Only fools will goÖ step 5 -
Gear Cutting etc.
Before I can really advance any further with finishing or decoration of the movement I need to have all the functions working OK. Right now I still have one function not working satisfactorily; winding. There are two things that are causing problems there as I see it.
The solutions are:
Letís start with the more fun part, my first attempt at cutting a gear.
The deal is this, to cut a gear you need to have two turning axes, one for the gear blank and one for the cutter. The cutter turns continuously, but the blank just needs to be indexed at certain positions. I want to be able to cut this gear without purchasing any new and expensive machinery (or old and expensive machinery either). So I set out to find a way of using the two turning axes I have, that of the lathe and the milling machine together to reach my goal. After a couple of weeks of mulling over possibilities of mounting the lathe on the mill, mounting portions of the lathe on the mill, mounting the mill on the lathe, etc., I found a solution. I made a mount for the lathe headstock on the table of the mill. You can see the whole setup below.
On the left is the lathe headstock bolted horizontally on the table of the mill. The drive pulley of the headstock has 64 (I thought) indexing holes and there is a spring-loaded lever for indexing with these holes. In the chuck I made a fixture to hold the gear blanks. The picture below was made on the lathe as I turned the blanks to size, before the headstock was fixed on the mill.
Actually I am showing this all out of order. First I made the cutter from a piece of 6mm tool steel. I will show you how I did it and then describe how I should have done it.
I started out by turning down the shaft to 4mm leaving 2mm in the middle, which I will form, to the cutter. I tried to turn the cutter by hand and without enough magnification, which is a mistake. I didnít see it at all, only after cutting the gear did I see that the form was a tad lopsided, cutting me lopsided teeth. I thought that having two concentric shafts I could turn the cutter in the lathe and end up with something symmetric, but it didnít work cutting free hand as I did.
After I thought that I had the cutter form OK, I turned down one of the shafts eccentrically, that is the shim I used in the chuck to get 0.5mm eccentricity. I then removed the non-excentric shaft and filed down about one half of the cutter profile, the one tooth of the cutter is at the most eccentric point. That gives the relief behind the cutting edge and will also allow some sharpening in the future. Then hardening and tempering to pale straw color.
With the blanks on the fixture all mounted on the milling table as above and after centering the cutter in the middle of the axle and at the right distance from the center the actual gear cutting started. Advance and return the table, then turn the headstock by 2 index holes. Of course I discovered just before starting that there are 60, not 64 positions on the index on the lathe pulley. I decided to cut a gear anyway, just reduced the diameter as required to compensate for the two missing teeth.
The gear cutting went fast and easy. Below you can see the results, the gear and the backup blank.
After a little cleaning up to remove the burrs the hole in the center is turned concentric with the teeth on the lathe.
Above you see the finished gear along with the ETA original gear that I manhandled, just a slight difference, huh! It is no wonder that the winding was so difficult.
What is there to say about this work? Of course this gear is missing 2 teeth. Here in the winding mechanism they will never be missed, but I will make an index wheel for 32 teeth and with those two additional teeth the winding force will be reduced unnoticeably. The tooth engagement will also be better. You can see the lopsided teeth in the picture above. No, it is not from placing the cutter too high or low compared to the axle of the blank. Under the microscope one sees that the cutter is also lopsided. I think that it is impossible to cut a good tooth form free-hand on the lathe. For my next attempt I will make a graver with the form of one side of a tooth and use that on the cross table to form the cutter. Using the two shanks as I had made it should be no problem to do that from both sides and get a truly symmetric tooth form. There is also the finishing of the cutter. I did not polish it and that can be seen on the surface quality of the cut teeth. Polishing is necessary.
Another problem that can be seen is that one tooth is smaller than all the others. That is the last tooth cut, you can see it at about 4 oclock on the picture above. Apparently the blank turned imperceptably during cutting. The pressure from a single screw in the middle is not enough to keep everything in line. In the future I will glue the blanks to the fixture with nail polish or something similar to prevent such movement.
In general I would say that this first attempt has shown that cutting gears in this way is quite possible, but there are several details that need to be cleaned up. I have only tried cutting brass, but this ratchet wheel should be in steel, so this is also another step to take.
But in any case, substituting this gear for the manhandled ratchet wheel from ETA makes a tremendous difference in winding. But the Breguet teeth on the sliding pinion are still jumping.
A short consultation with my mentor. He thinks it is simply that the clutch lever spring is not strong enough and that after a while the teeth have worm making it even worse. So I made a new clutch lever spring, which is twice as thick. It is also longer placing the pressure point farther down the lever and thus increasing the mechanical advantage. Doubling the spring thickness should make the force 8 times as much, there is a cubic relationship in the spring calculation formula there.
Now I have not only smooth winding from the new gear and after replacing the sliding and winding pinions with new pieces no more jumping either.
It all looks good to me.
So I decide that I donít need to wait until I have remade the ratchet wheel to say that all is OK for the watch functionality. I now claim that I have a working movement and will start the finishing next.
There it is, a complete watch. Here you see all the pieces:
For the statistics lovers among you:
I will now cut out the movement, relieve it of its ďearsĒ and head out to find out how to do finishing.
So now I will head back to the top of the page, make a good cutter as I have explained above and try cutting that gear again. But also start the finishing of the plate and bridges.
Copyright © 2005 Donald W. Corson