0025// Touch trigger probing strategy for flatness measurement -- BS 7172:1989

The current technology provides us plenty of choices to perform CMM measurement (such as scanning). However, in some circumstances, scanning for measurement is too luxury for the inspector. For example, quick measurement for a defective batch, CMM machine w/o scanning features, short takt time and etc.

This post will cover how to selecting the number of probing point and its distribution for surface flatness measurment (Don't mix up with the center plane flatness)

To design a probing straregy, we need to consider these few factors:

1) Ecomical consideration

2) Mathematical requirement

3) Fabrication process of workpiece

4) Fraction of form

1) Ecomical consideration

The more points you probe, the more reliable data you can get. However, the more probe you probe also implied the more cycle time you will consume. Therefore, by consider factors 2, 3 and 4, we should come out a strategy to use least points to provide sufficient information for the inspected workpiece.

2) Mathematical requirement (BS- 7172: 1989)

According BS-7172:1989, the recommended minium probing point for a surface is 9 points.

The 9 points should located randomly and nearly uniform on a 3x3 rectangular segmented plane. (In my opinion, this seem not suitable for large area or small tolerance workpiece. In that case, N = number of probing points, XxY segmented planes, XxY=N is recommended)

The sub-rectangle should approximate a square as possible. (as shown below)

However, equal spacing of probing points is not recommended because this will cause the measurement cannot detect periodical deformation such as waviness.

3) Fabrication process of workpiece

Fabrication process for that inspect workpiece also can provide us useful information to establish more meaningful probing strategy.

For example, for 3D printed plastic holder which shown below, we know that the surface come with regular unidirectional waviness due to its fused deposition modeling (FDM)'s nature. So, when design probing strategy, we will ensure the random distribution of probing points across the printing direction perpendicularity to detect its waviness.

4) Fraction of form

For the workpiece with incomplete or fractionated flat surface, the standard strategic in point 2 may become impractical. For example, Bulleid Firth Brown wheel which shown below contain a lot of lightening holes. By using strategy mentioned in point 2, you will found that a lot of sub-rectangles will fall on those lightening holes. So, in such cases, other method should use establish the probing strategy.

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