Shape and position tolerance
The shape and position tolerances are a sub-area of the geometric product specification ( GPS , English Geometrical Product Specification) and offer the possibility of tolerating the permissible geometric deviation of components by means of drawing entries. As a result, lower manufacturing costs can be achieved than with narrower dimensional tolerances without shape and position tolerance. This drawing entries and symbols are governed by the standard ISO 1101. In North America, these rules include GD & T (abbr. Of English Geometric Dimensioning and Tolerancing) and are by Geometric Product Specification standards ASME defined Y 14.5.
A distinction is made between shape, direction, location and running tolerances.
Symbols and their definition
Abbreviation: t = tolerance value (2 × deviation); Ø = diameter; Δr = difference between the two radii
| German name | symbol | definition | English name | Symbol (unicode) | |
|---|---|---|---|---|---|
| shape | Straightness |
|
For surfaces: The tolerated line must lie in each plane between two parallel straight lines with a distance t.
for axes: The toleranced axis must be in a cylinder (?? Ø = t). |
Straightness | ⏤ |
| Flatness |
|
The tolerated area must lie between two parallel planes (distance t). | Flatness | ⏥
U + 25B1 |
|
| Roundness |
|
The tolerated circumferential line must be perpendicular to the central axis between two concentric circles (Δr = t) in all cutting planes. | Circularity |
○
U + 25CB |
|
| Cylindricity |
|
The toleranced surface area must be between two coaxial cylinders (Δr = t). | Cylindricity |
⌭
U + 232D |
|
| Profile of a line |
|
The tolerated profile must lie in each plane between two equidistant envelope lines , the distance between which is defined by circles (?? Ø = t). | Profile of a line |
⌒
U + 2312 |
|
| Profiles of a surface |
|
The tolerated surface must lie between two equidistant envelope surfaces , the distance between which is defined by spheres (?? Ø = t). | Profile of a surface |
⌓
U + 2313 |
|
| direction | parallelism |
|
For surfaces: The toleranced surface must lie between two planes (distance t) which are parallel to the reference .
for axes: The toleranced axis must lie in a cylinder (?? Ø = t) whose axis is parallel to the reference. |
Parallelism | possibly U + 2225 |
| Squareness |
|
For surfaces: The toleranced surface must lie between two planes (distance t) which are perpendicular to the reference.
for axes: The tolerated cylinder axis must lie in a cylinder (?? Ø = t) perpendicular to the reference surface. |
Perpendicularity |
⟂
U + 27C2 |
|
| Angularity |
|
For surfaces: The toleranced surface must lie between two planes (distance t) which are inclined at the specified angle to the reference.
For axes: The toleranced axis must lie between two parallel planes (distance t), which are inclined at the specified angle to the reference. |
Angularity |
∠
U + 2220 |
|
| place | position |
|
The center of the hole must be in a square (a = t), the center of which corresponds to the theoretically exact position of the hole.
with Ø symbol: The center point of the hole must lie in a circle (?? Ø = t), the center of which corresponds to the theoretically exact position of the hole. |
position |
⌖
U + 2316 |
| Concentricity
Coaxiality |
|
The center of the tolerated circle must lie in a circle (?? Ø = t), the center of which is concentric to the reference.
The axis of the toleranced surface must lie in a cylinder (?? Ø = t), the center axis of which is coaxial to the reference. |
Concentricity |
◎
U + 25CE |
|
| symmetry |
|
The tolerated center plane must lie between two parallel planes (distance t), which are symmetrical to the reference. | Symmetry |
⌯
U + 232F |
|
| Run | Concentricity (radial)
Axial runout |
|
With one revolution around the reference axis, the runout deviation must not exceed t.
With one revolution around the reference axis, the axial runout may not exceed t. |
Circular runout |
↗
U + 2197 |
| Total concentricity (radial)
Overall axial runout |
|
With multiple revolutions around the reference axis and simultaneous axial displacement, the concentricity deviation must not exceed t.
In the case of multiple rotations around the reference axis and simultaneous radial displacement, the axial runout may not exceed t. |
Total runout |
⌰
U + 2330 |
Remarks
- ↑ Straightness on surfaces is measured in the direction of the line in the specified view. Surfaces can be straight in the X-axis although they are curved along the Y-axis .
- ↑ a b The center of the circle or sphere lies on the ideal line or surface.
- ↑ Square aligned according to theoretically precise dimensions
- ↑ Ø symbol in front of the tolerance value (see tolerance frame (picture))
- ↑ a b The coaxiality cannot be measured if the length of a cylindrical body is too short.
Drawing information
If no geometric tolerance is specified on the drawing, the values in the tolerance table according to the ISO 2768-2 standard generally apply .
Tolerance framework
The shape and position tolerances are indicated on the drawing in a tolerance frame. The specified tolerance value describes the entire width of the tolerance zone. In most cases, the maximum permissible deviation is only half in practice (tolerance zone Ø = 0.04, deviation r = 0.02). A reference arrow connects the tolerance frame on the left and / or right side with the tolerated element. It can split and point to multiple edges of material or extension lines to efficiently tolerate multiple elements. Depending on the position of the arrowhead, the area or center plane is meant (for round components: the lateral surface or center axis). If it is on the same level as an adjacent dimension, the center plane / axis is meant. The same rule also applies to payments.
reference
A reference is always a plane or straight line (axis) and is required as a reference for measurements. Often it is sufficient to press the reference surface with a certain force against a reference surface, but with very tight tolerances the reference surface must be measured and all roughness and unevenness removed using a measuring program. Especially when using several covers, where the component could stand out from the first cover when pressed against the third cover, it is important to minimize the human factor .
The order in which the reference elements are listed in the tolerance frame determines their priority. They are therefore not necessarily sorted alphabetically, but are simply arranged by indicating the references on the component for easier readability. The number of reference elements required depends on the type of tolerance. In some cases, however, more can be specified, so an area can be tolerated at right angles to A and B with just one tolerance frame.
For example, three references are specified for the position tolerance:
- the first can be fully defined using the three-point support.
- the second can only be defined with two points, since it must be at right angles to the first reference.
- the third can only be defined at one point, since it must be at right angles to both of the other references.
With the exception of shape tolerances, which apply in themselves, all symbols listed in the table above require at least one reference.
Multiple reference
In the case of a multiple reference, the specified references form a common reference which is used as a reference. This is particularly useful for elongated components with bearing surfaces at both ends. A small angle error in the individual axes can lead to a high running deviation, which is eliminated during installation by the two-sided mounting. With a common reference axis, the component is measured closer to the real installation situation.
Theoretically exact measure
The exact position of the tolerance zone without tolerances is determined by a theoretically exact dimension. It is given its own tolerance frame on the drawing, which excludes it from the previously defined general tolerance, i.e. quasi ± 0. Since a tolerance of zero is invalid, all holes, edges, etc. require their own position tolerance to a theoretically exact dimension. The same holes in a hole pattern are an exception .
literature
- Metal table book (without formula collection), ISBN 978-3-8085-1678-2 .
- Hans Hoischen / Andreas Fritz: Technical drawing , 36th edition, Cornelsen 2018, ISBN 978-3064517127 .