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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > Z AXIS - Moving bearings or moving rails ?
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  1. #1
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    Z AXIS - Moving bearings or moving rails ?

    I'm going to build my Z axis later.
    I've been looking at various machine builds and getting some ideas.

    There seems to be two ways to approach it.

    1. Fix the bearings to the gantry carriage, and have the rails move up and down with the router.

    2. fix the rails to the gantry carriage, and have the bearings ride the rails , up and down with the router.

    as far as I can see. with the bearings fixed, and the rails moving, it makes for a more compact machine, only being tall when the router is fully up.


    Are there any pros and cons to either method ?
    Rigidity ?

    Unless I hear otherwise, I'm moving towards the fixed bearings design.

    TIA for any suggestions

    Rich
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  2. #2
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    1183
    It depends on what kind of machine you're building and what
    it's intended purpose is.

    Mine is right for the design of machines I build. Reason is,
    the further the bit is extended, the more the bit can act as
    a lever. Leverage is what causes flexing or bit breaking in
    many machines.

    And that's why I always suggest use of layers...like MDF...to bring
    the work closer to the bottom of the Z axis.

    It can work like this. If you write your name with a pencil and hold
    the pencil normally, it's easy. But the further you hold the pencil
    from the writing end, the harder it gets to write clearly.

    It can also depend on lots of other things. Like your choice of rails.
    The uprights. How the X rails are oriented. The backing for your
    Y rails.

    If you want the bearings to carry the weight of the router, it's mount,
    whatever fixtures it has, and reach down a long way, then
    precision linear rails like Hiwins will handle it all better.

    With something like round shafts...that can be the cause of binding
    with the extra weight. Again, depending on how those bearings are
    oriented....and/or what kind they are.

    The further a Z needs to reach down, the more rigid and stable the
    design needs to be.

    I've built them both ways many times. For the small machines I make
    now, the way I do it is the best way for that design....and what it's
    supposed to do.


    John

  3. #3
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    My Z axis has the bearings moving. My carriages are only 200mm tall, but the rails are 380mm long. So if I had the rails moving, I'd need to double the height of my carriages.
    Also, if the rails move, when the Z axis is lowered all the way, a portion of your rails equal to the length of travel will be hanging down unsupported. When my Z axis is fully extended, it's just as supported as when it's all the way up.

    Jmo, of course.
    Attached Thumbnails Attached Thumbnails Z2_Z.jpg  
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  4. #4
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    Quote Originally Posted by ger21 View Post
    My Z axis has the bearings moving. My carriages are only 8" tall, but the rails are 15" long. So if I had the rails moving, I'd need to double the height of my carriages.
    Also, if the rails move, when the Z axis is lowered all the way, a portion of your rails equal to the length of travel will be hanging down unsupported. When my Z axis is fully extended, it's just as supported as when it's all the way up.

    Jmo, of course.
    let me just translate that to English, so I can picture it better in my head :P

    (yes, I used a calculator for the 15" bit )
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  5. #5
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    I specifically swapped it to metric, thinking it was better for you. Yes, it's definitely 8" and 15". (The rails are 380mm, though.)
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  6. #6
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    John

    You don't give any explanation as to which way would be more rigid or why.
    Or maybe I missed it ?

    I know about leverage and long Z travel. I still need to decide how much travel I need/want

    I have 6 1/2" clear under the gantry, with the thinking that I could carve 6 x 6 posts or corbels if I felt the need.

    Yes, I am thinking of making different sized raised beds, to bring the sheet stock up to the gantry.

    I'm not sure what I will use the machine for, as I'm a newbie, and the possibilities just seem endless !

    I have approx 25" x 25" X and Y, with the 6 1/2" clearance under the gantry

    Thanks

    Gerry

    Looking at your router mount, there is a lot of distance (leverage) from the bit, to the bearings. Does this show up in use ?

    also, as I see it, the bearing support would be the same (amount ) which ever way round you fixed them ?

    If your router mount was tall - 15", with rails,
    and the bearings were 8" tall on the carriage,
    that should work ? no ?

    that is what I am thinking of doing.

    I think Louie's is set up this way... but I've seen that many Z's in the last couple of days I might be getting confused !

    Thanks

    Rich
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  7. #7
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    Quote Originally Posted by ger21 View Post
    I specifically swapped it to metric, thinking it was better for you. Yes, it's definitely 8" and 15". (The rails are 300mm, though.)

    ha ha ... I thought that's what you had done.

    I don't get why the Europeans are trying to force metric on us... I still use Imperial / English.

    I mean, in the UK we all still measure our height in feet and inches
    our weight in Stones and pounds

    We drive in MPH although we now have to buy liters of petrol
    We work out fuel usage in MPG

    We go to the pub for a Pint !
    We eat 1/4 pound burgers !

    etc. etc. ... rant rant ... LOL I could go on forever
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  8. #8
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    Looking at your router mount, there is a lot of distance (leverage) from the bit, to the bearings. Does this show up in use ?
    I haven't finished building it yet.
    Lowering the bearings any further would result in a reduction of Z clearance.
    If tried to design it so the carriage / router mount will have next to no flex, with the bearings mounted to the carriage.

