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Chinese rail alignment/accuracy

Discussion in 'CNC Mills/Routers' started by Rangatang, Feb 28, 2017.

  1. Rangatang

    Rangatang Well-Known
    Builder

    Joined:
    Jan 26, 2017
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    Hey guys,

    I will be posting a build thread shortly, but in the meantime I have a few questions.

    I bought a Chinese motion kit, 1000 mm long ball screws and supported linear rail (sbr20), etc. I'm currently redesigning my machine to fit around these parts. My question is about alignment. The rail support is extruded aluminum, with holes in it to mount to whatever you want. After measuring these holes, they definitely are drilled by hand. This is fine, however, has anyone successfully used these bearings, and mounted them straight? I would like to set one up as a master rail, and align the other to it. But if the holes are randomly placed that makes it difficult. Not sure I can trust the edge of an extruded rail support either. If I could, I would use that edge and all would be well.

    That being said, say I was able to successfully mount my first rail as straight as possible. Since the rails are round, and the carriages are able to slightly rotate on them, how are you guys attaching indicators to the carriages to align the follower rail? What reference do you use to check that they are parallel to one another? I've seen how you can attach the two with a plate in between, and mount based on feel (binding vs smooth feel), but I feel there there should be a better way to do it. Obviously with a square rail you can attach an indicator to it since it doesn't swivel.

    Has anyone measured the straightness of these rails in relation to the support? For example the height difference between the support bottom and top of carriage across the length of the rail? Or, the distance from the side of the carriage to the side of the rail support across the distance of the real?

    If anyone has any input or advice I would appreciate it. I do have several indicators, a granite surface plate, a precision straight edge, and the rails and carriages. I guess long story short, how much can I trust the linear support of the rails to be parallel to the actual rail itself?
     

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