The use of material modifiers in GD&T may seem quite abstract for beginners. When should we use MMC (Maximum Material Condition), LMC (Least Material Condition) and RFS (Regardless of Feature Size)? What are the practical implications of these modifiers? This text emphasizes the relationship between material modifiers and fit conditions. Take note that this is just a conceptual introduction and that each application should be considered in its particular circumstances.
Fits between cylindrical features (holes and pins) define the proper assembly of many mechanisms. ISO standard  defines three main kinds of fits:
Clearance fit: There is always a clearance between the hole and the pin. The lower limit of the hole is always bigger than the higher limit of the pin.
Transition fit: Fit that can provide a clearance or an interference after the assembly.
Interference fit: There is always an interference between the hole and the pin. The upper limit of the hole is always smaller than the lower limit of the pin.
Material Modifiers and Fits
- MMC: More Clearance, Looser Fit
- LMC: Less Clearance, Tighter Fit
- RFS: No Clearance, Interference Fit
Hence, we could summarize the relationship between hole size and relative bonus tolerance as follows:
- MMC: As the hole gets larger, bonus tolerance increases,
- LMC: As the hole gets smaller, bonus tolerance increases.
- RFS: There is no relationship between the hole size and the bonus tolerance.
ASME Y 14.5-2009 , Dimensioning and Tolerancing. New York: American Society of Mechanical Engineers.
ISO 286-1:2010, Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits, http://www.iso.org/iso/catalogue_detail.htm?csnumber=45975