Engineering tolerance

The tolerance refers to the state of a system in the (still) makes a disturbing influence caused by a deviation from the normal state no counter- regulation or countermeasure necessary or a result. In a narrower sense, tolerance is the deviation of a variable from the standard conditions or standard size that is just not jeopardize the proper functioning of a system.

Manufacturing tolerance

By a suitable definition of the tolerances of related parts a complete interchangeability of every part, and thus the mass production and mass production is possible. Tolerances also permit targeted to achieve a desired clearance or interference fit ( interference fit ) between two parts.

For the production, the tolerance is not possible to be determined by the Committee - side. As manufacturing nominal size can, for example, a value at the upper limit of size, the maximum or maximum dimension, chosen to be very close within the tolerance ( for waves), which still allows the removal of material within the tolerance. When drilling the actual size may be closer to the lower limit of size, the minimum and minimum size, are to still remain at a later reworking in tolerance. This has the advantage that the predetermined by the designer to be observed and the function of the component tolerance of a non-attainment of the design nominal dimension, the workpiece in question may optionally be refinished better use of the safe side and.

Other options are suitable shaping ( rounded or sharp contacts), guide pins and guide rails, slots, adjustment and calibration equipment and the like.

Dimensional tolerance

The tolerance or " acceptable deviation " from the mean is a design and manufacturing -related yardstick. It refers to the difference between the upper and the lower limit of size, ie the maximum and the minimum. Within the tolerance the actual size of a workpiece or component from the corresponding nominal dimension ( zero line ) may vary.

Dimensional tolerances thus limit the permissible deviation of component dimensions.

Dimensional tolerances can be differentiated according

  • General tolerances,
  • ISO tolerances and
  • Freitolerierten dimensions

Differ.

Lengths and angles

General tolerances for linear and angular

General tolerances for linear and angular (ISO 2768-1 ) apply to all not separately tolerated dimensions and angles of a drawing. The general tolerances are divided into classes. The title block of a technical drawing is 2768- m set with the abbreviation ISO tolerance for the entire drawing. Furthermore, additional allowances for certain dimensions can then be entered in the technical drawing. The general tolerances are divided into:

  • F ( f) fine - for example, precision engineering
  • M (m) medium - for example, mechanical engineering ( workshop usual accuracy)
  • C ( g) coarsely - eg foundry technology
  • V ( sg ) very coarse - This tolerance class is used now little more, as the most modern manufacturing processes allow higher accuracies.

ISO tolerance system

ISO tolerance systems are for fits ( ISO 286, DIN 7154 and DIN 7155 ) and fit data to ISO. ISO tolerances with their defined tolerance classes (location and size of a tolerance zone ) should be only used in special function and Passungsforderungen.

Form and position tolerance

Form and position tolerances, with the help of the finished shape of a work piece in assembly or function context is tolerated.

Information

As tolerances can be specified as a factor of the setpoint, as maximum or minimum deviation from setpoint or certain permitted range.

Tolerances may be symmetrical, eg, 40 ± 0.1, or asymmetric, eg 40 0.2 / -0.1, can be specified.

General tolerances of form and position

General tolerances of form and position (ISO 2768-2 ) apply to all non- separately specified form and position tolerances in a drawing and are indicated above the signature field in the so-called drawing header. This shall not apply to the following characteristics: slope, position, total run -out, concentricity, surface shape, line and form a cylindrical shape.

Specific shape tolerances

Specific form tolerances limit the permissible deviation of an element of its geometrically ideal form. You determine the tolerances within which the element must be and must not have any shape and are indicated in the drawing. The shape tolerances include straightness, flatness, roundness, cylindricity, line profile and the surface profile. (Note: line and surface profile are to be regarded only as a form unrelated tolerance with respect is positional tolerances. . )

  • Symbols for form tolerances in the Technical Drawing

Flatness

Roundness

Cylindrical shape

Line profile

Surface profile

Rectitude

For flat components, the tolerance zone is limited by the distance T in the measuring plane by two parallel lines. In round components we distinguish two cases:

  • Case 1: The reference arrow points to the arrow head of the tolerance of the diameter dimension of the cylinder. The axis of the cylinder must be located within a cylinder of diameter t.
  • Case 2: the reference arrow on the outer surface of the cylinder. Each surface line of the toleranced cylindrical surface must lie between two parallel straight lines a distance t apart.

Specific position tolerances

Specific positional tolerances restrict each other, the maximum deviations from the ideal position of two or more elements or levels, of which usually a reference is fixed. They are indicated in the drawing. Position tolerances are: parallelism, perpendicularity, position, coaxiality, concentricity, symmetry and concentricity tolerances: runout, axial runout and total runout tolerances: Total runout and total runout.

