Fig. 2251.
The screw-driver to be used in saw slots should have its end shaped as at a in [Fig. 2251], which will tend to prevent it from slipping out of the saw slot, as it will be apt to do if wedge-shaped as at b, because in that case the action of the torsional pressure or twist is to lift the screw-driver out of the slot.
Scrapers and Scraping.—The process of scraping is used by the machinist to true work, and to increase the bearing area of surfaces, while the brass finisher employs it to prepare surfaces for polishing, applying it mainly to hollow corners and sweeps.
For scraping work to fit it together the flat scraper is used, ordinary forms being shown in [Figs. 2252] and [2256].
| VOL. II. | SCRAPERS AND SCRAPING. | PLATE IX. | |
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| Fig. 2252. | Fig. 2253. | ||
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| Fig. 2254. | Fig. 2255. | ||
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| Fig. 2256. | Fig. 2257. | ||
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| Fig. 2258. | Fig. 2259. | ||
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| Fig. 2260. | |||
That shown in [Fig. 2252] may be made of a flat smooth file, of about an inch wide, and 3⁄16-inch thick, which is large enough for any kind of work. Two opposite faces, one of which is shown at a, are ground beveled so as to leave the end face b about 1⁄16-inch thick. This end face is then ground square as denoted by the dotted lines, producing two cutting edges of equal angles, and therefore equally keen. If it were attempted to grind face b at an angle as denoted by the dotted lines g, in [Fig. 2253], the lower edge h would cut too keenly, causing the scraper to chatter and cut roughly, while the upper one i would not cut sufficiently easily.
For very smooth work the scraper may be formed as in [Fig. 2256], the front face e being ground slightly out of square as shown, and the bottom face f being given considerable angle to the body of the scraper. For very rapid cutting, however, the front face e may be at an angle of less than 90° to the top of the scraper.
The only objection to this form is that the eye lends no assistance in bringing the edge fair with the work surface. The scraper should not exceed about 6 inches in length, exclusive of the handle, for if longer it will not cut well or smoothly, and its end face should be slightly rounded as in [Fig. 2254]. Its facets should be ground square or straight and carefully oil-stoned after the grinding, the oil-stoning process being repeated for two or three resharpenings, after which it must be reground upon the grindstone.
The scraper should be grasped very firmly in the hands, and held as in [Fig. 2255]. It requires to be pressed hard to the work during the cutting and lightly during the backward stroke.