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Profile Tolerances
Profile of a surface is the most versatile geometric tolerance. The nominal perfect geometry controlled by a profile tolerance is called the true profile. Profile tolerances should only be applied to surfaces that are defined by basic dimensions, either by dimensions on the drawing or indicating that model geometry is basic.
The boundaries of a profile of a surface tolerance zone are called a Maximum Material Boundary (MMB) and a Least Material Boundary (LMB). The MMB is the tolerance zone boundary that allows the part to have more material and the LMB is the tolerance zone boundary that allows the part to have less material. MMB allows more material, LMB allows less material. Material is what the part is made from (e.g. metal, plastic, etc.).
The MMB and LMB are offset from the true profile. In some cases, the true profile is used as the MMB or LMB. See below. The MMB and LMB may be offset the same or different distances from the true profile.
There are two ways to indicate the MMB and LMB offsets: The graphical method and the unequally disposed method. The figures for questions 2-4 show examples of both methods. The graphical and unequally disposed methods shown in each figure are equivalent (they define the same LMB and MMB).
Graphical method to define unequally disposed profile tolerance zone
The graphical method is an older method to indicate unequal profile tolerance zone boundaries. Using the graphical method, the total profile tolerance zone width is indicated in the feature control frame, and one of the offsets is directly dimensioned on the drawing. For examples of the graphical method, see the figures for questions 2-4 below. The graphical method requires other information outside the feature control frame.
Figure 1 – Equally and unequally disposed profile tolerance zones
Unequally disposed method to define a profile tolerance zone
An unequally disposed profile symbol followed by an MMB offset value are placed after the tolerance value in the feature control frame to define an unequally disposed profile tolerance zone. Figure 2 shows how the unequally-disposed method works.
Figure 2 – Profile Tolerance Specified Using the Unequally-Disposed Profile Symbol
Note that the unequally-disposed profile symbol is only explained when used with an unequally-disposed profile tolerance. That is, the ASME Y14.5-2018 standard does not show examples of using the unequally-disposed profile symbol with an equal-bilateral profile tolerance specification. If the symbol is used to define an equal-bilateral profile tolerance specification, the MMB offset would be ½ of the total tolerance value. However, to specify an equal-bilateral profile tolerance, ASME Y14.5-2018 defaults to specifying a single tolerance value in the tolerance compartment of the profile tolerance feature control frame. See Figure 3 below.
Default method (equal bilateral profile tolerancing)
Figure 3 – Equal Bilateral Profile Tolerancing
Unequal-bilateral and unilateral profile tolerances may be specified using the older graphical methods from ASME Y14.5M-1994 and earlier versions, or they may be specified using the unequally-disposed profile tolerance symbol. See Figures 4, 5, and 6 below.
Note that it may be cumbersome to use the older graphical methods in certain CAD software. It may be necessary to draw a sketch of the graphically represented offsets in a drawing view. The challenge is compounded given that the graphically represented offsets are usually exaggerated on the drawing so they can be seen by a person reading the drawing, as some tolerance values are small relative to the part geometry. The unequally-disposed profile tolerance symbol was developed in the ASME Y14.41 standard as a means to represent unequally-disposed profile tolerances on annotated models. The graphical method did not work on models, as models are three dimensional and many 2D drafting methods don’t work well when applied to annotated models.
Figure 4 – Unequal Bilateral Profile Tolerancing
Figure 5 – Unilateral Profile Tolerancing – Full-Value MMB Offset (only adds material)
Figure 6 – Unilateral Profile Tolerancing – Zero-Value MMB Offset (only subtracts material)