tolerances, limits and fits -...

FEAR

Our greatest fear is not that we are inadequate,but that we are powerful beyond measure.

It is our light, not our darkness, that frightens us.We ask ourselves, Who am I to be brilliant,gorgeous, handsome, talented and fabulous?

Your playing small does not serve the world.There is nothing enlightened about shrinking

so that other people won't feel insecure around you.

~Quoted selectively from Marianne Williamson and Nelson Mandela

Tolerances, Limits and Fits

A succession of rude jokes

Tolerances, Limits and Fits

The Importance of Fitting In

Tolerances

• A convention to define leeway in manufacturing

Limits

• Maximum Thresholds that may not be exceeded in manufacture

Fits

• The manner in which items fit together, and the behaviour of this

Why Tolerances?

• No dimension can be “accurately machined”

• Thermal instability of material

• Greater accuracy requires more precision tooling and time, which is costly

• Function of fit must be considered

Conventions of Notation

Further Conventions of Notation

Conventions

• Nominal size is the “actual” vernier sizeWe tend to base our calculations on the hole size – tooling

• Capital ‘H’ refers to holes

• Lower case letters a to z refer to shaft

• Numbers refer to magnitude of displacement from nominal

Types Of Fits

• Clearance fit - A fit provides a clearance. The tolerance zone of the hole is entirely above that of the shaft.

• Transition fit - A fit provides either a clearance or an interference. The tolerance zones of the hole and the shaft overlaps.

• Interference fit - A fit provides an interference. The tolerance zone of the hole is entirely below that of the shaft.

• Source: Hong Kong University of Science and Technology

The MicrometreMeasured with a micrometer

Item

Anthrax Spore 6 μm

Diameter of Bacterium 1 - 10 μm

Discernable Gap 20 μm

Human Hair 70 μm

Sheet of Paper / Coat of paint 100 μm

Tolerance:

• Nominal : 48mm

• H8 f7

CalculationNominal Plus Minus

Hole 48 39 -25

Shaft 48 +0 -50

48+.039(-)0

48 - .025 - .050

Hole

Shaft

48+.039(-)0

48 -0.025-0.050