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Before considering the kinematic design on the cutting arm linkage mechanism, we had to learn how professional chefs cut vegetables. When using a large chef knife for relatively small vegetables, the knife blade is typically left in contact with the cutting surface. This blog was blog was helpful in deciding a knife path (and is the source of the gif shown right): http://www.hertzmann.com/articles/2001/knives/


Initially a fourbar mechanism was considered to drive more complex motion. However, this would leave the knife end unsupported, which could result in poor cuts or possible danger. To better constrain the system, a slider crank was chosen. This is also a good choice because the knife needs to stay in constant contact with the cutting surface, so the sliding axis will be parallel to the cutting board. Lastly, the knife itself can be used as the slider crank output coupler, further simplifying the design.


In order to choose the input coupler length, the effective cutting length of the knife was found. This is the length of the knife available for cutting, which is equal to the overall blade length minus the end clamp length, maximum vegetable diameter (chosen to be 1.5"), and tolerance for minor misalignment. This came about to 5", which should be set to the diameter of the input coupler's circlular path. This leads to the input coupler having a length of 2.5".


For safety reasons and practicality, machining modifications to the knife were as limited as possible. The handle connection was initially intended to be a drill and tap into the dowel pin; however, the knife was cheap and the "dowel pins" ended up being completely aesthetic, about 1/8" thick disks. Because of this, a portion of the rubber handle was removed and a custom part was made for the connection. At the knife end, a simple clamp was made to safely hold the blade. This allowed for small adjustments in case the knife CAD wasn't fully accurate. After these attachments were modeled, the knife linkage length from the handle pin to the clamp shaft was found to be 11.75".


The last step in completing knife kinematics was finding the position of the sliding axis relative to the driven axis. Since the knife has complex geometry, it is difficult to do a standard simulation to choose the best linkage ratios. Given this, it was important to create a precise 3D model of the knife. To do this, the knife was photographed next to a ruler. Using the ruler as a scale, the outline of the knife was traced in SolidWorks using splines, and extruded to measured specifications. After this step, the vertical distance between the driven axis and the slider rail was found by incrementally adjusting the value in CAD. A small notch was machining in the cutting board to account for slight errors, assist with alignment, and ensure the vegetable was fully cut.


Once the vertical distance was found, the slider crank was examined at its toggle positions to find the rail length. At this point, every dimension was fully defined, so dynamic analysis could take place. To do this step, a tool called PMKS (Planar Mechanism Kinematic Simulator) was used (

Diagram of analysis and CADImage Added