The main components of the machine are the cylinder, piston (with an inbuilt one-way valve) and lever. The lever provides the mechanical advantage required to generate the required pressure within the cylinder. Most commercial machines are calibrated to operate at 9 bar pressure, so this is the target pressure. The pressure can be easily calculated from the mechanical advantage provided by the lever and the piston area. The area of the piston is also determined by the volume of espresso desired, as is the piston stroke and therefore cylinder length. The piston stroke is a function of the minor lever length, which effects the mechanical advantage. All of this can be modelled with simple equations, as shown in the attached document. I have done my best to optimise the design based on my preferences but please do try out changing design aspects to meet your personal preferences.

Although I was trying to keep the design of the machine as simple as possible, I felt it was necessary to include a one-way valve in the piston. This allows water to flow through the piston when the lever is lifted but pressurise the water when the lever is pushed down. This enables the user to fill the cylinder with hot water without completely removing and reinserting the piston into the cylinder before the water goes cold: not something you want to be doing before your morning coffee! When the lever is lifted, a small piston inside the main piston moves upwards slightly which opens a pathway for water to run through the piston assembly. When the lever is pushed down, the small piston seals against the inside face of the main piston. This is easiest to understand from the CAD model (next step).

The prototype cylinder was initially made from one piece of aluminium. However, due to the thermal conductivity of the material, the water wouldn’t maintain its heat when brewing, unless the whole cylinder was heated beforehand. The final cylinder hence incorporated an acetal liner which insulated the hot water from the main body of the cylinder. (The thermal conductivity of acetal is approximately 700 times smaller than the thermal conductivity of aluminium.)

Several prototype designs were manufactured and are shown in the images - including disasters! A picture of my dad's version is also shown (last image). Rest assured that the final version has been making two espressos per day for the last few months with no explosions!