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Thread: Large Format 5-Axis Gantry Router

  1. #73
    Member jono5axe's Avatar
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    Default Re: Large Format 5-Axis Gantry Router

    Quote Originally Posted by jpvonoy View Post
    I'm enjoying this build thread, looking forward to seeing the next update! I used HIWIN brand rails on my machine and they publish the specs in their catalog. See attached image. This should give you a rough idea, but the manufacturer of your rails should have documentation for you.
    Thanks for that, a number of people have referred me to that type of information, but as previously posted - I have that information already and have looked at lots of other manufacturer technical guides and while they do provide parallelism and relative height (level) tolerances of one rail to another, they do not provide flatness/straightness information. For example, what deviation is allowable in a rail of a given size up or down over say 300mm. If, say, a 35mm rail is all good to toque down with 0.003mm low point between two level points 1.0m apart? This is flatsness of the mounting surface, and is not specified in the Hiwin installation guide. It will obviously affect operational life of the bearing (if the bearing moves at all). The Hiwin catalog only states "mount on a flat and straight surface".

    I have found some references to allowable deviations for 'out of alignment of blocks' on the same rail (ie blocks bolted to a carriage out of alignment) but this is only in a horizontal plane, and in any case is somewhat a different situation to 'mounting surface flatness and straightness'.

    I suppose another way to consider this, in the absence of other information, is to ask the question "what is a typical flatness and straightness achieved by a larger milling machine cutting operation" and presume that that is acceptable to work with.

    The flatness and straightness of a surface should be expressed as a deviation per unit distance, for example +/- 0.002mm per 300mm [0.002/300mm] or similar.

    Jonathon Clarke
    www.solpont.com


  2. #74
    Activation process peteeng's Avatar
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    Default Re: Large Format 5-Axis Gantry Router

    Hello Jono - The bearing companies specify the running parallelism of their bearings and rails. This is the tolerance of the height of the bearing land to the bottom of the rail along the rail and their width. They specify what tolerance the bearing geometry is expected to achieve over various lengths, see the PMI manual I sent for instance. They also infer that the tolerance of the bed is up to the user as they do not know what tolerance machine the user is building. So you have to figure what tolerance bed you need to achieve to get the operational tolerance you want. The bearings will function with a relatively poor surface flatness.

    The process is as follows:
    1) Decide the vertical and horizontal tolerance you need in the machine, these are usually the same.
    2) From this pick the bearing accuracy grade you need, also consider rail length
    3) from this specify the bed tolerance needed to remain within the bearing tolerance and
    4) this should then achieve the motion tolerance you need

    Cheers Peter

    Last edited by peteeng; Yesterday at 05:52 PM.


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