What is "General Tolerance"? How Does It Differ from Other Tolerances?

"General tolerance," also known as "common tolerance," refers to the tolerance applied when no specific tolerance is indicated for a dimension. On technical drawings, dimensions usually have specified tolerances, but if tolerances are marked for every single dimension, the drawings can become difficult to interpret. By using "general tolerance," it becomes easier to read the dimensions on a drawing and accurately identify critical dimensions.

Difference Between "Dimensional Tolerance" and "General Tolerance"

"Dimensional tolerance" is the allowable range clearly specified for dimensions with special precision requirements, such as 100±0.1. Since the tolerance value can be chosen arbitrarily, it allows the designer's intentions to be clearly communicated to the machinist.

On the other hand, "general tolerance" allows for specifying the allowable range for each dimension at once, reducing the time required to create drawings and making them easier to read.

What Are the Grades of "General Tolerance"?

When using "general tolerance," the grade is important. In the Japanese Industrial Standards (JIS), there are four grades: "fine," "medium," "coarse," and "very coarse," represented by the symbols f, m, c, and v, respectively. The types of dimensions are further subdivided into categories such as "length dimensions," "chamfer dimensions," "angular dimensions," and "perpendicularity."

What is the "Standard Size Classification" for "General Tolerance"?

The values of "general tolerance" are determined by the grade but vary depending on the dimension size. For instance, although the tolerance ranges of 1±0.1 and 100±0.1 are the same, their relative precision differs significantly when considering the ratio to the "length dimension." Therefore, in JIS, tolerance values are set for each "standard size classification" to manage the precision of dimensions on a drawing uniformly.

As shown in the table above, tolerance values change depending on the standard size. For example, if the dimension is 18mm and falls under the "fine" grade, even if "dimensional tolerance" is not specified on the drawing, a tolerance of ±0.1 will automatically apply.

How to Specify General Tolerance

When applying general tolerance to a drawing, it is necessary to indicate the "application of general tolerance and its grade" in the title block of the drawing.

Appropriate Use of Tolerance Grades

• Fine: Used for metal parts of precision equipment.
• Medium: Used for general metal parts.
• Coarse: Used for general plastic molded parts.

Note: Applying a fine grade to plastic molded parts can sometimes make molding difficult, resulting in increased part costs. Therefore, it is crucial to specify the appropriate grade.

To promote the globalization of manufacturing, it is essential to comply with international standards for "dimensional tolerance" and "geometric tolerance" (ISO). Overseas manufacturers may not interpret your drawings for you, so when sourcing parts that meet design requirements, it is crucial to create drawings based on a full understanding of "dimensional tolerance" and "geometric tolerance."