ISO 14405
DIN EN ISO 14405 has been the internationally valid drawing standard for linear dimensions since April 2011.
The main difference to the previous standard is that ISO 14405 does not use the envelope principle, but only the independence principle.
If the document is to be read and understood according to a different toleration principle, this must be expressly noted.
ISO 14405
Aim of the amendment
Areas of application for measuring coaxiality
In engine construction in particular, there are numerous shafts that have an irregular contour. This can hardly be realized otherwise due to the attachment of various components or the multifunctional tasks of the component.
A turbocharger, for example, consists of a continuous shaft that must have mounting points on both sides for the drive and delivery wheels.
In view of the enormously high rotational speed that prevails in a turbocharger, high-precision coaxiality is extremely important.
Another component for which the coaxiality must be 100% correct is the camshaft. It also consists of a monolithic turned part, but this is extended by elliptically shaped bulges.
The pivot point of these cylinders with an elliptical cross-section, the so-called cams, must always lie exactly on the imaginary center line of the axis of rotation, otherwise the component is likely to fail soon.
The old envelope principle
The “envelope principle” in force until April 2011 regards the specified tolerances as the limits of an envelope within which the manufactured molded element may move.
This normally applies to every tolerance specification. It becomes problematic if several tolerances are to apply to the same form element:
A hole has a tolerance specification in position, dimension and shape. The position tolerance specifies how far the hole should be from another dimension point, such as another hole or an edge.
As a rule, at least two specifications (x and y) are required. Secondly, the size of the bore must be specified. Thirdly, the shape of the bore (e.g. cylindricity) is an important specification for the manufacturer.
The envelope principle summarizes all three specifications and states that the envelope defines the limit range within which all tolerances relating to position, dimension and shape are allowed to move.
This can lead to considerable problems with fits, especially if the components to be assembled come from different manufacturers. For this reason, ISO 14405 was established.
Independence principle creates improved precision
It doesn’t take much imagination to realize that, for example, the size of a bore does not necessarily correlate with its shape: If a bore is outside the dimension but within the tolerance, but at the same time not circular but elliptical, this can be problematic for all kinds of fits.
For this reason, the approach “If nothing else is specified, the envelope principle applies” has been abandoned and replaced by “If nothing else is specified, the independence principle applies”.
The principle of independence considers each tolerance specification in isolation. The all-encompassing envelope no longer exists in this form. For each tolerance specification, a separate tolerance range that applies only to it must be defined and adhered to in production.
However, this regulation can be changed again by making a corresponding entry on the drawing or on the dimensioning of the form element itself.
Marking of the tolerance specifications on the form element
The standard specifies a series of assignment operators in order to correctly dimension a shaped element in accordance with ISO 14405.
The assignment operators are specified in an oval bordered text field and clearly define the meaning of a dimension.
If this information is missing, the two-point dimension automatically applies in accordance with ISO 14405. The two-point dimension can also be named separately to emphasize its application.
The assignment operators according to ISO 14405 are:
Two-point dimension LP
The two-point measurement requires that the diameter of a hole must be measured twice.
Ideally, the largest and smallest diameters of the bore are determined and checked to see whether both dimensions are still within the desired tolerance.
This dimension is particularly useful for bores that are not linear to the Z axis, but require additional movement of the tool in the X and Y axes. The two-point dimension has been criticized for being difficult to implement, but is still valid.
Gaußelement GG
The mapping operator “Gaussian element” of ISO 14405 is also called “mean dimension”. The application method is the “method of least squares” (MKG).
This method, which has been practiced for 200 years, always provides a clear measurement result with minimal measurement uncertainty. GG is particularly easy to implement in practice, especially for software-based measurements.
Horse element GX
The detected element is assigned to an ideal element. This ideal element is located in its perfect geometry within the captured geometry.
Envelope element GN
The envelope element serves as the counterpart to the pen element. In its ideal form, it encompasses the captured geometry.
Pen ball measure LS
According to ISO 14405, the pen ball dimension is the spatial extension of the pen element. It is the local dimension of a ball fitting.
The pen ball dimension is mainly used for drawing flexible pipes, hoses and ball joints.
Envelope condition E
The traditional envelope condition is not invalidated by ISO 14405, it is simply no longer given priority.
Where it makes sense, it remains permissible. The envelope condition is based on the two-point dimension and states that it must enclose a maximum material, inviolable dimensional envelope around the form element. Its most common application is a high-quality clearance fit.
Any cross-section ACS
ACS, “Any Cross Section” is based on the Gaussian element and transfers it to the depth of a longer, cylindrical bore cross-sectionally along a defined dimensional section. Instead of ACS, GX can also be extended with /0 in accordance with ISO 14405.
Any longitudinal section ALS
ALS is the counterpart to ACS. The geometric element is defined according to ISO 14405 in its depth along its cross-sections. It is the three-dimensional extension of the two-point dimension.
Special cross-sections SCS
SCS is a specification of ACS. Instead of tolerating all arbitrary cross-sections, SCS specifies defined cross-sections in which the given tolerance must apply. The distance between these cross-sections must be specified in the dimensioning.
Special longitudinal sections SLS
SLS is a specification of ALS. Instead of tolerating all arbitrary longitudinal sections, SLS in ISO 14405 specifies defined longitudinal sections in which the given tolerance must apply. SLS is indicated by arrows and angles.
Length restriction /xx
In ISO 14405, the length restriction, marked by a slash followed by a dimension, defines the section within which the tolerance applies.
If the length restriction is located on any section in the depth of a shaped element, i.e. does not start or end at its edge, the angle brackets are used to define the measuring section. The measuring points are additionally marked with A and B in accordance with ISO 14405.
Measuring and evaluating with ISO 14405
ISO 14405 also specifies methods that enable a statistical evaluation of the applied tolerances. Various measuring methods can be used to check compliance with the tolerances. These are particularly popular:
This is a very effective basis for quality control. In addition to the absolute tolerance at the defined point, a statement can also be made about the general course of a geometry. The measuring methods are
Largest/smallest value SX/SN
The form element is examined along its depth or length in cross-section or longitudinal section. The largest or smallest value is documented.
Arithmetic mean SA
The values determined from SX and SN can be summarized to SA using the arithmetic mean.
The tolerance can be reduced to SA, so that larger deviations are possible. This is used in flow systems, for example. However, SA is generally not applicable for fits.
SR span
The statistical range examines how far apart the minimum and maximum values of a measurement series are.
Span center SD
The range specifies how far apart a maximum and minimum value may be along a series of measured values.
The span center refers the span to a range of three fixed cross-sections.
Assignment operators for multiple form elements: x N
If a tolerance specification in accordance with ISO 14405 is to apply to more than one form element, the drawing can be simplified with a positive, integer factor before the dimensioning.
The prerequisite is that the shape elements are the same size and shape and are only repeated along a line (lengthwise, crosswise, diagonal) or an edge (circular, angular).
Common tolerance zone CT
If a group of differently designed form elements are to be considered with the same tolerance specification, they can be combined into a tolerance zone CT (Common Tolerance) in accordance with ISO 14405.
Free state F
The F condition (Free) requires a measurement of the manufactured part in an unstressed state. This is particularly useful for flexible materials such as rubber, etc. The F condition is therefore often used for sealing rings.
