Although just two .250" long gears are needed, several .4" long brass and 12L14 steel blanks were prepared. The longer blanks nearly doubled the machining time, but the longer spares might be more useful in some future project.
In order to set up the work offsets, I made the alignment tool George came up with for use with Chuck's helical fixture. It looks deceptively simple, but it took a couple tries to get one indicating true on a surface plate. The tool was installed on the rotary's mandrel and after indicating the two arms parallel to the mill's table, the rotary's DRO was zero'd. An edge finder was then used to indicate the spindle to the center of one of the arms so the X DRO could be zero'd. The edge finder was then replaced with the cutter which was visually aligned with the pointed end of one of the arms so the Z DRO could be zero'd. Finally, the alignment tool was replaced with a gear blank. After positioning the cutter at X=0, Z=0, it was touched off to the blank's outside surface, and the Y DRO zero'd.
An end point for the cutting passes was then chosen. With the work offsets completed and the rotary pointing up at 45 degrees, equal moves along the X and Z axes keep the cutter properly aligned to the blank. With cutting taking place on the front side of the blank, the cutter will travel upward at a 45 degree trajectory while the rotary turns CCW. In my particular setup, a suitable end point where the cutter just clears the blank turned out to be X=.250, Z=.250. For convenience, the cutter was moved to this location and the X and Z DRO's zero'd for the last time. A suitable starting point for the cutting operation turned out to be X=-.450, Z=-.450.
Between my particular start and stop points, the cutter will travel a total distance of .450" x 1.414 = .6363" along the .400" long blank. One last parameter needed for coding is the gear's lead which is the length over which one full rotation of the gear's helix will occur. Lead is a function of the circumference of the gear's pitch circle and helix angle. It's calculated as L = [pi * N]/[DP * sin(A)] where N is the number of teeth and A is the angle of the helix. For this particular gear, L = .8639". Therefore, in my setup, the cutter's .6363" travel will require a simultaneous rotary movement of 360 x (.6363/.8639) = 265.196 degrees.
A full listing of the g-code is included. Since I was a newbie with the helical gear making process and still troubleshooting my cutter debacle, I didn't attempt a universal program with parameters. I did include plenty of comments. With the machine in incremental mode (G91) and a starting point at (X-.450, Z-.450, Y-.015, A0), the single line of code that does the actual cutting is simply:
G1 X.450 Z.450 A-265.196
During testing, I found it best to machine the .030" teeth in two .015" passes. For reasons I still don't understand, there was a lot more noise and vibration while cutting the brass gears compared with the steel gears which machined dead quiet.
Due to burrs raised by the cutter, the resulting o.d.'s of both gear types consistently came out .007" greater than the starting diameters of their blanks. They were finished on a lathe by re-facing their ends and skimming their diameters back to their blanks' original values. This, of course, left yet another tiny burr on the inside edge of each tooth. These were manually cleaned off using a cobbled-up de-burring tool fashioned from a Nicole .040" diameter profiling insert. Drawing this tool just once through the space between each pair of teeth removed these tiny burrs without significantly affecting the tooth profiles.
The tooth profiles on the finished gears are different from that predicted by my cosine gear model. The difference is probably at least partially related to the non-zero diameter of the actual cutter. The final parts certainly look like helical gears and they do mesh smoothly, but I don't expect a true involute contact line.
One last sanity check is the measurement of the axial spacing between a pair of freely turning 90 degree meshed gears adjusted for minimum backlash. The theoretical number is .275" which is the pitch diameter. The measured spacing of the four gears in my first batch ranged between .276" and .277". After some thought, I decided it would be smart to have a few more options to choose from in case the still-to-be-done drilling operation for the distributor driveshaft bore veers off its course. The diameters of two more batches of blanks were tweaked plus and minus .0015", and I wound up with two more batches of gears with spacings ranging between .271" and .273" and between .280" and .281".
The little shop-made cutter held up much better than I expected - so much better in fact that I don't believe I'll be buying any more expensive commercial cutters. - Terry
