The Value of Scale Modelling.

I have my students make 1/4th scale models of their furniture designs before committing to the design and investing in a full size prototype. 1/4th scale is simply a standard I've settled on for evaluating all projects consistently.  The scale used isn't as important as trying to make the model realistic. For example: If the design is in wood then I ask them to use wood (and preferably the same species of wood) for the model. If the design is metal then they use the same metal for the model.

If the model is made accurately you can test it for stability and strength, predicted mechanical function  (if any), as well as for appearance realities.

Strength is how well the design resists both static and dynamic loads (forces). A static load is any non-moving load on the design, like a TV on a TV stand or the books on a book case.  Dynamic loads are active interactions with the design, like sliding a dresser across the carpet or propping your feet up on a coffee table.  Knowing the scale used is critical here, because you can also scale the loads applied for testing. If a chair is designed to support a 250lb occupant, a 1/4th scale model of it should support 250/4/4/4 or 3.9ish (4) lbs.  Use the same math for any 3D model.  A 1/5th scale model of the same chair should support 250/5/5/5 or 2lbs. On chairs more than any other type of furniture the dynamic loads can be unexpected, and far more than the average static weight of a seated occupant. We fidget and lean and slump. The momentary force of just sitting down can be double our actual force on the chair. Force = Mass x Acceleration.  Getting out of a chair can put larger forces on the arms and front legs.  Since some of us do push-up from the arms of chairs this suggests the arms of a design should support the entire weight of the anticipated occupant. You can test this in a model with some creative loading.

Stability asks: "Will the design remain steady and standing once loads are applied?". I often see student designs tip over when loaded.  Sometimes a load deflects a model in an unexpected way. Legs may wobble or twist under load.  These are NOT reactions most students can predict from a computer rendering.  It is far better to discover this in a scale model than in an expensive full-scale prototype. The model can also reveal weaknesses in joinery, hardware used, materials used, and the design geometry itself.

Any furniture that folds, opens, or adjusts has a mechanical component. When the design is mechanical a model can prove that the action works (or doesn't work).  In these cases a VERY accurate model is required, and my students typically cut parts for these models on a CNC router or CNC laser cutter. 

Models will also reveal appearance realities. This is the difference between how a design appears on a computer screen and how perspective, lighting, materials and textures appear in reality.  The visual qualities of real materials used will often differ from their computer rendered equivalents. Wood, for example, varies in color from board to board, and the grain pattern we see will change significantly depending on how the board was cut from the tree. That grain pattern can distract from or accentuate a design.  I have never seen a computer rendering display how quarter-sawn Oak or Ash can emphasize the linearity of a design. Face-cut boards of the same species will often obscure it.  At 1/4 scale this distinction can be lost since the grain pattern does not scale down on real parts. Yet many times this very distinction has proven itself in a model when the generic rendering of wood grain in software fails.

I had a student who designed subtle faceted surfaces on console cabinet doors to add visual interest. A computer rendering of this detail required exaggerating the lighting applied to make the facets show up. That should have been a clue.  The model showed such details became lost in shadows. They were minimized in the generality of real light.  The reality of light is rarely as specific as it can be rendered on a computer. The model suggested eliminating that detail as a waste of time, or at least altering it until the desired visual effect was achieved in more typical room light.

It is useful to have an accurate postal scale available, as it is hard to know the real weight of common objects that one might want to test their models with.  A bottle of water comes in handy as you can add or subtract water to arrive at the desired weight. Sometimes a handful of coins will be enough.  My students will put coins in a plastic bag to use for testing.

Access to CNC technology is also valuable, as perfect, accurate scales parts can be made from wood, plastics, man-made composites, and soft metals.  I've cut scale brackets from old aluminum license plates, and scale washers from plastic milk carton sides. With a small 1/16" or 1/32" straight bit in a CNC router fine joinery and details can be cut accurately.

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