_MILLING._
Milling relates to metal cutting with serrated rotary cutters,
and differs in many respects from either planing or turning. The
movement of the cutting edges can be more rapid than with tools
which act continuously, because the edges are cooled during the
intervals between each cut; that is, if a milling tool has twenty
teeth, any single tooth or edge acts only from a fifteenth to a
twentieth part of the time; and as the cutting distance or time
of cutting is rarely long enough to generate much heat, the speed
of such tools may be one-half greater than for turning, drilling,
or planing tools. Another distinction between milling and other
tools is the perfect and rigid manner in which the cutting edges
are supported; they are short and blunt, besides being usually
carried on short rigid mandrils. A result of this rigid support of
the tools is seen in the length of the cutting edges that can be
employed, which are sometimes four inches or more in length. It
is true the amount of material cut away in milling is much less
than the edge movement will indicate when compared with turning
or planing; yet the displacing capacity of a milling machine
exceeds that of either a lathe or a planing machine. Theoretically
the cutting or displacing capacity of any metal or wood cutting
machine, is as the length of the edges multiplied into the speed
of their cutting movement; a rule which applies very uniformly in
wood cutting, and also in metal cutting within certain limits; but
the strains that arise in metal cutting are so great that they may
exceed all means of resisting them either in the material acted
upon, or in the means of supporting tools, so that the length of
cutting edges is limited. In turning chilled rolls at Pittsburg,
tools to six inches wide are employed, and the effect produced is
as the length of the edge; but the depth of the cut is slight, and
the operation is only possible because of the extreme rigidity of
the pieces turned, and the tools being supported without movable
joints as in common lathes.
Under certain conditions a given quantity of soft iron or steel may
be cut away at less expense, and with greater accuracy, by milling
than by any other process.
A milling tool with twenty edges should represent as much wearing
capacity as a like number of separate tools, and may be said to
equal twenty duplicate tools; hence, in cutting grooves, notches,
or similar work, a milling tool is equivalent to a large number of
duplicate single tools, which cannot be made or set with the same
truth; so that milling secures accuracy and duplication, objects
which are in many cases more important than speed.
Milling, as explained, being a more rapid process than either
planing or turning, it seems strange that so few machines of
this kind are employed in engineering shops. This points to some
difficulty to be contended with in milling, which is not altogether
apparent, because economic reasons would long ago have led to a
more extended use of milling processes, if the results were as
profitable as the speed of cutting indicates. This is, however, not
the case, except on certain kinds of material, and only for certain
kinds of work.
The advantages gained by milling, as stated, are speed,
duplication, and accuracy; the disadvantages are the expense of
preparing tools and their perishability.
A solid milling cutter must be an accurately finished piece of
work, made with more precision than can be expected in the work
it is to perform. This accuracy cannot be attained by ordinary
processes, because such tools, when tempered, are liable to become
distorted in shape, and frequently break. When hardened they must
be finished by grinding processes, if intended for any accurate
work; in fact, no tools, except gauging implements, involve more
expense to prepare, and none are so liable to accident when in use.
Such tools consist of a combination of cutting edges, all of which
may be said to depend on each one; because if one breaks, the
next in order will have a double duty to perform, and will soon
follow--a reversal of the old adage, that 'union is strength,' if
by strength is meant endurance.
In planing and turning, the tools require no exact form; they can
be roughly made, except the edge, and even this, in most cases, is
shaped by the eye. Such tools are maintained at a trifling expense,
and the destruction of an edge is a matter of no consequence.
The form, temper, and strength can be continually adapted to the
varying conditions of the work and the hardness of material. The
line of division between planing and milling is fixed by two
circumstances--the hardness and uniformity of the material to be
cut, and the importance of duplication. Brass, clean iron, soft
steel, or any homogeneous metal not hard enough to cause risk to
the tools, can be milled at less expense than planed, provided
there is enough work of a uniform character to justify the expense
of milling tools. Cutting the teeth of wheels is an example where
milling is profitable, but not to the extent generally supposed.
In the manufacture of small arms, sewing machines, clocks, and
especially watches, where there is a constant and exact duplication
of parts, milling is indispensable. Such manufactures are in some
cases founded on milling operations, as will be pointed out in
another chapter.
Milling tools large enough to admit of detachable cutters being
employed, are not so expensive to maintain as solid tools. Edge
movement can sometimes be multiplied in this way, so as to greatly
exceed what a single tool will perform.
Milling tools are employed at Crewe for roughing out the slots in
locomotive crank axles. A number of detachable tools are mounted on
a strong disc, so that four to six will act at one time; in this
way the displacement exceeds what a lathe can perform when acting
continuously with two tools. Rotary planing machines constructed on
the milling principle, have been tried for plane surfaces, but with
indifferent success, except for rough work.
There is nothing in the construction or operation of milling
machines but what will be at once understood by a learner who sees
them in operation. The whole intricacy of the process lies in its
application or economic value, and but very few, even among the
most skilled, are able in all cases to decide when milling can
be employed to advantage. Theoretical conclusions, aside from
practical experience, will lead one to suppose that milling can be
applied in nearly all kinds of work, an opinion which has in many
cases led to serious mistakes.
(1.) If milling tools operate faster than planing or turning
tools, why are they not more employed?--(2.) How may the effect
produced by cutting tools generally be computed?--(3.) To what
class of work are milling machines especially suited?--(4.) Why
do milling processes produce more accurate dimensions than are
attainable by turning or planing?--(5.) Why can some branches of
manufacture be said to depend on milling processes?