Gaps Hypotheses

· [ plotting, out-damned-spot ] · @pentronik

Continuing the gaps analysis, in this one I will list all the ideas that I have thought of, quick reactions, and alternatives. It will likely turn into a sort of table of contents. Right, so the hypotheses!

First is the list after organizing it into groups of related issues. Below that is the original raw list.

Organized list

Issues related to the motion of the pen that is either due to my own programming or due to lack of understanding of motion library. It is also possible that there is a bug in that library.

  1. The problem is not being careful about the placement of the end of the loop.
  2. The problem is the pen is picking up too early.
  3. The problem is const_speed is affecting the precision.
  4. The problem is the pen speed is too high, which results in lower precision.
  5. The problem is with the way I have programmed the motion. The second to last step may be very small, and the final step is larger. There may be some problem here.

Mechanical issues

Issues related to the mechanics of the plotter.

  1. The problem is that the main rail is warped by the clamps that I have used to keep the machine in the same place.
  2. The problem is that the pen motion plain is not parallel to the paper surface. This could be caused by the rail warping, or it could be the pen in the holder, or the metal plate is warped.
  3. The problem is that the Y-arm is loose.


Potential issues related to the materials that I am using.

  1. The problem is that the pen is old and weak.
  2. The problem is that I am not using the same pen every time and this is confusing.
  3. The problem is that I am not putting the pin in the holder consistently.
  4. The problem is that the paper is not as absorbent as needed, and this makes it difficult to lay down the last bit of ink.


Thoughts on what might be done if the issue is not resolved. Let’s hope it doesn’t come to this.

  1. Simply do not close the loops, so it doesn’t show this problem.
  2. Overwrite the loops more than once so that the dots are not as evident.
  3. Hide the dots under overwritten lines; draw lines over the dots as part of the design.
  4. Leave it be, this is one of the things that makes plotted drawings special. I don’t totally agree.

Raw list

First hypothesis was an extension of what I talked about last time. Basically to put the last point at a precise distance from the start mark. This has been my working hypothesis for a while, but it can’t be the only thing; the gaps do not occur consistently.

Second hypothesis is that the pen is picking up too early and then moving away. It is not precisely hitting the mark.

Third hypothesis is that the long rail is being slightly distorted (warped) by the way that I have clamped it to the table. This seems really unlikely, it’s very rigid and the “table” is actually just a piece of plywood. I mean obviously there is some equilibrium between the board and the rail, but come on. Easy to test: just remove the clamps.

Fourth hypothesis is that the pen arm and the drawing surface are not parallel.

Fifth hypothesis is that the pen is getting old and weak, and even if the pen was in the right place, it may not be leaving ink. There are several variations here. Also easy to test: get a different pen.

5B: Different papers absorb ink differently, so it’s not a problem on my HP paper, but it is a problem on the Bristol paper I am using. Easy to test: try with a few different papers.

5C: I have not been careful about which pen I have been using. There are several practically identical pens. I have intended to use the same pen, and have tried to be consistent. But I always have doubt. Perhaps if I use stickers to be sure.

Sixth hypothesis is that const_speed is somehow interfering with fine (small) precise motions.

6B: is a variation: the small motions aren’t the last step, but the second to last step. This ties in with the first one, and I’ll elaborate on it.

6C: Faster motions are less precise, so reducing speed may help.

Seventh alternative is that the dots are obvious because the ink isn’t thick enough, so the bit that is overwritten stands out. Maybe draw slower so more ink can flow, or overwrite more than once.

Eighth alternative is to just not close the loops. Instead of a constant \(A\), I could make it a slowly increasing function of \(\theta\) such that it increases in radius just as the current function does. This could be interesting, but doesn’t solve my actual problem which may crop up later when I need lines to meet precisely.

Ninth alternative is to hide the dots under other crossing lines. Not always possible though.

@pentronik    :fountain_pen: