IV. Indexing Mechanism Design

The design goal of the indexing mechanism was to create intermittent linear motion of the vegetable, under the knife, from the continuous rotational motion of the same motor that powered the cutting mechanism. Additionally the indexing motion was designed to have 80% dwell (i.e. for one full rotation of the input motor the vegetable would have constant position for 80% of the cycle and would be moving to the new position for 20% of the cycle) so that the vegetable would only move when the knife was clear of it.

These design goals were accomplished using a barrel cam and follower. A barrel cam is designed such that the cam profile is cut into a cylinder and wrapped around it (similar to a lead screw) with a consistent pitch which defines the linear distance that will be moved with each rotation of the cam. The follower is pressed into a the delrin carriage of an aluminum linear rail that runs alongside the cam. This carriage has a part mounted to it with adjustable skewers that moves the vegetable over the cutting board.

The design constraints for the specific cam profile were to achieve 80% dwell and to have a profile that results in finite acceleration of the follower/carriage assembly over the whole motion; thus, the resulting profile has to be constant position around 80% of the rotation of the barrel to satisfy the first requirement and has to have a finite second derivative to satisfy the second requirement. The simplest profile which achieves these goals is a cubic spline as shown by the Matlab plots on the right. Multiple copies of this profile are wrapped around the barrel cam which has an 1" diameter and a 3/16" pitch (the desired thickness of the vegetable slices). The magnitude of the velocity and acceleration vary with the angular velocity of the motor.

Additional static analysis was done on the cam follower to determine if the forces exerted on it by the cam under normal operating conditions would be enough to shear the follower shank. It was determined that the follower would see ~17 N of force if the motor should stall. An FEA was performed and it was determined that the follower would have a yielding safety factor of 1.5 in these conditions. These calculations did not account for impact effects which could explain why the follower sheared while cutting a carrot that jammed and prevented the forward motion of the indexer.