A gear cutting machining uses a special tool, called a gear cutting tool, or hob, to cut gears. It's the way cogs are made for heavy power transmission applications; for example, cogs in automobile transmissions. It is the most accurate way to make teeth.
Below is a picture of a gear cutting tool, contrasted with a sprocket wheel and a cutting end mill. The cutting teeth are the mirror image of the cog teeth to be cut. It cuts the tooth a little bit at a time, leaving a perfect cog when it's done.
Aluminum sprockets are generally CNC'd with a cutting end mill. This process produces good teeth in aluminum. CNC cut teeth in titanium do not turn out as good (although they're still better than stamped teeth). As illustrated in the diagram above, the end mill required for bicycle sprockets is necessarily quite thin: it dulls quickly and then deflects when pushed against a hard metal like titanium. The variation in tooth profiles is slight, but does impact performance. A chain roller in a sprocket tooth is similar to a ball bearing in a cup: a poor fit, surface imperfections, or just slight looseness in fit rapidly accelerates wear.
Gear cut teeth provide the most efficient power transfer between sprocket and chain for two reasons. First, the tooth surface that contacts the chain roller is very smooth and perfectly formed. Second, teeth can me made to a tighter tolerance with the chain. Stamping, for example, gives teeth with a wide variation in shape. This variation is accommodated in the design of the stamp by making the teeth looser fitting. Looser fitting teeth sacrifice efficiency and durability, but these features are not readily noticed. Shifting and durability aspects of sprockets are discussed on our technical overview page.
This page updated June 1, 1997