I've always been fascinated by the trammel of Archimedes. This version consists of a base with four slots intersecting at the middle of the circle. Four sliders are confined within the slots and are attached to a handle by pivots (screws) at fixed positions along the handle. The mechanism can be used for drawing a perfect ellipse. This is done by positioning a writing utensil at the end of the handle while making a full revolution. For more information/history, see here.

With that said, it is often sold in novelty shops with interesting names like Bullsh*t Grinder, Bullsh*t Masher or Do-Nothing Machine. Once you have it in your hand, you will notice that it turns into a nice fidget toy and a definite time waster :).

This is actually my third iteration of the the trammel. The first one (two pass) was built from wood. As part of the same instructable, I included a 3D print. I then took it up a notch and made 3 pass version. This one was beyond my woodworking skills so I took the 3D print route. The one shown in this instructable is my most advanced yet at four passes. With each build, I've taken lessons learned and made improvements. For this build, it actually took me 3 prints to get all the bugs worked out.

As an added feature, I was able to convince my son to animate the CAD files in Blender. Besides showing how it works, I feel the results are pretty mesmerizing.

The actual part in motion is shown here:

Supplies

Tools:

3D Printer

Materials:

Filament: PLA used - 3 colors
Sand paper (optional)

Step 1: Base

The base is 100 mm in diameter and 20 mm tall. The 4 passes on the trammel are centered equally spaced around the circle. For the paths, I chose a dovetail groove. This groove was based off a lesson learned on my initial 2 pass print (T-Slot) where I had to remove the supporting structure above the slot. I added rounds to the center edges which help keep the slides on track if they enter to the middle section at an angle. The pockets around the perimeter are mostly cosmetic even though they do add a nice place to grab the trammel from underneath as you are spinning it.

Step 2: Slide

The slides were designed to match the grooves in the base with a 0.5 mm of clearance. I added chamfers and rounds on the slides to allow for cleaner movement. For each slide piece, the hole is centered on the top and is sized for a small wood screw.

Step 3: Handle

Being sort of lazy, I used the CAD tool to work out the dimensions for the pivot points. For the overall length, I just wanted something to clear the base while spinning. The hole in the end is for your finger.

Step 4: Parts

These are the three colors I had on hand. I suggest using contrasting colors if you print your own. Besides the print, you will need four screws. I used eye hooks to in lieu of screws to allow for quick adjustment. I also added flat washers but they aren't really needed.

Step 5: Assembly

The build is pretty straight forward. Place the sliders in the base. Line up the holes and attach the handle with the screws.

Step 6: Pictures

A few inside pictures. Banana for size reference.

Step 7: More Pictures

Here are the final pictures. It has been passed around the house quite a bit and will be a desk toy at the office once the Covid19 pandemic is over.

Step 8: STL Files

All files are included to build your own. You will need to purchase screws or hooks. Note that I can provide STP files if anyone would like to modify the design.

Thanks for viewing!