The tolerance describes the state of a system in which a deviation from the normal state caused by a disruptive effect does not (yet) require or result in any counter-regulation or countermeasures. In a narrower sense, tolerance is the deviation of a variable from the standard condition or standard dimension that just does not endanger the function of a system.
A suitable definition of the tolerances of parts that belong together enables complete interchangeability of each part and thus series production and mass production . Tolerances also make it possible to achieve a desired clearance or interference fit (interference fit) between two parts in a targeted manner.
For production , the tolerance should, if possible, not be determined on the reject side. As a nominal production dimension, for example, a value within the tolerance very close to the upper limit dimension , the maximum or largest dimension, can be selected (for shafts), which still allows the removal of material within the tolerance. In the case of bores, the actual dimension can be closer to the lower limit dimension, the minimum or smallest dimension, in order to remain within the tolerance for subsequent rework. This has the advantage that the tolerance specified by the designer and to be maintained for the function of the component can be better utilized from the safe side and, if the nominal design dimension is not reached, the workpiece in question can be reworked if necessary.
Further possibilities are suitable shaping (rounded or pointed contacts), guide pins and guide rails, elongated holes , adjustment and calibration devices and the like.
The tolerance or "permissible deviation from the nominal dimension " is a dimension that is determined by design and production. It denotes the difference between the upper and the lower limit dimension , i.e. the maximum and the minimum dimension. The actual dimension of a workpiece or component may deviate from the respective nominal dimension (zero line) within the tolerance . Dimensional tolerances thus limit the permissible deviation in component dimensions.
Dimensional tolerances are divided into:
- General tolerances,
- ISO tolerances and
- freely tolerated dimensions
General tolerances for lengths and angles
General tolerances for lengths and angles ( ISO 2768-1 ) apply to all dimensions and angles in a drawing that are not separately tolerated. The general tolerances are divided into classes. In the title block of a technical drawing , the ISO 2768-m code is used to define the tolerance for the entire drawing. In addition, further tolerances for certain dimensions can then be entered within the technical drawing. The general tolerances are divided into:
- f (f) fine - e.g. B. Precision engineering
- m (m) medium - e.g. B. Mechanical engineering (standard workshop accuracy)
- c (g) coarse - e.g. B. Foundry technology
- v (sg) very coarse - This tolerance class is rarely used today, as modern manufacturing processes usually allow higher levels of accuracy.
ISO tolerance system
ISO tolerance systems apply to fits (ISO 286, DIN 7154 and DIN 7155 ) and fit specifications according to ISO . ISO tolerances with their defined tolerance classes (position and size of a tolerance field) should only be used in the case of special functional and fit requirements.
Free tolerated dimensions
The free tolerance can be specified on the drawing according to three different systems:
- Dimensions (asymmetrical): + 0.1 / -0.2; or:
- Dimensions (symmetrical): ± 0.1
The maximum size, or the size with which the maximum size can be achieved, is always at the top / in the first place. The limit dimensions replace the nominal dimension on the dimension line, whereas the deviation is behind the nominal dimension. In the case of asymmetrical dimensions, both tolerance values may have the same sign or even be equal to zero.
Shape and position tolerance
Shape and position tolerances with the help of which the finished shape of a workpiece is tolerated in the assembly or functional context. As a result, lower manufacturing costs can be achieved than with narrower dimensional tolerances without shape and position tolerance.
Specific shape tolerances
Specific shape tolerances limit the permissible deviation of an element from its geometrically ideal shape. You determine the tolerance range within which the component must lie. The shape tolerances include: straightness , flatness, roundness, cylindrical shape , line profile and the surface profile. (Attention: Line and surface profiles are only to be assessed as form tolerance without reference. With reference, it is positional tolerances.)
Specific position tolerances
Specific position tolerances limit the permissible deviations from the ideal position of two or more elements to one another. The positional tolerances include: parallelism, perpendicularity, inclination, position, coaxiality, concentricity, symmetry as well as the runout tolerances: concentricity, axial runout and the total runout tolerances: total concentricity and total axial runout.
Attention: In the case of drawings without an indication of the tolerance principle, the time of creation must be observed or, if in doubt, the creator must be asked. For drawings before 2011, DIN 7167 still applied (envelope requirement without drawing entry). The independence principle according to ISO 8015 applies to new drawings (according to ISO 14405-1).
