Alignment systems: “geometric” or “three centres”
The referencing or alignment of a checking fixture can be done in two different ways: geometric system or 3 points system.
- GEOMETRIC -
This term refers to the alignment system composed of plan, straight line, point. On a mathematic point of view, this system is the most correct. For this system, you need to have enough elements to create a plan, a straight line and a point, and to define their nominal position. When using three sights or three rectified bushings, the centres of these elements have to be measured; the plan will be made thanks to these three elements, the straight line thanks to two of them, and finally one of them will be used as a reference point.
Normally, when making a checking fixture, we try to have the base plate parallel to the working axes; to do so, it should be sufficient to indicate the reference point position in X,Y,Z, to which plan “pln ref” is parallel, and to which staight line “rec ref” is parallel. Yet, small mistakes always occur during the manufacturing process; consequently, what should be parallel is not parallel, which is why the two inclinations according to the theoretic plan will have to be indicated as well, and such inclinations will represent minimal values that can be easily misinterpreted (a 0.012º can be misinterpreted as the opposite value, and trigger mistakes that do no exist).
- 3 CENTRES -
This system consists of three points in space. This system is practical, fast and multifunctional; this is why it is the most commonly used checking fixture referencing system. The only thing to do in this case is measuring the three centres and define their nominal position in X,Y,Z without having to make elements such as plans or straight lines.
This system is very practical since it does not imply making the base plate with a great precision; after all, the base plate is only a basis to set the checking fixture elements, and the only condition it has to comply with is to be constant, that is, to have no evolution in time. For a long time, the three plans system was very used; yet, for this system the base plate had to be perfectly squared, which means that it had to be made with as much precision as the most precise elements of the checking fixture. The new 3D measuring systems and specialised softwares (Metrolog, PC-DMIS, …) are already set with this more practical alignment system, which is why the three-sights system is used by VW, Renault, PSA, HONDA, Toyota, and so on…
As for Tecnomatrix, due to the constant demand for costs reduction, we have adopted the three points alignment system as a standard system, not only for dimensional checks – Tecnomatrix has standard CMM programmes for parts alignment – but also for the machining of mounted parts on the base plate for adjusting, and so on…
In the example of the picture, we have three centres; “rec ref” is build with two of them, and “pln ref” with three of them. In this case, the “pln ref” element has had to be adjusted until being completely parallel to the theoretic XY plans. If it had not been adjusted, the angle from the ZX plan and the angle from the YZ plan could have been indicated, a method making the alignment interpretation much more complicated.
This picture displays the same example, with the same position in space, but with a 3 centres alignment. In this case, it would not have been necessary to have the three centres with same Z, since by entering the different values of each one it would have been useless to adjust them as if it were an element of the checking fixture. I insist on the fact that the current market constantly demands lower prices, consequently, if one hour of work can be spared in the workshop by simply modifying values, then we will try to do it.
An important issue is to get values that can easily be interpreted; it has to be taken into account that checking fixtures are not only used by metrologs – who are used to coordinates systems and can easily interpret any alignment type – but also by engineers and operators from the manufacturing department who need to analyse a problem or may want to know an approximate position of the part in space, the orientation it has in the vehicle etc. Thanks to the 3 centres referencing system, they can even deduce the axes orientation and which position they have through three plates mounted on the checking fixture with each centre coordinates; compared to it, the geometric system makes it more difficult to deduce for someone who is not used to these working systems.
At Tecnomatrix, if a checking fixture needs to be modified and new parts have to be machined again, the operators save a lot of time if they work with coordinates instead of angles, and there are fewer possibilities of mistake or confusion.
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