoughts

Discussion in 'CNC Mills/Routers' started by John Meikrantz, Sep 25, 2015.

  1. John Meikrantz

    John Meikrantz Veteran
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    I've been working on my Ox based system - Cebu, for a few months now. Still trying to figure out ways to strengthen the Z axis, which is not as stiff as I'd like.

    I think the problem is in the original design, where the acme screw is held to the x plate via the nut block, leaving the V-Slot and router kind of "hanging", and subjecting the z axis wheels/screws to lateral force. It would be better I think to have the V-Slot attached to the x plate.

    So, wondering if there is a way to mount either C-Beam linear actuator or the new V-Slot linear actuator to the existing x plate. I think that might help to make a stiffer z axis.

    Can someone design an adapter plate to allow mounting the linear actuators to the existing x plate, or come up with a new x plate design that would support a linear actuator?

    Hopefully this description makes sense!

    -- John
     
    Hytech2k and dddman like this.
  2. Jonny Norris

    Jonny Norris Veteran
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    Should be fairly straightforward with combination of sliding t nuts and corner brackets.

    Best bet id say is give it a go. Worst case you would simply need to drill 4 holes to accept the bolts for the sliding t nuts.

    If you wanted to try improving the standard ox z. Doing something like this to the supporting wheels would help a lot:

    [​IMG]
     
    #2 Jonny Norris, Sep 26, 2015
    Last edited: Sep 26, 2015
  3. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    You might start by just switching out your short spacer blocks with a pair of @Chris Laidlaw's extended version. The extra set of wheels that was added above the original 3 sets really offers little strength due to the way the wheels cantilever so far off the surface of the plate. Even small amounts of flexure in the standoffs leads to noticeable play in the overall system. And honestly, I'd ask Chris for a custom set that moves the top wheels a full 1" higher than the amount he has already extended them.
     
  4. John Meikrantz

    John Meikrantz Veteran
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    I think both of those ideas definitely help the problem, but I'm still not comfortable with the nut block being a pivot point for the z axis. I think the more surface area that is attached to the x plate the better. Another thing that might help the original design is a stiffer threaded road, maybe go up to M10.

    Rick, you make a great point in that the z axis being cantilevered is not the most rigid way to attach.

    -- John
     
  5. Jonny Norris

    Jonny Norris Veteran
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    Rick is right in where the flex occurs.
    There actually isn't a lot of difrence in the cbeam design in that it has wheels that are unsupported at one end. So potentially can suffer the same flex.

    Supporting the wheel bolts both ends is the answer and bolting a block as shown in the picture correctly it wouldn't go anywhere. Infact I'd like to compare such mod with a cbeam actuator. My money would be on the mod being stronger. As let's face it is not hard to bend an m4 bolt.
     
  6. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    If you're flexing about the nut block something is seriously wrong. And a more rigid threaded rod is not the solution. Start by giving the wheels the most rigid grip possible. As for whether it is better to fix the Z extrusion to the X plates or to the router mount and let the Z axis slide, that is somewhat of a toss up. Ultimately you are going to have vertical motion somewhere between the X plate and the router mount and that motion will be achieved by wheels gripping the sides of the Z extrusion. That's why the wheels are the place to start.

    Edit: Oops, I see Jonny can type faster than me. There was no intent to be redundant here, it just happened.

    Oh and Jonny I like your idea, I just think implementing could be difficult. The slot width would have to be an absolutely perfect fit for the width of the wheels and the shims or the wheels would shift side-to-side. The problem lies that even if you were to match the width perfectly to the width of one wheel, due to manufacturing tolerances all wheel assemblies don't come up exactly the same so getting four wheels to fit exactly the same might be a challenge.
     
    #6 Rick 2.0, Sep 26, 2015
    Last edited: Sep 26, 2015
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  7. Jonny Norris

    Jonny Norris Veteran
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    As a kid I was into inline skating, Bauer bought a pair that similarly had the wheels bolted just one side through a single plate. My mate bought some and within months the bolts bent and the plate holes widened.

    Bauer soon dropped the design and As far as I'm aware no other company has attempted it since.

    For a good reason I think.

    Bmw achieve it with the single fork motorbike, but that's a different beast altogether. Where the bearing assembly is retained in the fork, this must be because simply a bolt just isn't up to it.

    I know these are difrent things all together but same principles apply.
     
  8. Jonny Norris

    Jonny Norris Veteran
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    Yes it would require a milling process as accurate as the original spacers.. You would still require shims so you could leave as much tolerance in there as you like as the intention is to simply support the bolt at both ends.

    And any difference in wheel width is always going to exist so wheel alignment will remain the same as standard.

    But yes it would still require shims.

    Alternatively am I right in thinking the spacers and shims all metric? 0.75mm 1mm etc so you could build one up from metric thickness sheets stacked. Bolted to the X plate with some m6 bolts, would be surfice.

