For general work the plain cylindrical cutter is the most effective and least expensive. As compared with a facing cutter, thero are teeth upon the periphery only; consequently it is less costly to make and keep in repair, also there are not any corners to chip or burr up by excessive stress of work, and facing cutters for gauge work are useless after re-grinding unless the patent adjustable split cutters are used. The ratio of the cost of labour brought to bear upon the material in producing a first-rate tool, does not in many cases admit of a comparison with cost of. material; consequently nothing but the best quality should be employed which is frequently obtained only by experimental data upon the work to be done. Experimental attention upon this point and also upon suitable angles for the teeth, both cutting and clearance, is bound to repay the operator to an almost unlimited extent. It appears that up to the present no acknowledged rule has been adopted regarding the pitch of teeth, which now simply rests upon the individual experience of the manager. For all practical purposes, whether for periphery or face, a pitch of about ⅜ inch or « inch will give satisfactory results with angles cut to 45° and 12°, but re-cut to 40° and 12°, with an angle of clearance or relief of about 3° to 5°, varying for different materials, and this angle may be taken about the same as that for a turning or other machine tool.

The angle of relief is maintained when re-grinding, by placing the centre of the emery wheel either a little below the centre of the cutter if upon one side, or a little to the left of the centre line if the emery wheel is placed above the cutter. A spiral groove or thread of about ⅝-inch or ¾ inch pitch, forming what is generally called a "lob" cutter, will enable it to do rougher work by relieving the teeth of the shavings in detachments; but a continuous tooth is preferable for finishing, the angularity of the spiral of which ranges from 1 in 30 to 1 in 60 inches, the finer pitch being the easiest upon the machine, its cut being more continuous and less intermittent, so that each tooth does its work gradually, and consequently there is less wear and tear upon the machine. The direction of the spiral is arranged either right or left hand, to force the work upon the table and the spindle against its bearing, the direction of rotation deciding this point. It is beneficial to use cutters with the least possible diameter consistent with the work they have to perform, because, naturally, they are less expensive to manufacture and maintain, also heavier cuts can be worked with less strain upon the machine, owing to the smaller leverage due to less radius. Again, as shank cutters ant "chambered" ones are frequently interchangeable with two or more machines of their kind, in order to get standard spindles, it is possible to get a set of cutters on a machine for which they have not been designed; and to use an extreme example, supposing a large diameter cutter gets into machine where the speed of feed is only designed for smaller diameter, that large cutter is handicapped, because having more teeth it should take more feed, but cannot because the machine will not admit, and therefore a more effective cutter becomes defective.

It is a small point, but nevertheless an important one, to bestow care upon the roots of the teeth, keyways, and all corners, in order that they have a good radius, for it is well-known fact that square corners will go in hardening Mild steel sections, with ordinary wear and tear, are not permitted to have square corners.

Granted a true spindle, then it is unnecessary to have bearing for the full length of the shank of the cutter, but a portion of its length should be backed or hollowed on the cones top and bottom only being a good fit. For large horizontal cutters, placed two or more upon one spindle, the cones are dispensed with, parallel bearings being used, and chambering is always resorted to, as the ends only are required to be a good fit, and it is a difficult matter to lap the middle out to a standard without enlarging the ends. This recess should be as little as possible, 1/32 inch in diameter being quite sufficient.

It is needless to make comparison between milling and other machine tools, or to discuss the detrimental effects of cutters being untrue, blunt, or many of the other defects which may present themselves to the practical mind. It is also difficult to estimate and compare the cost of work done by milling with that of other machines, because sometimes the milling machine embodies the work of various others. Radii, both inside and out, are formed by the cutter, etc, but where it replaces slotting or planing only, its superiority is at once recognised.

The machine itself must be of sufficient power in every respect to overcome a maximum exertion, and also perfectly rigid. In fact, too much cannot be said for rigidity. Let any part of the machine be defective, and it will rebound upon the work done. Some examples are, insufficient support of the spindles, or the table upon which the work is cramped only bearing upon one-half of one of the slides. This table should be so constructed that the work to be done never overhangs the bearing slides of the table whilst under operation of the cutter, as the author has observed it to do in some machines. Any improvement in driving gear to produce steadiness is at once adopted, and a worm-driven cutter would be an approach to perfection. The speed and feed should also admit of easy regulation for the cutters of various diameters. The feed should be adjusted in such a maimer that it may be easily altered if necessary without stopping, otherwise the surface will be injured. This is done mostly by the aid of cone pulleys, having three steps, which represent, say, ¾ inch, 1 inch, and 1¬ inch feed per minute, which does not perhaps achieve the above regulation. There are also various other designs which perhaps approach more perfectly the change of feed during operation, effected generally by a frictional agency, some having an index showing the number of revolutions of the cutter per inch of feed. Transverse cuts are also maintained by feed gear, but irregular profiles are produced by the use of a cone, kept by the aid of weights against a former. The cone is necessary, because any alteration in the depth of cut can then be obtained by an adjustment of the cone either up or down. Finally, a good lubricant, copiously applied to the cutter, is a necessity.