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Lesson 1.3: Sketching and Annotating Mechanisms by Hand

Technical Context​

A sketch is the fastest way to move an idea from one head into another without distortion. It is faster than CAD by an order of magnitude, and unlike a verbal description it produces the same picture in everyone's mind.

You do not need to draw well. You need to draw clearly, which is a different and much easier skill.

The Rules That Make a Sketch Useful​

One view is rarely enough. Draw the mechanism from the side and from the top. Most misunderstandings live in the dimension you did not draw.

Label everything that moves. An arrow showing direction of travel, with a label, removes the most common ambiguity in a mechanical sketch.

Put numbers on it. Even approximate ones. "About 6 in" is far more useful than nothing, because it tells the reader the scale you are imagining. A sketch with no dimensions can be read as any size.

Show the game element. Draw the thing the mechanism acts on, at roughly the right scale, in the position where the interaction happens. Half of intake design problems become obvious the moment the element is drawn in.

Mark what is fixed. Hatching or a ground symbol on the parts that do not move tells the reader which side of a joint is anchored, which is otherwise a guess.

A Worked Example​

Consider sketching a linear slide with an arm on the end.

A poor sketch shows a rectangle with a line coming out of it. It communicates almost nothing: the reader cannot tell how far it extends, what drives it, where it mounts, or what it is reaching for.

A useful sketch of the same mechanism shows:

  • The side view with the slide at its retracted and extended positions, one drawn in solid lines and one in dashed
  • The extension distance labeled, "24 in travel"
  • The motor and spool drawn where they actually sit, with an arrow showing string direction
  • The scoring target drawn at the height it needs to reach, "34 in"
  • The pivot at the arm base marked with a circle and an arc showing its range, "0 to 110 deg"
  • Hatching on the chassis rail where the slide mounts

Same mechanism, same amount of drawing skill, roughly ninety seconds more work. The second one can be built from.

Draw the extremes, not the middle

Sketch the mechanism at the two positions where it is most likely to have a problem: fully retracted, where packaging is tightest, and fully extended, where reach and loads are worst. The middle of the travel almost never causes trouble.

Annotating for the Reader Who Is Not There​

The person reading your sketch may be a teammate next week or a judge in February. Annotate as though the reader cannot ask you a question.

Useful annotations:

  • Dimensions on anything that had to be chosen: travel, reach, mounting spacing, wheel diameter
  • Materials where they were a decision: "1/8 in polycarbonate," "goBILDA 1120 series channel"
  • Callouts on the hard parts, with a leader line and a short note: "bearing here, otherwise the shaft walks"
  • Questions you have not resolved, written in the margin: "does this clear the frame at 90 deg?" Unresolved questions in a sketch are honest and judges respond well to them
Do not erase old sketches

When a design changes, draw a new sketch on a new page and date it. The old sketch is the evidence of iteration, and iteration is a large part of what the notebook is for. A notebook with one clean sketch of the final design proves nothing about the process.

Sketching as a Design Tool, Not Just Documentation​

The strongest use of sketching is during a disagreement. When two students describe different intake designs verbally and cannot agree, hand both a pencil and ask them to draw it with the game element in place. Three times out of four, one of them discovers the problem while drawing, because the sketch forces the geometry to be specific in a way that talking does not.

This is also why sketching precedes CAD. CAD makes you commit to exact numbers before you know which numbers matter. A sketch lets you find out which numbers matter first.

Fill-in-the-Blank Practice​

  1. Drawing the mechanism at both its retracted and extended positions catches problems that are invisible in the __________ of the travel.
  2. Hatching or a ground symbol on a part tells the reader that the part is __________.
  3. Including the game element in the sketch at roughly correct scale exposes __________ problems that a mechanism-only drawing hides.
Show answers
  1. middle
  2. fixed (anchored, does not move)
  3. geometry (interaction, clearance, or reach problems)

Exercise​

Sketch one mechanism from your robot in side and top view, with the game element drawn in, at least four dimensions labeled, and the moving parts arrowed. Then hand it to a teammate who did not build it and ask them to describe what it does. Every question they ask marks something your sketch needs to annotate.

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