This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Without the proper tools and material, the amateur electrician will find it quite difficult to construct a small armature for a battery motor that will run true, without vibration and have a neat appearance. Ordinary cast-iron gears or pinions, as shown in Fig. 1, make excellent cores for armatures on small motors. A gear of any number of teeth can be used for an armature with a smaller number of coils by cutting out a certain number of teeth. For example, a gear with 12 teeth will take 12 coils, but if every other tooth is cut out, it will take only 6 coils, etc. The teeth can be easily chipped out with a cold chisel.
Larger armatures can be made from gears with spokes, the spokes being cut out, if a ring armature is desired. The gear, when wound, can be mounted on a hub made of empty thread spools. The spool can be turned at one end to insert it in the armature, and if too long, one end will serve for the core of the commutator, as shown in Fig. 2.
Ill: Gear Used as a Core
This combination will make a neat, efficient little armature, which will run quite free from vibration. Only simple tools, such as a hammer, cold chisel, file, jackknife and a vise, are required. - Contributed by R. J. Nault, Hartford, Conn.