LMC concept does not have to be automatically used when control of what is commonly called "wall thickness" is required. Each positional tolerance, regardless of material condition applied (RFS, LMC, MMC) creates two boundaries - inner and outer boundary of a toleranced feature. In other words:
1. Current lower segment of composite positional FCF with tolerance value modified by (L) creates two boundaries for each of the holes within the pattern:
- outer boundary, which is virtual condition of the hole = .077 (hole LMC size) +.005 (pos. tol. at LMC) = .082. This outer boundary "is responsible" for "wall thickness" between the holes.
- inner boundary, which is worst-case resultant condition of the hole = .073 (hole MMC size) -.005 (pos. tol. at LMC)- .004 (bonus tol.) = .064
2. However, the lower segment of composite positional FCF could contain a tolerance value modified by (M), and it would also create two boundaries for each of the holes within the pattern:
- outer boundary, which would be worst-case resultant condition of the hole = .077 (hole LMC size) + [pos. tol at MMC] + [bonus tol.]. Now, to keep the same .082 as in case #1, [pos. tol at MMC] + [bonus. tol] would have to be equal .005, correct? This in turn would be possible only if the value of positional tolerance specified in lower segment of positional FCF was .001 followed by (M).
- inner boundary, which would be virtual condition of the hole = .073 (hole MMC size) -.001 (pos. tol. at MMC) = .072
3. The lower segment of composite positional FCF could contain a tolerance value at RFS as well, and it would also create two boundaries for each of the holes within the pattern:
- outer boundary of the hole = .077 (hole LMC size) + [pos. tol at RFS]. To keep the same .082 as in case #1, [pos. tol at RFS] would have to be equal .005.
- inner boundary of the hole = .073 (hole MMC size) -.005 (pos. tol. at MMC) = .068
The disadvantages of such "conversion" can be easily noticed:
A. Sizes of the inner boundaries are different in each case.
B. Total available positional tolerances as defined by lower segment of composite FCF are different in cases #2 and #3 comparing to case #1:
- case #1 = .009 = .005 (specifed on the drawing) + .004 of bonus;
- case #2 = .005 = .001 (specifed on the drawing) + .004 of bonus;
- case #3 = .005 (specified on the drawing)
In return in case #2 you gain a possibility of verifying the composite positional callout by using 2 hard gages.
At the end of the day it is up to you to decide which scenario fits the most to your application.