Accuracy -

Quantitatively, it is an assessment relating measured conformance or performance to nominally specified values. Accuracy can only be determined after measurement(s) have been made.

Qualitatively, it is used to compare the operation of similar instruments or machines or to comparatively classify in groups.  One may state that 'Machine A is more accurate than Machine B when boring holes because of a stiffer structural loop and lower spindle error'. Gage blocks are divided into accuracy or tolerance grades. These grades are for comparing how close the blocks were manufactured to the standard values.

• Workpiece or Standard Accuracy - Quantitatively, the amount that the measured value for any quantity of a workpiece or standard deviates from the nominal or specified value. Exact knowledge of the true workpiece accuracy cannot  be realized since the comparison measurement is limited by the  uncertainty of the measuring machine or etalon.  Thus, since there exists uncertainty in measurement, there will also be uncertainty in the quantified accuracy.
  
  
• Measurement Accuracy - For a reported measurement, the ISO VIM (Vocabulary in Metrology) standard has ascribed the word qualitative use only, employing the term  uncertainty as a quantitative assessment of the trustworthiness of the reported value.
  
  
• Machine Accuracy - (1. qualitative) a particular class of performance to which a machine belongs. (2. quantitative) The measured capability of a machine to produce a nominally specified operation.
  
  Example: The linear displacement accuracy (LDA) for  the machine is 5 micrometers plus 10 micrometers per meter per degree Celsius deviation of the ambient air from 20 degrees (can be written LDA= 5+10(T-20)L micrometers, where L is in meters). Because there is uncertainty in your knowledge of the machine state and related constant coefficients (such as thermal linear expansion), there will also be a related uncertainty in the assessed accuracy. Additionally, since there may be unknown or unconsidered affecting variables, accuracy is commonly limited to specified parameters. 

Related topics: geometric errors. precision, tolerance, repeatability, uncertainty, statistical confidence.

July 26, 2000