304 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. condition is not an acceptance boundary that can be used to check for hole position. The boundaries are important for design considerations where edge distances, potential effect on assembly variation, or other variation effects need to be analyzed. Least Material Condition The least material condition (LMC) modifi er is a letter “L” inside a circle. See Figure 8-12. The shown specifi cation is for a hole. The LMC modifi er indi- cates that the specifi ed tolerance value applies only when the controlled feature is actually produced at the LMC size. If the controlled feature is produced at any allowable size other than LMC, the position tolerance value increases an amount equal to the size departure from LMC. This is similar to the MMC concept, except it uses the LMC size as the size to which the original tolerance value is applied. When using the axis method of verifi cation, the allowable increase in positional tolerance is based on the size of the unrelated actual minimum material envelope of the hole instead of the size of the hole. For a hole, the unrelated actual minimum material envelope is the smallest circumscribing cylinder that is inside the material. Because the form of each hole shown in the following examples has no variation, the size of the hole and the size of the actual mini- mum material envelope are the same. A hole specifi cation and position tolerance are shown in the given fi gure. Limits of size for the hole are .500″ and .510″ diameter. The position tolerance is specifi ed as .025″ diameter LMC. Any hole produced at the exact LMC size of .510″ diameter is permitted a position tolerance of .025″ diameter. Any hole produced at a smaller diam- eter is permitted a larger amount of position toler- ance. For every .001″ diameter decrease in hole size, there is an allowable .001″ increase in the diameter of the position tolerance zone. Two examples of produced holes are shown. One hole is produced at .510″ diameter, the least material condition for the hole. The position toler- ance for this hole is .025″ diameter. The other hole is produced at .500″ diameter, the maximum material condition for the hole. The position tolerance for this hole is increased to .035″ diameter. The LMC modifi er is typically used for applica- tions where edge distance or remaining material is of concern. Examples are a clearance hole centered in a boss or a structural hole near the edge of a plate. Another example is a hole in a casting where the position of the hole must be adequately controlled to ensure subsequent machining operations will have material for removal. Two boundaries are created by a position toler- ance that is applicable at LMC on a hole. There is a virtual condition (outer boundary for a hole) that is equal to the LMC hole size plus the position tolerance. The virtual condition for LMC is a boundary inside the material, and this boundary must not be violated by any segment of the hole. A resultant condition (an inner boundary for a hole) is also created and its size is calculated using the MMC hole size minus the allowable position tolerance. The resultant condi- tion of a hole (for LMC applications) is the smallest boundary that must not have any material inside it. Figure 8-12 shows four circles at the LMC size of .510″ diameter at four different allowable locations at the extremes permitted by the .025″ diameter position tolerance zone located on the true position. A circum- scribing circle is drawn representing the virtual condi- tion. Any hole produced within the size and position tolerance will not violate the virtual condition. Produced hole sizes and allowable position tolerances Hole specification and position tolerance Virtual and resultant condition boundaries Ø.500 Hole produced at MMC size LMC specified Ø.510 LMC size Ø.500 MMC size Ø.535 Virtual condition Ø.025 Allowable position tolerance Ø.035 Allowable position tolerance Ø.465 Resultant condition at LMC size Ø.510 Hole produced Goodheart-Willcox Publisher Figure 8-12. The position tolerance zone increases in size when the LMC modifi er is applied to the tolerance specifi cation and the size of the unrelated actual minimum material envelope of the feature departs from LMC.