## ENG 004 Lecture 14, Nov 13, 2012

### Announcements

• Homework #5 due today
• Homework #6 posted after class
• Office Hours 5-6

• Tolerances

## Tolerances

Tolerances are the allowable deviation in size, location, and geometry of a feature.

The primary role of dimensioning is to control the size or location of a feature.

Every dimension has to have a tolerance associated with it: X.XX±0.02

Does not control for the form (geometry) of the feature directly, it only controls the size or location

## Material Conditions

### Maximum Material Condition (MMC)

Size of a feature at one end of its tolerance zone where there is the most amount of material.

### Least Material Condition (LMC)

Size of a feature at one end of its tolerance zone where there is the least amount of material.

## Tolerance Stackup

0.7±0.2 + 0.5±0.3 = 1.2±0.5

0.7±0.2 - 0.5±0.3 = 0.2±0.5

## Classes of Fit

An assembly of two parts creates a "fit" whose functional characteristic is determined by the differences in the parts' associate sizes.

### Clearance Fit

The MMC of the hole is always greater than the MMC of the shaft: MMChole > MMCshaft

### Interference Fit

Both the MMC and the LMC of the hole are less than the MMC and LMC of the shaft, respectively

MMChole < MMCshaft

LMChole < LMCshaft

## Metric Preferred Hole Based System

Minimum hole size is the basic size

## Metric Preferred Shaft Based System

Minimum shaft size is the basic size

## English Preferred Precision Fits

### Running and Sliding Fits [RC]

The loosest fits. Used when shaft must move freely in a hole. Always has clearance, position is not critical.

### Clear Locational Fits [LC]

Tighter than RC. Shaft and hole may be same size (line-to-line fit). Location is more important than RC.

### Transition Locational Fits [LT]

Transition between LC and LN. May have slight clearance or may have slight interference.

### Interference Locational Fits [LN]

Can be line-to-line, but almost always shaft is larger than the hole. Position/location very important.

### Force and shrink fits [FN]

Pure interference fits. Used to secure parts together.

## English Preferred Precision Fits

### Basic Size

Theoretical nominal size to which the tolerance limits are applied.

### Basic Hole System

Smallest hole is set as the basic size. Most common due to drill tooling

### Basic Shaft System

Largest diameter of the shaft is assigned as the basic size. Less popular.

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