Just about any drawing tool (pen, pencil, marker, engraver...) that is not too big or heavy, can be attached to the pen-arm and used for plotting on eggs and spheres. In choosing a pen, a number of questions come into play:
- Precision - How thin a line do you want? How much detail is needed, vs. bold lines?
- Permanence - "Forever" or erasable?
- Surface - What are you plotting on? Eggs, glass, ping-pong balls, etc. all have very different surface textures and ink absorption.
- Expense - Are you doing ultra-fine, archiveable work or rapid-fire demo's? Pen price and longevity vary tremendously.
For now, discussion is focused on pens used to put pigment upon a surface. The basic Eggbot pen-arm comes with a 3/8" diameter hole. Without requiring custom modification, a pen must fit in this hole, whose size was chosen to mate with ultra-fine point Sharpie brand pens. At least four commercially available pens that are "Sharpie-compatible" have been tested:
- Sharpie Ultra Fine point* Ubiquitous and inexpensive (~$1), this is a great pen to start with. It comes in a wide variety of colors, is permanent, and has a relatively fine tip. Larger point size Sharpie pens will not fit in the standard pen holder.
- Sharpie Micro* Same as above, but half as long. More expensive (~$1.40), not as much ink per pen (and fewer color choices), but the smaller pen will not bounce around as much as its bigger brother. (You may need to order these online.)
- Tul ultra-fine point* Similar to Sharpie in quality, tip size, color availability and price.
- Copic Multiliner SP series* Significantly more expensive (~$8), but offers a choice of much finer tips (nibs), plus both the nibs and the ink cartridges are replaceable. These are harder to find, though you may find them at local art supply stores like Dick Blick. They can be ordered online from Blick, Amazon, and others. There is also a cheaper, disposable ("non-SP") Multiliner series that is not Sharpie-compatible in terms of size, but will fit in the pen holder of the Egg-Bot 2.0 kit.
- Pigma Micron series* Like the Copic Multiliner SP, this is a nice, technical fiber tip pen, and offers a variety of nib sizes, though not as tiny as Copic. It is Sharpie-compatible, but not as "nicely" as the Copic. It mates to the pen-arm higher up on the pen, and requires more force. Easier to find than Copic, it is disposable and ~$3.
- Martha Stewart Crafts .1mm Writing Pen* These markers come in only a few colors. The marker body is pretty close to the Ultra-Fine Sharpies, so they work well for detail work in conjunction with those. These markers are very light and need a little extra weight to make them work right – it helps to put the cap on the butt of the marker. $8 for 4 pens.
All above examples are Sharpie-compatible (in terms of size), and permanent. Non-permanent and non-Sharpie compatible suggestions (with pen arm mods) welcome.
Nibs: Manufacturers' stated nib sizes are far from straightforward to understand. Terms like "ultra-fine" are subjective. But numerical specs should be regarded with some skepticism as well. Micron's size descriptors are not in mm ("005" = .2mm). And Copic's smallest nib is ".03mm" -- a quick check with a calculator reveals this is ~1/1000 of an inch. Which is finer than a human hair. It's tiny, but not anywhere near that small! Of course, these numbers refer to the width of lines the pen can make, not the nib width. This value depends greatly on pen pressure, surface texture, and feedrate (speed of drawing).
Extremely fine color plots: Replacement ink cartridges for Copic's Multiliner SP series come in two types (A and B). These are related to different pigment carrier formulation for different nib sizes (A for the smallest, B for larger). Unfortunately, they do not make "type A" color cartridges (the smallest nib is .3mm for color). However, tests mating color cartridges with .03mm nibs show good results with some, but not all colors. The "A" formulation inks require longer drying times: handle your eggs carefully when using the "A" formulation so as to prevent smudging.
Variable pen heights between Sharpie-compatibles: Different brands of pen will be held by the pen-arm at differing heights above the egg. Doing multicolor plots with tight color registration is far easier with all pens being the same brand.