Where amateurs experience inconvenience in making their tools from
the want of a forge, the use of detached cutters in a tool holder
will be found of the greatest advantage for outside work. Even in
plain turning there must always be some special forms for cutting
into odd corners and deep grooves; but with a good tool holder and
a grindstone, which is an indispensable piece of furniture in every
metal turner's shop, the cumbrous array of slide-rest tools may be
reduced to a few special forms and a very small box of cutters. These
also possess another great advantage; for the spirit of the old adage
quoted by Holtzapffel--
"He that would a good edge win
Must forge thick and grind thin,"
may be carried out far more conveniently than in the case of whole
tools which are generally filed into shape before tempering, and when
worn down must go to the fire again and have the process repeated. But
the detached cutter admits of being tempered evenly throughout its
whole length and ground up afterwards as long as it lasts, without
going to the forge again to the deterioration of the steel.
The patterns of tool-holders are innumerable, but very few are good
for general service, because most of them are arranged so that the
natural sides of the cutter are used for the face or faces of the
edge. Thus either the plan angle of the point is limited to the angles
presented by the transverse section of the cutter employed, or else
the section angle is fixed by the position in which the cutter is
clamped. Holtzapffel's arrangement is open to the first objection,
Babbage's to the second. To obviate this inconvenience, Prof. Willis
arranged a holder which clamps the cutter at an angle of 55° from
the horizontal line. Thus no side can be used either for the lower or
upper face of the edge, but any faces can be ground upon it; and the
plan and section angle of the edge may be varied at pleasure within
the whole range available for metal turning. Prof. Willis's holder
for the cutter is almost a facsimile of his admirable tool-holder for
the slide-rest, than which none is more convenient or can act more
perfectly. But the arrangement is a little complicated for a cutter
holder, and must be very carefully made with the knowledge of certain
laws, if it is to insure a perfect grip of the cutter. It is also
designed for the use of sound wire cutters which require filing flat
on one side.
[Illustration]
Adopting Willis's inclination for the cutter I have found that all its
advantages may be secured with a simpler form of holder and common
square for steel for the cutter. The holder is simply the modification
of an old pattern to suit the inclination of 55°, and the sketch needs
little explanation beyond saying that the nick in the solid part
should be rather less than a square angle, and made perfectly true all
the way down, or, if anything, rather hollowed in the middle, so as
to insure the greatest amount of pressure at the top and bottom, as
otherwise the cutter might not sit quite true and firm. The angle at
the end of the strap against which the cutter bears should be rather
_more_ than square, both to allow for any want of exact truth in the
squaring of the cutter and to avoid the wedging action which would be
set up on tightening the screw if this angle were _less_ than square,
as this could of course create a risk of splitting the strap. The end
of the screw and the cup in which it fits should be round, as this
allows of a little play and insures a truer grip in the strap than a
pointed screw working into a conical hole. A perspective sketch of a
detached cutter is added, with dotted lines to show how exactly the
arrangement of the faces can be accommodated to the positions which
have been shown to be the best in solid tools for the slide-rest.