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Herreshoff Steam Engine
September 2017 Bianchini
For my undergraduate thesis, I documented my process in manufacturing sand cast parts for a 1897 Herreshoff steam engine. The project got started through the Pappalardo Apprenticeship program I had joined the previous year. The final functional steam engine is part of a demonstration at an MIT Museum exhibit on Nathanael Greene Herreshoff that opened in fall 2018. I manufactured four of the engine components: the iron column, two iron gibs, and the bronze bearing crosshead.
I was able to work on this steam engine project through a Pappalardo apprenticeship. Linked to the mechanical engineering robotics course 2.007 Design and Manufacturing, Pappalardo apprentices spend half of their time mentoring students in 2.007 and the other half further developing their own design and manufacturing experience. Seniors in spring 2017 and 2018 collaborated on this steam engine recreation for the purpose of being a part of a Nathanael Greene Herreshoff exhibit at the MIT Museum. I started early in my fall 2017 semester, and I used this head start as material for my thesis.
Architecture of the recirculating steam engine
The recirculating steam engine is a double-acting steam engine. A double-acting steam engine is one whose valve allows the highpressure steam to act on both the up and down strokes of the piston. The entire system converts heat energy into mechanical work, beginning with heat in the form of steam and providing rotational energy of the crankshaft.
The original Herreshoff assembly drawing. The valve cylinder is on the right, and the power cylinder is on the left. The crankshaft is shown in profile at the bottom in the bed plate.
An "exploded view" of the Pappalardo apprentices' physical progress in April 2018. I made the column and bearing crosshead (labeled) as well as the gibs (on either side of the column).
Sand casting process
To sand cast an object, first the part design must be changed to better suit the sand casting process. This redesign depends on what is called the pattern design. The first physical step is to fabricate patterns, around which sand is packed and then the patterns removed to form the desired cavity shape. Because the patterns have to be removed from the sand before pouring the molten metal, their design has to be done carefully, including eliminating overhangs, incorporating generous drafts, adding thickness to surfaces that require a machine finish, and scaling by a factor that will compensate for the metal shrinkage as it cools to room temperature. Typically this means making multiple pieces for one pattern, and occasionally cores made out of resin sand are required to create interior geometries. I made each of the patterns out of rigid polyurethane foam on a CNC Prototrak mill, using HSMWorks as my CAM software.
After making the patterns, the sand casting flasks should be packed with sand around those molds. Next is the gating and risering strategy, which involves how the molten metal gets poured into the mold cavity. The outcome of the sand casting is highly dependent on this strategy, and there is no exact science. Thus, this was a cool learning process and required iterations.
When the sand cast part is poured, the last step is post-machining. This step means cutting off the gates, establishing datum surfaces, and machining any faces or features that need to be to a finished dimension.
Now that I've given a brief background of the sand casting process, I'll go into detail for each of the parts I made.
The column serves the purpose of holding up the cylinders (in combination with the stanchion) from the bed plate, as well as providing a smooth surface on which the bearing crosshead can slide.