In industrial metrology, actual physical bodies of known length which can be contacted by mechanical sensors have an important part to play as reference standards when measuring geometrical parameters. They have become increasingly important for assessing the accuracy of two and three-axis coordinate measuring machines (CMMs) which employ mechanical sensors. Checking the length measurements uncertainty has proved to be a highly informative and economical method for the acceptance testing and ongoing monitoring of CMMs. The Step-gage can be used for this in a great variety of ways given, for example, the advantages of uni-directional and bi-directional targeting and of measurements from all the gage faces along the line of measurement in succession while needing only a short time for preparation and measurement. Local errors can be detected in the CMM and characteristics can be derived for individual coordinate axes of the machine.

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With the aid of the length measurement uncertainty, the manufacturer or user can specify and check the accuracy of a coordinate measuring machine to establish its suitability for length measurement. This fundamental task in metrology is of particular importance due to the fact that in practice the majority of measuring requirements are for the measurement of lengths. "Length measurement uncertainty" is defined by VDI/VDE guideline 2617, part 2.1 as the uncertainty with which a CMM allows the precisely known distance between two points on two mutually parallel gage faces situated in succession along a line of measurement to be remeasured. Figure 1 sows a measurement of this kind being made, taking as an example an individual parallel gage block with an outside length Le which is arranged obliquely in three dimensions and whose length is remeasured by successive contacts with the block with the probe head in positions I and II.

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On the step gage, spacing of different kinds for making test measurement are all available simultaneously, as follows:

• Outside dimension Le, e.g. with the probe head in positions I and II (figure 2)

• Inside dimension Li, e.g. with probe head in positions III and IV (figure 3)

• Rear-face to rear-face dimension Ls, e.g. with probe head in positions III and V (figure 3

• Front-face to front-face dimension Lx, e.g. with probe head in positions VI and IV (figure 3)

• Positional length Lp of a gage face from the datum gage face, e.g with probe head in positions VI and 0 (figure 3)

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The illustrations show only one of many options available for each type and size of spacing. The value of the length measurement uncertainty is generally given in the form of a length-dependant formula: U = A + K x L </= B