Page:Practical Treatise on Milling and Milling Machines.djvu/68

62 will be cut by gears, with 48, 72, 32 and 40 teeth, the first two being used as driven gears? Spiral to be cut equals $$\frac{10\times48\times72}{32\times40}=27$$ inches to one turn.

This rule is often of service in determining what spirals may be cut with the gears the workman chances to have at hand.

The tables on pages 226 to 228 give the leads and approximate angles of some spirals produced by the gears furnished with our machines, and the combination of gears given in each case is such that they will properly mesh with one another. The tables on pages 229 to 247 contain all the leads that can be obtained with any possible combination of the change gears furnished, even though some of the leads are not available for use on account of the gears interfering or not reaching. Combinations of gears that are too small in diameter to reach for right-hand spirals, can generally be used for left-hand spirals, as the reverse gear is then required and will enable the gears to reach.

As we have already mentioned, the two driving gears, or the two driven gears of any combination can be transposed, but a driver must not be substituted for a driven or vice versa. Four different arrange- ments of the gears of any combination are thus possible, without changing the ratio, and when one arrangement interferes, or will not reach, the others should be tried. Thus, the gears to give a lead of 3.60" are: drivers, 100 teeth and 32 teeth; driven, 24 teeth and 48 teeth. By transposing the gears, the following four arrangements may be obtained.

The first arrangement, however, is found by actual test to be the only one available, owing to the interference of the gears in the other combinations preventing their meshing properly. When very short leads are required, it is preferable to disengage the worm wheel and connect the gearing directly to the spiral head spindle (using the differential indexing centre). This method gives leads one-fortieth of the leads given in the table for the same combinations of gears. Thus, for a lead of 6.160", the table calls for gear on worm,