This is a description of how I ground my own gear cutting tools use with my Sherline 2010 Mill.
I purchased Sherline's gear cutting attachment P/N 3217 when I purchased my rotary table. The attachment comes with a holder similar to a fly cutter but not at an angle. I also comes with a 1/4" X 1/4" HSS blank into which you grind the tooth shape. Grinding your own tool isn't hard, but there were a few tricks that I learned that might help someone attempting this themselves.
My first gears were made out of brass. Not knowing much about where to start, I merely ground a V into the sides of the tool into what looked like it might make a gear tooth.
I set the adjustable tool rest on my bench grinder to about a 5 deg. angle, the idea being to make sure only the leading cutting edge was hitting the work piece.
Then I held the tool at an angle and ground it at approx 20 deg. on both sides
I also angled the tip about 20 deg. so the heel of the tool didn't hit the work piece as it was rotating.
When I did this I also made a small flat on the tip about 0.5 mm wide so it would have a wider cutting surface and hold up better. Some experimentation was necessary to learn the proper tool shape and brass was ideal for this. I used brass to test the software as I was writing it. I made many really wierd looking things in the beginning that couldn't possibly be called gears. I finally had a gear that actually ran in the truck for a short period of time. I had made the gear out of brass and the outdrives out of 6061 aluminum and the shaft was 1018 steel. After a short period of time inspected the outdrives and gears and learned that they were wearing at an alarmingly fast rate. I guess because of the speed at which the differential was spinning it didn't take long to realize the shortcomings of my choice of soft, easy to machine materials. This is when I decided to make the gears out of something that could be heat treated. I didn't know much about metallurgy and when I went to the machine shop and asked Don for some steel that I could heat treat, he gave me what he called ETD 150.
I have since purchased and read Metallurgy Fundamentals by Daniel A Brandt and J. C. Warner and looked up ETD 150 in the internet. I have come to learn, ETD 150 is a trademarked version of 4140 steel by LaSalle Steel. ETD stands for Elevated Temperature Drawn. 4140 is known as chrome-moly steel. I tried to use the same cutter that I used to make the brass gears to make the 4140 gears, but the process was very slow and the edge would come off the tool very quickly and start making a loud banging sound as it struck the work piece. I spent the better part of the night (till 4:00am) fighting with this material. I don't know what made me think of this, but I finally though about how the bit was hitting the work piece and how to make it better able to cut it. I though at first there was something wrong with the way the program was running the cutter and that maybe there was a software fix. After visualizing the contact that the tool was making I finally realized that maybe a hook on the face of the tool would be better able to pull a chip. So I turned the face of the tool against the grinding wheel and sharply angled the tool away from me.
I ground about a 30 degree angle into the face of the tool making a nice hook at the point. This did the trick.
This grind cut the 4140 beautifully. I was able to make 2 rounds (72 teeth) without having to resharpen the tool! Previously I was having to sharpen the tool every other cut! You can tell if the tool needs to be resharpened when it starts banging when it strikes the workpiece. The workpiece will also start to vibrate. If you have just finished sharpening the tool and it is banging as soon as you make your first cut, you don't have the proper relief on the sides or the heel of the tip is hitting the work piece. Turn the machine off and look as you rotate the tool bit by hand to see where it might not have the proper relief. A properly ground tool makes a nice pleasant "chunk, chunk" sound as it removes metal. Be sure to use plenty of lube and reduce your speed to minimize tool wear (and smoke) when machining hard materials. A couple of the pictures show me holding a brush in the tool path to clean the cutter and apply new lube on every tool revolution.
I had nearly ground down the entire 3" HSS blank that evening trying to cut 1 gear and had only a few sharpenings left. As soon as I learned about the hook, I was able to finish the first of many 4140 gears in about 2 hours. After discussing my experience with a friend who years ago owned a machine shop, he confirmed my empiracle evidence. He also said that I didn't need or want a hook in the face when cutting soft materials like brass else the tool will grab and possible sink in and stop the machine. I still have much to learn about grinding my own tools, but I learned much from this experience, I hope someone else can learn from it also.