Planning
The first thing I did was sketch the build plan on paper to get a rough idea of dimensions and assembly. From the onset I knew that I wanted the platter to be driven from the same crank the controls the four bar linkage, however due to construct-ability concerns it was not clear until build time just how that connection would be made. At first the intention was to directly gear them together, however it became clear during construction that maintaing contact between the two disks would prove impossible, so I opted to connect them instead via a belt and pulley, this resulted in the platter rotating in the same direction as the crank rather than the opposite as I had assumed I would make it.
Figure 3. Preliminary Build Schematic, Planer View
Build
This mainly consisted of me cutting disks then forming and attaching them together to create the various components needed to construct the device. Everything used was constructed from raw materials except for the platter base. For that I sources an un-lubricated ball bearing square turntable from McMaster Carr. This was due to the fact that I needed to maintain a flat surface on the top for my drawing pad. For the rest of the pulleys I simply used the cutouts from various sized hole saws which I then routed down and sandwiched together so that I could keep my belt captive. The belt turned out to be necessary because using the turntable for the platter made locating the center of the hand cut "circular" platter an impossibility, these two issues lead to a run-out on the diameter of about 1/8" which was too much to make a friction fit gear mechanism maintain engagement. I subsequently opted for the pulley as mentioned which required I make a belt. The solution to that was liberating a bungee cord from its sheath then braiding it to keep the strands mostly in check and then whipping the ends together with nylon string. Due to the generous groves in my pulleys the belt runs well enough as is but this would be a good place to start should improvements become necessary.
Figure 4. Belt and Pulley / 4 bar crank closeup
Though this machine was conceived and will be modeled as a planer mechanism, it is obviously built in three dimensions. This is evident in Figure 4 as you can see the washer stack which was needed to get the coupler linkage of the four bar to the correct height such that it would clear the platter and be co-planer with the rocker linkage. This height discrepancy manifests in local off axis torques on the crank pivot, this is a tolerance and manufacture problem that causes roughness in the rotation and negatively impacts the output, effort was made to reduce this effect but without more sophisticated fabrication it will not be removed.
During the planning stages there was always an intention to include adjustability to the device such that link lengths and things of that nature could be readily changed to alter the output. During the build phase it became rapidly apparent that these settings would also make a big difference in machine performance. Certain configurations proved difficult or impossible to assemble, as well some lead to undesirable out of plane movements in the linkage, or at times would be close to binding making for smooth rotation difficult thus reducing the quality of the drawing. So some holes or positions that were initially planned for were either omitted or made obsolete based on heuristics.
Figure 5. Assembled Drawing Machine
Figure 5 shows the completed machine prior to any paper being mounted. The paper is simple placed on top of the steel platter and magnets were used to hold it in place. You can also see in this image the pen holder. This is another area of improvement that would be something work addressing. The intention was to mount it on a pivot point with a slip critical friction fit to the coupler linkage, the radius and angle of the holder would define the couple point path position. In its current state it is functional, however it needs refinement in order to hold the pen completely normal to the platter at all times. Typically humans write at an angle without much trouble, however we are able to adjust pressure easily and can do push or pulling strokes easily as a result, this machine does not have that so when the angle of attack of the pen is acute to the direction of the stroke, it has a propensity to cut the paper. Simply mounting it normal to the platter and in something with a loose slip fit on a prismatic writing implement would help keep the machine un-bias to the direction of stroke. Additionally a robust paper is preferable.