Principle of independence
Tolerancing principle according to ISO 8015
The tolerancing principle according to ISO 8015 defines how the tolerance of a component is to be described. Tolerances are divided into the following dimensions:
- Shape tolerances
- Dimensional tolerances
- Position tolerances
- Surface tolerances
The definition of a tolerance can be based on either the envelope principle or the independence principle. The envelope principle according to ISO 2768 describes a line as “within tolerance” if it is located between two imaginary tolerance limit lines.
Two measurements are therefore always required to determine compliance with a tolerance according to the envelope principle. This applies in particular to fits of long components. If, for example, a profile is guided through a matching hole, the need for this double measurement becomes plausible.
The round profile can be manufactured within the required tolerance at any point. However, a bend or a deviation in roundness can mean that it can no longer be guided smoothly through the hole.
The enveloping principle, which covers the entire component – in our example the profile – is intended to rule out such a fault.
In short: What has been checked for tolerance according to the envelope principle will fit together when assembled. However, there is the restriction that the envelope principle can only be applied to cylindrical or plane-parallel shapes.
The independence principle, on the other hand, considers each dimension and its associated tolerance separately.
This approach to tolerances eliminates the double check. Each parameter is checked independently and specified separately.
In practice, however, interdependent parameters are set in relation to each other. For example, the diameter of a round profile must always be set in relation to the roundness at the selected measuring point in accordance with the principle of independence.
The independence principle according to 8015 is the standard test method used today for shape tolerances.
Unless otherwise specified on the drawing, a test according to the independence principle in accordance with ISO 8015 is sufficient to ensure component quality. To define the envelope principle as a tolerance specification, a corresponding note, for example “ISO 2768 – mK – E”, must be entered on the drawing.
Possibilities from the independence principle according to 8015
As the envelope principle can only check plane-parallel and radial shapes for tolerance according to dimension and shape, the independence principle according to ISO 8015 is much more efficient.
The dimensional tolerances can therefore also be applied to irregularly shaped components. Restrictions such as roundness or plane parallelism do not occur with the independence principle.
Deviation in Austria
In Austria, the envelope principle for tolerances is the standard. This is the exact opposite of the German system.
If a component is to be manufactured according to the shape tolerances and dimensional tolerances in accordance with the independence principle 8015, this must be explicitly entered on the drawing. The following details are possible:
It is very important to be aware of this serious deviation, as the same language can otherwise easily lead to misunderstandings when interpreting a technical drawing.
Practical implementation of the specification of tolerances according to the principle of independence
The ISO tolerance system was developed so that the tolerance does not have to be calculated and specified separately for each dimension.
This designates the exact dimension that may exceed or fall below the ideal line with a letter and number code. The specification “Ø40 H7” for a bore on a drawing produced in accordance with ISO 2768 has the following meaning:
- Nominal diameter: 40 mm
- Position of the tolerance zone H: OPEN zero line
- Fit (quality or basic tolerance grade: 7 = 0.021 mm)
A bore of this fit may only be produced exactly on the zero line or exceed it by a maximum of 0.021 mm. Undercutting (therefore ON zero line) is not permitted.
The fits are indicated with the letters A to Z and with lower or upper case letters. In addition, there are the fits ZA to ZC. The variety of possible specifications for defining a tolerance is the greatest strength of the independence principle.
ISO 2768 therefore enables the dimensions of a component to be specified with extreme precision. The envelope principle is therefore no longer required as a basis for tolerances in many applications.
Conclusion: Independence principle for shape tolerances and dimensional tolerances to be verified economically
The principle of independence according to ISO 8015 has proven itself in practice as an indication of tolerances. Due to its efficient representation, it makes design and production considerably easier.
Both the CAD programs and the production machines have the fits according to ISO 2768 included in their programs. This makes the implementation of the required tasks and the subsequent quality control particularly simple and efficient.
