Page:Practical Treatise on Milling and Milling Machines.djvu/74

68 at c, where it may be increased or diminished according to the angle employed in the operation, the line of cutter action being represented by ii.

Before backing off, the surface of the smaller drills in particular should be colored with a solution of sulphate of copper, water and sulphuric acid. This solution can be applied with a piece of waste, and will give the piece a distinct copper color. The object of this is to clearly show the action of the mill on the lip of the drill, for, when satisfactory, a uniform streak of coppered surface the full length of the lip from the front edge g back to e, is left untouched by the mill.

The above-mentioned coloring solution can be made by the following formula:

It is sometimes preferred to begin the cut at the shank end. By starting the cut in at this end, the tendency to lift the drill blank from the rest is lessened.

The table given on page 326 is useful for determining the cutters, pitches, gears and angles for twist drills. Cutting Left-Hand Spirals. When giving directions for cutting spirals in any of the foregoing pages, right-hand spirals are at all times referred to. For the production of left-hand spirals, the only changes necessary are the swinging of the saddle to the opposite side of the centre line, and the introduction of an intermediate gear upon the stud, Fig. 12, to engage with either pair of change gears for changing the direction of rotation of the spiral head spindle. Cutting Spirals with an End Mill. When spirals cannot be conveniently cut with side or angular milling cutters, as previously described, it is sometimes convenient to use end mills, as for example, when the diameter of the piece is very large, or the spiral is of such a lead that the table cannot be set at the requisite angle, the work is so held that its centre and that of the mill will be in the same plane and the saddle is set at zero.