In last month’s column, I talked about the format of accuracy specifications for video measuring machines. The following is a typical format for a single axis accuracy specification:

Ex = ±[k + (multiplier * L)/1000] μm

And this is an example of an actual specification (linear accuracy in the X-axis):

(X) E1 = ±(1.5 + 6L/1000) μm

As you can see from the equations, there’s only one variable, L, which is the length of stage travel (how far the part or the optics have to move) in millimeters.

Standing still
Use basic algebra to solve the above equation where L = 0 (in other words, no stage travel). Because 6 x 0 = 0, and 0/1000 is undefined, that E1 specification when there’s no stage motion equals ±1.5 μm. This is a systemic value that is independent of any motion of the X-axis.

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