What Is Engineering Tolerance – Types of Tolerances in Engineering Drawing


The creation and use of tolerance in engineering is purposed to specify the boundary of qualified products, what is the definition of engineering tolerance? And what are the common types of tolerances used in engineering drawing? In this post, we are going to better understand the engineering tolerance.

What Is Engineering Tolerance?

Deviations around the target value often exist during the manufacturing of machined parts, based on the use of the product, tolerance is created to allow for reasonable variations. Engineering tolerance is a specified acceptable range of deviation in measurements relative to the base value. Different tolerances guarantee the products meet the required specifications and function properly.

Why tolerance is important in engineering?

What is an example of tolerance in engineering?
For instance, the hole diameter tolerance is the permitted deviation from the nominal diameter for a hole that a part must fit into. ±0.005mm is an example of a tight tolerance.

How to determine tolerance in engineering drawing?
Tolerances should be set in a way that does not significantly affect other factors or the outcome of a process. Process capability, measurement error, and statistical uncertainty should be taken into account when setting tolerances. The intended statistical sampling plan and acceptable quality level may also influence the choice of tolerances.

Types of Tolerances in Engineering Drawing

Tolerances in engineering drawing include dimension tolerance, shape tolerance, position tolerance, general tolerance, and more. Let’s dive into the common tolerances used in engineering drawing.

1) Dimension tolerance: refers to the acceptable variation in sizes like lengths, widths, thicknesses, radii, etc.

2) Geometric/shape tolerance: refers to the allowable variation in the shape of a part’s feature from its ideal geometric form. For example, if you have a flat surface that must be within a certain tolerance for the product to function correctly, you might specify a flatness tolerance.

3) Position tolerance: specifies the allowable deviation for the location of a feature from its exact position.

4) General tolerance: provided in standards like ISO 2768 for unspecified dimensions, provide acceptable tolerance ranges for unspecified dimensions on a drawing.

5) Fits: refers to the clearance between two mating parts.

6) GD&T (Geometric Dimensioning and Tolerancing): a comprehensive system that covers various types of tolerances.

List of Engineering Tolerance Symbols

Common engineering tolerance symbols:
∅ (phi): Indicates diameter dimension
+/-: Indicates upper and lower deviation from nominal
±: Indicates total toleranced width from nominal
Ø: Alternate symbol for a diameter dimension
×: Indicates multiplication
μ: Micron (prefix symbol for micrometers)
°: Degree
‘: Minutes (unit of angle)
“: Seconds (unit of angle)
mm: Millimeters
in: Inches
mas: Minutes of arc (unit of angle)
c: Clearance tolerance
L: Limit dimension
M: Maximum material condition
N: Nominal dimension
R: Reference dimension
PCD: Pitch circle diameter
P: Projected tolerance zone
W: Basic dimension
A: Actual dimension
BTM: Basic tolerance of manufacturing
BTR: Basic tolerance of repair
PT: Parallelism tolerance

GD&T symbols:
⊥: Perpendicularity
∆: Angularity
∥: Parallelism
=: Concentricity
C: Circular runout
TIR: Total indicator reading