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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 The Secret of Life is ..... Choice!
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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 The Secret of Life is ..... Choice!
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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 Veteran
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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 Veteran
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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 The Secret of Life is ..... Choice!
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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 Veteran
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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
  22. Jon Norris

    Jon Norris Journeyman
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    I'm in the middle of a build right now and I'm trying to figure out this exact issue. I'm with you on your reasoning. My question is this: Is either of these methods stronger? Does either have less deflection? My build at the moment has the entire Z axis moving, and on plunges drilling holes in the waste board, I'm getting noticeable deflection. I was wondering if rebuilding it where the axis was fixed and just the gantry moves would help with that, or if it would be exactly the same. Any thoughts?
     
  23. Jonathon Duerig

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    Odds are that the deflection you are seeing is a movement in the plastic wheels on your X-axis gantry. With everything off, apply pressure with your hand upward from the bottom of the spindle. It is likely that the moment you produce is causing the whole thing to rotate slightly as the wheels shift in their grooves. You might have some luck adding more wheels or having wheels on both sides of your X-axis.

    While it is hard to say precisely what your weakest link is, I have found that the weakest link on my own setups is rarely the Z-axis but is usually the X-axis.
     
  24. Jon Norris

    Jon Norris Journeyman
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    You're right -- it's definitely the wheels moving. I'm using a setup like the C Beam XL that has wheels and gantry plates on both the front and back of the X. Not sure what else I can do to modify that.
     
  25. Kevon Ritter

    Kevon Ritter Veteran
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    End mills aren't made for plunging. If you are getting deflection, then you are either plunging too fast, using a completely unsuitable tool, or have something else off in the build. Changing the type of Z motion might only be masking something else.
    What end mill are you using?
    What material are you plunging into?
    How fast is your plunge?
    What's your spindle speed?
    What machine?
    How large is the machine?
     
  26. Jon Norris

    Jon Norris Journeyman
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    It's a two-flute upcut Whiteside CNC router bit, 1/4", peck drilling holes in the MDF spoil board about 1/8" deep at a time. If I remember right, plunge rate was about 30ipm. Not sure about the rpm as it's a Bosch colt. Machine is similar to the Dollarz81 C-Beam XXL Tank using 1000mm C-beam actuators for X and Y and a 250mm for the Z.

    It's entirely possible that it's the wrong bit for the job.
     
  27. Jonathon Duerig

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    As @Kevon Ritter says, the solution might be to accept the limitations of the rigidity of your machine and change your cutting accordingly. Ramp into plunges and make plunges much more slowly.

    If you want to make your machine itself more rigid, there are three basic possibilities, IMO:

    (1) Figure out a way to make the wheels themselves more rigid. There are brass and/or steel wheels available. The trick is that over time harder metal wheels will dig ruts into the softer aluminum. So people who have done this successfully have had to take measures to prevent this. I think there are some items (open rail?) which could be added that would be suitable for metal wheels.

    (2) Add onto your x-axis beam. If you had two C-beams back to back bolted together, you could add another two sets of wheels to provide more support. Not sure how much this would help.

    (3) Add an x-axis on the other side of your Z-axis. The basic problem is that you are trying to support a force on a cantilevered beam. So if you have a second x-axis beam on the other side then you would instead be resisting a force on a supported span. If you do this, then your bit is no longer acting like the end of a lever against your wheels. I haven't really seen this be implemented in part because it can reduce the travel of a machine and costs more. But if I needed to make my machine more rigid, this is what I would do. Any reinforcement on just one side is reduced in value by the mechanical advantage of the lever.
     
  28. Jonathon Duerig

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    Generally speaking, it is better to use a bit smaller than the hole size you want and have it spiral in. If you really want to drill a 1/4" hole with a 1/4" bit, you might try using a drill bit instead.
     
  29. Kevon Ritter

    Kevon Ritter Veteran
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    30ipm is feed territory and is a bit too fast, even in mdf. That's 762mm/min for the fellow metric heads. Back that down to 8ipm (~200mm/min) with a speed setting of 2.5 on the router dial. Look at how the machine reacts, then increase the plunge speed from there. Once you know where it's comfortable, you can go much much deeper per peck.

    I think I only plunge at 250mm/min with a 1/4" corner radius 2 flute up spiral.
    I pre-drill all holes in aluminum with a proper 4mm drill bit before swapping to a dedicated 1/4" drill mill. You can do the same in any material to relieve stress from the center of the tool.
     
  30. Jon Norris

    Jon Norris Journeyman
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    Thanks! Definitely some options to consider. I'll see what I can figure out, and I'll definitely take it slow doing some test cuts as I make a dust shoe and some fixtures.
     
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