Fig. 32. Sectional Views of Quick Change-gear Mechanism
With this mechanism, eight changes for different threads or feeds are obtained by simply placing gear S into mesh with the various sized gears in cone C. As the speed of shaft T depends on which of the three gears d, e and f are locked to it, the eight changes are tripled by changing the position of knob K, making twenty-four. Now by shifting idler gear I, three speed changes may be obtained for gears a, b and c, which rotate together, so that the twenty-four changes are also tripled, giving a total of seventy-two variations without removing any gears, and if a different sized gear s were placed on the spindle stud, an entirely different range could be obtained, but such a change would rarely be necessary. As shown in [Fig. 30], there are eight hardened steel buttons B, or one for each gear of the cone C, placed at different heights in the casing. When lever L is shifted sidewise to change the position of sliding gear S, it is lowered onto one of these buttons (which enters a pocket on the under side) and in this way gear S is brought into proper mesh with any gear of the cone C. To shift lever L, the handle is pulled outward against the tension of spring r ([Fig. 32]), which disengages latch l and enables the lever to be lifted clear of the button; yoke Y is then raised or lowered, as the case may be, and lever L with the sliding gear is shifted laterally to the required position.
Fig. 33. Index Plate showing Position of Control Levers
for Cutting Threads of Different Pitch
The position of lever L and knob K for cutting threads of different pitches is shown by an index plate or table attached to the lathe and arranged as shown in [Fig. 33]. The upper section a of this table shows the different numbers of threads to the inch that can be obtained when idler gear I is in the position shown by the diagram A. Section b gives the changes when the idler gear is moved, as shown at B, and, similarly, section c gives the changes for position C of the idler. The horizontal row of figures from 1 to 8 below the word “stops” represents the eight positions for lever L, which has a plate p ([Fig. 30]) just beneath it with corresponding numbers, and the column to the left shows whether knob K should be out, in a central position, or in.
In order to find what the position of lever L and knob K should be for cutting any given number of threads to the inch, find what “stop” number is directly above the number of threads to be cut, which will indicate the location of lever L, and also what position should be occupied by knob K, as shown in the column to the left. For example, suppose the lathe is to be geared for cutting eight threads to the inch. By referring to section a we see that lever L should be in position 4 and knob K in the center, provided the idler gear I were in position A, as it would be ordinarily, because all standard numbers of threads per inch (U. S. standard) from 1/4 inch up to and including 4 inches in diameter can be cut with the idler gear in that position. As another illustration, suppose we want to cut twenty-eight threads per inch. This is listed in section c, which shows that lever L must be placed in position 3 with knob K pushed in and the idler gear shifted to the left as at C.
The simplicity of this method as compared with the time-consuming operation of removing and changing gears is apparent. The diagram D to the right shows an arrangement of gearing for cutting nineteen threads per inch. A 20-tooth gear is placed on the spindle stud (in place of the regular one having 16 teeth) and one with 95 teeth on the lead-screw, thus driving the latter direct as with ordinary change gears. Of course it will be understood that the arrangement of a quick change-gear mechanism varies somewhat on lathes of different make.