by Yun-Han Lin
Drawing geometric patterns using mechanical linkage can be fascinating. This type of plotter driven by a gear-link is called a Pintograph, whose origin can be dated back to The Marvelous Wondergraph in 1908, when the first model was driven by a belt and linked arms. In 2015, Joe Freedman optimized the design and raised funds at Kickstarter, putting the model into a new use.
Later on, there have been different companies launching their own designs, such as the foreign toy brand KiwiCo and the manufacturer Fun-Maker in Taiwan.
In the model structure, the most common type is the four-bar linkage, where the movement of one bar allows the designer to trace the movement of the other three. In comparison, my design is a five-bar linkage where the linear gearing constraint maintains the free movement of bars. This is called Geared Five-Bar Mechanism (GFBM).
GFBM features its movement resembling a bird’s flapping trajectory, and we make use of GFBM and a rotating chassis to complete the model.
* The design focuses on the structure of the arms, not the gear that rotates the drawing paper.
- The large gear (rotating the drawing paper) and the smaller one must be “relatively prime,” to prevent the trajectory from repeating, and the space between lines can thus be minimized. There are 60 dents in the large gear, 29 for the smaller one, and 20 for the one at the handle (the dent number for the handle gear does not affect the patterns drawn)
- Revolute joints are recommended for starter. Using prismatic joints adds to the pattern changes but can also complicate the design.
- The pen fixed on the arm must be fine and smooth (color pen, whiteboard marker). Pencils and rollerball pens that require pressing are not recommended.
- A gasket placed between the gear and the bottom plate can facilitate movements.
- Nylon screw nuts are recommended to prevent screws from loosening. The same is true of using regular nuts glued with hot-melt adhesive.
Place the two smaller gears and the large gear on the bottom plate. Although rotation can be done by simply placing a large gear in the middle, it didn’t work well for my design, so I used an extra screw to fix the shaft.
Place the drawing paper on the large gear. The size of the paper should be smaller than the gear, so it will not be stuck when the gears rotate.
Use a popsicle stick as linkage arms with a hole drilled at the end. In addition to popsicle sticks and screws, there are two 3D-printed parts. One is for stacking up the linkages and the other fixing the pen.
To better adjust the patterns drawn, linkage arms are placed directly on the gear and fixed with the screws below it. This makes it easier to adjust the patterns drawn but could lead to loosened joint due to too fast of rotation.
Rotate the handle, and the drawing begins!