edges are sharpened as indicated in the drawing by using a tapered oilstone or India slip. Should the tool become much worn with prolonged use, the metal behind the two cutting edges should be relieved by careful grinding.
The two-lipped wood countersink, illustrated in Fig. 33B> as made by Messrs. Starrett, is hardened and ground and will, therefore, but rarely require sharpening. This model has a rather wider gap than is shown in the drawing, and ample space is provided for using a triangular oilstone slip when resharpening the cutting edges.
The Rosehead countersink, Fig. 33c, costs but a few pence and is so roughly made that it does not invite resharpening.
The pattern shown in Fig. 33d, however, has fewer cutting edges, but these are accurately ground and the
teeth are not so liable to become clogged. To resharpen the tool, the leading faces of the cutting edges are honed with a triangular oilstone or India slip. Any attempt to grind the periphery of the cutter may result in damaging the following cutting edge, but careful hand stoning may be used, provided that the clearance or relief behind the cutting edges is not unduly reduced.
The spear-headed countersink depicted in Fig. 33e was formerly employed for metalworking, but as it is very liable to chatter it is now seldom used. Nevertheless, it can be very easily resharpened, as illustrated, and the included angle can be readily altered by grinding to suit any particular purpose.
The cutting lips are ground with a clearance of some 15 deg., by using the angular grinding rest, and the angle included at the point is checked with a suitable gauge or protractor. The cutting edges should be finished by honing the flat sides on the oilstone.
The four-lipped metal countersink, Fig. 33f, will be found prone to chatter unless firmly supported and run slowly.
The radial faces leading the cutting edges can be stoned to resharpen the tool, but as there is but little bearing surface towards the tip to guide the stone, great care must be taken to avoid rounding the cutting edges and thus impairing their cutting properties. The peripheral surface of the cutting edges can be ground free-hand, nearly up to the actual cutting edge, and this surface is then finished with a hand stone.
Care must be taken, however, to maintain the proper clearance angles and also not to upset the line of the cutting edges.
A non-chattering type of metal countersink is illustrated in Fig. 33g. Here, only one lip actually cuts while the other acts as a guide and steady.
Once the tool has beeri formed as shown in the drawing, resharpening is carried out by stoning the flat face, but should this surface be reduced to lie below the line of the diameter, the tool's non-chattering properties will be lost. In the drawing the dimension to which the point is . d ground is shown as —, where d is the diameter, to which is added an allowance of i/iooo of an inch for each i/io of an inch of the tool's diameter. This addition is made to allow for the removal of metal when resharpening, and in the case of a |-in. diam countersink it would, therefore, amount to 5/1000 of an inch.
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