    If you take the image I posted above and swap the rails and bearings, you'll need to slide the bearings up 7" in order for the Z to go up. Which would result in an additional 7" unsupported.

    Designing a Z axis can be very complex, especially as travel gets longer. The longer the travel, the harder it is to make a rigid Z.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  9. #9
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    Quote Originally Posted by ger21 View Post
    Designing a Z axis can be very complex, especially as travel gets longer. The longer the travel, the harder it is to make a rigid Z.

    hmmm.... I though it should be easy ... every machine has one !

    I'm sooo impatient now !

    I'll rig something up with a couple of inches travel, just to get me working, then I'll think on it some more.
    I'm thinking what I have laying about in the garage / workshop that will do as rails, I may end up with copper water pipe for my 1st Z rails, if I cant find any steel.

    I just want to cut something now

    I'll make it so I can easily swap Z's to the carriage, then I can experiment a bit.

    Thanks both

    Rich
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  10. #10
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    I would say,the longer the z travel you need, the better to put the rails on the carriage. The shorter the x travel, either way would work but might be better to put the bearing on the carriage for ease of fabrication. Reason being is that with moving bearings, you're limited by the length of travel by how far your router hangs below the bearing. This would normally not be an issue unless you had needs that require a longer z travel (tall fixtures like mill clamps, tall workpieces, rotary axis, 5-axis head...)

    Either way has advantages and drawbacks. I prefer to have moving rails, since the lower bearing will be closer to the work at all times, and that is the bearing that will take the grunt of forces. But then the gantry legs have to be taller. If the bearings move, then the spindle is always "hanging down" from the lower bearing, and it has to hang down the max z travel. But the gantry can be made lower. I wouldn't rely on rails to help support the carriage on a moving rail system

    Also a moving bearing system is less complicated to build. Mounting bearings on the carriage saddle may require a separate plate to do so, unless you had flanged bearings...

  11. #11
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    Quote Originally Posted by TrickyCNC View Post
    John

    You don't give any explanation as to which way would be more rigid or why.
    Or maybe I missed it ?

    I know about leverage and long Z travel. I still need to decide how much travel I need/want

    I have 6 1/2" clear under the gantry, with the thinking that I could carve 6 x 6 posts or corbels if I felt the need.

    Yes, I am thinking of making different sized raised beds, to bring the sheet stock up to the gantry.

    I'm not sure what I will use the machine for, as I'm a newbie, and the possibilities just seem endless !

    I have approx 25" x 25" X and Y, with the 6 1/2" clearance under the gantry

    Thanks

    Rich

    It's probably a safe bet to say that as a newbie DIYer, it'll be better
    to plan for the rails to be fixed and the bearings moving.

    Your X rails are fixed...Your Y rails are fixed. The Z is basically a smaller
    X or Y. Imagine the complications of trying to use your bearings as fixed
    and the rails moving on X and Y.

    That's a good sized machine you're building and it'll highly depend on
    how it's made....what it's for....materials and construction...etc...etc...

    The Good...and Bad...thing about DIYing a machine is...there are no rules.
    Good because you can break whatever rules someone tells you exist.
    Bad because it depends on how good you are at breaking the rules with
    mechanical stuff.

    Some drawer slide machines can machine metals. Some welded steel
    machines can barely carve wood. IMO...it can be a good idea to make
    anything at all that works to get the feel of the mechanics involved.
    If you see it working...or not...that's valuable experience. Verrrrrrry
    few people are going to design a winner first time out. The lowly MDF
    machine has it's place...


    John

  12. #12
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    Quote Originally Posted by TrickyCNC View Post

    I'll rig something up with a couple of inches travel, just to get me working, then I'll think on it some more.
    I'm thinking what I have laying about in the garage / workshop that will do as rails, I may end up with copper water pipe for my 1st Z rails, if I cant find any steel.

    I just want to cut something now
    Rich
    In my line of work we have an expression : Two weeks in the lab can save you 5 minutes work in the library.

    The Z is critical, and you should (IMHO) spend the time to think about it rather than just do something to get something quickly made. Copper pipe is just a bad idea as it will flex and worse, permanently deform quite easily. I would suggest that you make a design that makes the most of your carriage clearance, up to a max of 6" in Z. Try the stationary rails approach, and then build to the best of your ability. If you want to change it, your machine will then be good enough to help you out. If you build quick and dirty, it may well be inadequate to make parts for Mk II.

  13. #13
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    good replies from all there , thank you all very much.

    I'll keep this thread alive in parallel with my machine build so everyone who cares to search can see what is what.

    I'll leave the copper alone ... cash is short at the moment, so until my next order, I'm even scrating around for scraps of MDF ! I usually buy 10 sheets at a time for a better price, but it's all used up.

