Chapter 13: Geometric Dimensioning and Tolerancing

Tolerance definition and types: Bilateral and unilateral

Unilateral tolerances: A basic size is followed by a +- expression of tolerance where variations are allowed in only one direction from the nominal size. is 0.0 in one direction Bilateral Goes in both directions

HOLE BASIS SYSTEM

-Smallest possible hole (MMC) is the basic size -Hole tolerance added to basic size for largest hole -Allowance subtracted from basic size for largest shaft - Tolerance of shaft subtracted from largest shaft for smallest shaft

Tolerance stack up for different datum

-Tolerances taken from the same direction from one reference are additive -Tolerances taken to the same point in different directions are additive in both directions

Tolerance for 100% interchangeability: most extreme condition

Any random combination of mating parts will be guaranteed to assemble. The extreme or most difficult condition for assembly are used to find the unknown tolerance. In the extreme condition, internal dimensions are taken at the minimum value and external dimensions are taken at the maximum value.

LMC

Opposite of MMC, the feature contains the least material. Example 1: Minimum diameter for a shaft Example 2: Maximum diameter for a hole Symbol:

MMC, LMC, RFS

MMC: Maximum Material Condition LMC: Least Material Condition RFS: Regardless of Feature Size

RFS

Requiring that the tolerance of form, runout, or position be met regardless of where the feature lies within its size tolerance. It is the default unless MMC or LMC is mentioned. Limits: are the extreme allowable sizes for a feature.

Circular and total runout

Runout is a tolerance used to express relationships between surfaces of features in the many applications in which acceptable part deviation is best observed in terms of rotation about an axis. There are two types of runout: circular and total. Both are expressed in terms of limits of full indicator movement (FIN) Circular runout controls circularity and coaxiality. As shown in the following figure, the control would be measured +- 0.05 mm FIN of a dial indicator. Total runout controls the combined variation of circularity, straightness, and coaxiality.

MMC

The condition in which a feature of size contains the maximum amount of material with the stated limits of size Example 1: Maximum diameter for a shaft Example 2: Minimum diameter for a hole Symbol:

Determining fits and tolerance for shaft and hole

Tolerance of hole - difference between largest and smallest hole Tolerance of shaft - difference between largest and smallest shaft Allowance - difference between smallest hole and largest shaft (negative for interference) (minimum clearance)