GD&T Rules (Rule #1 & Rule #2)
Let us now discuss two fundamental rules of GD&T.
Rule #1 states that where only a tolerance of size is specified, the limits of size of an individual feature of size prescribe the extent to which variations in its geometric form, as well as its size, are allowed. No element of a feature shall extend beyond the MMC boundary of perfect form. The form tolerance increases as the actual size of the feature departs from MMC toward LMC. There is no perfect form boundary requirement at LMC
Let us now discuss rule #1 in detail. The simplest meaning of rule #1 is at MMC the feature is on perfect form. For example consider a shaft with diameter Ø5.000-Ø5.040. Then its MMC is Ø5.040. Now assume we have this part produced at Ø5.040 then according to rule #1 the part should be in perfect form. That is it should have perfect straightness, circularity and cylindricity.
Rule #2 states that RFS automatically applies, in a feature control frame, to individual tolerances of size features and to datum features of size. MMC and LMC must be specified when these conditions are required.
Now examine rule #2, here also lets assume a stepped shaft with Ø5.000 and Ø3.000-Ø3.004. The bigger diameter is assigned as datum (Say C) and the smaller diameter Ø3.000 is controlled by a concentric geometric tolerance and this tolerance is applied without any dimensional size constraints ie regardless of feature size (RFS)
Now in the next chapter we will discuss about bonus tolerance and its calculations in detail.