That number is the total number of teeth on a gear . As the diameter of a gear changes , the tooth profile does also . The cutters each cut a limited range of teeth . There's a book called gears and gearcutting by Ivan Law that's available on Amazon and maybe eBay . I bought it , and found it
*very* helpful . ISBN number is 978-0-85242-911-2 and I think I paid like 7 or 8 bucks for mt copy .
The number means it will cut a gear with 17-20 teeth total. The cutter should be labeled for diametric pitch (the number of teeth per inch of diameter) Cutters come in a set of eight for each pitch. Each one cuts a range of gear diameters to the same pitch as the others in the set.
Gears with 17 to 20 teeth total -- of the *one* diametric pitch and pressure angle for which that cutter is made. It is *useless* for other diametric pitches and pressure angles. (The best tooth shape is probably at 18 or 19 teeth total from that one.)
The standard involute cutters come in sets, with each numbered member of the set for a specific range of tooth counts -- the same range for the same number in each set -- the fewer the teeth, the narrower the range of counts because the angle from one tooth to the next changes more. The largest number is for something like 132 teeth (IIRC) up to a full rack gear of infinite length.
There are also extension sets with half numbers which cut a better tooth form at the crossover between one standard numbered cutter and the next.
Each set is made to cut a specific diametrical pitch, and a specific pressure angle. The most common ones are 14-1/2 degrees pressure angle and 20 degrees pressure angle.
Do you have a copy of _Machinery's Handbook_? It should have all you need to know about the sets, the way to calculate the necessary OD for a gear blank of a given number of teeth and a given diametric pitch, and the way to calculate the total depth of cut for an involute gear cutter.
(Just looked them up earlier today to calculate the figures for cutting a 32 DP, 14-1/2 degree pressure angle, 20 tooth gear to replace one which disintegrated in an old Tektronix 7A13 plugin.)
The general "Emily Postnews guide for Nettequette" should be somewhere on the net still. There is none specific to this group (and a large number of people who would not follow it even if it did exist. :-(
That varies with the individuals, but *I* certainly prefer interleaved answers/comments between parts of the properly trimmed quoted text. And totally detest top posting.