All gear hobbing machines have an Index Constant. This constant, along with the number of teeth to cut and the number of starts in the hob cutter determine the index gear ratio and the index change gears. Some typical Index Constants are: 36, 30, 24, 15, 12, 10, etc. Let’s take an example using a single start hob cutter. If we want to cut a 100 tooth gear on a machine with 30 Index Constant, the index change gears would have to be in a ratio of 30 to 100, or: 30 —— 100 Index change gearing usually uses at least four gears; two drivers and two driven gears. Using four gears, the example index gear set could be: 30 x 60 ———— 60 x 100 Notice that the ratio is still 30:100 which is correct to cut the 100T spur gear. Adding two more gears (usually found on an adjustable idler shaft) provides a way to mesh with the two primary gears to synchronize hob cutter motion with workpiece motion. But there is another reason for using four gears. Four gear trains permit matching decimal ra