  • Symbols for geometrical tolerances in the Technical Drawing

Squareness

Inclination

Position

Concentricity and coaxiality

Symmetry

Concentricity and axial run-out

Total runout and total runout

Tolerance principle

Note: For subscriptions, without specifying the tolerance principle of the date of origin is noted or if in doubt the creator. For drawings before 2011 was still DIN 7167 ( envelope requirement without drawing entry). For new subscriptions (according to ISO 14405-1 ) the independence principle according to ISO 8015th

Hüllprinzip (DIN 7167 (withdrawn) )

The form element must be made ​​within the geometrically ideal envelope. For a document, such as a drawing, which is passed from the customer to the supplier and the envelope requirement specifies a tolerance principle, the following applies:

  • Each cylinder shape, and all are of the opposite parallel surfaces of envelope if they are dimensioned.
  • The geometric ( shape ) deviations must be within the specified dimensional tolerances.
  • From the shape and position tolerances parallelism are on the envelope requirement only ( indirectly: flatness, straightness) and the cylindrical shape ( indirectly: straightness, circularity ) covered. Different shape and position tolerances must also be specified. (Note: the Hüllprinzip according to DIN 7184-1 (withdrawn, predecessor of DIN 7167 ) also included squareness )

According to DIN 7167 automatically was the envelope requirement when was (eg a drawing ) is entered in a document no tolerance principles. For you to override the envelope requirement, it was necessary to comply with EN ISO 8015 specify the document.

Since April 2011, however, the DIN 7167 has been withdrawn and replaced by EN ISO 14405. This stipulates that by default, the independence principle according to EN ISO 8015 applies. The Hüllprinzip so must now be specially marked, if it is to be applied, preferably with "Size ISO 14405 E " above the title block; alternatively: "DIN 7167 " or General tolerances according to "ISO 2768 - mK - E" by the abbreviation " E" ( see ISO 2768-2 ).

Independence principle (EN ISO 8015 )

Dimensions, shape, position and surface tolerances are to be considered independently. If, the independence principle was expressed as tolerance principles in a document, such as a customer's drawing, then the drawing header is the norm EN ISO 8015, which sets the envelope requirement repealed. In this standard, only the main geometry of the elements to be packed in a wrapper. This is done by the entry of an "E " to the respective measure for which the envelope requirement should apply.

According to ISO 14405-1, the independence principle applies when a document does not mention any tolerance principles. Since this regulation is the exact opposite to the old regime, drawings should be labeled according to the principle of independence with "Size ISO 14405 " or " ISO 8015 ". Old alike important information: " tolerance DIN 2300 " and " toleration ÖNORM M 1300 " ( Austria ).

Fit data according to ISO

The system of fit data in accordance with ISO is also called IT system. IT in this context means: " ISO tolerance ".

The Tolerance value with the tolerance abbreviation ø30 H7 in a technical drawing (dimensions entry to DIN 406-12 ) means:

  • Nominal size of the bore diameter 30 mm
  • Fit system: One hole
  • Tolerance position H ( zero line)
  • Standard tolerance grade (Quality ): 7

For ø30, this corresponds to:

  • Minimum 30,000 mm ( this measure must not be exceeded)
  • Maximum 30.021 mm ( this measure may not be exceeded )
  • Size of the tolerance zone 21 microns

Specifying ø30 m6 means analog:

  • Nominal size of the shaft diameter 30 mm
  • Tolerance zone position: m (above the zero line )
  • Standard tolerance grade (Quality ): 6

For ø30, this corresponds to:

  • Minimum 30.008 mm ( this measure must not be exceeded)
  • Maximum 30.021 mm ( this measure may not be exceeded )
  • Size of the tolerance zone 13 microns

Both components together would result in a transition that can be mounted in general still without special equipment.

Definition of tolerance

The tolerance is determined at the design of a component and specified in the design and manufacturing documents. You can, below or on either side of the nominal dimension. The designer specifies the tolerance directly in numbers to the nominal dimension to or uses depending on the tolerance of fits standardized symbols in the measurement method. For the analytical determination of the tolerances, the tolerance analysis and tolerance synthesis is used.

Even with nominal dimensions without direct tolerance indication ( clearances ) tolerances or specifications must be observed for dimensional accuracy, which must be considered when designing accordingly. Therefore, information on the general dimensional accuracy and surface quality are entered in the text field of technical drawing, while specific information on specific tolerances or surface finish are entered directly in the drawing.

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