Engineering Drawing for Manufacture phần 8 pdf

Surface finish specification 113

6.2 Measuring the surface finish

The most common method of assessing the SF is by traversing a

stylus across a surface. A typical stylus is shown in the scanning

electron microscope (SEM) photograph in Figure 6.2 (courtesy of

Hommelwerke GmbH). The stylus tip is made of diamond having a

tip spherical radius of 5um and an included cone angle of 90 ~ Styli

are available in a standard range of spherical radii of 2, 5 and 10um

and included cone angles of 60 ~ and 90 ~ (ISO 3274:1996). The

stylus is shown in contact with a ground surface that gives an indi￾cation of the scale of the surface features. The stylus is positioned at

the end of a mechanical arm that connects to a transducer such that

the undulations on the surface are translated into an electrical

signal. This signal is amplified and eventually displayed on a PC

screen along with the calculated parameters.

6.2.1 Sample length and evaluation length

Considering the case of a flat surface, the traverse unit drives the

stylus over a distance called the evaluation length (EL). This length is

Figure 6.2 A scanning electron microscope photograph of a stylus (courtesy of

Hommelwerke GmbH)