I agree with Swarf, if you purchase a full set of gears it is very likely that many will never get used. This, unless your activity is likely to frequently need a lot of differing threads to be cut and having odd pitches.
What is often overlooked is that for any relatively common pitch there are very many combinations that will achieve the required result. Typically, my changewheel charts,( using the Myford standard series of gears plus a 21 tooth gear for metric pitches) lists 45 combinations that achieve 14TPI, Of course, some are easier to set up than others, even so, there are still 12 combinations that use either two or four gears.
Even an odd number pitch, 13 TPI, lists 33 combinations.
The moral of this is that, if it is found one gear is missing that is listed on the lathe's screw cutting chart for the pitch required, almost certainly there are many other combinations that can still achieve the required pitch. This, even if a few gears are missing.
The charts can be found here,
However, it is impracticable to print the lists as there are 37,293 possible combinations between 6 and 50 TPI. Other than returning to the web page, the text files can be copied onto the PC's hard drive and kept for reference using a word proccessor
To use them for metric threads on an imperial lathe, convert the metric pitch to an imperial TPI and again you will find very many combinations that come very close. These are often more accurate than the combinations that the lathes standard chart lists.
Harold
Edited By Harold Hall 1 on 07/03/2012 20:31:52