Accuracy
RP: 0.005 to 0.030 in. (0.125 to 0.75 mm)
While it is possible to achieve dimensional accuracy better than 0.005 inch (0.125 mm) on individual dimensions, 0.005 to 0.030 inch (0.125 to 0.75 mm) is the typical range of deviation. Accuracy is a function of the RP system and the size of the dimension. As a dimension increases, so does the inaccuracy.

CNC: 0.0005 to 0.005 in. (0.0125 to 0.125 mm)
With the right equipment, it is possible to machine at high precision. While CNC in general is more accurate than RP, precision is usually related to the cost of the machine.

  Repeatability
RP: Low
Build any part on two different days and the results may vary. RP is sensitive to many factors that affect the quality of the prototype. Temperature, humidity, orientation and placement are just a few of the parameters that can affect the repeatability of the output.

  CNC: High
CNC is much more repeatable than RP. If the toolpath, tool and materials are unchanged, the output is highly repeatable. Note, however, that environmental conditions and the human factor can affect results. Temperature and humidity, for some materials, can alter the outcome as can the accuracy of the machinist's setup.         

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