I was the mechanical lead for the Ghana-team within the Olin College & Babson College Affordable Design and Entrepreneurship Program from August 2014 through January 2015. This project had been under development by previous teams for the past three years. The end-goal for the team was to increase income for rural women in Ghana by starting a social-venture to manufacture and sell cassava-processing equipment. Our mini cassava grater filled a need in the market for small, affordable, easy-to-use food processing equipment for small-scale cassava farmers and producers of Gari, a dry staple-food made from grated and dried cassava.
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Structure, Power, and Transmission
Previous teams had settled on the general specifications and layout of the grater and had put 3 pilot-machines into the field with customers in Ghana, however, there were critical issues for usability, durability, and manufacturability. Throughout my time on the project, I was able to significantly improve the machine from each of those perspectives then my team and I built and distributed 7 new pilot-machines in Ghana.
The older transmission design supported the grating-drum directly on the motor shaft. That design had very few moving parts, but showed a number of problems in the field. The head was supported only on one end, making it prone to excessive vibration and noise. The new design supports the grating drum on both ends using a separate shaft, connected to the motor using a coupling. This considerably improves rigidity and decreases risk of vibration.
We were able to reduce the cost of the machine by creatively combining or eliminating parts and reducing the number and complexity of manufacturing operations. The upper and lower flanges of the chute were redesigned to share a common shape and the shape was developed to minimize scrap production.
The greatest cost reduction came from sourcing a Chinese motor to replace the American motors we prototype with. This reduced the material cost per machine from about $140 down to $60. I worked with three Chinese motor manufacturers to identify and order sample motors that met all of our specifications including torque, mounting style, shaft style, and environmental tolerance.
User-interaction was greatly improved with a new grating-head design. Previous teams had manufactured heads using sheet metal and tube weldments with cutting-teeth riveted to the outer cylindrical surface. Those heads were signifigantly imbalanced, due to imperfections in the manufacturing processes, and would cause the whole machine to chatter and shake when spinning at 1800 rpm.
Switching to a head-design with a machined solid-plastic core allowed us to reduce the average imbalance from about 60 grams to less than 1 gram, which reduced operating noise by 20 decibels. Customers loved that the solid plastic head was easy to remove and clea, and allowed the machine to run quietly.
In addition to improving the design of the grater machine, I developed processes and fixtures to allow for quick, precise, and repeatable manufacturing.
Tooth Piercing Tooling:
One of the first processes I worked on was a precise tool for piercing the teeth that allow the cutting-head to shread cassava so effectively. Previous teams had used the Ghanian technique of piercing sheet metal with a sharp nail. I used a CNC mill to precisely machine a punch and die out of A2 tool steel, to use in a commercially available punching machine. With this improvement, manufacturing time for a set of teeth was reduced from 1 hour down to 10 minutes.
The frame of the grater is comprised of several weldments that hold the motor in alignment with the chute and ensure that the chute is straight and vertical. I devised a step-by-step manufacturing procedure for the grater which included fixtures for each welding stage to ensure proper alignment. Specific processes and order-of-operations were chosen to maximize the precision of critical dimensions while condidering the tools and skills available in Ghanian machine shops. Prototype fixtures were machined from MDF using a 3-axis CNC router, making them quick to manufacture and light enough check as luggage on an airplane.
Design Report: Download PDF