The popular 10" and 12" Atlas and Craftsman metal lathes were equipped with change gears made of cast "ZAMAC". These gears are much softer than steel or cast iron gears, and not nearly as durable.
Better quality change gears are available, but the information as to exactly what to buy, and where to buy seems a bit scarce. Here is my attempt to help.

Browning, Union Gear, and Boston Gear make a range of change gears that will fit these lathes. These gears often show up on eBay, but of course you can't expect the size you need to be available when you need it. Boston Gear products are sold by Amazon.com. A number of other industrial or specialty suppliers carry Boston and/or Browning and/or Union, and some time spent with a search engine will help to find these, but at the time of this writing (January, 2014) I've found Amazon to have the best prices.

When purchasing change gears, or any spur gears, for that matter, you need to know:

1. Pressure Angle. Either 14.5 degrees or 20 degrees.

2. Diametral Pitch. (DP) This is the number of teeth per inch of diameter at the "pitch line". You can determine the DP of an existing gear by dividing the number of teeth plus 2 by the outside diameter. *(see example at bottom)

3. Bore diameter and configuration (smooth bore, keyways, splined, etc.)

4. Gear face width and hub thickness

5. Number of teeth

Fortunately, there's a bit of standardization here:

1. Change gears for the lathes in question all have 14.5 degree pressure angle. You don't have to worry about that.

2. The Atlas / Craftsman 10" and 12" lathes all use 16 DP gears.

3. All of the 16 DP change gears from Union, Boston or Browning have a 3/4" bore with two keyways. This is the configuration that fits your Atlas / Craftsman lathes.

4. Likewise the hub and face thickness are standardized. The 16 DP gears are 1/2" thick.

With this in mind, all we really need to know is the numbering system used by the manufacturers. It's not very complicated, either.

Boston Gear:

"GBxx" The two letters, "GB" denote a 16 pitch gear with 14.5 degree pressure angle, 3/4" bore, two keyways, and 1/2" thick. This is followed by the number of teeth and sometimes a letter suffix. You can forget about the suffix when searching. So a 30 tooth gear would be "GB30." A 127 tooth gear would be "GB127A". Boston lists these gears in tooth counts from 20 to 128.

Browning:

"NCG16xx" The letters "NCG denote 14.5 degree change gear. 16 is the diametral pitch and xx is the number of teeth. So your 30 tooth change gear would be "NCG1630" And a 127 tooth gear would be "NCG16127". Browning lists these gears in tooth counts from 20 to 129.

Union:

"CG-16xx Style K". The letters "CG denote 14.5 degree change gear. 16 is the diametral pitch and xx is the number of teeth. Style K denotes the 3/4" bore with two keyways. Your 30 tooth change gear would be "CG-1630 Style K" And a 127 tooth gear would be "CG-16127 Style K". Union lists these gears in tooth counts from 20 to 129.

Now, knowing these numbering systems, you can search eBay or your favorite industrial parts supplier for high quality gears to fit your old lathe. These gears will be interchangeable with the old ZAMAC gears.

The part numbers for steel keyed bushings for these gears are:

Browning, CGB16

Boston, GGB16A

Unfortunately for the owners of 6" Atlas / Craftsman / AA / Dunlap lathes, your change gears are 24 pitch, and neither of these manufacturers make change gears to match.

*Calculating Diametral Pitch. Example: Your gear has 64 teeth, and measures 4-1/8" outside diameter. Divide the diameter into the tooth count + 2, thus: 66 divided by 4.125 is 16, and your gear is a 16 pitch.

One final note: the ZAMAC gears have 1/2" thick hubs and 3/8" wide faces. The steel and cast iron gears I've discussed have full 1/2" wide faces. This can cause some slight interference and noise in the gear train. The easy remedy is to put a thin, .010" or so shim washer behind the stud bushing of whichever gear pair has the larger gear in the "Front" position (away from the headstock.)