In the majority of cases, the fastening of parts on checking fixture is made as follows: the supporting of the part is made with a support block and a clamp, the centring with a conic centring pin in a circular hole, and the anti-rotation with a diamond-shaped pin in a slot. Yet, to do this fastening correctly, one of the most important information we need is the tolerance of the circular hole, and most of all, of the slot. Tolerancing a slot, however, is not always that easy. Fortunately, Tec Ease has prepared a video of GD&T explaining how to tolerance correctly a slot, providing us once more with useful information for the designing of parts and checking fixtures. (Continua leyendo …)

Sometimes, we receive from our customers industrial drawings where the datum features are not always correctly indicated. Datum features are essential for the designing and manufacturing of checking fixtures, and if they are not taken into account correctly, the mounting of the part on the fixture may be disastrous. To explain you the importance of datum features and how to set them correctly on an industrial plan, we have found this video from GD&T, presented by Don Day, who already provided us with very useful information in a previous article (GD&T - Datum Features Have Axes). You will find bellow the written transcription of what is said in this video.

“There continues to be the misconception that when fake features are labelled as datum features, their use is somehow implied.
Take the drawing shown. The position tolerance for the hole references datum features A and B, but not C. Therefore, the hole must be perpendicular to the plan established by datum features A at 20mm from the axis established by datum feature B. Datum feature C has nothing to do with the position of the hole.
Yes, the hole is shown on the centre line and is perpendicular to datum feature C, so we know what the goal is. But since the datum feature C was not referenced in the position tolerance, there is no limit to how far the hole can be off in a 90º angle. In order to avoid this, all you have to do is add datum feature C to the position tolerance.”

Since this video is the fourth one we make, I hope you will note an improvement in its quality. This video shows the example of a checking fixture for a large part, made of plastic, that can be easily deformed. The functioning principle of the checking fixture is to use the fixation points of the part to conform it correctly in its theoretical position, which enables us to check:

• The problems of fastening to the vehicle
• The deformation on the part ends
• The general contractions of the part
• The absence of holes, or diameters errors
• The shaping errors on the edge of the part, and so on.

In another video coming soon, you will see the manufacturing process of that type of checking fixtures, which is very interesting.

Today, we would like to present you the video of a cubing-style checking fixture for the checking of three different versions of a bumper assembly. Since all the versions have the same fixations to the car body, the same checking fixture will be used for their dimensional verification by only swapping the checking blocks varying from one version to another.

Contrary to the displaying fixture we showed you in a former article, that type of checking fixture is more stable dimensionally speaking. This one is made from a stabilized aluminium founding structure, ensuring the sufficient sturdiness to support all the checks that will be needed. It can weigh from 450kg to 400kg depending on the size of the bumper and the checking elements that are to be added:

• Bonnet
• Fenders
• Headlights
• Radiator grille, hubcap, and so on
• Dial indicator towers, sensors, and so on

(Continua leyendo …)

In this article we would like to show you one of the latest checking fixtures we made in Tecnomatrix that proposes the ideal solution to check plastic parts with clips. In this case, the part checked is a rear-view mirror cover, with 5 clips. In order to carry out repeatability studies with satisfactory results, the checking fixture must have an opening clipping system, as we have showed you in previous articles. This is the essential condition of a checking fixture: we must always obtain the same results, no matter who makes the measurements.

(Continua leyendo …)

In the industry, a lot of people have heard at least once the terms “Cpk study”. But when it comes to define it, not so much can provide a simple and clear definition, which is why we have decided to make our own inquiry.

The function of the Cpk study can be explained through this image, that was used in the forum of Six Sigma: let us say that the points for the Cpk study are darts. If your shots are falling in the same spot and form a good group, then you have a high Cp; when the sighting is adjusted so that this tight group of shots is landing on the bullseye, then you have a high Cpk.

In the video above, Keith Bower explains us the function of Cpk as well as the essential keys to be able to use this index in a relevant way. You will find bellow the written transcription of what is said in this video. (Continua leyendo …)

There are three types of checking fixtures for large parts:

• (1) Welded-structure display fixture
• (2) Aluminium-profiled-structure display fixture
• (3) Cubing fixture representing all the adjacent elements

The former two have the same function, their difference is in the type of manufacturing material used and production deadlines. Their structures have a similar resistance, but the second one is made from assembled aluminium profiles, and can be produced more quickly. Both are used to position the part on the tridimensional to enable its check. (Continua leyendo …)

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.