    My angle iron is pre-used, and has holes in. So OK for bearing trucks, but not for rails. I'll see if I can find some steel / iron that I can use for rails.... I must have some somewhere !

    thanks again, I'll keep you posted

    Rich
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  14. #14
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    Quote Originally Posted by ger21 View Post
    If you take the image I posted above and swap the rails and bearings, you'll need to slide the bearings up 7" in order for the Z to go up. Which would result in an additional 7" unsupported.
    In your image, the router bit is quite far bellow the bottom bearing .

    I had imagined mine being about the same height.

    so you either have

    7" unsupported rails / mount , or
    7" unsupported router / bit

    do they not equal out . More or less ? The distance fron the bottom of the gantry and the bottom of the bit will be unsupported which every way you make it ?

    you could make the router mount with the moving rails , easily as rigid as your router hanging down

    I am building this in my head (as I do everything) so I cant show drawings.

    Rich
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  15. #15
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    Quote Originally Posted by louieatienza View Post
    I prefer to have moving rails, since the lower bearing will be closer to the work at all times, and that is the bearing that will take the grunt of forces. But then the gantry legs have to be taller. If the bearings move, then the spindle is always "hanging down" from the lower bearing, and it has to hang down the max z travel. .
    see, this is the way MY head thinks too.

    My gantry legs are already tall, as I have under bed rails, and 7" gantry clearance.

    If I raise (space out) my bed to suit the work material, then this must be the best way ?

    Rich
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

  16. #16
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    TrickyCNC

    With having the rails move is better,( if your design can work with it) than if you have the bearings moving, with the rails moving you have the same support over the whole travel, with the bearings moving the support is changing with every Z axes move, making it not as ridgid, The rails make the moving plate stiffer as well

    One of my designs in the photos
    Attached Thumbnails Attached Thumbnails X & Z Axes Side-8.jpg   X & Z Axes Together-6.jpg   X & Z Top-9.jpg  
    Mactec54

  17. #17
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    Quote Originally Posted by mactec54 View Post
    TrickyCNC

    With having the rails move is better,( if your design can work with it) than if you have the bearings moving, with the rails moving you have the same support over the whole travel, with the bearings moving the support is changing with every Z axes move, making it not as ridgid
    Odd, in my mind the descriptions would be exactly the opposite. If the rails are fixed, and the bearings are on the moving Z stage, then the separation between the bearings and the spindle/tool is fixed, giving a constant support geometry, and constant 'stiffness'.

    If the bearings are stationary, and the rails are moving, then the distance between the lowest point of bearing/rail contact and the spindle/tool is changing with Z, leading to a variable 'stiffness'.

    Which is better ? depends on the build (easy way out), Build it as best you can.

  18. #18
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    so you either have

    7" unsupported rails / mount , or
    7" unsupported router / bit

    do they not equal out
    Not in my design.
    I don't consider the body of the router to be unsupported. The body of the router is not going to flex. The top clamp is inline with the bottom bearing, so there really can't be any flex above the clamp. And I tried to design the clamp so that the "saddle area" that the router rests in is tied in all the way to the top of the top bearing. While it is made of wood, it's all tied together with long screws and epoxy, and feels like a solid block. I feel it's as rigid as you can make a wooden mount (and there is a 1/4" aluminum plate screwed and bonded to the back for the bearings to mount to.

    Now, if you flip it around, unless you run the rails down to the very bottom of the router, you still need to add the length of the router to the extended rail length, which will increase the unsupported length.
    You can't count it one way, and not the other.

    Each method can work, but they need to be designed quite a bit differently. You should also consider that you'll want to mount some type of dust shoe at the bottom, and you need to leave some room for that.
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  19. #19
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    PaulRowntree

    No just the opposite of what you are thinking, the bearings are in a fixed position this gives more support over the whole travel look at the photos I just attached
    Mactec54

  20. #20
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    Quote Originally Posted by PaulRowntree View Post
    Odd, in my mind the descriptions would be exactly the opposite. If the rails are fixed, and the bearings are on the moving Z stage, then the separation between the bearings and the spindle/tool is fixed, giving a constant support geometry, and constant 'stiffness'.

    If the bearings are stationary, and the rails are moving, then the distance between the lowest point of bearing/rail contact and the spindle/tool is changing with Z, leading to a variable 'stiffness'.

    Which is better ? depends on the build (easy way out), Build it as best you can.
    Now that does make sense too....

    but !!!

    with moving rail, you can have the cutting bit very slightly below the bearing support.

    with moving bearing, you need the distance from the cutting bit to the bottom bearing support (bottom of gantry), to be the distance equal to the max travel

    with moving rail, the cut is 'stiffest' when the bit is up close the the gantry.
    (or equal to the moving bearing design when lowered to its lowest point)

    If you can raise the bed (with sheets of MDF ) then you can have a stiffer cut this way.
    My 1st Build (ongoing) http://www.cnczone.com/forums/diy-cnc_router_table_machines/134670-one_big_one_smaller_my.html

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