image Geometrical Dimension and Tolerances (GDandT)

                                                                    Geometrical Dimensioning & Tolerances (GD&T)

Tolerances - Allowance for a specific variation in the size and geometry of part.

Tolerances Specified – Size (length, width, height, diameter, etc), Geometry                  (Circularity, Cylindricity, Perpendicularity, Parallelism etc).

Value of Tolerance - The tolerance is total variation between the upper and lower  limits.

Tolerances specified for size - Limit Tolerances –example (12.75/12.25 )

Plus/Minus Tolerances –

               Unilateral Tolerances - (12.00 + or - xxx)

                Bilateral Tolerances - (12.00 +xxx/- xxx)

These tolerance values indicate the:

                   MMC: Maximum Material Condition

                   LMC: Least Material Condition

ALLOWANCE - an intentional difference between the maximum material limits of mating parts. Allowance is the minimum clearance (positive allowance), or maximum interference (negative allowance) between mating parts.

                ALLOWANCE = MMC HOLE – MMC SHAFT

Where,   MMC: Maximum Material Condition

 CLEARANCE - the loosest fit or maximum intended difference between mating parts.

                    CLEARANCE = LMC HOLE – LMC SHAFT

Where,     MMC: Maximum Material Condition

                   LMC: Least Material Condition

Types of Fit –

Clearance fit –

  • The parts are toleranced such that the largest shaft is smaller than the smallest hole
  • The allowance is positive and greater than zero

Interference fit

  • The max. clearance is always negative
  • The parts must always be forced together

Transition fit –

  • The parts are toleranced such that the allowance is negative and the max. clearance is positive
  • The parts may be loose or forced together

BASIC FITS OF MATING PARTS –

Running and Sliding fits (RC) - Running and Sliding fits (RC) are intended to provide a running performance with suitable lubrication allowance. The range is from RC1 to RC9.

Force fits (FN) - Force fits (FN) or Shrink fits constitute a special type of interference fit characterized by maintenance of constant pressure. The range is from FN1 to FN5.

A force fit is referred to as interference fit or a shrink fit. The smallest amount of interference is:-

                     MIN INTERFERENCE = LMC SHAFT - LMC HOLE

The greatest amount of interference is:

                    MAX INTERFERENCE = MMC SHAFT - MMC HOLE

Locational fits - Locational fits are intended to determine only the location of the mating parts.

 



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