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IndustryArena Forum > Mechanical Engineering > Mechanical Calculations/Engineering Design > Flatness requirement for linear rails... interpretation of GD&T?
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  1. #1
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    Flatness requirement for linear rails... interpretation of GD&T?

    I'm having issues with getting within tolerance on a project which have two linear rails. After spending loads of time on trying to get within tolerance i'm starting to think that i might be reading the documentation wrong and/or misunderstanding GD&T. So here i am, asking you guys for a sanity check before i go deeper into this rabit hole...

    The setup: I have two Hiwin RG35 linear rails spaced 400mm apart. Its these guys: https://www.hiwin.de/en/Products/Lin...0H/p/12-000287
    If one download the catalogue, it specifies the required properties for the mounting surfaces like this:
    Click image for larger version. 

Name:	RG35 rails2.PNG 
Views:	0 
Size:	47.9 KB 
ID:	481238
    All reference surfaces have been milled and now also ground...and i'm still way of...maybe...

    So the parallelism of the two shoulders P must be below 0,01mm according to the datasheet . If i mount one rail and put a dial on the oppesite shoulder i'm somewhere below 0,002mm worst-case over the entire length, so well within spec on the shoulder! But if i put the same indicator on the mountingsurface i'm hitting almost 0,04mm worst-case over the travel length. So i've been chasing this and ended up grinding the surface... i have a mirror finish and zero(!) flaws if i blue it up using a high precision granite "straightedge"... i can't do better than this and i'm still 4x of the tolerance... this can't be right! And now i'm starting to think that i'm misunderstanding these tolerances completely....

    Since the rail is only 35mm wide, even the smallest imperfections a magnified over the 400mm rail-distance with a factor of almost 12x. So me having 0,04mm on my dial from one rail to another tells me that i'm +-1,5µm off at one point below one of the rails....this seems unrealistic for me to correct...

    Is there any way i can verify that i am within tolerance on this using the rails themselfs? I don't have a surfaceplate nor a CMM large enough and i'm not even sure that it will account for the "twist" i'm seeing....

  2. #2
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    Re: Flatness requirement for linear rails... interpretation of GD&T?

    I realized i'm an idiot. Guess the wife is right then ;-)

    I think it is useless to make a measurement from one rail to the other, as i did. Instead the flatness and parallelism requirements for the mounting surface defines how big of a "twist" are allowed to be in the rail. So nomatter what i measure from one rail to another, it is always ok as long as flatness and parallelism is within tolerance. If this is true, then my current setup is pleanty fine...

    At least that is my conclusion for now, if i'm wrong, please correct me :-)

  3. #3
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    Jan 2005
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    14186

    Re: Flatness requirement for linear rails... interpretation of GD&T?

    Quote Originally Posted by badhabit View Post
    I'm having issues with getting within tolerance on a project which have two linear rails. After spending loads of time on trying to get within tolerance i'm starting to think that i might be reading the documentation wrong and/or misunderstanding GD&T. So here i am, asking you guys for a sanity check before i go deeper into this rabit hole...

    The setup: I have two Hiwin RG35 linear rails spaced 400mm apart. Its these guys: https://www.hiwin.de/en/Products/Lin...0H/p/12-000287
    If one download the catalogue, it specifies the required properties for the mounting surfaces like this:
    Click image for larger version. 

Name:	RG35 rails2.PNG 
Views:	0 
Size:	47.9 KB 
ID:	481238
    All reference surfaces have been milled and now also ground...and i'm still way of...maybe...

    So the parallelism of the two shoulders P must be below 0,01mm according to the datasheet . If i mount one rail and put a dial on the oppesite shoulder i'm somewhere below 0,002mm worst-case over the entire length, so well within spec on the shoulder! But if i put the same indicator on the mountingsurface i'm hitting almost 0,04mm worst-case over the travel length. So i've been chasing this and ended up grinding the surface... i have a mirror finish and zero(!) flaws if i blue it up using a high precision granite "straightedge"... i can't do better than this and i'm still 4x of the tolerance... this can't be right! And now i'm starting to think that i'm misunderstanding these tolerances completely....

    Since the rail is only 35mm wide, even the smallest imperfections a magnified over the 400mm rail-distance with a factor of almost 12x. So me having 0,04mm on my dial from one rail to another tells me that i'm +-1,5µm off at one point below one of the rails....this seems unrealistic for me to correct...

    Is there any way i can verify that i am within tolerance on this using the rails themselfs? I don't have a surfaceplate nor a CMM large enough and i'm not even sure that it will account for the "twist" i'm seeing....
    It will depend on the Linear Bearing Tolerance and ( Preload), the surface you are mounting them on has to be within this tolerance, or they will have premature wear, No you can not use the rail as a referance for flatness, they rely on the mounting surface to straighten them when the rail are bolted down

    Was the mounting material stress relieved before machining? it does sound like you are very close, and may be over thing this, the end result is what is important, checking the top of the Linear Bearings once everything is mounted is the next important step as there is another gotcha waiting
    Mactec54

  4. #4
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    Re: Flatness requirement for linear rails... interpretation of GD&T?

    Quote Originally Posted by mactec54 View Post
    Was the mounting material stress relieved before machining? it does sound like you are very close, and may be over thing this, the end result is what is important, checking the top of the Linear Bearings once everything is mounted is the next important step as there is another gotcha waiting
    The material wasn't stress relieved:-/ I should have done that, but they are now cast into epoxygranite, so that is not an option anymore.... i'll do that next time ;-)

    "[]...as there is another gotcha waiting"....huh? More of those? :-)

    /Thomas

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