Dear Sir,

Could you please let us know what could be the outcome of these two controls? What are the measurement techniques to measure parallelism at RFS condition?

Thank you.

blog-question-april2010a

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Thank you for the illustration with your question. Under ASME Y14.5M-1994 or 2009, these controls are very different. In the first illustration, concentricity has been used to relate a large outside diameter to a datum axis. In the second illustration, a parallelism has been used to relate a large outside diameter to a datum axis.

Concentricity is a location control that places the cylindrical tolerance zone uniformly about the datum axis. Once established, the tolerance zone remains about the datum axis while median points of the large outer diameter are evaluated for placement in relation to the cylindrical tolerance zone. The median points must be inside the tolerance zone for the part to be acceptable.

Parallelism is an orientation control. This means it limits the angle of one feature to another. The parallelism control may only be indicated to limit deviations from a parallel state. In the second illustration, the cylindrical tolerance zone is established parallel to the datum axis, but may be located anywhere in space, so long as it remains parallel to the datum axis. The axis of the actual mating envelope of the toleranced outer diameter must be parallel enough to remain within the cylindrical boundary.

Measurement techniques are nearly as varied as measurement tools themselves. The key is to know what the specification requires and that your measurement technique meets those requirements.

One accepted practice for measuring concentricity to the datum axis would be to place the dial indicators with their probes opposite each other. The dial indicators need to be centered about the datum feature simulator (a chuck or collet to simulate the axis of the datum feature). The separation of the dial indicators should be sufficient to allow the parts to assemble into the datum feature simulator with some depression of the probes (actual linear distance depends upon the size of the diameter to be measured and the stroke length of the dial indicator’s probe).

As the part sits in this fixture, each median point is accessed for acceptability. Remember, each actual local size generates a median point, and that tithe concentricity specification applies to the entire length, width, and depth of the feature to which it is indicated. You will have thousand of median points to access, which is why inspection planning is necessary when using concentricity.

An accepted practice for measuring parallelism of an axis RFS to a datum axis is to use a CMM. A CMM (coordinate measuring machine) will need to probe a sufficient number of points at numerous cross sections along the length of the cylinder to accurately calculate the axis of the actual mating envelope using its algorithms. Not all CMMs use the same algorithms, so detecting more points is a safeguard against the anomalies between different CMM software programs. Once the axis of the AME is established, a datum axis must be simulated using the same practice on the datum feature. The axis of the actual mating envelope is the datum axis where RFS is specified with a datum reference. Then a reference cylinder with a diameter equal to the tolerance specified (0.2 for your illustration) is set parallel to the datum axis. This tolerance zone may be moved, but not reoriented, to fit the axis of the actual mating envelope of the toleranced feature inside the tolerance zone. There will be only one value in this measurement, so measurement planning is a lot less critical than in the example of concentricity.

Remember, these are just examples of how the geometric characteristics specified could be measured, other practices are acceptable, as long as they meet the specified requirements.

Design safely,

Michael Adcock
ETI Dimensional Engineering Mentor