Page 101 - Robot Builder's Bonanza

70 WORKING WITH WOOD

Figure 7-1 Plywood is
made using several thin layers
of wood, with the grain of
each layer alternating to
provide added strength.

Planks are lengths of wood milled from the raw lumber stock. They are available in stan-
dard widths and thicknesses and come in either precut lengths (usually 4, 6, and 8 feet)
or are sold by the linear foot.
Plywood is made by sandwiching one or more types of wood together. The grain— the
direction of growth of the original tree— is alternated at each ply for added strength.
Both plank and ply are made from softwoods and hardwoods. Depending on where you
live, your local home improvement store is likely to have only softwood ply and just a few
types of hardwood plank (mostly oak). For a wider variety you need to shop at a hardwood
specialty store or mail order.

USING PLYWOOD
The best overall wood for robotics use, especially for foundation platforms, is plywood. Ply-
wood gets its strength by sandwiching two or more pieces of wood together, where the grain
(growth pattern) of the wood alternates direction— see Figure 7-1. Plywood is usually stronger
than the equivalent thickness of a single slice of wood. Each thin slice strengthens and rein-
forces the other.
Plywoods are commonly used in residential and commercial construction, but they’re made
of softwood and aren’t well suited for use in small robots. A better choice is specialty hard-
wood plywoods, available at craft and hobby stories. They’re more desirable because they are
denser and less likely to chip.
This type of hardwood plywood comes in two “grades”: aircraft and craft. A typical air-
craft plywood uses birch and consists of from 3 to 24 plies— the more plies, the thicker the
wood. It’s rated for use in model airplanes, where structural strength is critical. A less expen-
sive variation is craft plywood, which is not intended for use in model planes. It’s made of less
expensive woods that aren’t quite as strong. For most robotics use, the craft plywood is per-
fectly fine. Sheet size for both vary, depending on manufacturer and source. Hobby stores
commonly carry plywoods in 12″ 12″ squares.
Plywood comes in various thicknesses starting at about 1/4″ and going up to over 1″.
Thinner sheets are good for making a small robotics platform, but only if the plywood is made
from hardwood. Typical thicknesses are:

Thickness
Metric Inch Plies

2.0 mm 0.7874 3 or 5
2.5 mm 0.9843 5
3.0 mm 1.1811 7
4.0 mm 1.5748 8

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