Page:Practical Treatise on Milling and Milling Machines.djvu/64

58 Aliquant or Fractional Spacing.

Example 4:


 * Required: A Vernier to read to $1⁄12$ degree or five minutes, the scale being divided to degrees.
 * Each Vernier space can equal $11⁄12$ degree.
 * $$\frac{11\times1}{12\times360}=\frac{11}{4320}\text{or}\frac{4320}{11}$$ spaces in whole circle = $392 8⁄11$ spaces.
 * Assume H = 18, n = 2.
 * Then $$\frac{\left(392\tfrac{8}{11}\times2\right)-\left(18\times40\right)}{18}=\frac{720/11}{18}=\frac{720}{11}\times\frac{1}{18}=\frac{40}{11}=\frac{64\times100}{40\times44}$$
 * One idler is required.

CUTTING SPIRALS. Spirals that are most commonly cut on milling machines embrace spiral gears, spiral mills, counterbores, and twist drills. Worms are also cut with the aid of a vertical spindle or universal milling attachment. Examples of some of these classes of work are shown in this chapter, and in operations in chapters VIII and IX.

The method of producing the spiral movement of the work has been described before, and the manner in which the head is geared is shown in Figs. 11 and 12. The four change gears are known as: gear on screw; first gear on stud (as it is the first to be put on); second gear on stud; and gear on worm. The screw gear and first gear on stud are the drivers, and the others are the driven gears. By using different combinations of the change gears furnished, the ratio of the longitudinal movement of the table to the rotary movement of the work can be varied ; in other words, the leads of the spirals it is possible to cut are governed directly by these gears. Usually they are of such ratio that the work is advanced more than an inch while making one turn, and thus the spirals cut on milling machines are designated in terms of inches to one turn, rather than turns, or threads per inch; for instance, a spiral is said to be of 8 inches lead, not that its pitch is 1-8 turn per inch.

The feed screw of the table has four threads to the inch, and forty turns of the worm make one turn of the spiral head spindle; accordingly, if change gears of equal diameter are used, the work will make a complete turn while it is moved lengthwise 10 inches; that is, the spiral will have a lead of 10 inches. This is the lead of the machine, and it is the resultant of the action of the parts of the machine that are always employed in this work, and is so regarded in making the calculations used in cutting spirals.