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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > Moving table with 3 rails and offset drive
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  1. #1
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    Moving table with 3 rails and offset drive

    I am mulling over a moving table design and am wondering about the benefit vs effort/cost of using 3 rails on a moving table instead of the standard 2.
    I have been told it might be a good idea, but would like to hear from someone who has done it. Worth the cost and effort aligning everything?

    I assume the offset ball screw would not be a problem.

    Also, benefits of using 6 carriages on 3 rails, or even 9 carriages? Crazy, ( I know....)


    Machine would be thick RHS steel, brazed together. Table would be 480 x 720 x 16 mm Ali plate. Table would move on long axis.
    Steve

  2. #2
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    Re: Moving table with 3 rails and offset drive

    Quote Originally Posted by Sterob View Post
    I am mulling over a moving table design and am wondering about the benefit vs effort/cost of using 3 rails on a moving table instead of the standard 2.
    No, 3 rails would not be a good idea. Quote from a Nippon Bearing North America white paper:

    Because two parallel single-rail guides spread apart is the proper approach to carry large loads, many engineers assume it’s acceptable to use more than two rails to carry even heavier loads. One reason they’re tempted to do this is to avoid the cost of a larger rail. While increasing rail size may be more expensive, using three or more rails and carriages produces a statically indeterminate system and can lead to rough operation. Unless installation is perfect, rails and carriages will inadvertently be slightly out of line. The expense of installing such a system accurately enough to produce smooth motion will likely exceed the cost of using a larger rail.

  3. #3
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    Re: Moving table with 3 rails and offset drive

    Thank you sir.
    Makes sense.

  4. #4
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    Re: Moving table with 3 rails and offset drive

    Quote Originally Posted by CitizenOfDreams View Post
    No, 3 rails would not be a good idea. Quote from a Nippon Bearing North America white paper:

    Because two parallel single-rail guides spread apart is the proper approach to carry large loads, many engineers assume it’s acceptable to use more than two rails to carry even heavier loads. One reason they’re tempted to do this is to avoid the cost of a larger rail. While increasing rail size may be more expensive, using three or more rails and carriages produces a statically indeterminate system and can lead to rough operation. Unless installation is perfect, rails and carriages will inadvertently be slightly out of line. The expense of installing such a system accurately enough to produce smooth motion will likely exceed the cost of using a larger rail.
    This would be a fail if the Table was able to bend / flex in between the 2 Rails using larger rails would not solve this problem, there are many manufacturers that use 4 rails per Axis to spread the load and support the moving load

    They are right if the distance between the 2 rails can support the table on top without it flexing in between the 2 rails, this is not so easy to achieve if the rails are far apart like on a 24" wide table
    Mactec54

  5. #5
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    Re: Moving table with 3 rails and offset drive

    Quote Originally Posted by Sterob View Post
    Thank you sir.
    Makes sense.
    How wide is your table and how thick , this will determine how many rails you will need to support your table
    Mactec54

  6. #6
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    Re: Moving table with 3 rails and offset drive

    Quote Originally Posted by mactec54 View Post
    How wide is your table and how thick , this will determine how many rails you will need to support your table
    Machine would be thick RHS steel, brazed together. Table would be 480 x 720 x 16 mm Ali plate. Table would move on long axis.

  7. #7
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    Re: Moving table with 3 rails and offset drive

    If the rails are installed so that the movement occurs along the long axis,it suggests the centres of the rails will be less than 480mm apart.What kind of evenly distributed load will deflect a 16mm thick plate significantly over that span?My feeling is that it would be too big a piece for a router and would need to be sent out to a heavy duty machine.

  8. #8
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    Re: Moving table with 3 rails and offset drive

    Quote Originally Posted by routalot View Post
    If the rails are installed so that the movement occurs along the long axis,it suggests the centres of the rails will be less than 480mm apart.What kind of evenly distributed load will deflect a 16mm thick plate significantly over that span?My feeling is that it would be too big a piece for a router and would need to be sent out to a heavy duty machine.
    The load come's not only from the weight you put on it which in most cases on a machine like this would not be very much, the Z axis is what will bend the table, the force the Z axis can use could will easily deform/ bend a 16mm aluminum plate in a gantry mill like this if the plate is not supported

    A good test for this is to support a piece of plate that wide at the ends, and drill a 1/2" hole in the middle and watch the plate flex, that is why most machine tables are in the range of 3" plus thick
    Mactec54

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