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Helical gear cutting setup

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gbritnell

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Since Chuck Fellows published the drawings for making and using the helical gear fixture I have made quite a few of them. The problem with the whole setup is the indexing/cutting fixture is always at some angle to cut the required gears. At the cutting end of the mandrel is an arbor to hold whatever blank you're going to be cutting. I have posed the question to machinist friends and thought about it myself but couldn't come up with a reasonably accurate way of centering the cutter both vertically and transversely relative to the centerline of the gear blank.

I'm in the process of starting a new project with the completion of the Galion road grader and thought I would like to make another transmission, this one for my flathead engine. My son knows a fellow that has several of them and he offered one to me so that I could take the necessary dimensions. When I built the Borg-Warner T-5 transmission I used spur gears instead of the normal helicals because of the large amount of gear cutting templates required to make all the gears. With this one which has fewer gears I thought I would make the helicals.

Now back to the problem at hand. The gears will all be .250 wide so I made a setup gauge, if you will, that mounts onto the end of the holding arbor. I started by turning a piece of stock that fits the arbor and is .25 wide, the same as the gears. I cross drilled it .093 diameter and inserted a pin in each side, one with a point and one flat ended. I moved the shaft of the fixture to approximately halfway of it's travel and locked it with the locking screw. The next step is to secure the gauge to the arbor and align it horizontally by eye. I then used the dial indicator to get it completely flat. Step three was to use the edge finder to get to the center of the pin. This was done and -0- set on the readout. (X travel)
 

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gbritnell

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The home-made gear cutter was secured in the holder and the table (X) was moved to center. The spindle was turned so that the edge of one of the flutes on the cutter was lined up with the point on the gauge. Now the Z was set with a magnifying glass. This is probably the only inaccurate part of the whole setup but for model work it should be very close. This give me the center of travel of the fixture shaft, X position and Z depth, which should be the centerline of the gear blank.
I made some test blanks out of Nylon to test out the setup. The gears are 24 DP, 14 teeth, .660 P.D. and 28 helix angle. A spread sheet was made by Don1966 and it is a great tool when working with gears.
My shaft center spacing is .875 so finding a DP and tooth count for spur gears isn't that big of a problem but when going to helicals the tooth count changes relative to the helix angle. With Don's worksheet you can plug in the variables and keep changing the helix angle until you come up with the proper center spacing. He also expanded on Chuck Fellows contribution by giving the numbers required to create the helical templates.
For a pair of helical gears 2 templates are required, on left hand and one right so this gear box will need 9 templates. Now you see why I didn't go that route with the T-5 trans.
I cut the nylon gears just to verify angles and mesh somewhat then cut one steel gear. I didn't check center spacing but that can be adjusted when I get into making all the steel gears.
 

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