Dimensioning and tolerancing leave a lot to be desired! I.e. the technical drawings are not unambiguous. Experience shows that the average costs involved amount to as much as 20% of the production turnover.

The reason is not that the designer does not know what he or she wants, or that the workshop does not know how to comply with the drawing. The reason is lack of effective communication, resulting in misunderstanding from idea to the real thing. The poor communication arises from the fact that the parties do not know the "grammar" of the drawing and in particular from the fact that the available standards have not adequately kept pace with development.

The technical drawing serves its purpose only if it is unique and results in the production of one single type of identical products with one single type of functional characteristics. The designer is responsible for ensuring that the technical drawing is unambiguous. The standardizers are responsible for ensuring that the designer has available to him the proper tools - namely the standards.

There is an apparent need for improving the communication between the designer and the workshop. Several elements of the communication can be improved:

  1. The human understanding and knowledge of the symbol language used on the drawing which expresses the functional characteristics of the workpiece by means of geometry.
  2. Drawings shall be made more precise and unique which means that they shall specify all requirements that are essential for the function.
  3. A complete, highly developed, systematic and standardized "language" is needed to express and translate the function of the workpiece into geometrical requirements on the drawing.

re 1):

The first improvement can be obtained only in part through improved and more deliberate standardization and will therefore to an increasing extent have to be based on more stringent requirements from the industry to the higher technical educational institutions.

re 2):

The second improvement would be an urgently required updating and modernizing of the existing standardized dimensioning and tolerancing system available. Technological development since the Second World War has practically not been taken into account.

Surveys show that dimensional tolerances (e.g. diameters, distances, etc.) as a main rule are reduced by an average factor of 10 over 50 years and this law of reduction can be traced back 200 years. It will probably not change - on the contrary. The present and future production and process methods can control these tolerances as well as the required measurements - during and after production - which are necessary for obtaining the smaller and smaller tolerances.

Other geometrical characteristics, such as form, orientation, location and run-out and the macro and micro geometrical form characteristics of surfaces, are quite another story. These characteristics cannot be controlled during the process as they usually depend on parameters which cannot be controlled during the process. It is often the choice and order of the process and the material which have a decisive influence. Today the resulting deviations are relatively larger than the dimensional deviations, the difference being greater than before - the deviations are in fact so considerable that they obstruct the function of the workpiece as well as dimensioning and tolerancing. Deviations of form must consequently be limited to a higher degree than before by means of tolerances to ensure correct function of the workpiece and the relevance to dimensioning and tolerancing.

The situation is that a marked shift has taken place between dimensional deviations and deviations of form, orientation, location and run-out. The problem is that the way of drawing and tolerancing has not changed. The ISO dimensioning and tolerancing system is based on the old ISA system which dates back to the 1940s, the ISO roughness system is from the 1950s. And because the ISO dimensioning and tolerancing system works only on theoretically correct workpieces - and not if deviations of form occur - the result is a drawing which does specify sufficiently precisely what is required of the geometry to obtain the desired function. As a consequence, far too much is left to the (random) decisions of other departments within the organization than the design/engineering department.

It is therefore perfectly permissible to maintain the following:

  • a) It is not the designer who decides the end function of the workpiece;
  • b) It is the measuring and gauging people who decide the end function of the workpiece by their (random) choice of measuring methods and equipment or the function is decided by the software integrated into the equipment;
  • c) Where coincidence reigns, quality cannot be controlled;
  • d) Subcontractors are at a loss.

re 3):

The task of standardization is to provide design/workshop/quality management with the tools necessary for producing workpieces with optimum function and the characteristics desired by the designer. The tools are the following:

  • Unambiguous and precise rules for "translation" from function to "geometry".
  • Elaborate rules allowing all specific "geometry" requirements to be expressed precisely and unambiguously.

Optimum implementation of such standards is conditional upon the following:

  • The designer knows the problems and the standards and learns the "geometry language" to perfection.
  • Personnel involved in production, inspection and measuring also know the problems and the standards and they know the "geometry language", offering them the possibility of producing the desired geometrical characteristics.

If the tools or any of the other elements fail - the system falls apart. Failure is most often caused by:

  • Limited knowledge and understanding of the correlation between workpiece function and workpiece geometry
  • Limited knowledge of the possibilities of expression and of essential details of the standardized drawing system
  • Limited knowledge of the "geometry language" on the part of workshop, inspection and measuring personnel
  • An inadequately standardized drawing system.

The fault on the part of the standards is that until now the characteristics indicated on drawings were only defined in theory. As soon as the real life reveals itself with e.g. deviations of form etc., the theoretical definitions no longer apply. The requirements lose their meaning or become "opaque" and/or ambiguous.

The immediate result of the failure of the standardized tolerance definitions in the "real world" is that in practice the choice of measuring principle and/or equipment will decide which result is obtained by the measurements. In addition, the value of the measurement result will in many cases be heavily influenced by the errors of form etc. of the workpiece - with an influence from other geometrical characteristics than the characteristic which is actually measured.

As a consequence, tolerances on drawings are not necessarily unambiguous. The function of workpieces is not under control. It means in practice that it is necessary to apply far too narrow tolerances to ensure the function of the work piece. If unambiguity is achieved between the indications on the drawing and the verification result, all tolerances could be considerably larger - and a lot of money could be saved.

Hans Henrik Køster
Secretary of ISO/TC 213

Updated 1996-07-01