Page:Practical Treatise on Milling and Milling Machines.djvu/162

156 pitch. In the case of 6 pitch s + f = 0.1928 inch. The thickness of the cutter at the pitch line is then found to be 0.1745 inch. This dimension will vary with different cutters, and will vary in the same cutter as it is ground away, since formed bevel gear cutters are commonly provided with side relief. Substituting these values in the formula, the following result is obtained:

Set-over = $0.1745⁄2$ - $0.280⁄6$ = 0.0406 inch, which is the required dimension. After selecting a cutter and determining how much to set it out of centre, proceed as follows:

Set the cutter central with the spiral head or universal index head spindle, as the machine may be equipped. Set the head to the proper cutting angle.

Set the index head for the number of teeth to be cut, placing the sector on the straight row of holes that are numbered to start with.

Set the dial on the cross feed screw to the zero line. Scratch the depth of both the large and small end of the tooth to be cut in the blank.

Index and cut two or three grooves or centre cuts to conform to the lines in depth.

Set the cutter out of centre the trial distance, according to the formula on the previous page, by moving the saddle and noting adjustment on the cross feed screw dial.

Rotate the gear in the opposite direction from that in which the table is moved off centre (Fig. 66), until the side of the cutter nearest the centre line of the gear will cut the entire surfaces of the approaching sides of the teeth.

After making one or more cuts in accordance with this setting, move the table the same distance on the opposite side of the centre and rotate the gear in the opposite direction from that in which the table is moved until the cutter just touches the side of a tooth at the small end and cuts the entire surface of this side the same as the other.

Cut one or more spaces and measure the teeth at both large and small ends, either with a gear tooth vernier or with gauges made from thin pieces of metal and having a slot cut to give the correct depth and width at the pitch line.

If the teeth at the large end are too thick when the small end is correct, the amount to set the table out of centre must be increased. On the other hand, if the small end is too thick when the large end is