Dividing head & rotary table indexing
Tells you how many whole turns of the crank, and how many holes on which circle, for each move. Works for equal divisions (gear teeth, bolt holes, flutes) or a set angle.
How it works, guides and notes
How it works
The crank turns the worm, and a full turn of the work needs as many crank turns as the worm ratio (40 on most heads). Divide that by the number of divisions. Any part-turn is made up by holes on a plate whose hole count divides evenly into it. Set the sector arms to the number of holes so you can’t lose count.
Simple indexing: turns per division = R / N. Write the remainder as a fraction in its lowest terms, a/b. Any hole circle that is a multiple of b works: move a × (circle / b) holes. Angular indexing does the same with turns = angle × R / 360.
Differential indexing covers numbers with no suitable circle (such as 127). Index as if for a nearby number A that the plate can do, and gear the spindle to the plate in the ratio R(A − N)/A so the plate creeps round to make up the difference. If A is larger than N the plate turns the same way as the crank; if smaller, the opposite way. Add an idler to change direction. The plate locking pin must be withdrawn and the plate free to turn.
Marking out instead? See marking out a PCD and the PCD calculator.
Spotted a mistake? Report an error.