Filters

A filter is a means of separating roughness from waviness. Mummery (1990) gives the useful analogy of a garden sieve. A sieve

Feedrate iness ■i h

Waviness

Figure 6.3 The effect of different sampling lengths separates earth into two piles. One could be called rock and the other dirt. The sieve size and therefore the distinction between dirt and rock is subjective. A gardener would use a different sieve size in comparison to a construction worker. With reference to machine surfaces, a sieve hole size is analogous to the filter. Figure 6.4 shows the results of different types of filters.

The simplest filter is the 2CR filter. It consists of two capacitors and two resistors. With the 2CR filter, there is 75% transmission for a profile with a 0.8mm wavelength. This is because all filter design is a compromise; 100% transmission up to the cut-off value and nothing after is impractical. In practice, the 2CR filter produces a phase shift and overshoot because it cannot read ahead. The 2CR filter is not mentioned in the latest standards.

The phase corrected (PC) filter (ISO 11562:1996) overcomes some of the disadvantages of the 2CR filter in that it can look forward. It does this by the use of a window or mask similar to that used in digital image processing. The mask or window of a PC filter is called a weighted function. The mask is ID and consists of a series of weights arranged in a Gaussian distribution. Each weight is applied to each profile point over the length of the window. Shifting the mask step by step scans the profile.

Unflltered profile

2 RC filter

Unflltered profile

2 RC filter

Phase Corrected filter (DIN 4777)

Valley Suppression filter (DIN 4776)

Figure 6.4 The effect of 2CR, phase corrected (PC) and valley suppression (VS) filters on a profile

Valley Suppression filter (DIN 4776)

Figure 6.4 The effect of 2CR, phase corrected (PC) and valley suppression (VS) filters on a profile

The PC filter will still produce errors particularly with the highly asymmetric profiles. For example, deep valleys will cause a distortion because of their comparative 'weight' within the mask. To overcome the above disadvantage, a double filter is applied which has the effect of suppressing valleys even further. This is called the valley suppression (VS) filter or the double Gaussian filter. It is defined in ISO 13565-1:1996.

Figure 6.4 (Mummery, 1990) shows a comparison of the 2CR, PC and VS filters when applied to a plateau-honed surface. The 2RC filter produces a 'bump' distortion in the region of the centre-left deep valley. This distortion is reduced but not eliminated by the PC filter in that a slight raising of the profile can still be seen at the same centre-left valley. The double filter reduces this to an almost negligible amount.

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