We proceeded with developing concepts based on the second solution outlined in the proposal, rotating the knife around an axis just ahead of the edge. Figure 1 shows a concept sketch of what a mechanism to drive the knife movement could look like. Some key features of this first concept include:

Revolute joint aligned with the knife rotation axis, which the knife carrier will rotate around
Whetstone riding on linear bearings
possibility to sharpen knife on both sides, by switching the knife carrier driving link to a different mounting point on the input rocker 180 deg offset, reversing the direction of rotation of the knife

Figure 1

At this stage, we had not considered how the motion of the stone and knife could be linked. Because we chose to approximate the knife edge as a straight edge followed by a section of an oval, our first plan was to use a geneva drive, cam and follower, or a similar discontinuous mechanism to accomplish the transition from straight to curved edge. However, we decided (with the help of the TAs) it was more in the spirit of the project to attempt a solution by optimizing plain linkage movement. Therefore, we began to conceptualize linkage layouts that could approximate the straight-then-curved motion profile. To accomplish this, we would need coordinated movement that would angle the knife minimally while the straight edge was being sharpened, then rotate the knife a significant amount (about 37 deg) to reach the curved edge. We generated a few concepts, with general goal of a mechanism where the angular rotation of the knife was low when the linear movement of the stone was high, and vice versa. These are shown in Figure 2. It should be noted that these figures do not represent actual dimensions or angles, but both give us the ability to control the coordination of the knife rotation and whetstone movement.

Figure 2

(a) (b)

We chose the concept in Figure 2b for further development, as it seemed to provide for more straightforward tweaking, due to the more decoupled motion of the stone movement and knife rotation. We also have an easy way to adjust the lag/lead between the two motion cycles by adjusting the position of the joints on the input crank. CAD for this concept is shown in Figure 3a, but again dimensions and angles are not accurate.

Figure 3

(a) (b)

We manufactured this design for our first prototype (shown in Figure 3b). This first prototype serves as a very rough proof of concept. Aside from modifying for correct kinematics, this design is still missing:

The rest of the mechanism to control the stone movement
Realistic Knife holding clamp (grabs better location on knife, some way to ensure alignment)
Spring loaded vertical position of stone, which is necessary to sharpen the correct profile
Adequate rigidity in most joints

On the bright side, all of the hardware used here can be reused for subsequent prototypes.

A video of the prototype moving:

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