MILLING MACHINES are a highly important class of machine tool which will perform almost all the operations neces sary in metal-cutting, by means of revolving cutters past which the work is fed. Milling was first done in the lathe, the cutter being rotated between the centres, while the slide traversed the piece of metal across and beneath it. There are still many milling ma chines, termed Lincoln millers, in which the resemblance to the lathe is evident, and which were developed at an early period to mill parts of small arms and sewing machines, when the inter changeable system of manufacture began.
The great advantage of the milling machine is that cutters can be made for all sorts of shapes, to mill corresponding profiles, which they will do on tens of thousands of pieces alike. Hand production by files is thus eliminated and great accuracy ensured. Cutters can also be multiplied, several being used on one spindle, so that a complicated surface or a number of duplicate pieces may be milled at one pass. Machines with multiple spindles, each carrying a cutter or a set will surface off very complex shapes of castings, e.g., castings for parts of engines and machinery, etc., thereby saving a lot of piecemeal cutting which would otherwise be necessary to finish the outlines.
There is a system termed profile milling, by means of which a cutter will produce curves and flats, the work being coerced dur ing the traverse by a copy of like shape. The constantly-changing curves of cams are also milled by a similar device. Screw-threads and worms are cut by a milling cutter of suitable shape, revolved and fed laterally whilst the screw rotates.
The Lincoln machine mentioned is built in small and in large sizes for what is termed manufacturing, that is the milling of large numbers of similar shapes for small arms, machine-guns, sewing machines, cycles, parts of cars, etc. Its constructional features are relatively simple, and the pieces of work are either gripped in a vice, or in a special clamping unit, termed a fixture, which automatically gives the correct attitude in relation to the cutter.
Often several articles are clamped in line for the cutter to mill them in succession, so that at one traverse of the table three or four or more receive treatment.
When a varied range of milling has to be performed, this model is not sufficiently adaptable, and what is termed a plain or a uni versal machine is chosen. This has a column near the top of which a horizontal spindle is driven by belt or motor, and usually a change-speed gearing to obtain the various speeds. A triangular shaped knee is adjustable up and down, slideways on the face of the column, and across the flat face at the top of the knee a saddle is adjustable. This saddle, in turn, carries the long sliding table for the objects to be milled. The operator has thus the choice of three adjustments with which he can bring all sorts of castings and forgings into correct position for mill ing, and then feed them by hand or auto matically. Some cutters, termed end mills, cut mainly by their ends and effect surfac ing and the milling of slots and grooves. By putting a pair of index centres, i.e., a couple of heads with point centres between which work can be swung, on the table circular dividing may be effected. This
action enables flats, squares, hexagons, etc., to be finished, or slots cut around a cyl inder or disc. Gearwheel teeth are also made in the same manner. The difference between a plain and a universal miller is that the latter has a swivel-mounting for the table, so that it may be slewed at a suitable angle, and spiral grooves be cut, or the helically set teeth of milling-cut ters or gears be produced, as well as screws and worms that may be required.
Vertical-spindle milling machines handle a varied range of cutting by means of the end mills previously mentioned, or of cylindrical mills cutting on the periphery, for edges and sides. The profiling of contours which cannot be milled by the linear movements of the slides is effected by controlling the slides through the medium of a copy of the shape desired. Somewhat recent developments of the vertical-spindle designs are the con tinuous millers ; a revolving circular table holds a set of work holding fixtures so that each in turn passes the cutter. The at tendant can remove the finished specimen from the fixture farthest from the cutter and put in a fresh one for milling, so that no stop need occur in the running. Station milling is another highly pro ductive method ; the rotary table feeds straight in towards the cutter; then it reverses, and makes a portion of a revolution, bringing another piece into line for the feed. The operator is able to remove finished components and to re-load the fixture that is furthest away from the cutter. Still higher rate of production is practicable in the drum continuous machines which have a set of fixtures spaced around a big horizontally set cylinder, and cutters attacking from each end of the work.
The principal difficulties in milling are connected with the results of vibration, which leaves a bad finish, and with the quantity of chips produced. The first dif ficulty is remedied chiefly by strong con struction of the machine; the second by effective lubrication with a heavy volume of oil, which washes the chips rapidly away from the machine. (F. H.)