    But it wouldn't phase a good machinist with a mill.

    Even better, I wonder if there's an aluminum Channel that would work.
     
    #8 Jonny Norris, Sep 26, 2015
    Last edited: Sep 26, 2015
  9. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Oh, I follow the principles I just think the level of precision required would be difficult for the average person to achieve especially with the limited manufacturing tolerances of the supplied parts. The best solution is to drill out the near side for a snug fit dowel spacer. That way the bolt head would draw everything tightly to the nut side but provide lateral support on both ends.
     
  10. Jonny Norris

    Jonny Norris Veteran
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    Yes that would work, the nut could even go inside the channel, with just the tip of the bolt in the block.

    Not as neat but it would work to same effect
     
  11. John Meikrantz

    John Meikrantz Veteran
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    Let's take the wheels out of the equation for a moment. In the existing design, the z axis is attached to the x plate with an 8mm rod and a single nut block. The long z axis is a lever, with the nut block as a fulcrum. Now put the wheels on the sides. This helps restrict the movement of the lever, but the wheels are not optimally designed for lateral forces. That and the method of attachment, where the wheel axle/bolt is only supported on one end allow for a certain degree of movement. If the z axis is attached to the x plate, it transfers that movement to the x axis wheels with a more axial force. See comments from @James Gao in this thread, where I was discussing the same topic - http://openbuilds.com/threads/c-beam-z-axis.1686/#post-16544

    I think if you were to take the existing z axis design in a current Ox build, and push straight back on the top of the z axis motor you would see a certain amount of movement. I'm thinking that if you had a C-Beam or V-Slot linear actuator attached to the x plate, that movement would be less because the forces would be distributed differently to the x axis wheels, and the y axis carriages.

    Also the smaller diameter wheels in the C-Beam design are less susceptible to lateral force. This is part of the reason that the C-Beam Plate Maker design is more rigid than the Ox. The C-Beam gantry plate is a much smaller lever, and the C-Beam or V-Slot attached to the x axis would support the threaded rod better.

    -- John
     
  12. Jonny Norris

    Jonny Norris Veteran
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    Not quite sure Im with you on the lever, the thread nut is in the middle so you could say it would pivot around it. But this is the same with ball screw and rack and pinion. Or do you mean simply the way the z is in respect to X creating a cross, which will act as a lever. Only workaround would be a completely different machine all together.

    bolting the z to the x, so you don't have a z? Yes would transfer the flex into the X.

    So what you are really saying is you need to swap out your X and Z for cbeam? And I suppose without modification to the X you would as the flex will simply move to the next weakest link.

    Yes the cbeam wheels may have less flex. But still has flex, so my money would still be on a design where the wheels have no flex.

    With a bit of ingenuity, one could apply the blocked wheel idea (the new name I just made up for them) to the X as well as the z.

    But you have raised a valid point, the X will also require attention.

    You are talking in triple figures with the c beam swap, I'd say the blocked wheel idea you would need 6 - 4 for the X, 2 for the z. And I'd hope to see a solution for the blocks at around £50-80 and you already have the wheels.

    Also need to Bear in mind, you could fit square rail and carriage to your existing profiles for £150-200. Which is going to be better than cbeam, I'm not sure how that compares in cost to cbeam so it may be that it's not really a cost effective solution as an upgrade.
     
    #12 Jonny Norris, Sep 27, 2015
    Last edited: Sep 27, 2015
  13. John Meikrantz

    John Meikrantz Veteran
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    @Jonny Norris and @Rick 2.0 ,

    Have been digging a bit more into the lateral wheel movement. When examined closely, it appears as if the wheel itself is sliding back and forth on the bearing, and the axle/bolt stays stationary. Because of that, I'm not sure if supporting the axle/bolt on both ends will fix the problem completely. It makes sense from a design standpoint, but won't stop the movement of the wheels back and forth.

    To be clear, this is a problem that is manageable, just need to adjust speeds/feeds/depth of cut appropriately. Where I see the most movement is usually on initial plunge, which needs to be ramped and slowed considerably.

    Appreciate everyone's input!

    -- John
     
  14. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    You might try tightening the nut a bit more. I've had to really crank down a few to take out the slack. You may also have a bad bearing.

    Adding a bit more pre-load on the wheels (i.e. tightening the eccentric spacer) may also be beneficial in general. (Won't help with with the sliding wheel but may help with looseness in general.)
     
  15. Jonny Norris

    Jonny Norris Veteran
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    Yeah i think they have a bedding in period, I found this happen too, only on one occasion though. Simply tightened up and not suffered it since.

    It could do with a few wheels at 90degrees to existing which would correct the issue I must admit. Not sure where you could put them tho.

    Suppose the cbeam mini vwheels would also suffer this.
     
    #15 Jonny Norris, Sep 28, 2015
    Last edited: Sep 28, 2015

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