6.3 Design Process
We began by looking at existing methods of grinding food materials and supplements, examining both automated and manual systems. These included different traditional mortar and pestle types, the Chinese tea grinder, and the Kharal machine. We considered the problems of each product: primarily ease of use, cost and materials. We determined that we would make an automated version of one of the manual mortar and pestle types, and ultimately landed on the Chinese tea grinder. The Chinese tea grinder offered a relatively simple design solution that would be effective and easy to operate.
We started off with a simple slider-crank prototype, inspired by the first build assignment. In this initial design, we wanted to drive the grinding wheel by a concentric axle that would ride along a pin slot. We used MotionGen to simulate the motion of our system, as seen below.
In this animation, the mechanism passes through the center of the mortar quickly, but moves more slowly as it approaches the ends of the mortar. The slower velocity in these regions is indicative of higher forces in these regions. This is crucial to the effectiveness of our design, since the ends of the mortar will have the most contact with the wheel and may require high forces.
Ultimately, we recognized that this solution was not apt for high forces and was susceptible to reliability issues and would result in a tall mechanism. As a result, we modified the design to drive the rails of the machine as seen in the CAD drawing below.
This design allows us to reduce the height of the mechanism while increasing the reliability. To test our design, we began by laser cutting acrylic wheels and a mortar. Using an axle, we hand-tested the mechanism to estimate how many passes required to crush a Smartie candy and get an idea of how much force would be required.
From our experimentation, it required 4 to 5 passes with light pressure to thoroughly crush a Smartie candy and between 2 and 3 passes with heavy pressure. We then constructed a low-resolution prototype of the slider mechanism to validate the method for driving the rails. The CAD model below shows the pieces we laser cut in order to test. Unfortunately we forgot to take pictures of the actual model so they are not included in this document.
From our testing of the low-resolution prototype, we discovered that having tight tolerances would be essential for having a mechanism that doesn’t toggle at the critical points. We then began our final iteration of the design as seen in the CAD model below
We chose to use a simple gear system to transmit power from the motor to the wheel, connecting one small gear directly to the motor shaft and one large gear to another shaft. The large gear also acts as the “input arm” as the long arm directly links it to the slider. Attached to the two parallel slider rods is another rod running perpendicular to the slider rods with a set of three wheels mounted to it. We determined that the mechanical advantage would be at a maximum at either end of its range of motion, so the mortar (bowl) has curved walls that the wheels fit into, crushing the pill (or in our case, Smartie) at either end. To mount everything together, we used boxes placed on a single board so that the machine would be one solid unit. This allowed us to use the gears and crank arms vertically without interference from the ground.