An intermittent dial needs a servo rotary table. The quote comes back and the customer balks. A cam indexer does the same job for a third of the price. The catch is you have to spec it right.
What a cam indexer is
The input shaft rotates continuously. The output shaft indexes: moves, dwells, moves, dwells. The cam profile fixes the move and dwell times. It’s not programmable. Change the cycle time and you either swap cams or change the motor speed.
What you get back is rigidity, repeatable stop position, and a low price. What you give up is flexibility.
Indexing accuracy
A cam indexer repeats its stop position to about plus or minus 30 arc seconds, around 0.01 degrees. That beats a pneumatic index table by an order of magnitude. It’s slightly below a servo with a direct drive, but good enough for most assembly stations.
Mount the dial balanced on the output flange. An unbalanced dial slams the cam on acceleration and wears it out fast.
Drive motor sizing
The input torque isn’t just steady friction. At acceleration the cam demands several times the running torque. Undersize the motor and you get a jerky motion and a scored cam.
Mount a gearmotor straight on the input. Don’t use a belt drive; belt slip kills the index accuracy.
When not to use a cam indexer
If you need to change the cycle often, stop mid-index, or rotate both directions, a cam indexer is wrong. Once it starts a move it finishes the index. If you need that kind of flexibility, pay for the servo.
