This is a remix of the Pen Arm Assembly of thing 20398. Thanks to Glasswalker for the original design.
This remix exchanges the single pivot arm with a four-bar linkage assembly. The original single pivot arm design works well for spherical objects because the object radius is constant, but when the single pivot arm is used on an egg with variable radius, the pen traces an arc on the egg rather than a straight line. The benefit of the four-bar linkage is that the pen tip traverses straight up and down as it rides on the surface of an egg, enabling the Eggbot to draw a straight line on the egg. This results in significantly improved image quality.
I printed this in PLA on my Monoprice Maker Select (0.2mm layer height, 3 walls and 25% infill). The part that holds the pen is the only piece that needs support. There are two versions of this part, PenLinkageConnector_Tapped if you have an M4 tap, and PenLinkageConnector_Nuts if you don’t have an M4 tap and want to use M4 nuts instead.
After printing, use a 4mm or a 5/32” drill to ream the through-holes IN PenLinkageLowerArm, PenLinageUpperArm, Pen_Arm_Pivot_EG, and PenLinkageConnector. Then, lightly sand the outside of the linkage arms to remove high spots that might drag. Likewise, sand the inside faces of Pen_Arm_Pivot_EG and PenLingageConnector where they interface with the linkage arms.
If you used the tapped version of the pen holder, tap the holes for M4-0.7. If you used the nuts version, then insert M4 nuts in the hex pockets and draw them in with a short M4 screw.
To assemble, I used M4-0.7 x 35mm screws with three washers and a nylon insert nut as shown in the attached picture. I tightened each screw to minimize the clearance between the parts will still allowing free rotation of the assembly. During the assembly, note the location and orientation of each of the parts in the labeled assembly drawing (attached).
I used M4-0.7 x 18 mm screws to clamp the pen in place. The opening is large enough to hold thin and fat markers.
Pen_Arm_UpFixture is a tool to lift the assembly up while you change pens during a print. Manually lift the assembly by hand and slip the fixture into place. Don’t forget to remove it when you resume printing.
The assembly was designed for optimal straight line performance with the pen tip 44mm below the top of the pen holder. The part PenLocator slides on the lower portion of PenLinkageConnector, and can be used to repeatedly place the pen at the optimal location.
I used Solidworks. The upper linkage arm is shorter than the lower linkage arm. As the linkage rotates, the four-bar linkage keeps the pen angle nearly constant. Because the upper linkage arm is slightly shorter than the upper arm, a slight rotation of the pen keeps the pen tip on a straight line for a large portion of stroke.