Page:Practical Treatise on Milling and Milling Machines.djvu/156

150 Care should be exercised that the teeth of a cutter selected are ground radially and equidistant, for the teeth are so formed that unless ground in this manner, the correct shape is not produced in the work.

If a universal milling machine is employed, the table should be set at exact right angles to the arbor by the graduations on the saddle. This precaution does not have to be taken on plain machines, as the table is fixed at right angles to the spindle or arbor.

Set Cutter Central. It is essential that the cutter be exactly central with the axis of the gear blank, especially when the gear is to be run fast, otherwise the gear will be cut "off centre," and will run more noisily in one direction than in the other. It may be set centrally as follows: Set the table or the cutter on the arbor as nearly as possible in position ; fasten the gear blank, or preferably an odd blank of about the size of the gear to be cut, on an arbor and lock it in position on the centres. Take a single cut, then remove the blank from the arbor, turn it end for end and put it back on. Permit the blank to remain loose on the arbor, and see if the cutter will pass through the groove already cut without taking any stock off on either side. If the cutter is not exactly central, stock will be cut from the upper part of one side of the groove and from the lower part of the opposite side of the groove. If this is found to be the case, the table can be slightly adjusted to compensate for the error and another trial cut taken.

Some of the gear cutters made by us have a line on the tops of the teeth that is central with the form, and for ordinary slow running gears, the cutter may be centred by bringing this line to coincide with the centre in the spiral head or foot-stock.

Measure Blanks. Measure all gear blanks carefully. It is impossible to cut correct running gears from blanks that are of the wrong diameter unless the error is small. The amount of error allowable in the diameter depends upon the pitch of the gear; the heavier the pitch, the greater the allowable error. It is better to return to the lathe any blanks that are oversize and throw away those that are turned very much undersize. If blanks are only slightly undersize, they can be cut by making allowance for the error in setting for depth of teeth, and the resultant gears will run satisfactorily, though not perfectly.

Secure Blank on Arbor. The next important step is to see that the work arbor runs true and that the blank does not spring it when