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Is a center mounted Z-axis Overkill?

Discussion in 'CNC Mills/Routers' started by Robert Johnson, Nov 29, 2015.

?

CenterMounted Z-Axis?

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  1. Robert Johnson

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    Two C-beams would "sandwich" the Z-Axis. The reasoning is to eliminate z-deflection that would occur when spindle is pushing lower into material. This would allow higher Z gantry and longer Z-axis travel if desired. Also potentially increased rigidity of X-axis with second c-beam. Due to lack of CAD/Sketchup skill here are a couple of pictures to hopefully illustrate what it would be.

    2015-11-29_095158828_EB610_iOS.jpg 2015-11-29_100321322_1D782_iOS.jpg
     
  2. Indieflow

    Indieflow Veteran
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    I agree this concept would definetly increase the rigidity of the Z-Axis, though I believe it could be overkill dependant on what you are after achieving with your machine!
     
  3. JustinTime

    JustinTime Master
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    It's a good idea, Bob, and could work fine. You'll lose some x, or y depending on the position, movement but it can be compensated by making that axis a bit longer. But, as Indieflow said, could be overkill.
     
  4. Jonny Norris

    Jonny Norris Veteran
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    I'd say it would make toolchange and zeroing off a little trickier with the reduced access.
     
  5. gotswrv

    gotswrv Journeyman
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    Bob,
    Great to see someone else thinking along the same lines as me. I've gotten most of the mechanical stuff in house now (from open builds) and have been looking into ironing out the details (in SW). Need to create provisions for belt tension, wire routing, limit switches, etc.
    I plan on sharing in my own build thread once I've finalized the design and start machining out the plates.
    Really thinking about belt-belt rack and pinion for the X. I'd like to mount with the X motors vertical, which has a couple advantage, but adds more plates, and complexity. Ohh, and the travel as currently drawn is X extrusion minus 240mm, more than some, but not horrible. The Y axis has the belt wrapping around the pair of top wheels and the stepper inside the C.

    My setup is intended to house an aircooled 1.5kw spindle using a C-Beam Z. Been thinking a version 2 will likely go to more expensive linear solutions with a ball screw. Would cost more, but offer a stiffer and lower profile. This would fit larger spindles into the same space.

    Also need to work out dust collection. This is likely going to be more complex than typical, do to the limited area around the spindle. Might be looking into forming some lexan or something to snake around the spindle motor.

    -Sam

    OB CNC 6a.png
    OB CNC 6b.png
     
    snokid and Rick 2.0 like this.
  6. Florian Bauereisen

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    HI,
    mixed feelings about it eventhough in general it is a good idea.

    Ok in general it is always good with cnc to stiffen up the machine... till you end up with machines weighting tons -LOL

    Looking closely you will find that your spindle-mount should not rotate forwards and back any more (y), but to eliminate sidewards flex one has to space the wheels further apart (like you did) to prevent flex in x. But than one needs longer extrusions (x) to compensate the distance lost - which will flex more the longer they get.
    Also these will bend more with the weight than shorter ones - to know if you gained or lost one really needs data and math- ore trial and error.

    What happens if one extrusion would ever so gently slope uphills at center while the other downwards? Think this improves the outcome?

    The ox works well cause all of its components are about the same in terms of stiffness (or weakness -depends on ones standpoint)
    So it is balanced well. All components work in harmony within the design parameters..
    NO money and no effort spend on a single apect only marginally improving the product and hardly affect cutting times in the end.

    I have posted some pics in a threat here somewhere of a machine using double x...
    And as mentioned there too: the z-x setup alone of that machine probably costs more than a complete ox.

    greets
    and a Happy new year

    Flo
     
  7. gotswrv

    gotswrv Journeyman
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    Hey Flo,

    I've gotten my X reworked and I'm very happy with it. So much so that I started looking at the Y (Z carriage) again. I plan on starting a thread this weekend to get some feedback specific to my machine.
    Thought I'd mention that at 240mm lost for the X brackets I think it is not too much more than some of the other designs currently accepted. However, if one does not have a well supported X then the added weight will be troublesome.

    I've gotten the extrusions in house and they seems to be of good quality. No detectable warping. At any rate, it is just as likely for an extrusion to be twisted as bent. Frankly, with a single beam it will twist more with the carriage in the center vs outboard. Particularly with the large moment realized in most designs currently being shared.
    If one beam is curved up and the other down, I'd suggest flipping the one that is down over..

