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Being Clear is as Easy as Being Cloudy - 1994, 2009, 2018 (#42)

(In accordance with the ASME Y14.5-2018 standard)
(In accordance with the ASME Y14.5-2009 standard)
(In accordance with Y14.5M-1994 standard)

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A faithful reader of these Tips called with a question that had caused controversy at his company. The question dealt with the most appropriate way to measure the 16±0.2 dimension. Some thought that the measurement should be oriented relative to Datum A as is shown in Figure 1. Others thought that since the dimension is shown from the top of the part, the measurement should be made oriented relative to the top as shown in Figure 2. The approach taken would make the difference between acceptance or rejection of the parts. The real answer is the Standards don’t clearly define what toleranced dimensions mean when used to locate surfaces like this. It could be interpreted differently depending on whether you are buying or selling. Figure 1 is not appropriate since there isn’t anything to relate the dimensions to datum A. Figure 2 is a possible interpretation, but it isn’t the only one. The variation of the measurement can change depending on which feature is used to establish the orientation of measurement.

One way to clarify the design intent would be to use a dimension origin symbol, as shown in Figure 3. For the 16 ±0.2 dimension, the dimension origin symbol identifies the top surface as the measurement reference that acts like a datum feature. The measurement would be defined as shown in Figure 4.

 

This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009 and Y14.5-2018.
This tip was originally released in April 2001.

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Back to Tips Tip added Jun 2021