What Is Engineering Tolerance?- Definition and Types

Tolerances establish the permissible deviation from the assigned dimensions or measured value in mechanical engineering. Manufacturers often use tolerances in product engineering to ensure the interchangeability of components in mechanical manufacturing.

A product may become unusable based on the design intent when the production parts’ dimension is not within the preferred tolerance limits since each fabrication process has a certain degree of inaccuracy.

Therefore, it is pivotal to understand engineering tolerances and their types to achieve quality and usable products.

This article discusses what tolerance engineering is and its different types. You’ll also learn about fits and helpful considerations for part tolerancing. Let’s dive in!

What is Tolerance in Engineering?

Engineering tolerance is the permissible variation in measurements deriving from the base measurement.

Tolerances can apply to many different units. For example, the working conditions may have tolerances for temperature (° C), humidity (g/m3), etc. In mechanical engineering, we are mainly talking about tolerances that apply to linear, angular, and other physical dimensions.

But regardless of the unit, a tolerance states an acceptable measurement range from the base point (nominal value).

Let’s say you are designing a sieve to separate 3.5 mm pebbles from 2.5 mm pebbles. You want the smaller pebbles to fall through the holes while keeping the larger ones on the sift.

The larger pieces of rocks vary in size between 3.3 mm and 3.7 mm. The smaller ones are in the range of 2.3…2.7 mm.

To ensure that only the smaller ones, all of them, will actually fall through the holes while keeping the larger ones on the sift, you can set the nominal value for the hole diameter as 2.8 mm. At the same time, manufacturing accuracy will mean that you may end up with some holes at 2.6 mm.

Adding a lower limit of -0 mm and an upper limit of +0.3 mm guarantees that all the holes will be between 2.8 and 3.1 mm in diameter.

Dimensions, properties, or conditions may have some variation without significantly affecting the functioning of systems, machines, structures, etc.

A variation beyond the tolerance (for example, a temperature that is too hot or too cold) is said to be non-compliant, rejected, or exceeding the tolerance.

Considerations when setting tolerances

A primary concern is to determine how wide the tolerances may be without affecting other factors or the outcome of a process. This can be done by the use of scientific principles, engineering knowledge, and professional experience.

An experimental investigation is very useful to investigate the effects of tolerances: Design of experiments, formal engineering evaluations, etc.

A good set of engineering tolerances in a specification, by itself, does not imply that compliance with those tolerances will be achieved. The actual production of any product (or operation of any system) involves some inherent variation of input and output.

Measurement error and statistical uncertainty are also present in all measurements. With a normal distribution, the tails of measured values may extend well beyond plus and minus three standard deviations from the process average. Appreciable portions of one (or both) tails might extend beyond the specified tolerance.

The processing capability of systems, materials, and products needs to be compatible with the specified engineering tolerances.

Process controls must be in place and an effective Quality management system, such as Total Quality Management, needs to keep actual production within the desired tolerances.

A process capability index is used to indicate the relationship between tolerances and actual measured production.

The choice of tolerances is also affected by the intended statistical sampling plan and its characteristics such as the Acceptable Quality Level.

This relates to the question of whether tolerances must be extremely rigid (high confidence in 100% conformance) or whether some small percentage of being out-of-tolerance may sometimes be acceptable.

Types of Tolerances

Today, there are 14 types of geometric tolerances by the number of symbols, and 15 types based on classification.

These are grouped into Form tolerance, Orientation tolerance, Location tolerance, and Run-out tolerance, which can be used to indicate all shapes.

Following are the three types of tolerances used in measurements:

• Unilateral tolerances
• Bilateral tolerances
• Compound tolerances.

#1. Unilateral Tolerances.

A unilateral tolerance shows that a value’s acceptable range of variance extends in only one direction. A unilateral tolerance value is either positive or negative relative to the nominal value.

While not as common as equal bilateral tolerances, unilateral tolerances are very useful for closely mating parts. For example, a machined hole that accepts a pin with a maximum diameter of 1” must have a minimum diameter of 1”.

By ensuring the tolerance ranges for these mating parts do not overlap, you can guarantee they will fit together.

#2. Bilateral Tolerances.

Bilateral tolerance describes the acceptable range of variance around a base value in both the positive and negative directions. You can have a bilateral tolerance that is either equal or unequal.

