DIMENSIONS AND TOLERANCES:
Dimensioning is a numerical way of representation with appropriate values such as inches, meter, millimeter, etc. and are used to display the size, location, orientation, form or other geometrical characteristics of the part to the audience.
A tolerance is a numerical value under which the basic or nominal value is allowed to deviate. In other words, the difference between maximum and minimum limits is known as tolerance.
TYPES OF TOLERANCE:
The two most common methods of tolerances are:
1. Limit tolerance.
2. Plus-minus tolerance.
In limit tolerance, the higher and lower limits are mentioned.
In this type the higher tolerance is placed at the top while the lower one at the bottom.
As we can see in the fig, the higher limit is 12.5 which is at the top.
The lower limit is 12.0 which is placed at the bottom.
The tolerance for this dimension is, 12.5-12.0 which is equal to 0.5.
When limit tolerances are defined in a single line always the lower limit comes first and then the higher limit for e. g. (12.0-12.5).
In this type of tolerancing scheme, first the nominal/basic value is stated followed by a plus-minus expression of a tolerance.
An example of plus-minus tolerance is shown in the fig, where the nominal value is 12.25. The plus-minus tolerance is 0.25.
The total tolerance is 0.5.
There are certain ways in which the plus-minus tolerances can be expressed.
First we’ll understand what bilateral tolerance is,
When the plus-minus tolerance vary in both the directions it is known as bilateral tolerance.
EQUAL BILATERAL TOLERANCE:
In this type, the limit values are similar in both the directions.
In this type, one of the limit value is 0 and the other limit value is defined as per the requirement.
UNEQUAL BILATERAL TOLERANCE:
The limit values in this case are not the same.
METRIC DIMENSION SPECIFICATIONS:
Mostly the Metric International system of units (SI) are used.
The millimeter is the limit of measurement widely used in the engineering drawings.
For the parts in which the dimensions are greater, unit of measurement is inches.
In Industry drawing, a note stating “UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN MILLIMETERS” is given to use the metric system.
Three conventions are widely used when specifying the dimensions in metric units.
1. Decimal point and zeros are not present when the dimension is a whole number.
2. When the dimension is less than 1, then a 0 is preceded before the decimal point. For example the dimension 0.2 has 0 at the left.
3. When the dimension is not a whole number, the portion of millimeter (10ths or 100ths) with a decimal point is specified.
The standard of dimensioning and tolerancing is ASME Y 14.5M-1994.
The full form of ASME is American Society if Mechanical Engineers.
There is no meaning to Y14.5, it’s just a standard value.
M stands for metric and 1994 is the year when this standard was approved.
Number of revisions are created after the approval year 1994.
United States has adopted ASME Y14.5M-1994 for dimensiining and tolerancing.
There is another standard used in different parts of the world.
The INTERNATIONAL STANDARDS ORGANIZATION (ISO) has also created a standard for dimensioning and tolerancing.
The ISO dimensioning and ASME Y14.5 M-1994 has 90 percent similarity in themselves.
ENGINEERING DRAWING STANDARDS:
An effort was made to standardize the dimensioning and tolerancing standards to improve the quality of the drawing.
After years of discussion in 1935, AMERICAN STANDARDS ASSOCIATION (ASA) published the 1st standard for dimensioning and tolerancing, “American Drawing and Drafting Room practices”.
There were 18 pages out of which 5 pages discussed about dimensioning and tolerancing.
During the World War II, many parts were rejected due to quality issues.
This lead to improper assembly of the parts.
The conclusion was made that the tolerances given to the parts are not appropriate and the standards need improvement in order to enhance the quality of the process of producing these parts.
FUNDAMENTAL DIMENSIONING RULES:
The fundamental dimensioning rules are a set of instructions for developing the drawings.
ASME Y14.5M-1994 has certain rules developed.
The 10 rules are as follows:
1. Every dimension must have tolerance except if it is a reference, maximum, minimum or stock size.
2. Entire dimensions and tolerances should be provided so that there is full definition of each part features.
3. The mentioned dimension must suit the function and mating relationship of the part.
4. The drawing should focus on the part and not on the manufacturing method.
5. A 90 deg angle is assumed where the centerline and lines depicting features are shown on a drawing, when no dimensions are mentioned.
6. All dimensions are applicable at 20 deg C, unless otherwise specified.
7. In free state condition, all dimensions and tolerances are applied.
8. The geometric tolerances apply to the full depth, length and width unless otherwise specified.
9. Dimensions and tolerances are applicable to the drawing where they are mentioned.
10. A 90 deg basic angle is assumed when the centerline of the feature is shown at right angles on a drawing, and no angle is specified.