Hi Timo,
OH BOY! Gear cutting! Books, no make that Tomes, have been written on gear forms, cutter creation, it's a lovely can or worms. But - like screw cutting, if you just ignore all the drama it falls into place well enough to get perfectly usable results without undue stress. Perfectly usable is defined as gears that run well with others you make, or others purchased from commercial gear makers. It does not mean the gears are so utterly perfect that they will run silently at 500 RPM or be perfectly formed with no variance from the theoretical ideal form.
My usual fly cutters were used for cutting clock wheels, which are typically cycloidal rather than involute in form. I machined the cutters from A2 tool steel, then hardened after they were correctly shaped and relieved. An end mill the same or slightly smaller in radius than the radius of the tooth tip is used to rough out the cutter, with the piece of A2 inclined 7 degrees in the mill vise. Remember to do a cut across the end of the cutter to be, inclining the cutter blank generates all the relief needed for clock work in thin stock. If you need side clearance as well it can be added with a needle file or a small grinding point as you wish.
KEY POINT: People talk about cutting gear teeth. I never cut a gear tooth unless I was removing it to repair a gear. We cut gear spaces. We all know this, but thinking teeth while making gear space cutters has caused more than one person to get turned around and make a perfect single tooth on the end of the cutter, rather than a perfect copy of the space to cut.
For involute gears you can always use a gear of the desired pitch and number of teeth as a template and just layout the cutter by scribing from the gear space. There are quite a few online guides to making gear fly cutters, or for making form tools to create more conventional gear cutters. I believe Ivan Law's excellent book in the Workshop Series (number 17 I think) has been mentioned. If you start with hardened lathe tool blanks, grinding is about the only way to get the cutter made. I'd rather cut than grind, it's quicker and when done in mill a more controlled process for me, as I tend to have a hard time getting dead on the line of concave curves with a grinding point. Use the largest diameter end mill or grinding point you can that still fits within the curve. Let the tool generate the form as much as possible, leaving only refining the small detail work to hand work. In smaller module or higher pitch gears, you can usually find an end mill that is close enough to the theoretical size to work just fine. Let it be easy when you can.
Here are a couple of videos that might be of interest:
Both of these are using grinding and hardened cutter blanks. Lots of work and lots of mess in my book, but if you imagine the grinding tool being an end mill of close to desired diameter it will all fall into place. Depending on metal availability grinding may be the only way to go. Lathe bits are all over, good tool steel in annealed state may not be.
Here's a time consuming way to generate gears with minimal equipment. A perfect time vs money trade off option:
Here's a better way to create multi tooth involute approximating cutter that looks like a gear hob but does not require synchronized cutter and work spindles:
The interesting part starts around 2 and half minutes in, the beginning is a recap of traditional methods. There are several videos in this series, gears are actually made in the follow on video.
If you find it easier and the material you are making gears from is a wee bit less demanding, perhaps aluminum, brass, or leaded steel, you may find it easier to cut the tooth space profile on the end of round stock, then mill away half the diameter, just as when making D bits. Sounds iffy, but I used this method a lot for the nasty little stubby odd form teeth on some French round movement spring barrels.
Here is a pic of a cutter made this way, the cutter on the right is a fly cutter, the others are piloted oil sink and bushing cutters. These are probably oil hardening drill rod rather than A2. Quickie stuff knocked out to do a job, it isn't fancy. The bushing and oil sink cutters are hand turned in use, no effort to make them perfectly symmetrical as they will never spin fast.
You asked about indexing? However you can hold and index the blank in your shop is the way to do it. I made dividing plates that can be mounted on one of my spare Taig (Peatol) lathe headstocks that are great for quick jobs. For larger work there is a dividing head. A rotary table with dividing plates works well, as does the cute little automated rotary table Sherline in the US sells. I believe on your side of the Atlantic it's knows as the Division Master. A four axis CNC mill is lovely, lets you do other stuff while the machine does the work. Once again, if you take away the "complicated gear cutting" terms, and I asked you how cut 24 reasonably equally spaced slots in the edge of a disk, I bet you would have an easy answer for your shop and tooling. Just replace the slitting saw with a form tool and you're in the gear business.
Indexing and gear cutting are in the category of things that can be done with time or money. If you are doing it as part of a business, you throw money at it. Doing two gears a year at home, get a nice cup of tea and spend a few hours per gear playing in the shop. Or buy the gears when you can, even if they need some rework for your application.
Last : PINIONS. Pinions can be a pain to deal with. As the number of teeth decreases, the form of the space gets more and more extreme. As you have probably found, using a higher count cutter to cut a pinion results in very skinny tooth bottoms, and big fat teeth at the top. Nothing quite fits, but each tooth in isolation looks just fine until you notice how top heavy it is. The cutter becomes more and more "V" shaped, and also becomes a fatter "V" as the tooth count gets lower. If you are forming the cutter on a lathe, you need to swing the compound to an increasingly more shallow angle with respect to the spindle centerline. On a mill, you need to move the vise to an angle with respect to the axis, and do it twice, once in each direction for the two flanks of the cutter. Yup, pinions are a pain. For clock work I either used commercial cutters to get them almost right then refined the form as needed or cut them with multiple passes of a slitting saw and refined the form with files. Funny, cutting a 9 tooth pinion could take longer than letting my little cnc mill knock out a 96 tooth wheel!
Should you have the misfortune of becoming a gear cutting addict, particularly for clocks or watches, J Malcolm Wild's book "Wheel and Pinion Cutting in Horology" is a classic. For lots of gear cutter manufacturing tooling set ups and such, Robert Porters "The Clock & Watch Makers Guide to Gear Making" is wonderful. Once more, the Ivan Law book is just about a necessity for the home shop person getting started in gear cutting. Just don't get bogged down in all the gadgetry in some chapters, it's the forms and dimensions that are everything in gear cutting.
Hope this gives you some help and not a headache,
Stan
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