Tolerance management
When tolerance management is a department of quality management . This forms the bridge between the requirements for production quality on the one hand and their consistent implementation along the entire process chain on the other. The goals are preventive error avoidance, the early safeguarding of geometric quality features as well as the functionality and production capability of constructions. In this way, costs can be minimized in product development and production and time can be saved without having a negative impact on the quality of the product.
Deviations from the nominal size in the manufacture of a product are unavoidable. Due to the contrary requirements of individual interest groups, such as assembly , development, suppliers, design, etc., on the product, it is important to find overall optimal solutions. The most important tool for this is the implementation of an arithmetic and statistical tolerance analysis along the product development process (PEP).
Basics
The basic principle of tolerance management is "avoiding errors instead of eliminating errors". Accordingly, possible product defects are eliminated or avoided in the course of the product development process before the start of series production. A clearly defined concept is used for this.
Reference point system
The reference point system (RPS) serves as the basis for tolerance concepts. This enables a clear and reproducible positioning of individual parts , assemblies or complete systems. As a result, the tolerances of the components can be precisely matched to one another and a continuity of the position can be achieved over the entire manufacturing / testing process.
The 3-2-1 rule
In order to be able to determine the position of a free rigid body in three-dimensional space, a reference system must be defined for it that restricts all six degrees of freedom (three translational and three rotational directions of movement). For this purpose, a rigid body usually requires six mounting points , which should be as far apart as possible in order to achieve the greatest possible stability. Exceptions are rotationally symmetrical bodies and systems with joints. Building on this, the reference point system should be used on the one hand to define a zero position for measurements and on the other hand to be used as a basis for construction and assembly concepts. The 3-2-1 rule creates a primary, a secondary and a tertiary level. The primary level is defined by three, the secondary level by two and the tertiary level by a point. The resulting surfaces should ideally be perpendicular to each other.
A general distinction is made between three types of application of the 3-2-1 rules:
- Area principle
- The pick-up points are represented by areas in the x, y and z planes and determine the position of the component.
- Hole-elongated hole principle
- A secondary and a tertiary reference is mapped by a pin in a hole (2-way reference) and the second tertiary orientation by a pin in an elongated hole (1-way reference). The elongated hole compensates for a position tolerance to other references.
- Translation-rotation-stop principle
- On the axis of rotation , the bearings each form two primary and secondary references. A rotation stop forms the third primary reference and a translation stop forms the tertiary reference.
3-2-1 rule using the area principle as an example
3-2-1 rule using the example of the rotation / translation / stop principle
In addition to the main mounting points, which guarantee the defined position of the components, there are so-called auxiliary points, which serve to reduce the instability in the component mounting in the case of pliable parts.
When applying the 3-2-1 rule, the following aspects must be taken into account:
- Ensure consistent retention of the reference points from the individual to the assembly part for the components and assemblies to be joined
- Position reference points in stable areas
- Lay out the mounting holes and surfaces parallel to the component coordinate system or to local reference points
- Changes to the component reference system are to be avoided
- Avoid positioning the components across multiple interfaces
- Allow tolerances to have an effect at those points that are neither functional nor customer-specific
- During assembly, the reference points should be easily accessible for reasons of measurement and assembly
Reference point designation
In order to be able to guarantee an exact identification of the points in referencing systems, the individual reference points are named according to DIN EN ISO 5459. Regardless of this, many companies, for example in the automotive industry, have their own reference point nomenclature.
Tolerance analysis
The tolerance analysis is an important part of the tolerance management. A distinction is made between arithmetic and statistical calculation.
Functional dimensions
Components and modules are to be dimensioned with regard to the fulfillment of superordinate quality and functional characteristics in coordination with the departments and suppliers involved. The dimensions derived from this are called functional dimensions.
The relevant functional dimensions are summarized in a " catalog of requirements ". This is used for production for statistical process control (SPC) and error analysis. Functional dimensions for individual parts or assemblies are documented in the corresponding drawings. This results in tolerance data sheets and functional dimension catalogs that can be used for coordination between development, assembly, production and suppliers.
