Sterling Engine for Students

    I built 6 of these sterling engines to help a professor to prepare for a class. This professor in the Industrial Engineering department was ramping up for a new class on CAD, CAM, and simulation. Typically the students learn Creo (or OnShape), but if they so choose, they can now take a class to learn SolidWorks modeling, MasterCAM, and SolidWorks simulation. A class project was selected by the professor on the criteria of it needing to be complex enough to last a semester, have moving parts for animation, have a thermal element for simulation, and have some pieces simple enough that a student could program and machine them.

A finished engine. The legs and flywheel are made by the class students.

   
    The idea was the students could progress through the project, modeling and assembling components in CAD, running motion and thermal analysis on the model, and programming and making a few parts. The remaining parts would be made be me and at the end of the year, the students could assemble the engine in real life with their parts and see it run. The parts of the engine the student didn't make would be reused year to year.

    Chosen was the "Vickie" Victorian styled sterling engine designed by Jerry Howell; a beautiful and elegant model engine. This project was really doomed from the start due to the choice of this engine. The plans from Jerry Howell contain no tolerances and the instructions intend a lot of hand fit and finish, creating bespoke parts. This doesn't jive well with the fact that 6 needed to be built, all with interchangeable parts, and several of the parts had to be made by inexperienced students. Add to this some on the fly design changes by the professor that override my better judgement and we ended up with 6, pretty, but non functional model engines.

Detail of the linear guide for the power cylinder.

    I came into this project half way though. The professor and another student worker had already made many of simpler parts of the engine. As I progressed through the remaining parts and sub-assemblies, I ran into fit issues. Going back and measuring the parts the professor and other student had completed, I found every single part was out of tolerance (self imposed +/- 50 um), or in some cases totally missing features. This revelation essentially doubled my workload on the project. While some parts were only off by a small amount or had a bad surface finish, I saw no excuse for missing features, or dimensions off by more than 1 mm. Some parts I reworked or shimmed to fit, most others I totally remade.

    This is what I believe to be one of the larger contributors to the engine not working. The other is on the fly design changes the professor wanted to make the engines easier to build. These included replacing some of the custom fasteners with OTS parts (minimal impact) and replacing some of the weight reduced parts with heavier ones. When I was reading through the assembly plans, a great deal of mention wad given to what components had to be very light weight. Many of those components involved soldered assemblies of machined parts and brass tubes. It was decided to replace the brass tubes with solid steel shafts to simplify. I think this extra weight contributed to the engines not working. 

    In the end, these engines are only good for display. Maybe some student or professor in the future will find these and be able to get them working.

6 completed engines. Note the oxidization on the hot ends from testing.

One Day Builds: Kinetic Necklace Update

       This is somewhat of a follow up without an original so bear with me. I started making kinetic jewelry around 2012 after being unable to find any satisfactory steampunk jewelry. Everything I found was just plastic gears or old watch cogs glued onto some sort of base. As en engineer, I couldn't stand it. I have always enjoyed the work artists like Chris Fitch, Arthur Ganson, and Theo Jansen; for me, if art has gears they have to move.

The updated, machined necklace (not when I was 12)

       This was when I was 16. I had no experience with gear design, no machining or jewelry experience, and I decided I would have to make what I wanted. I used Matthias Wandel's Free Online Gear Generator to generate the involute patterns to which I added spokes to by hand. Frames and bridges were all drafted by hand. Using a my grandfather's old files and improvised tools, I cut the gears and frames out of unfolded copper water pipe and riveted the parts together with recycled building wire.   

       I only made 5 or 6 pieces, only one of which has left my possession. Looking at them again they are far less precise that I remembered them, but this combined with their recycled nature gave them a certain style. 

       How does this relate to a one day build? Fast forward 4 years and I chose to make a few more, and this time all out on the technology. The involute gears I used before would jam frequently due to low their low precision, even with a very high pressure angle. The high pressure angle also resulted in pointy teeth. I designed a totally different type of gears for the new pieces, based on a ring of evenly spaced semicircles. Alternating semicircles would be peaks and valleys of the gear. These are not suitable for power transmission but mesh very smoothly and are do not pinch or poke in any way. They also can be machined easily. 

The design of the new gears in 2D CAD

       This post really contains two one day builds. The first was the machining of the original necklace. The four pieces were machined on a Tormach and programmed with Fusion360. As much as I criticise the small Tormach mills, they do get the job done for small, slow jobs. I avoid our's when possible but if its the only machine open, you make do. I had intended to make a post about machining the components at the time but I never got around to it. The final piece measures only 20mm wide and is made out of 7075 aluminium and 932 bronze.

       Finally to the actual title of the post, the update. The client this was made for enjoys fiddling with necklaces, well suited for my work. I unfortunately underestimated just how much they would twirl the gears and the loctite joints holding the cage together came loose. Thrice. I decided to drill out the integral pins and use thread to stitch the cage together. I had originally intended to drill the pins and use them as integral rivets, but didn't have the proper tools at the time. 

Indicating the integral pins. 2mm dia. 1mm tall stepped to 1.5mm dia. 0.5mm tall

1mm drill 

       While I had the necklace I cleaned up some of the corrosion and took some better photographs. Speaking to this design and project overall, I am very please with the result. The scale especially makes it attractive, but it also means I probably wont make pieces this small again. If I did this again in the same or similar size, I would use a more durable material for the cage and use a more permanent connection.