In this post, we will briefly cover 5 common GD&T mistakes. This post is in accordance with ASME Y14.5-2009[1], additional Y14.5 or related standards may conform to the featured content. GD&T examples herein may be intentionally incomplete. This post is reference and for educational purposes, always refer to the appropriate standards.
- Placing Datum Feature Symbols on Center Lines
Although this may be intuitive, placing a datum feature symbol on a feature of size center line is incorrect and may cause interpretation issues. See Figures 1 and 2 for an example of a part where more than one diameter shares a common axis.
Figure 1: Datum feature B is diameter 1. When a datum feature symbol is placed on a center line, the interpretation is unclear. Is diameter 1, 2, or 3 the datum feature?
Figure 2: In this case, an acceptable method is placing a datum feature symbol “on the dimension line or an extension of the dimension line of a feature of size when the datum is an axis or center plane.”[2]
- Incorrect Leader Attachment Methods for Surfaces
When attaching a leader intended for a surface within the part outline, common mistakes are terminating the leader with a wave or an arrowhead, see Figure 3. The correct method is terminating the leader with a dot[3], see Figure 4.
Figure 3: Terminating the leader with a wave or an arrowhead.
Figure 4: Terminating the leader with a dot.
- No Drawing Interpretation Standard Specified
Surprising, there are still engineering drawings in industries today that are missing a drawing interpretation standard. This is a big problem as Engineers, Manufacturing, and Quality do not know how to interpret the GD&T on the drawing, is it ASME? ISO? What revision? No one knows. This leads to a subjective interpretation which may cause miscommunication and costly mistakes! Reference to ASME Y14.5-2009 or other shall be specified on the drawing.[4] A common practice is to specify, “DRAWING INTERPRETATION IN ACCORDANCE WITH ASME Y14.5-2009.” in the tolerance block or in a form of a general note on the drawing. - Incorrect Usage of True Position
Even though this mistake can cause little to no harm, referring to a position tolerance as true position is incorrect. When having in depth technical conversations about GD&T, knowing the difference is important. The definition of true position is “the theoretically exact location of a feature of size, as established by basic dimensions.”[5] This misconception likely originates from early dimensioning and tolerancing standards such as USASI Y14.5-1966[6], where a position tolerance was formally called a true position tolerance.
Figure 5: A position tolerance or positional tolerance feature control frame.
- The Usage of the Notation “TYP” or “TYPICAL”
The practice of using the notation “TYP” or TYPICAL” when indicating the “number of places” for repetitive features or dimensions is not covered in ASME Y14.5-2009[7].
Figure 6: Chamfers using the notation, “TYP”.
Figure 7: Chamfers specified correctly by “number of places” plus an X.
References
1. Dimensioning and Tolerancing, ASME Y14.5-2009. NY: American Society of Mechanical Engineers. 2009.
2. See item 1, ASME Y14.5-2009, Section 3.3.2(b).
3. See item 1, ASME Y14.5-2009, Section 1.7.4.
4. See item 1, ASME Y14.5-2009, Section 1.1.3.
5. See item 1, ASME Y14.5-2009, Section 1.3.64.
6. Dimensioning and Tolerancing for Engineering Drawings, USASI Y14.5-1966. NY: American Society of Mechanical Engineers. 1966.
7. See item 1, ASME Y14.5-2009, Section 1.9.5.
Thx!
Sir mujhe bhi drawing padna sikhna h
Good information, I did not know true position was from y14.5 1966.
Excellent