This video is in response to a question that we received on our question line from Amar. Amar’s question is as follows: “If we are given a BASIC radius of 10mm, and on the radius, we are applying a Profile of a Line Tolerance of 0.6mm, what will be the max & min size of the radius & what will be the tolerance zone? I read somewhere that if Profile of a Line is applied to a BASIC size, then it controls the size as well. Thus, the radius can now have a MAX size of 10.6/MIN size of 9.4, and at these sizes, the tolerance zone will be equal bilateral of 0.6mm. Is this correct? Another interpretation is that the true size is 10mm, and my radius can vary from 9.7 to 10.3mm. These are the MAX and MIN size. Please tell (me) which is correct.”
In the example that Amar has outlined, the second scenario is correct. The maximum size allowed is 10.3mm, and the minimum size is 9.7mm. To better understand why this is, we can start by consulting our GD&T wall chart. We recommend that you download this chart and print it in 11×17 for a quick reference document to keep near your workstation. The chart provides a wealth of information, including tolerance zone diagrams for all fourteen GD&T symbols. A line section’s profile on our wall chart includes a drawing showing a curved surface with a tolerance of 0.03. The tolerance zone diagram indicates that the 0.03 tolerance value provided on the drawing represents the total amount of tolerance.
The definition for the profile of a line that we use in our training course states, “This control specifies how far from the true profile the cross-sectional surface is allowed to deviate. Unless otherwise specified, the tolerance zone is a uniform (or equally disposed) boundary about the true profile.” Also, a line’s profile is a 2-dimensional tolerance, meaning that the tolerance is evaluated for a given cross-section. This tolerance is applied perpendicular to the true profile at each point along the cross-section. All points along the actual profile of the part must be within the tolerance zone. Finally, note that the profile of a line, when used with a full DRF (Datum Reference Frame), will restrict the size, location, orientation, and form (SLOF).
Returning to Amar’s example, we can see that the 0.6mm tolerance stated in the GD&T callout is the total amount of tolerance. We assume that the tolerance is equally disposed bilateral, or 0.3mm in each direction, since it is not otherwise stated. Therefore, the radius can vary between 9.7mm and 10.3 mm.
When inspecting parts with a profile of a line tolerance, each cross-section must be evaluated independently. GD&T does not define the number of cross sections to be inspected and the location of each section, but it may be outlined in an inspection plan or left to the inspection or quality group’s discretion. Since the profile of a line does not relate one cross-section to another, it is not controlling the surface profile. The profile of a line tolerance could be used on an extrusion or other long part, where the flexibility of the part makes a surface constraint impractical.
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