Fig. 1739.
In producing holes of above or about two inches in diameter, cutters such as shown in [Fig. 1739] may be employed. a is a stock carrying a cutter b secured in place by a key c. Holes are first drilled to receive the pin d, which serves as a guide to steady the stock. The amount of cutting duty is obviously confined to the production of the holes to receive the pin and the metal removed from the groove cut by the cutters, so that at completion of the cutter duty there comes from the work a ferrule or annular ring that has been cut out of the work.
Fig. 1740.
Fig. 1741.
For use on wrought iron or steel the front faces of the cutters may be given rake as denoted by the dotted line at e, and smooth and more rapid duty may be obtained if the cutter be set back, as in [Fig. 1740], the cutting edge being about in a line with line a, in which case the front face may be hollowed out as at b, and take a good cut without the digging in and jumping that is apt to occur in large holes if the cutter is not thus set back. The larger the diameter of the work the greater the necessity of setting the cutting edge back, thus in [Fig. 1741] the cutter is to be used to cut a large circle out of a plate p, as, say, a man-hole in a boiler sheet. The cutter c is carried in a bar b secured in the stock a by a screw, and unless the cutter is set well back it is liable to dip into the work and break.
It is obvious that the pin e in the figure must be long enough to pass into the hole in the plate before the cutter meets the plate surface and begins to cut, so that the pin shall act as a guide to steady the cutter, and also that in all cutters or cutter driving stocks the shank must be either of large diameter or else made square, in order to be able to drive the cut at the increased leverage over that in drilling.