The front mounted distributor is driven from the camshaft through a helical gear set. Specialized equipment is used to manufacture commercial helical gears, but usable one-offs can be made in a home shop using conventional or, if necessary, shop-made gear cutters.
Involute cutters are designed to cut accurate (although not necessarily perfect) tooth profiles square to the face of the cutter. This isn't how helical gear teeth mate, and a mismatch occurs that grows with the angle of the helix. For the same DP, the pitch diameter of a helical gear is greater than the pitch diameter of a spur gear by a factor of 1/cos(A) where A is the helix angle. That is, PD = N/DP for a spur gear, and PD = [1/cos(A)] * N/PD for a helical gear where N is the number of teeth. The remainder of the spur gear equations apply so long as the PD is modified accordingly.
When cutting a helical gear with a conventional involute cutter, common shop practice is to use a cutter designed for a somewhat greater tooth count. The usual multiplier is 1/[cos(A)]^3. For example, to cut a 16 tooth 48DP 45 degree helical gear, a #3 48DP involute gear cutter intended for 45 teeth would be used.
Early modeling which focused on coming up with a faithful reproduction of the engine's front end eventually arrived at a suitable solution based upon a 48 DP involute cutter that I owned. The solution was a pair of 16 tooth 45 degree helical gears with a center-to-center spacing of .470". If a suitable 48 DP solution hadn't been possible, a custom DP button cutter would have been machined as was necessary for the gear set in my Ford Inline six build.
The biggest drawback to machining helical gears in the home-shop has been the need to coordinate the movement of the cutter along and around the longitudinal axis of the gear blank. Chuck Fellows solved this problem for us several years ago when he published his design of a fixture capable of coordinating these moves on a lathe or on a manual mill. In my case I used a four axis setup on my Tormach with the rotary positioned under the spindle at a 45 degree angle.
The form factors of the two gears are not the same (different shaft i.d.'s, lengths, and end features). A number of identical brass and steel blanks were prepared whose ends would be finished after cutting the teeth. The camshaft gear was machined from steel while brass was used for the distributor gear. The gear cutter was mounted in the spindle in a commercial holder, but a custom mandrel was needed to hold the gear blanks while staying out of the way of the gear holder. A shop-made alignment tool was also needed to align the rotary to the cutter.
One of the photos is a screenshot of the scratch-written g-code which is heavily commented with descriptions of the cutting parameters. For anyone interested, my earlier Inline six build on this forum contains more information.
The machining of the very first pair of gears went without a hitch, and a measurement of their spacing matched the target pitch diameter. The next step was to verify their fit inside the block.
With the design of only the lower half of the distributor completed, a dummy lower section was run with the camshaft to test the gear set inside the block. The gears' end features were finish machined, and a reducer was pressed into the brass gear to accommodate the distributor shaft. The gears turned smoothly inside the block with no tight spots and a backlash that appeared to be some 2-3 degrees.
The two tenon'd camshaft sections were then permanently joined together inside the steel drive gear with Loctite 638. Fortunately, I remembered to capture the front ball bearing and its retainer between the two camshaft sections during the assembly. This was needed because both the gear and cam lobes are too big to pass through the inner race of the front ball bearing which otherwise couldn't be installed in the block.
I wasn't expecting the very first pair of gears to wind up inside the engine. A precise measurement of the spacing inside the block had been difficult to make beforehand, and I was prepared to do some tweaking. Before taking down the machining setup the left over blanks were turned into helicals for future use. - Terry
Involute cutters are designed to cut accurate (although not necessarily perfect) tooth profiles square to the face of the cutter. This isn't how helical gear teeth mate, and a mismatch occurs that grows with the angle of the helix. For the same DP, the pitch diameter of a helical gear is greater than the pitch diameter of a spur gear by a factor of 1/cos(A) where A is the helix angle. That is, PD = N/DP for a spur gear, and PD = [1/cos(A)] * N/PD for a helical gear where N is the number of teeth. The remainder of the spur gear equations apply so long as the PD is modified accordingly.
When cutting a helical gear with a conventional involute cutter, common shop practice is to use a cutter designed for a somewhat greater tooth count. The usual multiplier is 1/[cos(A)]^3. For example, to cut a 16 tooth 48DP 45 degree helical gear, a #3 48DP involute gear cutter intended for 45 teeth would be used.
Early modeling which focused on coming up with a faithful reproduction of the engine's front end eventually arrived at a suitable solution based upon a 48 DP involute cutter that I owned. The solution was a pair of 16 tooth 45 degree helical gears with a center-to-center spacing of .470". If a suitable 48 DP solution hadn't been possible, a custom DP button cutter would have been machined as was necessary for the gear set in my Ford Inline six build.
The biggest drawback to machining helical gears in the home-shop has been the need to coordinate the movement of the cutter along and around the longitudinal axis of the gear blank. Chuck Fellows solved this problem for us several years ago when he published his design of a fixture capable of coordinating these moves on a lathe or on a manual mill. In my case I used a four axis setup on my Tormach with the rotary positioned under the spindle at a 45 degree angle.
The form factors of the two gears are not the same (different shaft i.d.'s, lengths, and end features). A number of identical brass and steel blanks were prepared whose ends would be finished after cutting the teeth. The camshaft gear was machined from steel while brass was used for the distributor gear. The gear cutter was mounted in the spindle in a commercial holder, but a custom mandrel was needed to hold the gear blanks while staying out of the way of the gear holder. A shop-made alignment tool was also needed to align the rotary to the cutter.
One of the photos is a screenshot of the scratch-written g-code which is heavily commented with descriptions of the cutting parameters. For anyone interested, my earlier Inline six build on this forum contains more information.
The machining of the very first pair of gears went without a hitch, and a measurement of their spacing matched the target pitch diameter. The next step was to verify their fit inside the block.
With the design of only the lower half of the distributor completed, a dummy lower section was run with the camshaft to test the gear set inside the block. The gears' end features were finish machined, and a reducer was pressed into the brass gear to accommodate the distributor shaft. The gears turned smoothly inside the block with no tight spots and a backlash that appeared to be some 2-3 degrees.
The two tenon'd camshaft sections were then permanently joined together inside the steel drive gear with Loctite 638. Fortunately, I remembered to capture the front ball bearing and its retainer between the two camshaft sections during the assembly. This was needed because both the gear and cam lobes are too big to pass through the inner race of the front ball bearing which otherwise couldn't be installed in the block.
I wasn't expecting the very first pair of gears to wind up inside the engine. A precise measurement of the spacing inside the block had been difficult to make beforehand, and I was prepared to do some tweaking. Before taking down the machining setup the left over blanks were turned into helicals for future use. - Terry
) am puzzled by the choice of CDI, neither D.K.Grimm nor anyone else has been able to come up with any compelling reason or evidence that it works better than LDI, either way you charge up a device (either capacitor or inductor) with a certain amount of energy and then let that go through a coil and some of it appears at the spark gap. So IMHO CDI offers additional complexity with no additional advantage (most even believe that given equal amounts of energy at the start the CDI delivers less to the spark, but my reading of D.K.'s experiments is that that seems un-verified, at least to this date).
