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Why I Believe a Moving Spindle is Better Than a Moving Z Axis

Discussion in 'CNC Mills/Routers' started by Kevon Ritter, Dec 7, 2017.

  1. Jon Norris

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    Thanks Rick. That's what I was wondering -- if the fixed Z beam was known to be stronger than the moving Z beam before I tear this thing apart.
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    It's not specifically stronger, it just allows you to keep the bit extension distance from the main support beam shorter thus reducing the leverage on the system and subsequently reduces the amount of flex.
     
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  3. Gary Caruso

    Gary Caruso OpenBuilds Volunteer
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    With a moving axis, I can engrave or cut on a 2" thick part, then with the same short bit reach down and engrave a 1/16" thick part without adding a bunch of spoilboard to boost up the stock.
    So even though my overall travel might be shorter, my useable reach is greater than a moving spindle.
    That's the main advantage for me with a 32x51" cut area I don't want to store a bunch of spoil lift plates, or have to lower my z-axis manually.
    make sense? I agree the moving spindle can be stiffer, but all things are not equal in that either, no way 8 mini wheels inside of a C beam is stiffer than the 8 standard wheels on the outside of the beam.
     
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  4. fwm891

    fwm891 Well-Known
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    What you really need is a Z-axis that moves to just above the material and is then locked with the spindle alone then able to descend into the material for the full depth of the cutter. I see the principle there but not sure about stability
     
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  5. Robby Candra

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    I also Believe a Moving Spindle is Better Than a Moving Z Axis.

    Here is my Z Axis with screw protector.
     

    Attached Files:

  6. Rob Taylor

    Rob Taylor Master
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    This is a weird thread. Neither is "better", because machine design is so nebulous and context-specific.

    A "moving spindle", as it's termed here- "fixed rail", as it's normally called- provides consistent- not necessarily consistently higher, though it usually is because it can be built so, but consistent- torsional and flexural rigidity because the spindle always has the exact same support structure throughout its range of motion and the rail is fully supported by the gantry. Typically the spindle can be clamped a little lower on the body, which also improves rigidity and vibration performance at the tool bit. The big downside is that material clearance height is fixed at whatever the lowest point of the linear bearing structure happens to be- any higher and you'll crash the z axis into the stock. But for short-travel builds like platemakers, luthiers, cabinet routers, etc, this setup is ideal.

    A "moving z axis", normally called "fixed bearing" (the motor doesn't necessarily have to (ideally shouldn't) be part of the moving mass) provides significantly improved clearances; your clearance height is effectively whatever your maximum toolbit +Z position is. This would be great, except by definition you have a now-variable axis rigidity; the lower you're working in Z, the less rigidity you have. Depending on what you're doing, this may not be an enormous problem; lasers and 3D printers shouldn't see much of an issue except for very extreme geometries, and if built correctly- and CAM'd appropriately- certain types of mill should also be perfectly happy with this arrangement. This system is probably less ideal for extrusion-and-wheel builds; the leverage is going to be much more of an issue with plastic wheels on narrow shafts. SBR and HGH rails are more the way to go here if you're planning on any kind of significant sideloading, extreme gantry heights, or high inertial (acceleration/head mass) loading processes. Other options like double-dual rail (parallel pairs to form a square) or telescopic overlapping can also help, depending on the machine requirements.

    The answer to "which is best" is, as always, simply whichever is necessary for your specific requirements, convenient to build, practically feasible, or appropriate for the general class of machine.
     
  7. Joe Santarsiero

    Joe Santarsiero OB addict
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    Getting confused again.
     
  8. David the swarfer

    David the swarfer OpenBuilds Team
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    Joe, Sphinx vs LEAD Z axis styles.
     
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