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So what are compounded gears, well you should now be happy with what a train of gears is, and most importantly that there is only one change of ratio. Well compound gears cause more changes of ratio, as many changes as there are compounded gears, so before you say that's fine but what is it, take a look at this picture with one set of gears compounded between the first and final gears.

You can see that like the simple gear train we had before we have a direct ratio between the first gear and the last, but in between we have two gears mounted together and turning together, what is meant by this is that neither of these two can turn unless the other does so also and one revolution is the same for them both.

So how does this particular ratio work out? Well what do we have here, let's break it down into it's respective parts. We have two gear trains actually, the first train is 20/50 and the second train is 38/55. You will have seen how to reach this stage the normal way, if you read the other pages. We are going to reverse the process here, something you would not normally do but it may well help the understanding process. Due to the fact that both the 38 and the 50 are linked together they must be rotating at the same speed, this means that every gear here is affecting the ratio. But it is still just two gear trains linked together, so....

20/50 = 0.4 the ratio of the first train.

38/55 = 0.69 the ratio of the second train.

So if we now multiply them we get the total ratio 0.4 x 0.69 = 0.276

All this states is that for every complete rotation of the mandrel the leadscrew will rotate 0.276 of a revolution. But we are not finished yet because we want to know how many threads per inch this will turn. All we know is that the lead screw has turned partially, so to get the final figure we need to know how many threads per inch are on the leadscrew and in our example we settled for 8, so the pitch of our leadscew is 1/8 which = 0.125 and this times our ratio of 0.276 = 0.0345 TPI, which of course is not much good for thread cutting as it is way to fine, but it would do for a bit of coarse turning.

So that is a compound, of course you can have several compounds together, but it is the same thing from here on in, and no matter how many gears are there, you now have the ability to work it out both ways.