Reference Point System

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Dominic Waters
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1 Reference Point System Well, I'll measure the hole from edge a. I think it's more convenient to measure from c. Service Information

1 Introduction Introduction When parts in different situations and with different measurement strategies can be measured at other positions, we talk about changing the reference.

2 1 Introduction Introduction When parts in different situations and with different measurement strategies can be measured at other positions, we talk about changing the reference. This can lead to different results arising in the quality assurance process. Processing with a reference change (red arrow) Step 1 Plate will be picked up at A. Holes B and D will be drilled. Tolerances: AB = ± 0.1 AD = ± 0.1 Step 2 Plate will be picked up at D (reference change) Hole will be drilled at C Tolerances: DC = ± 0.1 BC tolerance result: AB = ± 0.1 AD = ± 0.1 DC = ± 0.1 BC = ± 0.3 The example shows: If the reference change (pick up also in second step at A) does not occur (blue arrow), only the tolerances AB and AC have to be added for tolerance BC. BC tolerance result = ± 0.2 A comparable unsatisfactory situation arises, if various tolerances sum up as a result of the reference change. A solution to overcome this problem shows the reference point system (RPS). The basic concept behind this RPS is to determine the same positioning and same dimension reference data for the parts that have to be measured. This means that the tolerances will be minimised and reliability such as the workpiece fitting accuracy will be guaranteed. Consequently, RPS-points create the basis for uniform pick-up situations for measurement. The system is operated in compliance with the consistent VW Standard v

2 The 3-2-1 rule 2.1 Location In order to clearly position a rigid body you have to define six possible movement directions.

The location is ensured the 3-2-1 rule by the following pick-up situations: 3 pick ups in Z-direction 2 pick ups in Y-direction 1 pick up in X-direction 3 rotational degrees of freedom 3

3 2 The rule 2.1 Location In order to clearly position a rigid body you have to define six possible movement directions. These 6 movement directions can be determined by: three translational movements along the axes and three rotational movements around the axes. The location is ensured the rule by the following pick-up situations: 3 pick ups in Z-direction 2 pick ups in Y-direction 1 pick up in X-direction 3 rotational degrees of freedom 3 translational degrees of freedom Pick ups These pick ups are the reference points. The reference points have to be clearly named (see details given in Chapter 2.3 as well with regard to this). The 6 pick ups can also be selected differently. You will be able to realise the best possible location if you also take the function of the part into consideration and not just the shape. You will find them in the picture below: 3 pick ups in Y-direction, 2 pick ups in Z-direction and 1 pick up in X-direction v

The most important point regarding this is that this distribution always follows the 3-2-1 rule, only then is the absolute

The reference points should lie as far apart from each other as possible in order to achieve the greatest possible stability.

2 Exceptions The exceptions are: A ball (three main pick ups suffice), Symmetrically rotating body (five main pick ups suffice)

4 The most important point regarding this is that this distribution always follows the rule, only then is the absolute location of the part ensured. The reference points should lie as far apart from each other as possible in order to achieve the greatest possible stability. In principle, the rule applies - with just a few exceptions - to any part. 2.2 Exceptions The exceptions are: A ball (three main pick ups suffice), Symmetrically rotating body (five main pick ups suffice) Part with articulated joint, which requires more than six main pick ups. Note Additional main pick up Naturally, intentional function related reference changes are possible here. The additional tolerances that arise should be taken into consideration already at the design phase. 2-4 v

5 2.3 Naming The reference points are numbered consecutively as follows: RPS 1, RPS 2,... RPS n. The main pick ups contain additional upper case letters which correspond to the type of pick up: H for pick up hole / pin F for surface / edge / ball or tip T for theoretical point (determined from two support points; additional support points are indicated by lower case letters [e.g. t or f]) Lower case letters are also added to the location direction for each type of exception: x, y and z for a parallel reference system a, b and c for a rotated reference system Examples are shown in the picture below: By the way: a round hole can locate two directions v

Dowelling joints with VS 600

No. 112 Dowelling joints with VS 600 A Description Dowelling joints with round dowels (in addition to flat dowels) are part of the standard wood joints in furniture manufacture. This joint is very stable.