    Not sure what the expenses are you are mentioning. Having more plates and extrusions cost more, for me I think they will be offset in ability and size gains. Most of what I'm after is the increase of size. Of course I have the benefit of being able to make the parts on a CNC mill, so I may be underestimating the costs some.

    Anyway, I look forward to sharing more with you soon!

    Sam
     
  8. JustinTime

    JustinTime Master
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    A loss of 240mm is not trivial on a distance of 1000-1500 mm.
     
  9. gotswrv

    gotswrv Journeyman
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    What is typical?

    Edit:
    Got home and pulled up a couple OX designs. Looks like 180mm is typical, so I've given up 60mm more. This is an easily reduced number, but I'm not sure if I'm concerned at this stage.
     
    #9 gotswrv, Dec 31, 2015
    Last edited: Dec 31, 2015
  10. Florian Bauereisen

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    HI,
    i was replying generally.
    Could have used the phrase: a machine is perfectly constructed once you cannot leave out another part without impairing funktionality..

    greets
     
  11. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    I just wanted to throw in an additional concept. This is an attempt to simplify as much as possible, using as much off the shelf as I could. Please note it is not fully detailed, merely a quick concept. General View.jpg

    Side plates are the same width as the original Ox plates but squared off and 40mm higher to create additional space beneath the gantry.

    Front Elevation.jpg
    Exploded View.jpg

    Exploded view showing the X-axis and Z-axis carriages. The belt at the top of the Z-axis carriage is to raise and lower both sides of the unit simultaneously. The router is held in place with the stock Router/Spindle Mount for easy removal.

    Note there is an error in the attached drawing, showing a lead screw in the near side of the gantry which shouldn't be there. Like I noted above, this is merely a quick concept.
     

    Attached Files:

  12. gotswrv

    gotswrv Journeyman
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    Pretty cool!
    I have no idea, how close the being the same length are the extrusions? Can a guy count on +/- on overall C beam length?
    My thought is the vertical plates would not be square if one C-Beam was longer than the other. This would put the X? bearings in a misaligned state.
    I would suggest putting the cross beams under the X? axis to create the space rather than making the vertical plates taller. Personally I'd rather keep the vertical plate as short as possible and put risers under the entire machine as needed.

    I've not been able to find any info (minus one source) on what rapids can be achieved with DIY cnc systems (leadscrew, ball, or belt). Of course loading changes everything, but even non-system numbers are not around. For instance, how fast can a C-Beam (leadscrew), that is 1000mm, just setting on a bench, not in a machine, go?
    I plan on doing some testing with me little 250mm C-Beam this weekend (be using a 282oz nema 23 on 24v). However, my experience with a 4ft lead screw(1/2") hot wire(8020 based) was not great. Trying to run at any reasonable speed would cause the screw to whip. I know a larger diameter screw would help, but the C-Beam is 8mm if I remember right. How does the C-beam screw behave at 1000mm or even 1500mm? The last thing I build was actually my first belt system, been really happy with the belts. However, it a very simple down cut hot wire, so no real harmonics to deal with. Really hoping the next router machine with a belt on belt works as good as the hot wire.

    Looking at the above again, it is defiantly worth a closer look!
     
  13. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    You should pretty much assume that you will need to file one of them down to match the other. Even 1/2 mm makes a substantial difference.

    Yes. The concept above was just a quick makeover of a previous single rail concept. If I were actually building this I would take 4 pieces of 20x40 laid flat and mounted equally spaced to the bottom side of the C-beam rail and not change the height of the side plates. For someone who does want the additional height however, increasing the plate thickness up to 3/8" should more than compensate for extending the side plates. With a Dewalt 611 and leaving the spindle lock accessible, the normal profile will only have 2.5" below the collet nut which is not a lot of depth.

    Revised sketch attached.
     

    Attached Files:

  14. JustinTime

    JustinTime Master
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    Rick, I'm wondering how the eccentrics for the x axis will be adjusted. I looked at the .skp file and there is not room to access them.
     
  15. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Install the inner wheels first and adjust them the same way you would normally adjust the wheels on the C-beam system. You'll just have more plate extending out the end where you are making the adjustments. Alternatively you could also install just the outer wheels and then use a set of calipers to set the inner wheels to match the outer wheels or even more simply, just match the orientation of the eccentric nuts.
     

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