Generally, equal bilateral tolerance is the most commonly cited engineering tolerance. Its range extends an equal amount in both directions from the nominal value.

When designers need to specify ranges that are unequal about the base value, they use an unequal bilateral tolerance. Sometimes referred to as unequally disposed tolerances, they include positive and negative values that are different from one another.

#3. Compound Tolerances.

Compound tolerance is determined by the established tolerances i.e., the combination of more than one type of tolerances are called compound tolerances, the different types of tolerances may be angular, lateral, etc.

For Example: In figure tolerances on dimension/are dependent on tolerances of L, h, and θ. This compound tolerance on ‘l’ is the combined effect of these three tolerances. The minimum tolerance on ‘l’ will be corresponding to L-b, θ+∝ , and h+c.

General Tolerances

General tolerances may be included in an engineering drawing as a table or a note (e.g., “ISO 2768-m”) somewhere on the drawing. These tolerances are applicable in different conditions such as chamfer heights, linear dimensions, external radius, angular dimensions, etc.

ISO 2768 is an example of an international tolerance grade commonly used in Europe. ASME’s Y14.5 is the US variant of the same general tolerance standard but doesn’t encompass general tolerances.

However, what is the interpretation of the “ISO2768-m” note on an engineering drawing?

The note informs the manufacturer to use the m (medium) tolerance class when manufacturing the parts. It applies to all dimensions unless the client states otherwise on the drawing. Therefore, a specified tolerance in engineering a hole overrides the general tolerance requirements.

Below is a linear dimension table for further explanation:

Linear Dimension Range (mm)	Tolerance Class
	F (fine)	M (medium)	C (coarse)	V (very coarse)
0.5 up to 3	±0.05	±0.1	±0.2	–
over 3 up to 6	±0.05	±0.1	±0.3	±0.5
over 6 up to 30	±0.1	±0.2	±0.5	±1.0
over 30 up to 120	±0.15	±0.3	±0.8	±1.5
over 120 up to 400	±0.2	±0.5	±1.2	±2.5
over 400 up to 1000	±0.3	±0.8	±2.0	±4.0
over 1000 up to 2000	±0.5	±1.2	±3.0	±6.0
over 2000 up to 4000	–	±2.0	±4.0	±8.0

As you can see in the table above, the permissible deviation is +/- 0.2 mm if a linear dimension is in the 6 to 30 mm range according to the m (medium) column.

Also, +/- 0.8 is the permissible tolerance for dimensions between 400 to 1000 mm. Hence, 25.2 mm is allowed for a 25 mm cut, while 599.2 is permissible following the standard 600 mm nominal value.

GD&T

Geometric dimensioning and tolerancing (GD&T) is a superior and more intricate system that adds another aspect to engineering tolerances basics. It is a universally standardised way of indicating design requirements, even though it is initially daunting and complex.

GD&T describes the geometric tolerances for engineering parts using in-part references. It points out the precise geometric characteristic of the part where the tolerances apply.

GD&T goes beyond standard dimensioning and tolerancing (SD&T) by covering geometric characteristics, including concentricity, flatness, and true position.

Fits

Shaft and hole mating come with a lot of different options and always require tolerances to obtain the right fit. But what is a fit in short?

Limits and fits describe the allowance between the shaft and the hole. Allowance, in turn, is the maximum dimensional difference between the diameters of the two.

Types of Fits

There are three types of shaft-hole engineering fits.

Clearance Fit

Clearance fits allow for loose mating, where free movement is important and a certain amount of play is desired.

We see clearance fits called for where elements should be able to slide in and out without obstruction, and where alignment can be loosely guided but does not require tight precision. Examples of clearance fit might include bolt/shaft holes where an element will slide freely through another feature.

Interference Fit

An interference fit will be much tighter than a clearance fit. Also referred to as a press fit or friction fit, the interference fit requires some degree of force to join two components.

Pressing a bushing, bearing, dowel pin or other items into their mating components are all examples of how an interference fit can be used. Once joined, this creates a relatively solid union that would require substantial force or potential machine operations to uncouple.

Transition Fit

A transition fit would fall between a clearance and interference fit. Transition fits are called for when accurate alignment is critical, and mating parts must join with greater precision. You may also see these referred to as a slip or push fit.

There will still be a greater degree of clearance than a press/interference fit, but it will be substantially smaller and should remove excess play or movement in the joint.

FAQs.