To bring perfect contact between two metal surfaces, each is coated with a fine film of red lead and oil, and the surfaces are then rubbed together. Upon separating them, the high spots may be plainly seen, and are removed by hand scraping. The scrapers used are very hard pieces of steel, not unlike files without teeth, as shown in Fig. 213. The faces are ground true to form, either flat or curved, and the ends are ground blunt, except that the end of D is at more or less an acute angle to the surface. This grinding makes sharp edges at the end of the scraper and the scraping is done with these edges.

Fig. 213.   Scrapers.

Fig. 213. - Scrapers.

Bearings are scraped to fit journals by this method, and plane surfaces which move upon each other or must form a close joint for steam or other pressure connections, are brought to close contact by the same method. Scraping is always done for such fitting after the parts have been finished as accurately as can be done by machining, so that the amount of scraping needed will be reduced to a minimum.

Modern machines for grinding plane and curved surfaces are so perfected as to make hand-scraping unnecessary.