Tolerance and allowance 2

Tolerance and

Allowance

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Introduction

Modern industry has developed on the basis of interchangeable manufacturing.

Interchangeable manufacturing means production of parts to such a degree of accuracy as is necessary to permit assembly and proper functioning of parts without further machining or fitting.

To set the dimension for many product we make certain gages that measure dimensions. So gages used for interchangeable manufacturing.

Tolerance

• “The total amount by which a given dimension may vary, or the difference between the limits.”

Design Specifications and Tolerance

Develop from quest for production quality and efficiency.

Early tolerances support design’s basic function.

Mass production brought interchangeability.

Affected Areas

Product Design Quality Control

Manufacturing

EngineeringTolerance

Product design

Product Design has the responsibility of designing with consideration for the maximum possible working tolerances compatible with the functional requirements of the design.

Manufacturing

Manufacturing has the responsibility of bringing the design into a physical entity with consideration for process methodologies, capabilities and economics.

Quality control

Quality control has the responsibility of measuring and evaluating the final product to ensure the integrity of the product tolerance.

Variation is Unavoidable

• No two manufactured objects are identical in every way. Some degree of variation will exist.

• Engineers apply tolerances to part dimensions to reduce the amount of variation that occurs.

Tolerances

A tolerance is an acceptable amount of dimensional variation that will still allow an object to function correctly.

Tolerances

A tolerance is an acceptable amount of dimensional variation that will still allow an object to function correctly.

Tolerances

Three basic tolerances that occur most often on working drawings are: limit dimensions, unilateral, and bilateral tolerances.

Limit Dimensions

Limit dimensions are two dimensional values stacked on top of each other. The dimensions show the largest and smallest values allowed. Anything in between these values is acceptable.

These are limit dimensions,because the upper and

lower dimensional sizes arestacked on top of each other.

Limit Dimensions

Unilateral Tolerance

A unilateral tolerance exists when a target dimension is given along with a tolerance that allows variation to occur in only one direction.

This tolerance is unilateral, becausethe size may only

deviate in one direction.

Unilateral Tolerance

Bilateral Tolerance

A bilateral tolerance exists if the variation from a target dimension is shown occurring in both the positive and negative directions.

 Allowance

 Allowance is a planned deviation between an actual dimension and a nominal or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension

example

Outer dimensions (such as the length of a bar) may be cut intentionally oversize, or inner dimensions (such as the diameter of a hole) may be cut intentionally undersize, to allow for a predictable dimensional change following future cutting, grinding, or heat-treating operations.

For example:the outer diameter of a pin may be ground to 0.0005 inches (0.013 mm) oversize because it is known that subsequent heat-treatment of the pin is

going to cause it to shrink by 0.0005 inches (0.013 mm).A hole may be drilled 0.012 inches (0.30 mm) undersize

to allow for the material that will be removed by subsequent reaming.

Tolerance ALLOWANCE

A tolerance is the limit of acceptable unintended deviation from a nominal or theoretical dimension. Therefore, a pair of tolerances, upper and lower, defines a range within which an actual dimension may fall while still being acceptable.

an allowance is a planned deviation from the nominal or theoretical dimension

whereas allowance is one that we give to the material to undergo some process in the future.

Difference