GTDGTD

Geometric Tolerances and Geometric Tolerances and DimensionsDimensions

Why Geometric Tolerances and DimensioningWhy Geometric Tolerances and Dimensioning To ensure interchangeability of mating parts To ensure interchangeability of mating parts

during assemblyduring assembly

To eliminate controversy and guesswork To eliminate controversy and guesswork when drawing is interpretedwhen drawing is interpreted

To ensure the drawing reflects the form and To ensure the drawing reflects the form and function requirements of the manufactured function requirements of the manufactured partsparts

Principles of datum specification

Three perfect plans used to locate an imperfect part.

a. Three point contact is used on the primary plane.

b. Two point contact is used on the secondary plane.

c. One point contact is used on the tertiary plane

datum specification

One-Plane Datum Reference Frame

Two-Plane Datum Reference Frame

Three-Plane Datum Reference Frame

Symbols Advantages:1. The symbol has uniform meaning.

2. Symbols are compact, quickly drawn, and can be placed on the drawing where the control applies.

3. Symbols are the international language and

surmount individual language barriers.

4. Geometric tolerance symbols follow the established precedent of other well known symbol systems, e.g., electrical and electronic, welding, surface texture.

Using Symbols

Using Notes

Symbols/Notes

Material Condition

•

HOLE

PIN

MMCMMC

The actual local size of the hole The actual local size of the hole

atat . 245 . 245 and the pin at and the pin at ØØ .240 .240

of the figure are the Maximum material of the figure are the Maximum material conditioncondition

Ø

LMCLMC

The actual local size of the hole The actual local size of the hole

at at ØØ .255 .255 and the pin at and the pin atØØ .230 .230

of the figure are the least material of the figure are the least material conditioncondition

TERMINOLOGY

• VIRTUAL CONDITION - A constant boundary generated by the collective effects of a size feature’s specified MMC or LMC and the geometric tolerance for that material condition.

VIRTUAL CONDITION

• Virtual condition, based on MMC or L M C is a feature‘s extreme boundary; it represents

the “worse case”

• For MMC, “worse case” concerns fits and/or clearances with mating parts For LMC, “worst case” is concerned with strength, alignment, wall thickness, etc. with reference to mating parts

VIRTUAL CONDITION (MMC- PIN)• Virtual Condition for a Pin

(Based on Maximum Material Condition) =

Maximum Material Condition + the Stated Position or Orientation Tolerance

• VC = MMC + Tolerance

VIRTUAL CONDITION (MMC- PIN)

VIRTUAL CONDITION (MMC- Hole)

• Virtual Condition for a Hole

(Based on Maximum Material Condition) =

Maximum Material Condition - the Stated Position or Orientation Tolerance

• VC = MMC - Tolerance

VIRTUAL CONDITION (MMC- Hole)

VIRTUAL CONDITION (LMC- PIN)

• Virtual Condition for a Pin

(Based on Least Material Condition) =

Least Material Condition - the Stated Position or Orientation Tolerance

• VC = LMC - Tolerance

VIRTUAL CONDITION (LMC- PIN)

VIRTUAL CONDITION (LMC- Hole)

• Virtual Condition for a Hole

(Based on Least Material Condition) = Least

Material Condition + the Stated Position or Orientation Tolerance

• VC = LMC + Tolerance

VIRTUAL CONDITION (LMC- Hole)