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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. Kevon Ritter

    Kevon Ritter Veteran
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    A moving Z axis is something that is seen very often, but I can't quite figure out why it's so popular. The only thing I can think of is, "it's like this on the [machine name], so I should do it too."

    After looking at the pictures, do I really need to say more? In these pictures, there are two layers of v slot (40mm total) and another two layers of 3/4" (19.05mm) MDF, as well as the ~3mm tall screw heads. With the pictures tool, that workable area would go from 80mm to about 42mm.

    Both setups would have the same rigidity if all else is equal, such as wheel spacing. But a moving gantry cuts your available working height by the length of your endmill that projects below the Z axis. In my picture, I would have lost a good 1.5" of usable material height. So instead of having a machine that can cut a maximum thickness of A, it would be a machine that can cut A minus tool length. Why do people go that route?

    The counter argument would be that you can simply mount the spindle higher. If you do that, you no longer can use the full depth of every end mill you use.


    I'd say a height controlled X gantry is something entirely different and would not be appropriate to bring up in this discussion.

    Kev
     

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  2. Joe Santarsiero

    Joe Santarsiero OB addict
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    I'm confused.
     
  3. Kevon Ritter

    Kevon Ritter Veteran
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    What do you need clarity on?
     
  4. GrayUK

    GrayUK Master
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    Sorry. Me tooooo! I feel I've come in half way through a conversation. :rolleyes:
    I thought for a moment you meant a stationary spindle, and a bed that moves in the X and Y axis? Or even a bed that goes up and down, therefore eliminating the Z axis? :)
    Now there's a thought!! :rolleyes:
     
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  5. Kevon Ritter

    Kevon Ritter Veteran
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    Ah I see. By "moving spindle," I'm referring to a setup where ONLY the spindle is going up and down. The Z axis that holds the spindle remains fixed to the X gantry. The position of the end mill relative to the rest of the Z axis stays varies. An example of this would be the C-Beam Machine.
    [​IMG]
    With a "moving Z axis," everything, goes up and down. The position of the end mill relative to the rest of the Z axis stays the same. An example of this would be the Ox.
    [​IMG]
     
  6. GrayUK

    GrayUK Master
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    You mean, I think, the spindle moves along the X axis, as usual, but the X axis goes up and down, therefore becoming X and Z axis?
    Like you showed before, and I think I might use the principle in my next build.
    Please forgive me if I have it wrong again. :banghead:
     
  7. Joe Santarsiero

    Joe Santarsiero OB addict
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    So fixed gantry vs. fixed bed? They both have the same problem of clearing the bottom of the Z slide.
     
  8. Flash22

    Flash22 Veteran
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    A moving Z axis imho is more stable and able to with stand more forces when cutting, over a moving spindle - moving the whole X axis up and down would be a non starter as you will need to keep the motors in step with a very high chance of it dropping tolerance unless you drive the axis with servos with encoders - 0.5-1mm out on 1 side could mean the bit missing its cut to hitting the bed and snapping a end mill on a x axis anything over 500mm

    As for cutting depth the limitation in not in the Z movement but the clearance of cutter to the base of the spindle, the more tool length you have out of the collet (chuck) the slower you will need cut, As well as reducing the cutting depth per pass to stop it loading the spindle and dropping tolerance

    As above it seems like this is half a conversation, don't try and reinvent the wheel if it was a usable method It would be of been done - you only need to look a industrial grade machines to see what works

    If you haven't built a machine before, stay with the tried and tested methods

    My machine started out as a standard C beam - It has since evolved and I have recently upgraded it to over come some of its limitations - its now a very solid machine with some very fine cutting tolerance's (currently awaiting parts, and calibration again)

    My double wide CBeam
     
  9. JustinTime

    JustinTime Master
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    Kevon, looks like I'm the only one who understood from the get-go what you mean and now that you mentioned it, I agree, the spindle moving up and down is a better solution. My PhlatPrinter III works just like this and I never gave it a second thought.
     
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  10. Flash22

    Flash22 Veteran
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    Looking at the thread again if its a OX, just change the fixed bed position by changing the front and rear supports to drop the bed down

    with a fixed gantry and moving Y (bed) its not a issue
     
  11. Kevon Ritter

    Kevon Ritter Veteran
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    I'm not at all trying to look for opinions that align with mine, but Justin is the only one that actually understood my post.

    The only reason why a rising/lowering x gantry was mentioned was to keep it out of the conversation.

    At no point was a moving bed of any kind mentioned either.

    I can't sit down and dissect replies right now, but there is absolutely no wheel reinventing going on.
     
  12. Jonathon Duerig

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    I think that you have accurately pointed out one of the main drawbacks of a moving Z-axis. You are giving up Z-travel. And you are also requiring a stronger motor on the Z-axis since it has to move more stuff up and down. Rigidity is a wash.

