Page:Practical Treatise on Milling and Milling Machines.djvu/164

158 correct, the amount the table is set out of centre is too great. In either case, the settings must be changed, and the operations of cutting repeated, remembering that the blank must be rotated and the table moved the same amount each side of centre, otherwise the teeth will not be central. It is well to bear in mind that too much out of centre leaves the small end proportionately too thick, and too little out of centre leaves the small end too thin.

The adjustment of the cutter and the rotating of the blank are shown in Fig. 66, which shows the setting, so that the right side of cutter will trim the left side of tooth and widen the large end of the space. The table has been moved to the right and the blank brought to the position shown, by rotating it in the direction of the arrow; the first out of centre cut was taken when the cutter was set on the other side of the centre.

After determining the proper amount to set cutter out of centre, the teeth can be finished, without making a central cut, by cutting round the blank with the cutter set out of centre, first on one side and then on the other.

To prevent the teeth being too thin at either end, it is important, after cutting once around the blank with cutter out of centre, to give careful attention to the rotative adjustment of the gear blank, when setting the cutter for trimming the opposite sides of the teeth. If by measurement, both ends are a little too thick, but proportionately right, rotate the gear blank and make trial cuts until one tooth is of the correct thickness at both ends. The cutting can then be continued until the gear is finished. Teeth of incorrect thickness may be more objectionable than a slight variation in depth. The finished spaces, or teeth, as already mentioned, are of the correct form at the larger ends, and the teeth are of the correct thickness their entire length, but the tops of the teeth at the small ends are not rounded over enough. It is, therefore, generally necessary to file the faces of the teeth slightly above the pitch line at the small ends, as indicated by the dotted lines F F, Fig. 67. In filing the teeth, they should not be reduced any in thickness at or below the pitch line.

When cutting cast iron gears coarser than five diametral pitch, it is best to make one central cut entirely around the blank before attempting to find the correct setting of the cutter or rotation of the blank for correct thickness of teeth; and it is generally advantageous to take a central cut on nearly all bevel gears of steel.