Specification of Isostatisms and Checking Points

To prepare correctly the quotation for the production of a part or of a checking fixture, the technical specifications must indicate the isostatisms and checking points, areas or characteristics.

It is not sufficient to tell us: “Look, here is the part, and I want a checking fixture”. Each part has its characteristics and problems, and it is crucial to have them specified when holding and checking the part.

This example is the only documentation received from a customer for the preparation of a quotation, we were sent a 3D plan with an email saying: “red areas = fixation; green areas = check.”

This type of 3D is fine to specify clearly the fixation and checking areas to be taken into account, but it is not sufficient: we need to know if the customer has already thought about how to hold his part in a clearer way than with a red mark! In the same way, we need an explicit identification of the points to be checked, to avoid confusions and omissions in the quotation, that will result into further problems once the project has begun.

The solution? A mere document explaining in written the characteristics the customer wants:

• ISOSTATISMS:
  - Holding and fastening on RPS1
  - Centring in RPS2
  - Locating in RPS3
• CHECK:
  - Position points 1 and 2
  - Surface 3

etc.





On the contrary, this example provides us with a very clear vision of the customer’s requirements: the green squares detail the part isostatisms, while the numbers set the different checking points. Next to this picture were two paragraphs indicating on the one hand the isostatisms and the part fastening method, and on the other hand the checking points and measurement methods.

Technical specifications total pages number: 1, and it is more than enough.

What is important is not to have very long technical specifications, specifying all the standard points required by the final customer, but to only have a few clear and useful elements:

• Part isostatisms (and fastening method if you already have an idea of the solution)
• Checking points (and checking solution if you have already thought about one)
• Material if you require something different from the aluminium and steel that is usually used in those cases.

The key is to take well into account this data before writing them, and to study the potential problems the part may trigger so as to not check whichever characteristic, but only the important ones. Checking fixtures are expensive investments and have to be well thought about to be as efficient as possible.

When we explain this to our customers, some of them do not understand our reaction, and we get comments such as: “But this is your job, not mine”, or “I do not see why I should know how to fasten and check the part!” We do not ask them to describe in detail the checking fixture design process, but to write into two or three lines more information about what they want.

But in the end, customers see that when they detail well these points, they enable to limit the risks of misunderstanding, omissions and mistakes during the fabrication of the checking fixture, and this is as much important for the customer as for his provider

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