Improving precision

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This wiki page is part of the documentation for the Egg-Bot kit.
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If you are producing sloppy drawings, or good ones that you want to be better, you may need to do one or more of the following:


  • Check head and tailstock alignment. The head and tailstocks should be square to the chassis' side walls. Use the ruled measuring guages on each side wall to ensure that the headstock is adjusted to the same measurement on each side. Repeat for the tailstock. If the headstock is not square to the chassis, then the egg motor shaft will not be parallel to the egg's rotational axis.
  • Check egg alignment. Make sure that the object (which we'll call an "egg" even if it's a light bulb or a golf ball) being plotted is very well centered on the egg axis. Use the "Raise pen, turn off motors" setup mode to turn your object by hand, and make sure that it is as concentric as possible.
  • Check pen alignment. The pen should point directly towards the egg-motor axis, and directly towards the pen-motor axis as well. There are a number of places that you can make adjustments to ensure that these are correct. For example, the screw at the top of the proximal pen arm can be used to adjust the angle between the pen point and the pen-arm axis.
  • Increase tension. Compressing the plunger spring further pushes the "egg" against the egg-cup of the egg-motor, improving contact. This generally helps up until the tension starts to create excess drag-- for example when the spring is fully compressed.
  • Check egg alignment, part 2. Turn off the motors, raise the pen, and move the pen all the way left and right around the egg. You're looking to have the pen tip be the same distance from the egg surface all the way round.

Machine setup:

  • Adjust motor current. The page on making your first plot explains how to adjust the motor current. Skipping this step is a common cause of various problems when setting up a new Eggbot. If the current is too low or too high, you may find the pen arm falling over, the egg not turning reliably, or the pen jumping in overly-large steps.
  • Check contact. The "egg" needs to make good contact with the egg-motor. There should not be any slippage between the two, or the egg will gradually work its way out. Even if there is not slippage, there can be apparent "backlash" (direction-dependent lag) when the egg-motor turns back and forth. This is a common cause of poor accuracy when setting up a new Eggbot. It may help to clean and dry the "egg" and/or the polyurethane egg cup.
    • The Precision Egg Coupler is an Eggbot add-on (included with the deluxe kit) that can help to improve precision. It's large, sandpapery surface grips very well on eggs, ping pong balls, and other soft or porous surfaces.
    • For hard surfaces like glass and stone, the sandpapery surface will not offer much grip, so use a rubber washer between the precision coupler and the object, or use the original polyurethane egg cup.
    • For super-precision work you may want to experiment with different egg cup designs, or even screw down or glue your object to the egg-motor shaft. (See also Hacks.)
  • Tighten screws. A properly set up Eggbot has very little flex in the chassis. Firmly tighten all of the stainless steel screws that hold the chassis together, and all of the thumbscrews on the sides of the chassis. Also check to make sure that all screws on the pen arm are firmly tightened.
  • Check for drag on the plunger. If the plunger is not able to turn easily, then it will provide drag to the "egg." If the tension is too high, the O-ring is rubbing on the back bushing, or the thin nylon washer is not present, there can be increased drag.
  • Use a smaller/shorter pen. The pen arm grips your pen very close to the bottom of the pen. While this generally gives better precision, gripping the pen below the center of mass means that there will be more sway, particularly when the pen has been lifted or is being lowered.
  • Check for drag on the pen. The pen should only lightly touch the surface that it's drawing on. If you use a stiff hinge, or otherwise arrange things such that there's heavy downward force, it can create excess drag at the egg-pen interface.
  • Reduce lateral play in the plunger. Appendix B of the assembly instructions shows how to do this if you would like to do so. In practice, we have not found this to have nearly as much effect in improving precision as the other suggestions listed on this page.

Surfaces and Pens:

  • Clean your medium before plotting. If you remove surface dust and oils before drawing, you'll get a sharper plot. In particular, certain types of pens will show fingerprints-- areas where the pen does not make as strong as a mark, where the object has been handled.
  • Maintain your pen(s). Clean them frequently (if applicable.) And, don't let them dry out.
  • Use finer point pens. The "ultra fine" Sharpie is not nearly as fine as pens come. Check out pen choices for hints about other types of pens that you may want to consider.
  • Try a different pen. Some pen tips, particularly the tips of extremely fine point felt-tip pens, can "wobble" when dragged one way and then another against the egg surface. This can lead to an apparent backlash in your drawing.
  • Warm your eggs up. Moisture may condense on cold eggs while plotting. Not only will this ruin your plot it may also damage your pens. Be sure to allow cold eggs time to warm up a bit before drawing on them. Placing them in lukewarm water is not only a good way to take the chill off of them, it also prepares them for cleaning. (Do not soak eggs in water if you will be dyeing them: the dye may not take evenly.)

Drawing setup:

  • Use a smoother toolpath. If your pen travels in a path with sharp corners, you may see ringing at the corners, where the pen vibrates after quickly changing direction. Using a smooth path that provides built-in deceleration and acceleration can prevent this. For example, when filling regions the plots made with the "hatch" method generally provide excellent toolpaths with minimum ringing.
  • Check aspect ratio. If you're plotting on eggs-- or anything else taller than it is wide, your shapes will need to be elongated horizontally to give "square" output.

Eggbot Control Settings:

  • Slow down the pen speed. The faster the pen is moving, the more vibration (and thus slop) you'll have when the pen stops moving. By slowing the speed at which the motors turn, you'll improve your drawings. Conversely, it's okay to speed up until the quality begins to suffer. The correct speed will depend upon your medium (egg shells, glass, etc), your drawing, and your pens -- you'll need to experiment.
  • Lower the pen more slowly. While you can usually raise the pen quickly without trouble, it is generally important to lower the pen carefully. Hammering the pen down by quickly lowering it can "mushroom" the tip out over time, or cause it to simply bounce in place.