Cutting gear blanks woodworker style?

dewbane

Michael McIntyre
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Now that I have accepted that you don't make a 3.5" diameter gear by slicing off a hunk of exorbitantly expensive round bar, I am starting to accept that my future is going to involve cutting a lot of circles out of brass sheet. I'm not excited by the prospect of doing it the usual way, where you rough cut it with a saw.

I started wondering about the "fly cutter" I used to use on my drill press when I did woodwork. That would work on brass, wouldn't it? Hmmm.

Something like this:

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That thing sure was some ragged piece of misery, but it worked. Seems like the exact same tool would probably work on brass, but I might like to try to make something fancier. The thought is only half formed, but it seems like if I planned it right, I could mount the cutty bit on a screw, and give the thing a nifty little dial that could make exact cuts with micrometer level precision. I wouldn't want a twist drill making a wreck of the center. How about a center point, and a feed mechanism for plunging the cutty bit. Run a few passes, stop the machine, feed, repeat. It would be even better if I could engineer some kind of inertial-mass powered whirly gig doodad to advance the feed screw automagically, but that is probably aiming too high. Another possibility is to just drill and ream the center, and use a bit of drill rod. It would work harden the center, but that might not be a bad thing.

I'm just kind of thinking out loud, and thought I'd see what you guys have to say. Does this seem like a ridiculous waste of perfectly good time, or a nifty outside the box approach to a tedious problem?
 
I've used those "fly cutters" or "trepanning tools" to cut both holes and discs in wood, aluminum and steel. It can be done but it's a finicky process and can be quite dangerous if not set up properly. As far as accuracy, I would say maybe +- 5 thou at best, and that's with some fussing. Best to cut them slightly oversized and turn them to final dimension on the lathe.
Yes a smooth pilot is better than a drill, use lube go slow and make sure the work is extra secure.
Here's a link to a video clip I did cutting some aluminum discs a while back:
 
Clockmakers often cut them by hand with a coping saw, but not too practical for large quantities. What about a small vertical bandsaw with a very narrow fine tooth blade? Also, if the gears will eventually have spokes you could fasten the square blank to a piece of wood, fasten that to a faceplate and turn them round on the lathe.
 
I have done them all the different ways mentioned. They all work. Cutting them out of a large round bar seems like the most straightforward way, but any wandering of the saw really wastes expensive material. Parting them also works, but it is dicey on the larger sizes. Cutting out with a coping saw is faster than one would think, especially on soft metal, but it is tedious for a large quantity. Turning an octagon (square with corners trimmed) also works.
 
What about a hole saw or annular cutter for cutting out the blanks?

BTW: You probably know this but the "fly cutter" you are showing in the pic is setup to cut a hole. Rotating the fly cutting bit 90 degrees will yield a much better cut if you intend to keep the round piece as a blank.

I had a buddy that used to cut his round blanks with a plasma torch and a circle jig then turn them on his lathe. I don't know if that would be easier or not?
 
I've used those "fly cutters" or "trepanning tools" to cut both holes and discs in wood, aluminum and steel. It can be done but it's a finicky process and can be quite dangerous if not set up properly.

Ah, trepanning tool! I knew there had to be a machinist name for this thing. I guess all fly cutters have the common property that some kind of single tool is spinning, and there are more uses for fly cutters in metal, like form cutting and surface finishing. Anyway, finicky and dangerous is definitely an accurate description. Just cutting circles out of plywood on a drill press was plenty sketchy. Thanks for confirming that it would work on soft metal.

I use my rotary table and an end mill. I can make surprisingly accurate blanks that way with minimal fuss.

I don't have a rotary table, or cutting blanks with an end mill would be an attractive option.

I have done them all the different ways mentioned. They all work. Cutting them out of a large round bar seems like the most straightforward way, but any wandering of the saw really wastes expensive material. Parting them also works, but it is dicey on the larger sizes.

I gave that approach a try for my first test blank, and it went pretty badly indeed. Even though my bandsaw cuts pretty straight, "pretty straight" is a lot to face off on a blank of any size. I don't notice it so much on normal sizes of round bar. Just one light pass with a facing tool is all it ever takes. Getting just a 1.5" blank trued up was such a chore I ended up facing off one side, and machining the other face parallel in the mill.

This problem would be even more pronounced on the larger sizes, and I've also found larger material is just generally sketchy to get running in the lathe. The slightest bit of tool deflection, say when trying to part a 5" hollow cylinder in a benchtop lathe that isn't amazingly rigid. Well, that can go south all kinds of ways, and it went pretty badly. I spent hours on that project, and turned a lot of good money into huge piles of chips, but ultimately failed to produce the parts as designed.

No, cutting discs out of sheet is the way to go. Centuries of clockmakers aren't wrong on that score. In the time I've spent scratching my head here, I probably could have cut out six of them with a basic coping saw. I keep forgetting that I'm going to be working with a thickness that I could almost just cut out with shears. I don't actually have any of the material on hand yet, and the only plate I've ever worked with is steel of about 1/8" and thicker, which would be just inconceivably miserable to cut with my scroll saw. It would take days and eat 42 blades.

What about a hole saw or annular cutter for cutting out the blanks?
Both could produce good results if I had the right sizes on hand. These aren't available in especially fine increments, and it seems like I would waste more material by having to shave off 1/2" or 3/4" or whatever. Plus annual cutters are pretty expensive in the larger sizes.

BTW: You probably know this but the "fly cutter" you are showing in the pic is setup to cut a hole.
I do. It was just some random web picture.
 
I've done the plasma method too, it also works well.

Good excuse for a rotary table... Even a small import would do the job if you put a little thought into dealing with the plate overhanging.
 
I've done the plasma method too, it also works well.

Good excuse for a rotary table... Even a small import would do the job if you put a little thought into dealing with the plate overhanging.

I'm starting to think about it, but after I had a look at the options, I realized it's a whole new research project. I need to think beyond cutting circles, and consider what I would do with a rotary table. Buy nice or buy twice, right?
 
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