The strokes should not exceed for the roughing courses, say, half an inch in length, the first course leaving the work as represented in [Fig. 2257].
The second course should be at a right angle to the first, leaving the work as in [Fig. 2258], and after these two courses the work should be tested by surface plate, or with the part to which it is to fit, as the case may be. Previous to the testing, however, the work must be carefully wiped clean with old rag, as new rag or waste is apt to leave ravelings behind. The surface plate should be given a light coat of red marking, and then moved backward, forward, and sideways over the work, or, if the work is small, it may be taken from the vice and rubbed upon the surface plate, and the high spots upon the work will be shown very plainly by the marks left by the plate. The harder the plate bears upon the work the darker the marks will appear, so that the darkest parts should be scraped the heaviest.
After applying the plate, the scraper may again be applied, the marks being at an angle to the previous operation, the testing and marking by the plate and scraping process being continued until the job is complete, appearing as shown in [Fig. 2259].
It will be noted that the scraper marks are much smaller and finer at and during the last few scrapings; and it may be here remarked that the scrapings are very light during the last few finishing processes.
The strokes of the scraper being made of a length about equal to the acting width of its edge cuts, makes the scraper mark approximately square, on which account it is sometimes termed “block” scraping. It gives an excellent finish, while not sacrificing the truth of the work to obtain the finish.
The scraper will not remove a quantity of metal so quickly as a file, and on this account it is always preferable to surface the work with a file before using the scraper, even though the work be well and smoothly planed. Not until the file has almost entirely removed the planer marks, and the surface plate shows the surface to be level and true, should the scraper be brought into requisition, the first courses being applied vigorously to break down the surface.
It would appear that scraping might be more quickly done by taking long scraper strokes promiscuously over the work, but in this case the bearing marks are not well defined and do not show plainly, which leads to confusion and causes indecision as to where the most or heaviest scraping requires to be done, whereas in the block scraping the marks are clearly defined and the high patches or spots on the work show very plainly, and the workman is able to proceed intelligently and with precision.
[Fig. 2260] represents a three-cornered or “three-square” scraper, which is used principally upon hollow or very small flat surfaces. The half-round scraper is employed upon holes, bores, or large concave surfaces, such as brasses. Both these tools are for vice work, used in the same manner as described for flat scrapers, while all scrapers cut smoother when the edge is kept wetted with water, as is essential when used upon wrought iron, copper, and steel.
Hand Reamers or Rymers.—The hand reamer is employed for two purposes, first, to make holes of standard diameter and smooth their walls, and second, to bring holes in line one with the other.