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. (Continua leyendo …)

This video, presented by Don Day from Tec-Ease Inc., shows very clearly the importance of referencing correctly the plans of parts according to their future function. This case shows a common mistake, a design mistake that meant huge costs for the company that made it. Customers often make the same type of mistakes for the production of checking fixtures, because they tend to underestimate their usefulness. Choosing as a basic principle to invest in staff training for the use of checking means is not a cost, it is an INVESTMENT !!! Here is the written transcription of what Don Day says in this video.

You know that some companies continue to work with some house standards that refer to the part underline. And I continue to see the datum features simply placed on underlines under the drawings.
The standard allows only the datum targets used to indicate the datum features.
That was not the case for a company designing lense systems to be used in aerospace.
Here is a simplified drawing of one of their lenses bearers. They applied a total runnout to a critical diameter within the face of a lense.
This means that when checked with an indicator, the full indicator could not be greater than 0.0006 when the part was rotated around axis A.
A customer placed the datum features symbol on the underline. The supplier used it to continue features, such as the largest tool to check the runnout. Unfortunately, the customer held it in the way it was assembled in the lense mark, and the parts failed.
The result cost the customer nearly $8,000,000.
The problem is that each feature has a different axis. Holding the part differently gets different results. Hence, if parts can be made perfect, there would be no need for tolerances underlines.
So, it must be made clear on the drawing which features to use to establish a datum.

This way, the drawing makes clear the design intent. This problem is easily avoided by following the standard. The datum feature symbol must identify a feature to use to establish a datum.
Hereby associating the feature symbol with a size dimension, everyone knows what the drawing means. The supplier knows how the part will be inspected and has a better chance of making the part right the first time. Or, your company can live with their old addage that engineers do not make mistakes, they make revisions. This one could have been avoided.

Last week, as I was delivering a set of checking fixtures to a customer, he told me that it was the first time that he received the checking fixture before the injection mould. Although this is how things should be, since it is a basic and essential point, this is scarcely the case, which is a big mistake. One of the main advantages of checking fixtures, as we explained in former articles is to enable the checking of the first parts manufactured with the definitive production means; this enables to take the relevant measures as for the adjusting of these means, and even more so in the case of parts forming an assembled part or mechanism, for which adjustment is crucial.

Through our experience, we have seen that in nearly 30% of the cases, checking fixtures get delivered when the production process has already been started, and there are fewer possibilities to pay off the checking fixture. Another customer justified this fact by saying that they would rather launch the manufacturing of a checking fixture once they are sure that parts can be correctly manufactured and will not have any design modifications. In that case, what is the point in having a checking fixture manufactured? The cost of possible modifications on a checking fixture is minimal compared to the utility it will have in the production process. Let us learn from our mistakes.

HandyScan is one of the tools for digitalizing parts and surfaces that have been most successful and gone most popular over the past few years, and have even managed in some cases to overthrow the traditional tridimensionals. It is a hand scanner, totally portable and user-friendly.

It has very different uses: reversed engineering to display on screen a physical object, dimensional checking, rheology, animation, multimedia, medical applications, and so on…

In our field, one of the most typical applications is the digitalizing of foundry parts or structures that have to be machined but have an excess of matter or can be deformed. To determine exactly where this excess of matter is, the foundry part can be digitalised and overlaid on the 3D geometry that will be machined, to exactly calculate the optimal machining process. (Continua leyendo …)

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). (Continua leyendo …)