Considering the case of a flat surface, the traverse unit drives the stylus over a distance called the evaluation length (EL). This length is
divided into five equal parts, each of which is called a sampling length (SL). In ISO 4287:1997, the sample length is defined as the 'length in the direction of the X-axis used for identifying the irregularities characterising the profile under evaluation'. The evaluation length is defined as the 'length in the direction of the X-axis used for assessing the profile under evaluation'.
The SL length is significant and is selected depending upon the length over which the parameter to be measured has statistical significance without being long enough to include irrelevant details. This limit will be the difference between roughness and waviness. In Figure 6.3, the waviness is represented by the sine wave caused by such things as guideway distortion. The roughness is represented by the cusp form caused by the tool shape and micro-roughness by the vees between cusps caused by tearing. The SL over which the profile is assessed is critical, if it is too large (LI) then waviness will distort the picture, if it is too small (L2) then the unrepresentative micro-roughness will only be seen. The correct SL is that length over which the parameter to be measured is significant without being so long as to contain unwanted and irrelevant information. The length (L3), containing several feed-rate cycles, would be a suitable representative length. The drift due to the wavelength would be filtered out.
The default SL is 0,8mm. This is satisfactory for the vast majority of situations but for processes that use a very small or a very large feed, this is inappropriate. Information on how to determine the correct SL for non-standard situations is given in ISO 4288:1996.
Was this article helpful?
Realize Your Dream of Becoming a Professional Pencil Drawing Artist. Learn The Art of Pencil Drawing From The Experts. A Complete Guide On The Qualities of A Pencil Drawing Artist.