This mechanism consists of a base with two slots intersecting at right angles. Two 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. Interestingly, most of the names are preceded with state names such as Arkansas, Kentucky, etc. I guess that’s how locals make fun of their neighbors.

I made two versions of the Trammel. The first version is made from wood. This build is similar to LeumasYrrep’s version as shown here. I decided to take a different approach by making the slides from dowels instead of the layered approach. The second version is a 3D print. I’ve included all STL files in case you would like to print your own. The wood version was made from scrap wood; so my cost was minimal. The 3D printed version cost ~ $15 at my local library. The library charges $0.25/gram so I tried to reduce the weight by removing the four corners. I feel I could have reduced the weight by another 25% with a little more CAD work. If anyone is interested, I’ve also included STP files which can be modified or scaled in most CAD packages.

Trammel in Motion.

This video shows the actual build which should help in conjunction with the build instructions.

Step 1: Tools/Materials

Tools:

Table or Miter saw
Drill Press
Drill bits: 13/16" & 7/32"
Vice or wood clamps
Wood rasp file or sandpaper
Screw Driver
Tape Measure/Ruler
Pencil

Materials:

2x6 x 5.5” long
¾” diameter dowel x 3” long
½” thick wood (1/2" x 3/4 x 3”)
Glue
Screws for wood version (x2): Screw for Plastic and Particleboard, Pan Head Phillips, 8-11 Thread, 1" Length
Screws for 3D Print version (x3): Thread-Cutting Screw for Soft Plastics, 6-20 Thread, 1/2" Length

Step 2: Wood Version Build

You will be building to this drawing.

Step 3: Base - Step 1

Cut 2x6 to a 5.5” x 5.5” block as shown.

Step 4: Base - Step 2

Mark centers for holes on the sides of the blocks. Use a 13/16" bit to drill the holes through the base. Depending on your drill press travel and the length of your bit, drilling the holes might take a few steps. My low end drill press only has a two inch travel. I used blocks to extend the depth - see video. I also had to drill from both sides. Take your time measuring both sides since this must be a smooth pass.

Step 5: Base - Step 3

The groove was cut using a table saw. Mark the locations for the cross slots as shown on the drawing. Set the blade height to ~ 3/4". Make two passes to remove the slot material. Rotate the part and cut the second slot. To allow for smoother operation, place a slight chamfer on the four inner corners. I did this with a wood rasp file although sandpaper will work.

Step 6: Slide - Step 1

Two slides are required for the trammel. The slides are made from two pieces. The first piece is made from a 3/4" dowel. Use a table saw to make a flat section as shown. Although the drawing shows 1.5" long, I suggest cutting this longer (over 3") to make two pieces at once - see video.

Step 7: Slide - Step 2

Cut a 1/2" thick piece of wood as shown. Again, make the length longer than 3".

Step 8: Slide - Step 3

Center the block on the dowel and attach with wood glue. After drying, cut the parts into 1.5" long sections. You might want to round the sharp edges to allow for smoother motion as the slides cross the center section. Drill a hole in the center of the slide assembly. Note that this hole is for a #8 wood screw.

Step 9: Handle

Cut the handle from 1/2" thick wood. Drill the holes as shown. Again, the clearance holes are for a #8 screw. The large hole was cut using the same 13/16" bit from before. Note that a knob (see 3D print) could be used in lieu of the hole.

Step 10: Assemble

Attach the handle to the slides with #8 pan heads. This should be a loose fit to allow the slides to pivot around the handle. Try it out. If it binds, use sand paper to open up the slots. Adding rounds/chamfers to the slides also helps keep the trammel on track.

Lesson learned: I tried to jazz it up with paint which turned out to be a mistake. It worked perfectly until I added the paint. The extra thickness and sticky surface caused friction which prevented it from moving easily. I recommend keeping it bare wood or a stain without polyurethane.

A video of the trammel in motion is shown in the last step of this instructable.

Step 11: 3D Printed Version

Since my woodworking skills were not needed for this version, I was able to add features and achieve tighter tolerances which allows for smoother operation.

Step 12: 3D Printed Parts

All the parts required for the build are attached. I also included STP files for those wanting to modify the design.

Instructables.rar
Download

Step 13: Assemble

The assemble sequence is the same as the wood version. This version does have a knob at the end of handle. Use a screw from the bottom side to attach.

Step 14: Video of Trammels in Motion

Final Product

Step 15: Comparison in Size

Step 16: Update (Wood / 3D Print Hybrid)

I wanted to resurrect the painted version but didn't feel like building new wood sliders. Attached is a 3D print that will drop right into the wood base. As you can see from the picture, I added chamfers and rounds in the model which allow for smooth operation in the slots.