It very much depends on the chosen scale of the model and the pitch of the original gears.

In a lot of cases the commonly available DP series gears work out nicelyfor the commonly used scales eg 1/4, 1/8, 1/12 particularly when the original had an even numbered DP for example

20T 4DP original has 5" PCD,

1/4scale = 20T 16DP (4×4) and has a PCD of 1.25"

1/12scale = 20T 48DP (12×4) and has a PCD of 0.416

So in all the above they scale exactly

Now if you picked an odd scale of say 1/5th and the original had 3DP gears to keep the ctrs and tooth count the same you would need a 15DP cutter (5×3) so not an off the shelf item but if you wanted to make your own cutters then not a problem as you just work it out for the chosen DP.

The other option is to fiddle it by going for 16Dp which is the nearest and adjusting teh tooth count and maybe ctr distance to suit.

The other thing to watch if you want "realistic gears" is that the commonly available 20pa gears and even the 14.5pa ones tend to have the wrong profile when compared to the older gears which tended to be longer in the tooth in more ways than one. So another case for making your own cutters.

Edited By JasonB on 03/06/2015 08:17:18