involute gear cutters

Pressure angle
Diametral pitch (can calculate from diameter and number of teeth) or module if metric.
Number of teeth
for the gears you need to cut in order to select a cutter.

Unless your gears are close to the same tooth count, you will need one cutter for each gear size.

The shape of the involute curve varies for each combination of those three parameters so each size gear requires a different cutter to get the correct profile though in practice the number of teeth is divided into eight groups and the same cutter is used for a whole group to get results that are good enough.

Start by looking up the number of teeth on a particular gear and use the “Range of Cutters” chart near the upper right corner to convert that to a cutter number (1-8). That determines which column of the main table. Then select the table for the pressure angle and select the row which corresponds to your diametral pitch (which they list as pitch diameter). The part number at the intersection of those two rows and columns is the one which works for that particular gear. Then you pick an arbor that matches the hole size on the cutters.

CTC tools sells some cheap involute cutters and sets of 8, for metric gears.
http://www.ctctools.biz

Using this method requires a staggering number of cutters to be able to cut any arbitrary gear.

Instead of buying expensive cutters that are specific to one gear, you can use a custom ground lathe tool in a flycutter. Here is an example (not R8).
http://www.sherline.com/3217pg.htm
You could use a regular fly cutter but you have to compensate for the angle of the tool when grinding. This will not cut as fast for a given level of smoothness because you have fewer teeth. Trick is, you need some sort of reference to grind the tool too, like an existing gear with the same specs or an actual size drawing of the correct profile.

There are other ways to cut gears if you have CNC, see the “gears, gears, and more gears” thread for an example. It is also possible to do this manually if you do the math, though it will take a long time and you really don’t want to screw up.

You can also use the hobbing method. One hob will cut all numbers of gear teeth for a particular pressure angle and diametral pitch or module. However, you have to tilt the spindle and synchronize the rotation of the hob and gear cutter using change gears or CNC. Another method is to pre-gash the gear blank and let the hob turn the blank, though this is less accurate and can fail entirely.

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