machining

Here are some miscellaneous things I have made in a machine shop. Machining is an excellent skill and hobby; whether you crank handwheels or watch a CNC system sling hundreds of pounds of metal around, it is a fun and practical way of realizing your ideas. Many people now prefer to use other fabrication systems like 3D printers and laser cutters, but ultimately they all fall short of the combination of flexibility, precision, and final quality that milling machines and lathes provide.

rubik's cube
experimental whistle
lock stamp
medallion
'mishap'

aluminum rubik's cube

I enjoy playing with Rubik's cubes, and after examining the internal structure I figured I could make one. With the help of Ray Harlan, a friend and mentor of mine, we produced a fully-functional cube from solid 6061 aluminum. Yes, I can solve it, though not with notable speed (around 1' 30"). Interestingly there are only five unique parts: eight corners, 12 edges, six two-piece face centers, and a central hub. It is basically an exact copy of the usual plastic Rubik's cube, though substantially heavier at about 500g (1.1 lb), about four times that of the real one. Its weight makes it very satisfying to play with.

	The original cube. Mmm, shiny.
	Since the grooves engraved into each face are hard to see, they were colored in with various colored permanent markers and cleaned up with some alcohol. It should be fairly durable, though filling it with hard colored wax, paint, or epoxy would be better.
	The other three faces of the colored cube; one face had no design engraved in it and was left uncolored. Yes, it really works.
	A checkerboard pattern, showing which faces oppose.
	The classic 'dots' pattern.
	Here it is partially disassembled to show the internal structure and constituent pieces. Note the grey arcs on the inner faces; rubbing pieces of aluminum together will leave this grey oxide crud, which only serves to further abrade the aluminum. This is one of the drawbacks of aluminum, and we would have made it out of stainless steel but for the ridiculous cost and difficulty of machining.
	It is the same size as a real cube. (The cube in the photo was swag from a trade show.)

sound-tron: experimental whistle

This unusual whistle was inspired by the cavity magnetron, the device in every microwave oven that generates microwaves. Inside the cylindrical core of a magnetron is a heated cathode along the axis that emits electrons, and an axial magnetic field that causes the electrons to swirl around in a central cavity. Surrounding this cavity are a number of resonant cavities that are energized by the swirling electrons, generating a rapidly-alternating electric field whose frequency is governed by the size of the cavities.

Again with the help of Ray and his shop, I produced a rough air-driven analogue to the magnetron. Inside, there are eight resonant cavities surrounding a central cavity, where air is allowed to enter and directed to swirl around. As the air passes over the opening to each cavity it causes the cavity to resonate, just as blowing over the top of a bottle will cause it to resonate. Sound is emitted from the ring-shaped opening.

As is, the whistle is rather temperamental and especially sensitive to the angular position of the central stator. It also appears to offer no discernible advantage over more typical designs. Maybe further experimentation will improve its efficacy or efficiency, but as it stands, it is more a curious object than anything else. Still, I think it is neat.

Here are the components of the whistle. The air-directing stator was made as a different part since the original (non-functioning) stator did not work and was milled away.

I asked how one would make something like the stator before CNC. The answer was basically 'you don't'. Back in the day people tended to avoid making parts with lots of curves or unusual geometry; the power of CNC is that it opens up a whole new set of possibilities. This part would probably have involved liberal use of a rotary table and some special fixturing.

Here is how it goes together. Air comes in from underneath through the small hole in the center of the large block (visible in the image above), swirls around in the ring-shaped area, and exits through the top along with the sound.

lock stamp

In 15-410, the operating systems course at Carnegie Mellon, students are taught a great deal about concurrency and system programming through various projects. Unfortunately, robust concurrent design is difficult and many students overlook aspects of it. As a TA, I have read and graded many such projects that are 'excessively concurrent'. We use this stamp to helpfully indicate where threads need to be blocked from concurrently executing some code, perhaps with a lock...

The stamp blank was made on the CMU Robotics Club manual mill, and the stamp face and side inscriptions were done on the club's CNC mill. Being aluminum, the stamp lacks flexibility and does not make as good of an image as a quality rubber stamp, but man does it ever make a satisfying thunk when used.

	The stamp blank, measuring 0.95" x 0.95" x 2". I apologize for the poor photo quality.
	The completed stamp. The engravings were colored in with Sharpie and the excess filed and rubbed off. The CNC machine is a benchtop mini-mill with tiny steppers and leadscrews with significant backlash, lacking rigidity and power for anything large and precision for anything small. If you look carefully you will find evidence of the backlash in the engravings and in the stamp face. Unfortunately, quality and precision are rarely found in the Robotics Club.
	Here is a closeup of the surface. Due to the reflectiveness of the surface and the tool marks, it is hard to tell exactly what is going on. It was cut in three passes: one deep cut with a 1/8" end mill, a shallower cut with a 1/16" end mill for more detail, and a 60 degree engraving bit at about .015" depth for the final detail.

medallion

This is a decorative medallion I made. It was engraved and cut out of stainless steel plate on the CMU Robotics Club CNC mill, then colored with Sharpie and sanded to a satin finish. Stainless steel is more tarnish- and scuff-resistant than aluminum, which makes it a far better choice for decorative items like this. Of course, it is correspondingly more difficult to work with; this piece already pushes the limits of the puny mill.

	The medallion. Most people say it looks like a sheriff's badge, but I see only a small resemblance at best.
	It is currently mounted to my bag using the PALS webbing on the front, with six screws holding it to three plates that pass under the webbing. This is not going anywhere.

'mishap'

People make mistakes. When a mistake involves a powerful mill you are bound to be in for an interesting time. I was milling the edges of a stack of aluminum plates with a 3/4" carbide end mill when it was discovered that the vise was not tightened. There was a loud boom and a small explosion as the little plates flew all over the shop. I later found one with deep gouges where the cutter had grabbed it and thrown it out of the vise, which must have dragged the rest along. Luckily I was not hit by anything, but since then I always make extra sure the workpiece is secure before taking a cut.

This sort of thing seems to happen at least once to everyone. One or two 'big' mistakes seems to be the norm with anything relatively new.