Okay Kel.
Get the book mentioned in the previous post. I've never seen it, but it seems to be the one people like, judging by how often it's mentioned.
These are the basics I use for my gearcutting.
Diametral Pitch, what we usually just call "pitch". You need to now this in order to size your gears for your projects. Different pitches produce different sized gears for a given tooth count. All gears in a train have to be the same pitch to mesh. Common pitches are 16, 24, 32, 48, 64, and there are lots of others. The smaller the pitch number, the larger the teeth on the gear will be.
The pitch of a gear is a constant, so you can't just pick some random diameter and decide to put a certain number of teeth on it. The pitch and number of teeth you need for a given gear will dictate the diameter of the gear. What that means in real life is that when you design an engine, you don't get to pick the distance between things like a camshaft and a crankshaft. The gears will determine that distance, and you design around it.
To get a smaller or larger gear for a given amount of teeth, you choose a pitch that fits. You don't always get to pick your exact diameter. You pick the pitch that most closely matches the diameter you need. All other gears in that train have to be the same pitch.
Some examples;
Say you want two gears, one about 1/2" diameter with 20 teeth, and one about 1" diameter with 40 teeth.
If you choose 24 pitch for your gears, you won't even get close.
A 24 pitch gear with 20 teeth will be .916", and you can't change it. A 24 pitch gear with 40 teeth will be 1.75".
So, to get closer to the size you want, to go to a higher pitch, which will make the teeth smaller, reducing the size of the gear for the same amount of teeth.
If you choose a 32 pitch, the 20 tooth gear will now be .687" diameter and the 40 tooth gear will be 1.312". The gears are getting smaller, but still have the same amount of teeth because you are changing the pitch.
There are other pitch numbers you might try, to see if they give you the exact diameter of gear you want, but it usually doesn't work out that way. You just have to pick what is close, and build your project around those measurements where they are concerned.
To figure out what the diameter of a gear will be for a given pitch and fixed tooth count, use this simple equation.
Diameter = Number of teeth, plus two, divided by the diametral pitch (DP, or pitch)
For a 40 tooth gear with a 24 pitch it looks like this:
40T+2/24DP = 1.75"
You can use that to find out how large your gear blank needs to be so when you cut the appropriate number of teeth on it, they will come out spaced exactly right. No left over spaces, or half teeth, etc.
To figure how deep to run your cutting tool into the gear blank when cutting teeth, use the constant of 2.157. Divide the gear pitch into that number and it will give you the depth of cut for gears of that particular pitch.
Learning to make your own cutters may well prove worth your while, once you've seen what follows.
Each pitch of gear takes a different size of cutter, of course, since different pitch means different sized teeth. The big deal about this comes from the fact that a cutter is not only unable to cut any pitch other than what it was designed for, but that it will only cut a certain tooth count range on a gear. To be able to cut all the gear sizes in one gear pitch, such as 24DP, you need eight cutters. If you want to cut 32 pitch gears, none of your 24DP cutters will work, and you will need another set of eight cutters to cut 32DP. It can add up quick.
The cutters are made to cut a certain range of tooth numbers. A 24DP no. 8 cutter will only cut 12 or 13 tooth gears. If you want to cut 14 to 16 teeth of that same pitch, you have to buy another cutter. If you buy a whole set of eight, you can cut from just a few teeth, (12, I think) to as many as you want. But only in that one pitch, unless you buy another set.
You have an RT, so you can divide a circle into any number of teeth you want, for cutting your gears. Make sure you know before hand what all the different settings on the RT should be for all the tooth locations on the gear, before you start. Work it out on paper, and keep a list of subsequent settings near the mill as you cut.
Make sure your gear cutter is set dead center to the gear blank. If you don't have it right, it will show up in the gear having teeth that appear to lean to one side, and they will mesh tight, or not at all.
If you need to know more about this, ask. The few basic things here will tell you how big to make your gear blank, how deep to cut the teeth, and how to figure different sized gears for a given pitch.
Read up on it some, and don't get freaked out by the stuff you don't really need to know. There seems to be more math theory to it than actual machine shop practice. You don't need to know the same stuff as an engineer who gets his jollys drawing out involute curves, but many books include that type of thing. Learn how to calculate the proper diameter for a given gear pitch/tooth count, and the correct depth of cut, and you will get along.
Dean
