tailieunhanh - Engineering Drawing for Manufacture phần 8

Các phương pháp phổ biến nhất của việc đánh giá SF là vượt qua một bút stylus trên một bề mặt. Một bút điển hình được thể hiện trong kính hiển vi điện tử quét (SEM) bức ảnh trong hình (do Hommelwerke GmbH). Đầu bút stylus được làm bằng kim cương có bán kính cực hình cầu của 5um | Surface finish specification 113 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 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 lOum and included cone angles of 60 and 90 ISO 3274 1996 . The stylus is shown in contact with a ground surface that gives an indication 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. 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 A scanning electron microscope photograph of a stylus courtesy of Hommelwerke GmbH 114 Engineering drawing for manufacture 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 the waviness is represented by the sine wave caused by such things as guideway distortion. The roughness is represented by the