Burrs are a raised edge or an excess of material on the edges of an object triggered by the mould juncture (plastics), or the gap between the punch and the matrix on the cutting area of a metal part. If their presence is not taken into account, they may be very disturbing to get repeatability in a checking fixture. On the first picture bellow is displayed the section cut of a matrix with an excessive gap between the cutting punch and the matrix, which triggers some tear in the cutting area, and thus a burr. This burr is dangerous, as it can hurt the operator manipulating the part; what is more, in this very case it could be a real brain teaser if it has not been taken into account when designing the measuring tool.

Unfortunately, the CADs of a part never show where the burr could be, because there is no possibility to know it until the production mean – mould or matrix – has been designed; consequently, when we design a checking fixture and have to centre holes, we will have to carefully consider how to avoid them, to prevent them from having a negative impact on the checking fixture repeatability. There are various ways to avoid them, and you will find an example of one of them bellow.

Let us say that we are going to design a checking fixture or a displaying fixture for a part that has one of its principal isostatisms points located on the axis of a hole. As this hole will have a diameter tolerance, we will have to use a conic centring system to get adapted to the existing diameter, so that it will set in all cases the hole axis to is alignment position. To do so, we will use a retractable conic centring pin or a conic pin. Each system is as effective as the other one, yet, we will have to choose the correct one according to the position of the burr.

In this picture, we can see how wrong it would be to set a retractable conic centring pin mounted on a spring here; indeed, in this case, as the burr is located on the lower part of the hole, the centring part would lean on the burr instead of on the metal: this would result for sure in the loss of the checking fixture repeatability, since the part would be aligned on a different point every time we would be position it.

We would probably have to ensure the part support with a clamp that would put some pressure on the metal. This would modify the shape of the burr every time it would be positioned on the conic pin, generating consequently different results. The best solution in that type of case is to lean the conic pin on the part from its burr-free face, that is, the upper face. To do so, we would have to use the conic pin that would be centered through a guiding bushing. Moreover, to prevent the burr from leaning on the base plate, we would also set at the same time a supporting ring, with a larger diameter than the burr’s; this supporting ring would lift the part up enough to fix the problem. If applied, a solution of that type can make the checking fixture function correctly; otherwise, it would be a total failure.

The retractable conic centring pin would only be used if the burr was positionned on the upper face. These details, that are not displayed in the 2D plan of the part, can only be solved if the customer is fully aware of their importance. This is the type of mistake usually found with the (ii) and (iii) customers defined in our former article “An Incorrect Design: Ennemy #2.

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