Exercise: Kinetic Toy

In this exercise we will design and fabricate a simple kinetic toy out of laser-cut plywood. The overall project objective is storytelling, but in the hands of a child anything can support narrative and role-play. So our goal is simply to create an object with a compelling movement which supports narrative play.

The main objective of this exercise is developing your parametric design skills in 3D CAD. The scope and size of the design idea should be chosen in keeping with your existing CAD skills. If you are a novice to parametric CAD, then please keep your priorities centered on creating a simple structure built from several parts. If you have prior CAD experience, please choose a scope that includes more elaborate construction.

Objectives

The overall objective of this exercise is to design and fabricate a hand-held toy with at least one degree of independent motion. The goals of this exercise are that you should be able to:

  1. Create 3D parts from 2D CAD sketches:

    • using parametric constraints and dimensions to capture design intent

    • supporting iterative design modification

    • compatible with the limits of the laser-cutting process

  2. Design a multi-part assembly including joinery.

  3. Export laser-cuttable part files.

  4. Laser-cut parts from 6 mm plywood, assemble, and test.

Reference Guides

Please review the following reference guides as needed:

Design Rules

Please observe the following:

  1. Use only 6 mm laser-cut plywood.

  2. No glue.

  3. No fasteners (e.g. no screws).

  4. No shafts, bearings, motors or other standard components.

  5. No electronics.

You may optionally include one or more:

  1. piece of string

  2. rubber band

  3. paper doll

Your device should:

  1. be capable of some form of internal movement

  2. combine three or more individually laser-cut parts

  3. join at least two parts using friction fits (e.g. tabs and slots, half-laps)

  4. be child-portable and child-safe

If there is something else you wish to include within the spirit of the assignment, please ask. The point of these rules is to keep our focus on dynamic behavior created using form and not get distracted by components, but also to make something playful.

Example Ideas

Following are few examples of possible approaches:

  1. Free-standing base with a part hanging from a laser-cut hook feature.

  2. Free-standing base supporting a wooden mobile.

  3. Free-standing base with a linear rail and a piece which can slide along it using pull strings.

  4. Handheld object with an rubber band pivot holding a mount for interchangeable paper puppets.

  5. Rocking base with curved track supporting roller.

  6. Advanced: free-standing object supported on bouncy wooden flexure joints.

Sample Design

../../../_images/rocker.PNG

Example solution: a rocking base with a detachable rocket toy. This design has five parts cut from four designs (the brace is duplicated), and demonstrates slot and tab joinery (aka mortise and tenon) in the base and half-lap joinery for the rocket. The overall center of mass is calculated to be below the rolling center so the base should rock but not tip.

This sample solution for this exercise can be browsed on the course site:

Or as a single zip file:

Sample files are provided on the assumption that students will go farther given a stronger foundation, so these are provided for you to examine and use as starting points. But if you use one as a template, please be mindful that you add meaningful development, not just tweak it trivially.

Preparation

The first rule of CAD: always make a paper drawing first.

The second rule of CAD: always make one more paper drawing before approaching the software.

An observant student will note that strictly following this rule will never lead to a CAD design, but the central idea remains sound. Iteratively drawing by hand remains the most efficient way of developing the core logic of a design idea. It is a generative process that raises essential questions and clarifies your design intent. It takes some discipline to avoid leaving contradictions and impossible requirements, but this can be accomplished in a much shorter timeframe than using CAD.

Drawing in CAD is time-consuming. With care, a problem can be modeled well-enough that necessary refinements and adjustments are quick. But in general, a poor understanding of the problem leads to a poorly structured model which cannot be easily and logically iterated.

Well-done, a design process in CAD will raise and answer all needed questions, resulting in a design which can be fabricated, assembled, tested, and solves the problems. At minimum, the process results in a concrete design, but it still takes human understanding to make sure that result is also compatible with physical reality.

Material and Tool Constraints

Every practical design needs to consider the tools and materials at the outset. The abstracted idea may be amenable to implementation using different fabrication methods, but the specific design invariably makes deep assumptions about the means.

For this assignment, please fabricate all parts from laser-cut 6 mm plywood. This is a versatile material which we use a lot for physical prototyping. If you are not familiar with laser-cutting, please see the IDeATe Laser Cutter Guide. Here are specific assumptions:

  • The laser cuts along any 2D path, all the way through a flat piece of material. All parts can be drawn as a planar sketch extruded 6 mm thick.

  • The laser cuts with a kerf approximately 0.2 mm wide. There is no offset compensation, the beam travels straight down the middle of edge geometry, so all holes are approximately 0.2 mm overside, all outside diameters approximately 0.2 mm undersize.

  • My suggestion is to draw all parts using the desired final dimensions, but with features which are tolerant to cutting variation. This is generally a good practice which accommodates both normal material and fabrication tolerance.

  • My recommendation for making tabs which fit into slots:

    • Draw rectangular slots 6.2 mm wide. These will cut approximately 6.4 mm wide to accommodate variation in material thickness. My own habit is to draw the length as an integer, e.g. 15 mm long.

    • Draw the tabs as a trapezoidal shape to wedge into the slot. E.g., I draw my tabs for 15 mm slots as a trapezoid which is 6 mm high, with each edge sloped 2 degrees off the perpendicular (total of 4 degrees included angle), and the tip dimensioned 15.2 mm. The tip will cut at approximately 15.0 mm wide and freely fit into the slot, but the wider root of the tab will be a a press-fit. Adjacent to the tab (coming off the root) should be at least a few millimeters of edge which will make face contact around the slot to set a definite insertion depth.

      ../../../_images/laser-cut-tab-15mm1.png

      Trapezoidal profile for a laser-cut tab to fit into a laser-cut 15.0 x 6.2 mm slot, both in 6 mm plywood.

    • Include a tab centerline to use for locating the tab. I recommend center-to-center dimensioning for both tabs and slots to keep the details of the tolerancing out of the design logic.

  • Laser-cut edges are not perfectly vertical, there is a subtle slope whose exact shape and angle depend on the focal length and calibration of the lens. A well-designed part has a shape and fit which tolerates slightly non-square edges.

  • The laser can cut sharp inside corners (unlike milling tools). For our purposes, this is generally fine, but sharp corners are stress concentrations which can induce fracture under high loading. This does become a concern when creating press-fits using brittle acrylic parts.

Deliverables

  1. Live in-class demo of your toy

  2. A short report posted to the shared drive including:

    1. a short text statement reviewing your intent and outcomes

    2. a photo and/or brief video

    3. your SolidWorks files