03 - Project Proposal

Introduction

Our proposal is to design a knife sharpening mechanism that imitates the motion of hand-sharpening using a whetstone. Whetstone sharpening involves following a specific path and maintaining specific angles that are determined by the knife geometry, which can be tricky to learn to do manually. Accomplishing this by hand successfully is an intuition/”feel” driven process, which will make reproducing the motions with a mechanism an interesting challenge. It should be noted that there are existing, much more practical knife sharpening processes that do not imitate manual whetstone sharpening, so this project isn’t really solving a real-world problem.

Project Complexity

The motion profile of a knife being sharpened on a whetstone has components in 3 translational axes and 1 axis of rotation. The edge must travel along both the length and width of the whetstone to ensure that the entire blade is sharpened in the correct orientation. The vertical position of the blade must also be taken into consideration when raising and lowering it to ensure constant contact with the whetstone. For the vertical axis, it is also important that we consider the force between the knife and the stone. Finally, we need rotation in at least one axis to maintain the correct angle between the whetstone and the blade. The combination of lateral translation and rotation must match the predetermined geometry of the knife, and to fully imitate the motion of manual whetstone sharpening, we will need coordinated motion in all 4 degrees of freedom.

Scope of Work

Another important consideration for our project is to what degree of functionality can we realistically design to. For instance, designing a robot that could sharpen blades of variable lengths and edge profiles would significantly increase the complexity of our design. Similarly, designing the robot to flip the knife and sharpen both sides may overcomplicate our design. These design features may be worth considering, but they would likely come at the expense of other aspects of our project, such as the motion and force profiles. As we design, we will need to weigh the pros and cons of each feature to determine the best and most sensible final design.

Proposed Mechanism/Preliminary Designs

Note: axes definitions showed in Fig. 1.

Fig. 1

The x-axis of the knife could be accomplished by a rotation-to-translation mechanism such as a rack and pinion or lead screw. Because linearity of the path is not critical, we could also potentially drag the knife in an arc across the whetstone with a single large-radius rocker. The y-axis translation and rotation of the knife is more complex. There are two primary options to achieve the motion profile that we have considered so far. In option 1 (Figure 2), the knife is rotated about an instant center positioned behind and above it (-x). The motion of the knife would stay in a plane at a constant angle relative to the stone, determined by the angle of the knife edge. However, the instant center would need to move to follow the shape of the knife, which would likely be accomplished with a linkage. Option 2 would be to have the knife rotate around an axis just in front of the blade edge and perpendicular to the blade body. This is shown in Figure 3 with a simplified blade that has two discrete edges rather than a continuous curved edge. This could be accomplished with a pin joint collinear with this axis and above the knife and stone, or with a linkage.  we have more complex mechanisms controlling the lateral and rotational motion of the knife, it may be pragmatic to mount the motor on this assembly rather than trying to transferring power through the x-axis mechanism.

Fig. 2

Fig. 3

Considering that we will need some sort of mechanism controlling the lateral and rotational motion of the knife, it may be pragmatic to mount the motor on this assembly rather than trying to transferring power through the x-axis mechanism. A motor mounted on the carriage could drive the sweep of the blade while also powering a pinion on a rack or a nut on a lead screw.

The most straightforward way to maintain contact in the vertical direction would be to leave either the knife or the stone free to move along this axis with spring preload pushing them together. Even if we attempted to fully constrain the vertical motion, some amount of compliance is necessary to avoid large spikes in contact force or total loss of contact.