Envelope principle [DIN 7167 (withdrawn)]
The finished form element must lie within the geometrically ideal shell. For a document, e.g. B. a drawing that is passed on from the customer to the supplier and defines the envelope condition as a tolerance principle, the following applies:
- Every cylindrical shape and all opposing parallel surfaces are subject to the envelope condition, provided they are dimensioned.
- The geometric (shape) deviations must lie within the specified dimensional deviations.
- Of the shape and position tolerances, only the parallelism (indirect: flatness, straightness) and the cylindrical shape (indirect: straightness, circular shape) are covered by the envelope condition. Deviating form and position tolerances must also be specified. (Note: the envelope principle according to DIN 7184-1 (withdrawn, predecessor of DIN 7167) also included perpendicularity tolerances)
According to DIN 7167, the envelope condition automatically applied if no tolerance principle was entered in a document (e.g. a drawing). To override the envelope condition, it was necessary to specify the EN ISO 8015 standard in the document.
Since April 2011, however, DIN 7167 has been withdrawn and replaced by EN ISO 14405. This stipulates that the independence principle according to EN ISO 8015 applies as standard. The envelope principle must now be specially marked if it is to be used, preferably with "Size ISO 14405 E" above the text field; alternatively: "DIN 7167" or for general tolerances according to "ISO 2768 - mK - E" with the appended "E" (see ISO 2768-2).
Principle of independence (EN ISO 8015)
Dimension, shape, position and surface tolerances are to be considered independently of one another. If in a document, e.g. B. a customer drawing, the independence principle was specified as a tolerance principle, then the standard EN ISO 8015 is in the drawing header, which overrides the envelope condition. In this standard, only the most important geometrical elements are packed into an envelope. This is done by entering an "E" for the respective dimension for which the envelope condition is to apply.
According to ISO 14405-1, the independence principle applies if a document does not mention a principle of tolerance. Since this regulation is the exact opposite of the old regulation, drawings should be marked with “Size ISO 14405” or “ISO 8015” according to the principle of independence. Old synonymous information: "Tolerance DIN 2300" and "Tolerance ÖNORM M 1300" (Austria).
Fit information according to ISO
The system of fit specifications according to ISO is also called the IT system. IT means: "ISO tolerance".
- Nominal dimension of the bore diameter 30 mm
- Fit system: standard bore
- Tolerance field position: H (on zero line)
- Basic tolerance level (quality): 7
At ø30 this corresponds to:
- Minimum dimension 30,000 mm (this dimension must not be undercut)
- Maximum dimension 30.021 mm (this dimension must not be exceeded)
- Size of the tolerance field 21 µm
The specification of ø30 m6 means analogously:
- Nominal dimension of the shaft diameter 30 mm
- Tolerance field position: m (above the zero line)
- Basic tolerance level (quality): 6
At ø30 this corresponds to:
- Minimum dimension 30.008 mm (this dimension must not be undercut)
- Maximum dimension 30.021 mm (this dimension must not be exceeded)
- Size of the tolerance field 13 µm
Both components together would result in a transition fit that can usually be assembled without any special aids.
Establishing the tolerance
The tolerance is in the structure determined of a component and defined in the design and manufacturing specifications. It can be above, below or on both sides of the nominal dimension . The designer specifies the tolerance directly in numbers for the nominal dimension or, depending on the fit system, uses standardized tolerance symbols in the dimensions . Tolerance analysis and tolerance synthesis are used for the analytical determination of tolerances .
Even with nominal dimensions without direct tolerance information (free dimensions), tolerances or specifications for dimensional accuracy must be adhered to, which must be taken into account when designing. For this reason, information on general dimensional accuracy and surface quality are entered in the title block of the technical drawing, while specific information on special tolerances or surface quality must be entered directly in the drawing.
In addition to the manufacturing tolerances, tolerances of the environmental conditions, such. B. Determine operating temperature or humidity in order to avoid dangerous situations and their risks and to take safety measures into account during construction .
In the vehicle industry, the effects of the individual tolerances on the overall system are examined and statistically evaluated as part of tolerance management . This reduces the error rate in the early phases of the development process.
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