Toler Ancing

Copyright ©2010 by K. PlantenbergRestricted use only

Chapter 4Tolerancing

Topics

Exercises


Tolerancing: Topics

Summary

4.1) Tolerancing and Interchangeability

4.2) Tolerancing Standards

4.3) Tolerance definitions

4.4) Tolerance Types


Tolerancing

4.1) Tolerancing for Interchangeability


Tolerancing / Definition

What are tolerances? Manufacturing processes are not

precise. There is always a dimensional error.

Tolerance is the leeway allowed on a dimension for accommodating manufacturing errors.


Tolerancing / Interchangeability

A tolerance is the amount of size variation permitted.→ You can choose a tolerance that specifies a

large or small variation.

1.005

.994

Tolerance = 1.005 - .994 = .011

Size limits =


Tolerancing / History The need for GD&T was identified during

second world war. Because of the mass production, different

parts were manufactured by different vendors.

But these parts would not fit together because of dimensional errors.

So this laid foundation for the development of a standard for tolerances. This standard is meant to define tolerances and their applications.



Choosing a tolerance for your design.

→ Specify a tolerance with whatever degree of accuracy that is required for the design to work properly.

→ Choose a tolerance that is not unnecessarily accurate or excessively inaccurate.



Choosing the correct tolerance for a particular application depends on:

→ the design intent (end use) of the part→ cost→ how it is manufactured→ experience


Tolerancing

4.2) Tolerancing Standards


Tolerancing Standards

Standards are needed for

→ Defining consistent definitions and rules for tolerances

→ Helps in interchangeability of parts.


Tolerancing Standards

The two most common standards agencies are;

→ American National Standards Institute (ANSI) / (ASME Y14.5)

→ International Standards Organization (ISO).


Tolerancing Definitions

4.3) Tolerance definitions


Tolerance types and methods

The tolerancing methods presented are:→ Limit dimensions → Plus or minus tolerances → zones


1. Limit Dimensions

Limits are the maximum and minimum size that a part can obtain and still pass inspection.

→ For example, the diameter of a shaft might be specified as follows.


2. Plus or Minus Tolerances

Plus or minus tolerances give a basic size and the variation that can occur around that basic size.


3. Zones

Zones give the distance between two parallel features, between which the variations are allowed.


Shaft-Hole Assembly

Used to illustrate concepts and definitions. Both the shaft and the hole are allowed to

vary between a maximum and minimum diameter.


Tolerances Definitions

Limits: The limits are the maximum and minimum size that the part is allowed to be.

Basic Size: The basic size is the size from which the limits are calculated. → It is common for both the hole and the shaft

and is usually the closest fraction.


Exercise 4-2

Fill in the following table.

Skip to next part of the exercise

Shaft Hole

Limits

Basic Size

Tolerance

.47 - .51 .49 - .50

.5 or 1/2

.04 .01


Inch Tolerances Definitions

Maximum Material Condition (MMC): The MMC is the size of the part when it consists of the most material.

Least Material Condition (LMC): The LMC is the size of the part when it consists of the least material.


Exercise 4-2

Fill in the following table.

Skip to next part of the exercise

Shaft Hole

MMC

LMC



Maximum Clearance: The maximum amount of space that can exist between the hole and the shaft. → Max clearance = LMChole – LMCshaft



Minimum Clearance (Allowance): The minimum amount of space that can exist between the hole and the shaft.

→ Min. Clearance = MMChole – MMCshaft


Exercise 4-2

What does a negative clearance mean?


Exercise 4-4

Tolerance Types


Tolerance Types There are 6 types of tolerances defined in the

current standard (ASME Y14.5)


Size Size is applicable on features of size. It is a limit

dimension. Features of size are those features on which a

diameter or thickness can be applied. ex. hole, with a size tolerance of 6 ± 0.3 mm:

Picture from “http://gdtseminars.com/2008/02/13/what-is-resultant-condition/”


Form Form tolerances are used to control the surface

characteristics of a feature. Flatness controls the surface variation for a

plane. Straightness controls the variation of line

elements on a plane or cylindrical surface or axis of a cylinder.

Circularity is used to control the variations of a circular element on a cylindrical surface.

Cylindricity is used to control both the circular and line elements on a cylindrical surface


Form

Flatness Circularity

Straightness

Figures from “http://www.roymech.co.uk/Useful_Tables/Drawing/draw_geom_ex.html”


Orientation Controls the orientation of features with respect

to each other 3 types – parallelism, perpendicularity,

angularity Features are controlled with respect to datums


Orientation

Parallelism

Perpendicularity

Angularity

Figures from “http://denisekitchencad.weebly.com/gdt.html”


Location Controls the position of features. 3- types - position, concentricity, symmetry Applied to features of size only Need datums


Location There

Location



Runout Controls the surface variation with respect to a

datum Applied to cylindrical features or circular

elements 2 types- circular and total Circular runout controls one circular element. Total runout controls the entire surface.


Runout There


Circular

Total Runout


Profile Controls size, form, orientation and position. Two types – line and surface Line profile controls one element at a time. Surface profile controls the entire surface.


Profile There


Line Profile

Surface Profile

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Toler Ancing