But you don’t want to develope a measuring system, design and build a measurement device, and write the measuring instructions only to find that the instrument itself could have never been capable of such a measurement. If you decide to include the instrument in a measurement system that you will include in the control plan, only then you make the r&R with the full system, the real process parts and the full measurement variation. The idea of Cg/Cgk is to have this information in advance to use it for the dessition about if the instrument would be “selectable” for a given measurement. Not such a crazy number!Ĭgk, as Cpk, takes into account the possition (which would be the bias): Cgk=(0.5 of the allowable variation – |bias|) / (0.5 of the actual variation) Then, the r&R using this instrument will never be better that 13% of the tolerance (in fact, it will allways be worse). Now, no measurement system that uses this instrument will have a Sigma due to r&R lower that this, because you have to add all the other variations to the instrument alone. Imagine an instrument that has a Cg of 1.33, that means that the Sinst is 0.025 x Tol. If you think that the 30 Sigma in the denominator here is ridicously excessive, you are just wrong. The allowable variation is taken as 1/5 of the process variation (6 x Sproc) or 1/5 of the product specification range, deppending on whether you want the instrument to controll the process or to check conformance of the parts, then:Ĭg=(0.2 x 6 x Sproc)/(6 x Sinst)=Sproc/5Sinst, or As said in a previous post, about 50 measurements are made consecutively on a master or “best available” part, and allways on the same point of the part, and by the same person, and in a controlled enviroment (ussually the metrology room). No between parts, within part, between operators or along time variation is included. For Cp, the actual variation is the process variation, and the allowed variation is the specification range.įor Cg, the actual variation is the INSTRUMENT ALONE variation (6 x Sinst). In Tolerance, select Upper spec - lower spec, and enter 0.0007. Choose Stat > Quality Tools > Gage Study > Type 1 Gage Study. The differences are what we take for allowable and actual variation. Open the sample data, HardcoatThickness.MTW. The idea for Cg is the same than for Cp, the “conceptual” formula for Cp and Cg is:Ĭg=(allowable variation)/(actual variation). I have heard about Cg/Cgk which are Instrument Capability Indexes.
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