d 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 29-1/2 degrees and the pitch, measured as before, is 12
per inch. For fine knurls, the spiral angle is 25-3/4 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.

[Illustration: Fig. 38. Hendey Relieving Attachment applied to a Lathe]

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 to