Knurling in the Lathe.—Knurling is done either to provide a rough surface which can be firmly gripped by the hand or for producing an ornamental effect. The handles of gages and other tools are often knurled, and the thumb-screws used on instruments, etc., usually have knurled edges. A knurled surface consists of a series of small ridges or diamond-shaped projections, and is produced in the lathe by the use of a tool similar to the one shown in [Fig. 37], this being one of several different designs in common use. The knurling is done by two knurls A and B having teeth or ridges which incline to the right on one knurl and to the left on the opposite knurl, as shown by the end view. When these two knurls are pressed against the work as the latter revolves, one knurl forms a series of left-hand ridges and the other knurl right-hand ridges, which cross and form the diamond-shaped knurling which is generally used.
If the surface to be knurled is wider than the knurls, the power feed of the lathe should be engaged and the knurling tool be traversed back and forth until the diamond-shaped projections are well formed. To prevent forming a double set of projections, feed the knurl in with considerable pressure at the start, then partially relieve the pressure before engaging the power feed. Use oil when knurling.
The knurls commonly used for lathe work have spiral teeth and ordinarily there are three classes, known as coarse, medium and fine. The medium pitch is generally used. The teeth of coarse knurls have a spiral angle of 36 degrees and the pitch of the knurled cut (measured parallel to the axis of the work) should be about 8 per inch. For medium knurls, the spiral angle is 291/2 degrees and the pitch, measured as before, is 12 per inch. For fine knurls, the spiral angle is 253/4 degrees and the pitch 20 per inch. The knurls should be about 3/4 inch in diameter and 3/8 inch wide. When made to these dimensions, coarse knurls have 34 teeth; medium, 50 teeth; and fine knurls, 80 teeth.
The particular tool illustrated in [Fig. 37] has three pairs of knurls of coarse, medium and fine pitch. These are mounted in a revolving holder which not only serves to locate the required set of knurls in the working position, but enables each knurl to bear against the surface with equal pressure. Concave knurls are sometimes used for knurling rounded edges on screw heads, etc.
Relieving Attachment.—Some lathes, particularly those used in toolrooms, are provided with relieving attachments which are used for “backing off” the teeth of milling cutters, taps, hobs, etc. If a milling cutter of special shape is to be made, the cutter blank is first turned to the required form with a special tool having a cutting edge that corresponds with the shape or profile of the cutter to be made. The blank is then fluted or gashed to form the teeth, after which the tops of the teeth are relieved or backed off to provide clearance for the cutting edges. The forming tool used for turning the blank is set to match the turned surface, and the teeth are backed off as the result of a reciprocating action imparted to the toolslide by the relieving attachment. The motion of the toolslide is so adjusted that the tool will meet the front of each tooth and the return movement begin promptly after the tool leaves the back end of the tooth.
Fig. 38. Hendey Relieving Attachment applied to a Lathe