Focus: Machining and Manufacturing Processes
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This project was the entirety of the intro to machining course, titled MEAM 201: Machine Design and Manufacturing. In this class, we each designed, modeled, and manufactured our own functioning Stirling heat engine. Many of the parts were already designed and each student only had to machine them, and many of the more aesthetic parts or custom parts were designed and built by each student. In total, there were about 20 different parts, not including fasteners or gaskets.
I will briefly describe how a stirling engine works, for clarity going forward. A stirling engine is a type of heat engine that uses the gas law to create rotational motion from heat. The full cycle is seen on the left. A hot end, in the air cylinder at the bottom, is heated using a blow torch. The air in the cylinder expands as it is heated. This air pushes a displacer which spins a flywheel through a connecting rod. The flywheel’s high moment of inertia, in turn, pushes back against a piston. The piston forces cooler air back into the air cylinder. The air cylinder is reheated, so the cycle continues, creating rotation motion from heat.
In order to accentuate the mechanical aspect of the engine, I decided to design my engine around a train theme. The ‘bedplate’ on which all of the components would be fixed to would resemble the silhouette of a 19th century steam locomotive. The flywheel would be one of the train’s wheels with a second flywheel connected to the powered one via a coupling rod.
These CAD model screenshots show the overall design of the engine. The picture on the left shows the two coupled flywheels, while the picture on the right shows the functional parts of the engine: air cylinder and heat sink to the left and connecting rods and shafts to the right.
As mentioned earlier, this design has many, many parts machined over a couple months. This would be far too much to cover here. Instead, I will walk through one of the more complex parts which use a variety of machining techniques: the flywheel.
The wheels of a train are relatively complicated parts for a few reasons. First, the outer edge of their cylindrical shape has a taper to it. The wheel runs on a rail and the angle along that edge allows for smooth and safe turns around bends in the track. Second, the inner spokes of the wheels are not fully radially symmetric. Because of the added mass of the coupling rod between the two wheels, there must be a counterweight added to the opposite end of the wheel. This is compensated for by adding a flat to the inner edge of the wheel, essentially filling in some of the spokes.
The wheel was machined in two stages: first, the outer profile was machined on a manual lathe; second, the inner features were machined on a CNC mill. While the outer cylindrical and conical faces of the wheel seem relatively simple to machine, looking ahead to the following operations led me to take a more complex route in this first operation to make the later ones more precise.
The illustrations below represent the lathe operations as viewed from above. The blue on the left represents the 3-jaw chuck used to hold the 3.5″ Al round stock, and the red dotted line represents the actual material. The yellow right-handed cutting tool takes passes to form the shape of the part, then the parting-off tool removes the part from the stock.
As seen in the 1st drawing, machining the outer profile from the front (coming from the right side) would work. However, once the part-off tool removes the part from the stock, the back face would be rough and inaccurate and, thanks to the tapered profile, there would not a good way to hold the part to machine the parted-off face smooth. So, I opted for a different method to turn the profile of the wheel. The 2nd drawing shows the part flipped around. The flanged face of the wheel can be faced smooth in the lathe first. Then the taper can be cut and the part removed from the stock. The key difference is that the rough face to be re-machined is at the taper: this way, I can hold the part by the flat flange face to mill away the rough top face. Also of note: the flipping of the cutting tool to machine the flange. Since I did not have any left-handed cutters and a right-handed cutter cannot reach the inner corner of the flange, I had to flip the cutting tool over and reverse the spindle direction.