Page:Practical Treatise on Milling and Milling Machines.djvu/157

Rh forced or tightened. A good method of holding blanks is on arbors, such as our milling machine cutter arbors, that have a taper shank to fit the index spindle; the outer end of the arbor being supported by the foot-stock centre. Another way of holding blanks is by means of a shank arbor with expanding bushing, such as our gear cutting machine "work arbors." A nut is located on the arbor at each end of the bushing, one nut forcing the bushing up on the arbor and holding the blank, while the other pushes the bushing off the taper and releases the gear when finished.

If a common arbor and dog are used, care should be taken that the tail of the dog is fastened between the set screws provided on the spiral head, so there will be no backlash between the index spindle and work; also see that the dog does not spring the arbor when it is clamped.

Set Knee for Depth of Cut. The depth of cut is regulated by the height of the knee of the machine. To make this setting, the knee is brought up until the cutter just touches the blank. Then the blank is moved out from under the cutter and the knee is raised the number of thousandths of an inch required for the depth of tooth, which can be ascertained from the tables on pages 321 to 324, or by dividing the constant 2.157 by the diametral pitch.

When raising the knee, use the graduated dial on the vertical hand feed screw for a guide to get the required depth, but be sure to take out any backlash that may exist before making an adjustment.

Testing for Correct Depth. To make certain that the depth of groove cut is correct and the size of teeth accurate, cut two grooves into the face of the blank far enough so that the full form of the tooth is produced, and then measure the resultant tooth at the pitch line for thickness and the depth of the tooth to the pitch line. The correct thicknesses of spur gear teeth of different pitches at the pitch line are given in the tables on pages 321 to 324, or can be found by dividing the constant 1.57 by the diametral pitch. By cutting only part way across the face of the blank the trial grooves can be quickly made and measured. If, on the other hand, the grooves are cut across the full width of the face, there is liability, under some conditions, of more stock being taken from these grooves when the actual cutting is commenced and the cutter is allowed to pass through the same grooves a second time, thus making these grooves too deep.

Chordal Thickness of Gear Teeth. When accurate measurements of gear teeth are required, it is necessary to work to the chordal