    What you gain in exchange for the reduced Z-travel is:

    (a) Fewer gantry plates. With a moving Z-axis, you can have the X-axis gantry plate on the side closer to the spindle also act as the Z-axis gantry plate at the same time. With a static Z-axis, you need to attach the C-beam to the X-gantry plates then you need an additional gantry plate to move the spindle. If you are using custom plates, this can reduce your costs and complexity a fair amount.

    (b) Easier mounting. Typically it is easier to mount things to V-slot than a gantry plate with holes. There is a lot more flexibility here. So whether you are mounting cable clips, dust boots, a pressure foot, laser guides, cameras, sensors, or anything else it is easier to mount it to the V-slot part of the Z-axis. A lot of this stuff you want to go up and down along with the spindle. So if you have a lot of stuff you want to mount, it may be worth the loss of Z-axis travel.

    At the end of the day, Z-axis travel is not that important for a lot of people. Many people only ever cut thin stock. In my case, I've only got about an inch and a half of Z-travel in my current setup. And it is plenty for everything I've needed. The trade-off is that I was able to easily mount a combination dust shoe and pressure foot that makes cutting thin stock a lot easier. But for other people, the trade-off would not be worth it. They need to be able to put a huge chunk of wood on the table and engrave a sign front into it.

    This is why it is important to look ahead at what you will use a CNC router for. A lot times people will learn about CNC and want a router that can do everything. And they learn that by not specializing, their router isn't particularly good at any one kind of task. A router with a huge Z-axis might be less suited for cutting thin stock or hard materials. While a router stiff enough to cut aluminum might not be large enough to cut out a big sign.

    -D
     
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  13. SugarJ

    SugarJ Journeyman
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    I'll agree. I've built an Ox-Metal variant, and one of my planned upgrades is to switch the Z-axis to a C-Beam style to give me more Z-travel and plunge depth. One of my planned uses is for cutting mortise and tenons for furniture, and I'd like to be able to cut pockets into a dressed 4x4 table leg. I can't do it with the way my Ox is set up now with a moving Z-axis. I've just got to nurse it through cutting some new gantry plates. And find the time to rebuild it. :)
     
  14. Flash22

    Flash22 Veteran
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    the first post was a bit strange that's what lead to confusion

    I see the issue you have being a lack of clearance on the Z - raise the machine and lower the bed then your not getting involved with any of the axes
     
  15. Joe Santarsiero

    Joe Santarsiero OB addict
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    So this is about a moving Z platform vs a moving Z slide?
     
  16. Giarc

    Giarc Master
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    I went with a moving spindle on a plate rather than moving the extrusion up and down, as you suggested. I did this because I gained a little more depth of cut. In fact, I gained enough depth of cut that I had to buy extra long end mills to drill the holes in my spoil board for threaded inserts for clamps (an extra $40 in cost to the build) :banghead:. The only time I needed that extra depth was cutting a 2 inch piece of foam to make Zero's grave stone. That is also the only time i needed those extra long end mills. Glad I had them.:thumbsup:
     

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  17. Sprags

    Sprags Journeyman
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    Though I'm not new to CNC machining I'm new to OpenBuilds and I'm confused about the terms used to define the various machine configurations. What is meant when the machine is an OX machine? A Cartesian machine? A fixed bed vs. a moving bed is easily recognized but some of the other terms and the way things are defined don't always seem to follow the same definitions as industrial commercially built machines such as Mazaks, Okumas, etc...
     
  18. SugarJ

    SugarJ Journeyman
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    The Ox is a Cartesian build designed by Openbuilds. OpenBuilds OX CNC Machine

    And I'm sure you're right in that most of us are hobbyists and don't use the same terms and nomenclature as the commercial CNC industry does.
     
  19. Sprags

    Sprags Journeyman
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    That's an interesting idea...to have the bed move up and down in Z ...but if it also moved in X and Y the beam assemblies would get complex and of course more prone to being loose and less accurate.
     
  20. GrayUK

    GrayUK Master
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    Yea, I re-thought that comment. Up and down wouldn't be too hard to organise, but, up/down/side to side and forward and back would be a whole new ballgame. You would probably have to build your up/down section, and then add it, as a cassette onto, or into, the other two axis' mechanism.
    Maybe a 2018 thing?
    Anyway, Happy New Year - Sprags.

    Gray
     
  21. Sprags

    Sprags Journeyman
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    I thought back in all my years working in manufacturing and thinking back at all the different machine configurations I've ever seen at all the machine design shows and IMTS's and I just don't remember any being like that.

    Of course the probable reason for not configurating the machine that way is to not have to lift the weight of the part plus the machine bed.
     
    #21 Sprags, Dec 26, 2017
    Last edited: Dec 26, 2017

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