Rule 1 of GD&T (ASME Y14.5 – 2018 : Perfect Form at MMC

👉 RULE-1 of GD&T is also known as “Perfect Form @MMC”

👉 It states – “Where only size tolerance is specified, the limits of size of an individual feature prescribes the extent to which variation of it’s form as well as in it’s size are allowed”

e.g. If the shaft has a size of Diameter 10.2 – 10.8 mm.
MMC = 10.8 mm

So, if shaft is produced at dia 10.8 mm, zero form error is allowed.
But if shaft is produced at dia 10.2, form error allowed is 0.6 mm.

Form Error @LMC = 10.8 – 10.2 = 0.6 mm

👉 If Design Engineer needs to control the form tighter than prescribed by the size tolerance, he / she could use appropriate form controls such as Straightness, Flatness, Circularity or Cylindricity with the tolerance value lesser than size tolerance.

👉 In other words, provided form control could be refinement of the form value prescribed by the size tolerance of the Feature.

👉 We use this kind of arrangement, when our design allows us to provide higher size control but requires us to control surface variations tighter.