07-04-2012, 03:53 PM
Metrology
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Definitions
❖ Accuracy
– a quantitative measure of the degree of conformance to
recognized national or international standards of measurement
❖ Repeatability
– a measure of the ability of a machine to sequentially position a
tool with respect to a work piece or produce a part within small
range under similar conditions.
❖ Resolution
– is the least increment of a measuring device; the least significant
bit on a digital machine.
Establishing Machine Capabilities
❖ Cut part
❖ Inspect part for dimensional accuracy
❖ Infer from data overall machine accuracy
❖ Advantages
– machine can be optimized for a particular part
– simple purchase and acceptance specifications
❖ Disadvantages
– machine accuracy masked by other error sources (e.g.,
inspection errors, process errors, etc…)
– individual sources of error are not identified
– the accuracy data cannot be extended to other part geometries.
Determinism
❖ The basic idea is that automatic machine tools obey cause and effect
relationships that are within our ability to understand and control and
that there is
nothing random or probabilistic about their behavior.
(J. Bryan)
❖ “Random” implies
– the causes of the error cannot be determined
– nothing can be done about the causes of the variations
– the best one can do is try to statistically assess the range of variatio
❖ Machine tools have apparent non-repeatable errors rather than random
errors
❖ Statistics