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A few closely related questions about the straightness of linear rods. The 'straightness' of linear rods is vital for a 3D printer to function correctly and prevent things like Z-Banding or Z-Wobble. So, how do we measure the 'straightness' of linear rods? What measurement method or metric can be used?

I can imagine using some reference material (a piece of wood or a table), but then again, how would we know the reference material is perfectly straight? Lastly, how straight does it even have to be? What's a standard or acceptable range they should be in, and is there a standard that defines, classifies, and indicates this?

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    $\begingroup$ It is hard to het a very straight object to verify your rods against. A regular household measure is to use a metal ruler on its side; not everybody has access to a perfectly flat slab of granite/steel as I had at my previous employer (e.g. used for mounting optics). $\endgroup$
    – 0scar
    Dec 7, 2023 at 16:21

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A common metric for straightness of shafts in industrial equipment is Total Indicated Runout (TIR). The shaft is placed in V-blocks or ball bearing rollers at its bearing journals. Runout is checked at various positions along its length with a dial indicator as the shaft is rotated.

This method could be easily applied to guide rods supported at the same axial locations where they are held in the printer.

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