Group 8 focused on designing a fully-functional espresso machine which is capable of producing a total of 100 cups of espressos a day before bean refilling is required and before the waste bin needs to be cleared. As the target audience of the machine is towards departmental staff, the machine has been accommodated to allow a total of 10 cups of espresso to be produced in 10 minutes peak periods between classes, allowing staff to grab a quick cup of coffee to boost their productivity throughout the day. The project has been split between three sub-assembly groups, where Group 8a focused on designing a suitable pump to allow 9 bars of water pressure to be inputted through the coffee bed, Group 8b focused on designing mechanisms which allows a fixed amount of beans to be fed into the machine, tamped and finally disposed, and Group 8c focused on designing a pressure vessel to produce 60ml of hot water (90 oC) to be inputted.
The result is a semi-interactive approach, allowing the user to appreciate the beauty and elegance of brewing their own cup of coffee. The machine is separated into the front and back end, allowing the users to oversee their coffee-making process and to interact with the machine at the front end, where the background work of producing and pumping the hot water through the machine can be seen through the back end of the machine. The machine also offers simplicity in design, requiring only two buttons and one switch. Red and green L.E.D. lights have also been installed to communicate with users.
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DMT08A Pressure Vessel / Heating Assembly
A cup of espresso is made when a small volume of 85 to 95 °C water at a relatively high pressure (7 to 15 bar) is forced through a puck of tamped coffee grounds in a relatively short time. This process translates to the anatomy of a classical espresso machine: a pump provides pressure, a boiler provides heating, and a separate assembly houses the coffee grounds. Our team is concerned with the temporary storage and heating of the pressurized water, before and during the time a shot of espresso is pulled.
The final vessel is made up of two stainless steel parts, clamped together using six nuts and bolts, and sandwiching an O-Ring for a reliable water-tight seal at high pressures. The water is heated by an immersed 1100W electrical heating element, attached and sealed to the base plate of the vessel using a suitable nut and gasket. Temperature regulation during operation is carried out by a thermostatic switch mounted on the top outer face, simply wired in series with the heating element. The inlet and outlet ports are made easily accessible using standard BSPP adaptors, featuring bonded seal washers in order to keep the assembly leakproof. Finally, non-return valves are fitted on each port, preventing water to flow back into the pump or grounds preparation assembly when it isn’t meant to.
The team also worked on the implementation of the user interface logic for the completed machine, where an Arduino registers the user’s button presses, timing and relaying them to the relevant assemblies (not shown here).
Espressos are popular drinks in the Mechanical Engineering Department, with two thirds of staff members regularly drinking them and 75% who would drink them if they had easier access. As part of a larger assembly that would make up the Department’s first student made semi-automatic espresso machine, the grounds preparation and feeding sub-assembly had to accomplish the following: ‘to feed a measured quantity of pre-ground coffee into a filter and to dispose of the used grounds afterwards’. An espresso is produced by passing a stream of hot water (provided by two other sub-assemblies) through a disc of compressed coffee grounds.
The resulting design is a novel, partially manual system divided into four parts. A hopper is used for the storage of pre-ground coffee beans which are then fed into a dropping mechanism. A stepper motor is used to turn a spindle in the dropping mechanism by 90° to dispense a fixed quantity of the coffee grounds into a cup. User interaction is required in the next step to provide compression of the coffee grounds in the cup by means of a lever and piston. Water is then passed through the cup and filtered by a stainless-steel mesh to provide a quality cup of espresso. The final part of the system is a partially manual mechanism to dispose of the used coffee grounds, whereby a linear actuator lowers the base of the previously mentioned cup for the beans to be manually swept into a bin.
The purpose of a pump is to transport fluid from one location to another. In this way, the rotary vane pump of an espresso machine allows for the transport of water from the water reservoir to the coffee grounds. It is a positive displacement pump and works via trapping a volume of water between the vanes of a rotor positioned in an eccentric tube. The volume increases and then decreases within the chamber as the rotor spins to create flow. The rotary vane pump is the hallmark of any modern high-end espresso machine. In blind taste tests, rotary vane pump espresso machines are known to deliver a better-quality coffee than its counterpart, the vibratory solenoid pump. Its advantages involve its quiet operation, long life and the minimal required maintenance.
The aim of this design, make and test project was to create a rotary vane pump to be placed within a medium sized espresso machine. This pump was designed to produce flow which can deliver water at 9 bars at a flow rate of 200ml/min to the inlet of the coffee grounds from a water reservoir. Our main objective was to produce a rotary vane pump which experienced minimal leakage and maximum volumetric efficiency while in operation. We aim to achieve a similar volumetric efficiency to that of the average rotary vane pump on the market which is 95%.
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