One of the sides which forms the right angle represents the circumference of the piece in inches, and the hypothenuse represents the
line of the spiral. The angle between the lines representing the
path of the spiral and the lead of the spiral is the angle of the spiral.
This angle can be transferred from the drawing to the work by a
bevel protractor, or even by cutting a paper templet and winding
it about the work as shown in Fig. 14. The machine is then set
Fig. 15
so that the spiral or groove as it touches the cutter will be in
line with the cutter. Or the angle may be measured and the
saddle set to a corresponding number of degrees by the graduations on the base.
The natural tangent of the angle of the spiral is the quotient of the circumference of the piece, divided by the lead of the spiral. Accordingly, the second method of obtaining the angle of the spiral is to divide the circumference of the piece by the lead, and note the number of degrees opposite the figures that correspond with the quotients in the tables of natural tangents, pages 309 to 317. The angle having been thus obtained, the saddle is set by the graduations on the base.
This second method is more satisfactory, as it is Fig. 15 more accurate, and there is less liability of error than with the first. The saddle can be set to the proper angle, but before cutting into the blank, it is well to let the mill just touch the work, then run the work along by hand and make a slight spiral mark, and by this mark see whether the qjiange gears give the right lead.
Special care should be taken in cutting spirals that the work does not slip, and when a cut is made it is well to drop the work away from the mill while coming back for another cut, or the mill may be stopped and turned to such a position that the teeth will not touch the work while the table is brought back preparatory to another cut.
Setting Cutter Central. In making such cuts as are alike on both sides, for instance, the threads of worms or the teeth of spiral gears, care must be taken to set the work centrally perpendicular with the centre line of the cutter before swinging the saddle to the angle of the spiral.
Cuts that have one face radial, especially those that are spiral, are best made with an angular cutter of the form shown in Fig. 15, as cutters of this form readily clear the radial face of the cut, keep sharp for some time and produce a smooth surface.
