# More Gear Cutting



## francist (Sep 13, 2019)

My foray into gear cutting continues although this is really the reason I embarked on it in the first place — a set of change gears.

I had already figured out how do the double keyway and that had worked fine, and the cutting of the gears themselves was going equally well until about two-thirds of the way through the set. Then I discovered, of all things, that I didn’t  have enough travel in the knee to cut the last three large sizes of gear!

I was (and still am) a bit dumbfounded as to how this could be, the gears are not that large. But regardless of whichever way I held my tongue I still could not get the mill table down far enough to obtain the proper tooth depth. Hmmm....

After much head scratching I came up with the plan to make a secondary pair of indexing centres with a lower centre height and move the gear cutting arbour further away from the cutter arbour on the machine. In order to keep the indexing feature of my original centres, I could drive this auxiliary pair of centres off the first pair with, what else, a pair of gears. So that’s what I did.

I turned two 60-degree centre points out of some 4140, hardened them over a propane torch, and supported them on a couple pieces of aluminum angle I had kicking around. Then made up two matching 32T acetal gears: one to press onto the end of the gear blank arbour and be the “driven” gear; the second or “driver” gear would mount on a stub arbour held in an ER collet that I could mount to the original indexing snout. It really helped that the 1”-10 thread on the indexing centre is the same as my lathe spindle which is also the same as the mill spindle so I already had a collet chuck and collets I could use. A second stub arbour was nothing fancy and easily done.




All in all it took about four times as long to build and figure all the new parts than it did to cut the entire set of gears, but isn’t that always the way. The setup does work, fairly well actually, although I can see areas to improve on. The mesh between the two driving gears is critical for accurate tooth spacing, and any amount of play can introduce a slight variance. For my purposes here I don’t think it’s significant, but if it were for a tighter expectation I’d want to revisit that part.




The gears themselves turned out fine. This set is kind of experimental anyway being out of Acetron, and I’m not sure how durable they’ll be. But it was a fun exercise that’s for sure, and I have a few new tricks in the bag for next time.







Thanks for looking.

-frank


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## HarryJM (Sep 13, 2019)

Creativity - love it!


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## brino (Sep 13, 2019)

That was some lateral thinking......great stuff!

My mind was going to a vertical gear arbour, mounting the gear parallel to the table and using the "side" of the cutter instead of the "bottom".
Then you'd feed with Z instead of X....I think....

Those gears look perfect.
I am very interested to hear about their longevity.

...and if you ever need to replace them with aluminum, you're all tooled up.

Thanks for showing this, Frank!

-brino


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## francist (Sep 13, 2019)

Yeah, lateral thinking, LOL. Most times I just call it off on a tangent....

Thanks for the kind words, I’ll let you know how they fare.

-frank


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## benmychree (Sep 13, 2019)

How did you go about the double key issue?


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## francist (Sep 13, 2019)

I made a broach bushing with two guide slots, 180 degrees opposed. The first slot is proper depth to cut the first keyway, but the opposite slot is finished keyway depth only. In use the first keyway is cut, the bushing rotated 180 degrees and a short length of key inserted into the shallow slot and the keyway just cut. This aligns the bushing exactly at 180 ( assuming the bushing was slotted accurately, that is ) so that the second keyway can be cut. 

It works very quickly because the one key sets the index for the other, and there is no need for repeated setup. I detailed the bushing and concept in an earlier thread here:









						Broach Bushing
					

Continuing along with my gear making expedition, I decided to try some broaching. Never done it before, and although the connection to gear making may not be readily apparent there actually is one.  I have two Atlas lathes, and in order to make up the compound change gear configurations they use...




					www.hobby-machinist.com
				




Thanks for asking.

-frank


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## silverhawk (Sep 15, 2019)

francist said:


> I made a broach bushing with two guide slots, 180 degrees opposed. The first slot is proper depth to cut the first keyway, but the opposite slot is finished keyway depth only. In use the first keyway is cut, the bushing rotated 180 degrees and a short length of key inserted into the shallow slot and the keyway just cut. This aligns the bushing exactly at 180 ( assuming the bushing was slotted accurately, that is ) so that the second keyway can be cut.
> 
> It works very quickly because the one key sets the index for the other, and there is no need for repeated setup. I detailed the bushing and concept in an earlier thread here:
> 
> ...



Brilliant!  Did you use a rotary table to get the 180-degree slots in the broach bushing?


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## francist (Sep 15, 2019)

No I did not, no rotary table nor spindexer here although I'd love to have one sometime.

My method was pretty crude actually: made a couple of V-groove liners for my milling vise to hold the arbour up near the top of the jaws, milled the first guide slot (the deep one), then slacked the vise enough so that I could turn the arbour 180 degrees as measured by viewing from the end next to a small machinist square (actually my favourite little 3" Moore & Wright adjustable machinist square, but that's beside the point ). Retightened the vise, and made the second cut.

That obviously isn't a terribly precision way to go about it, but weighing my various options at the time I figured it would give me as good or better results than some other methods I thought of. I did have a slight error although it wasn't in the angle of rotation, but more the centering of the slot on the arbour. I was off by a couple thousandths and I could see it once the second cut was made. The parts still fit, but they fit even better when I use a 0.002" shim against one side of the broach to bring the two slots into better line.

-frank


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