0199// Common Mistake for True Position Dimensioning

What is wrong in this drawing?

In this post, I will discuss one of the most common mistake in positional tolerancing.

Back to Y 14.5 M- 2009 standard. According 7.2.1 Components of Positional Tolerancing, there are three components in positional tolerancing.

Use of features control frame

Identifying features to establish datums

Dimensions for True Position

So, in order to avoid ambiguity, positional tolerancing at least need to meet these three points.

Checkpoint #1: Use of features control frame

Yes, this drawing clearly specify the tolerance zone size and it's shape.

Checkpoint #2: Identifying features to establish Datum

Yes, this drawing is simple and clearly specify the Datum use to establish the true position without ambiguity. However, sometimes you can argue that the drawing is self explanatory, specify Datum is not a big deal. Maybe you can have a look the example below, without specify Datum features you cannot tell whether internal diameters or external diameter should use to establish true position.

However, this checkpoint is not applicable when use positional tolerancing for coaxial relationship control. Detail please refer to 7.6.2.3 Coaxial Features Without Datum References. Below show two of the examples.

Checkpoint #3: Dimensions for True Position

Omg, this is the most scary mistake. That is unbelievable that, still have tons of drafters use toleranced dimensions to specify a true position. In the drawing at the beginning of the post, true position is specify by toleranced dimensions. 😣

According to 7.2.1.1, Dimensions for True Position, Dimensions used to locate true position shall be basic and defined in accordance with para 2.1.1.2. See Fig. 7.1. For applicable notes in digital data files, see ASME Y14.41.

Therefore, in this case, you can specify your true position with basic dimensions in rectangular coordinates or by drawing notes.