Handling results in the product development process
With the help of tolerance management, errors can be avoided before they arise. This leads to a reduction in development, production and reworking times, to the detection of styling-related weak points, concept errors and process risks. Required quality features can be adhered to, bottlenecks and functions secured.
Quality and functional features secured by statistical calculations are used to validate the internal component and concept requirements in the product development process. The result of the tolerance management shows whether the target parameters are met with the existing assembly or construction concepts and the available component properties, or whether optimizations are necessary to achieve the target.
Influence of the quality characteristics
In addition to the functional dimensions already described, it is also necessary to understand how quality features affect the results of tolerance management. The characteristics of a quality feature result from the various requirements for a component or a component group.
From the customer's point of view , this can be:
- technical function
- Geometry (bottlenecks)
- styling
- safety
In addition, the production u. a. Requirements regarding:
- Manufacturability
- Mountability
The derivation of these customer and company-relevant requirements results in connection with tolerance management, among other things
- Joint dimensions
- Offset dimensions
- Spacing
These in turn arise from a mostly complex interplay of individual tolerances, each of which can have a decisive influence on the quality feature or on the tolerance specification. The ability to meet the tolerance specifications for the quality feature can only be achieved if the essentially effective influencing variables are known and are taken into account in the calculation. For this, the teamwork of several specialists from different areas of the product development process is of great importance.
Process participants
- design
- Development / construction
- Assembly planning / production concepts
- production
- Quality control
- Suppliers
Carrying out a tolerance analysis assumes that the development and planning department has initial concepts both in terms of construction and assembly. Only when this information is available can exact tolerance relationships be shown and meaningful analyzes carried out. Conversely, the results of the tolerance analysis are used in the early development phases to plan initial concepts, both in terms of construction and assembly, as orientation and additional support. These results are mostly based on empirical values and are usually constantly changed and adapted as part of the process cycle.
Results of tolerance management
-
The statistical calculation result
predicts the tolerance range within which 99.73% * of the results lie. -
The arithmetic calculation result
provides the tolerance value - the so-called "worst-case" - within which 100% of the results are -
The direct run quota
requires a quality specification and provides information about the percentage of the results that will be within this specification. From a statistical point of view, the difference to 100% must be corrected by reworking. -
The Pareto analysis and the identification of the main contributors
shows the percentage influence of individual tolerances on the result. This shows the main contributors. -
The concept decision and variant
calculation Different construction / assembly variants are calculated and can be compared with one another. This process contributes to the concept decision.
System sizes in the tolerance management cycle
In the following figure, all of the above-mentioned system variables are summarized and represented as a planetary gear unit with input variables, a tolerance management cycle as a processing process and output variables. The main pillars of tolerance management: the reference point system, the functional dimension catalog and the tolerance analysis drive the tolerance management as planets, in a constant working cycle. Conversely, however, this tolerance management is also influenced and, in a figurative sense, driven.
Factors influencing the result
- Optimization of the recording concepts for individual parts
- Change of the assembly or construction concept (reduction of contributors, optimization of alignment concepts, etc.)
- Adaptation / restriction of the individual tolerances (optimize the manufacturing process, etc.)
- Increase in quality capability indicators of individual contributors (process capability cp or process capability indicator cpk)
- Styling adjustment (defusing critical joints etc.)
- Expansion of the quality specification (tolerance specification)
- constructive changes (increase spacing etc.)
* Value varies according to the respective guidelines.
literature
- Martin Bohn: Tolerance management in the development process: Reducing the effects of tolerances on assemblies of automobile bodies. Dissertation U Karlsruhe 1998
- Christoph Germer: Interdisciplinary tolerance management . Logos, Berlin 2005, ISBN 978-3-8325-0954-5 .
- Bernd Klein: Tolerance management in machine and vehicle construction . Oldenbourg, Munich 2006, ISBN 978-3-486-57850-8 .
- Bernd Klein, Frank Mannewitz: Statistical Tolerance: Quality of the Structural Design. Vieweg, Braunschweig 1993, ISBN 3-528-06563-X .
- Roland Leuschel: Tolerance management in product development using the example of the body in automobile construction . Freiberg 2010, urn : nbn: de: bsz: 105-qucosa-68799 (dissertation at the TU Bergakademie Freiberg ).