This section is from the book "Machine Shop Work", by Frederick W. Turner, Oscar E. Perrigo, Howard P. Fairfield. Also available from Amazon: Machine shop work.
When two flat or curved surfaces are to be worked together, and close contact over the surfaces of both is desired, they are hand scraped. Scraping removes less metal than filing and also enables the workman to confine the removal to limited areas. The scraper, which should be made from a very close-grained tool steel, is nearly 2 feet long exclusive of the handle. The general shape is shown in the upper view of Fig. 58. The cutting edge is about 3/32 of an inch thick and 1 1/2 inches wide. It is ground on an emery wheel or grindstone and carefully oilstoned, leaving the cutting edge as straight as possible. Scrapers are sometimes made from old files, the teeth being ground off and the end drawn out wide and thin. Sometimes the end is bent at right angles to the shank, as shown in the lower view of Fig. 58. The cutting done by scrapers should be perfectly smooth and free from scratches.
Fig. 5S. Straight and Bent Hand Scrapers.
In using the surface plate as a test for the trueness of a plane, such as a valve or its seat, the plate is covered with a very thin coating of red lead and then rubbed over the valve or seat. The latter should have previously been finished as smoothly as possible. The spots where the red lead shows contact are scraped off and the process continued until contact over the entire surface is obtained. During the last part of the operation, alcohol should be used instead of red lead, as it leaves clean bright spots to indicate where the scraper must be applied. Small pieces of work are rubbed over the surface plate, and in any case care should be taken to distribute the wear uniformly over the plate in order to prolong the trueness of the plane. The scraper for concave surfaces, such sis bearings, is of the general shape of a half-round file without teeth. In such cases, the spindle itself takes the place of a surface plate. The method of holding and using such a scraper is shown in Fig. 59.
Scraping is sometimes done as a matter of finish, and not for the purpose of getting an accurate surface. It is then termed "spotting". A spotted surface, therefore, does not always indicate accuracy. Many machine parts can be more cheaply finished by scraping than by polishing.
The prickpunch, Fig. 60, is made of tool steel with a hardened conical point of about 60 degrees. It is about 3 1/2 inches long and 1/4 inch in diameter. It is used for making very small indentations at intervals on a line, or at intersections of lines.
The center punch, also shown in Fig. 60, is made of the same general appearance as the prickpunch, but is about 5 inches long, 1/2 inch in diameter, and has a point angle of about 90 degrees. The principal use of this punch is to make center holes, marking the centers on the ends of pieces to be turned.
Fig. 59. Scraping Spindle Bearing.
Fig. 60. Hand Punches. Forged Center Punch Above; Prickpunch Below.
Ordinary forged center punches are usually made of hexagonal steel; but if round stock is used, the grip should be fluted or knurled to prevent slipping in the fingers.