High Torque Design
I love an out of the ordinary design and manufacturing challenge. So naturally I jumped at the chance to solve a research colleague’s problem a few years ago - he needed a stator mount that could support massive torque loads and hold a cylindrical stator the diameter of a basketball.
I designed, simulated and fabricated a solution using aluminum sand casting. I built a propane-fired casting forge that could hold 4 liters of liquid aluminum, fixed up some green sand using cat litter and 100 lbs of casting sand, designed an 8 part, 3D printable pattern, and ran simulations to confirm that the finished structure could support the required loads.
Next, I used Finite Element Analysis (FEA) to predict the deflections and stresses in the part while it reacts a huge magnetic torque load.
This allowed me to iterate on the dimensions of the mount, changing wall thickness, web dimensions, and other feature geometry.
I took special care to account for shrinkage in the cooling metal part - the mold pattern is oversized by about 7% to account for this size change during the casting process.
Because there are six keyways machined into the inner diameter of the mount after casting, I had to be very careful to leave enough material to allow the net shape to emerge.
I designed the mold pattern with generous 10 degree draft angles - this helps the pattern, which will be printed from PLA using fused deposition modeling 3D printing, to release from the mold.
I’m no expert in mold design - this is a whole field unto itself. I designed the mold with a central sprue (entry point for the molten metal), and six runners that allow metal to flow to the outer circumference. The runners will be cut away post-casting, allowing for the final machining operation.
We made the mold by pounding green sand around the pattern and then gingerly removing it… leaving behind a cavity for the metal to be poured into.
The pour went great! On the left side you can see the two halves of the mold (under inspection), in the center is the cooled aluminum part after casting. The pool of aluminum at the top of the mold is important - it applies a small fluid pressure to help fill the mold, and it supplies extra material as the aluminum undergoes massive shrinkage during cooling.
I designed an fabricated a double scissor linkage from steel stock to hold the crucible from two sides while maintaining a safe distance.
The casting was finished in a CNC machine shop at the University of Colorado Boulder - the excess material was cut away, the lower surface and inner diameters were machined to the correct dimensions, and the keyways for the stainless steel stator were cut.
Three years later and it’s still operational! The part was originally quoted above $10,000 for a single unit - the work here produced a functional solution using design, simulation, and fabrication technologies.
