Sandrag-
Cutting gears on a vertical mill is anything but a lost cause. It is done all the time. I have done it (and most assuredly will be doing it again) on my manual vertical mill with a rotary table and hand-ground gear shaper in a fly cutter. It just takes some patience and some care with the machining operations. With a four-axis CNC system, you are making it even easier. The only real question you have to answer before you jump in is: is it worth it for my application?
If you are just knocking out one gear, or you need it very soon, or if the price is a lower-order priority than immediate availability, then just go to a shop and have the gear EDM'd for you. It will be quicker, and you will be able to move on with your project. However, if you want to manufacture your own gears because you want to learn something new, or time is not the most pressing issue, or to save a few bucks, or just because it would be cool to say that you made your own gears, then go for it!
The specific step-by-step instructions are slightly more involved than we want to go in to here, but the quick summary is:
- determine the gear size and shape you want. There are any number of on-line and printed references for doing this. The best resource that I have ever found for a really good introduction to gear design is the technical section in the
Handbook of Gears by
Stock Drive Products. You can download the whole thing as PDF files from the SDP web site. There is also a lot of good stuff on the Martin site, but they don't get into the background knowledge you really need to understand what is going on and why the formulae work. The Handbook Of Gears is a bit surprising because it is tucked away inside the SDP catalog. But it is a great reference for understanding both introductory gear theory and application considerations.
- Obtain your source stock material and turn the gear blank on the lathe to the outer diameter of the gear (pitch diameter + addendum for the chosen tooth profile).
- Mount the stock material on a vertical rotary table (if being done manually) or indexing head or CNC rotary table. An example of how to do this is
shown here (near the bottom of the page).
- Mount your gear cutter in the mill. You will typically use a Thornton gear cutter, a hob, or a single-tooth cutter ground from a toolbit. The cutter should intersect the center line of the workpiece. Position it to just contact the outer diameter of the gear blank, and set your index to that point.
- Start the mill and move the table to cut the first tooth. Depending on the tooth size, cutter type, and materials used, multiple passes will be needed. The depth of the final pass will be dependent on the tooth size and profile you have selected.
- Move the table so the cutter is beyond the gear blank. Rotate the table or indexing head to the next position, and cut the next tooth. Keep repeating until you are done.
There are frequently good discussions on this topic in places like rec.crafts.metalworking (
one example is here and
here) or on
Metal Web News or on the
Machinists Workshop BBS. Also, there are many, many books on the subject of both gear design and fabrication. For the one-off type of situation you seem to have here, look through some of the home/hobbyist metalworking materials such as
Home Shop Machinist or
Machinists Workshop, or any of the books by Rudy Kouhoupt or Phil Duclos. There are also lots of other links to additional information on the
Metalworking Web Sites Index.
-dave