WOODEN PLATFORMS
Wood may not be high-tech, but its an ideal building material for hobby robots. Wood
is available just about everywhere. It's relatively inexpensive, easy to work with, and
mistakes can be readily covered up, filled in, or painted over. In this chapter, using wood for
robot structures will be presented and how you can apply simple woodworking skills to construct
a basic wooden robot platform. This platform can then serve as the foundation for a
number of robot designs you may want to explore.
There is good wood and there is bad wood. Obviously, you want the good stuff, but you
have to be willing to pay for it. For reasons you'll soon discover, you should buy only the
best stock you can get your hands on. The better woods are available at specialty wood
stores, particularly the ones that sell mostly hardwoods and exotic woods. Your local lumber
and hardware store may have great buys on rough-hewn redwood planking, but its
hardly the stuff of robots.
The best overall wood for robotics use, especially for foundation platforms, is plywood. In
case you are unfamiliar with plywood (Fig. 9-1), this common building material comes in many grades and is made by laminating thin sheets of wood together. The cheapest plywood
is called shop grade, and it is the kind often used for flooring and projects where looks
aren't too important. The board is full of knots and knotholes, and there may be considerable
voids inside the board, all of which detract from its strength.
The remaining grades specify the quality of both sides of the plywood. Grade N is the
best and signifies natural finish veneer. The surface quality of grade N really isn't important
to us, so we can settle for grade A. Since we want both sides of the board to be in good
shape, a plywood with a grade of A-A (grade A on both sides) is desired. Grades B and C
are acceptable, but only if better plywoods aren't around. Depending on the availability of
these higher grades, you may have to settle for A-C grade plywood (grade A on one side,
grade C on the other).
Most plywoods you purchase at the lumber stores are made of softwoods usually fir and
pine. You can get hardwood plywood as well through a specialty wood supplier or from
hobby stores (ask for aircraft-quality plywood). Hardwood-based plywood is more desirable
because it is more dense and less likely to chip. Don't confuse hardwood plywood with hardboard.
The latter is made of sawdust epoxied together under high pressure. Hardboard has
a smooth finish; its close cousin, particleboard, does not. Both types are unsuitable for
robotics because they are too heavy and brittle.
Plywood comes in various thicknesses starting at about 
in and going up to over 1 in.
Thinner sheets are acceptable for use in a robotics platform if the plywood is made from
hardwoods. When using construction-grade plywoods (the stuff you get at the home improvement
store), a thickness in the middle of the range—
or 
in—is ideal.
in and going up to over 1 in.
Thinner sheets are acceptable for use in a robotics platform if the plywood is made from
hardwoods. When using construction-grade plywoods (the stuff you get at the home improvement
store), a thickness in the middle of the range— or in—is ideal.Construction plywood generally comes in 4-by-8-ft panels. Hardwood plywoods, particularly
material for model building, come in smaller sizes, such as 2 ft by 2 ft. You don't need
a large piece of plywood; the smaller the board, the easier it will be to cut to the exact size
you need.
Remember that most home improvement stores will cut wood for you—saving you the
cost of buying power saws and doing it yourself. The professional-grade saws available at
home improvement stores will make a straight, even cut and the staff is usually trained in
making the cuts very accurately. Along with cutting the wood to the size you want, they may
have leftover pieces for just a dollar or two that can be further cut down to the exact size
you want.
An alternative to working with plywood is planking. Use ash, birch, or some other solid
hardwood. Stay away from the less meaty softwoods such as fir, pine, and hemlock. Most
hardwood planks are available in widths of no more than 12 or 15 in, so you must take this
into consideration when designing the platform. You can butt two smaller widths together
if absolutely necessary. Use a router to fashion a secure joint, or attach metal mending
plates to mate the two pieces together. (This latter option is not recommended; it adds a lot
of unnecessary weight to the robot.)
When choosing planked wood, be especially wary of warpage and moisture content. If
you have a planer, which cuts down planks of wood into absolutely flat and square pieces
and the unit you have can handle the relatively thin and small pieces to be used in robots,
then you can select the wood based on its grain and quality. If you don't have a planer, take
along a carpenters square, and check the squareness and levelness of the lumber in every
possible direction. Reject any piece that isn't perfectly square; you'll regret it otherwise.
Defects in premilled wood go by a variety of colorful names, such as crook, bow, cup, twist,
wane, split, shake, and check, but they all mean headache to you.
Wood with excessive moisture may bow and bend as it dries, causing cracks and
warpage. These can be devastating in a robot you've just completed and perfected. Buy
only seasoned lumber stored inside the lumberyard, not outside. Watch for green specks or
grains—these indicate trapped moisture. If the wood is marked, look for an MC specification.
An MC-15 rating means that the moisture content doesn't exceed 15 percent, which
is acceptable. Good plywoods and hardwood planks meet or exceed this requirement. Don't
get anything marked MC-20 or higher or marked S-GREEN—bad stuff for robots.
Yes, you've read correctly, balsa wood can be useful as part of a robot structure. Balsa is
very light (and strong for its density), easily shaped, and readily available in different sizes in
virtually all hobby shops throughout the world. It has a number of other properties that
make it well suited for robot applications, not only for prototyping complex parts due to the
ease in which it can be cut and formed and glued together in other pieces.
You might think that its understated to say that balsa is a softwood; you have probably
pressed your thumbnail into a piece at some time and been amazed how easy it is to put an imprint into the wood. This property along with its ability to absorb vibration make it ideal
as a lining for fragile parts mounts. By buying a thick enough piece of balsa, chances are
that you can replace the entire hold down with a piece having similar strength, as well as
being much lighter.
The major consumer of balsa is not the model airplane industry, as you might have
thought, but the liquid natural gas (LNG) industry as an insulator for large LNG tanks. There
may be cases where parts of your robot are hot and placed in close proximity to other parts
that are temperature sensitive. Before trying to come up with a fan or redesign solution,
why don't you try a small piece of balsa, cut to size. You will probably discover that the balsa
will help separate the different temperature areas effectively for a small amount of work and
cost.
Note that an insulator is not a heat sink or a heat removal device like a fan. If you are
using balsa to separate a device that has to stay cool beside one that becomes very hot,
make sure that there is a way for the heat from the hot device to leave the robot. Encasing
the part in a balsa box to keep heat from affecting other parts will result in the part becoming
very hot and eventually burning out or catching fire.
Wood dowels come in every conceivable diameter, from about 
to over 11.2 in. Wood
dowels are 3 or 4 ft in length. Most dowels are made of high-quality hardwood, such as
birch or ash. The dowel is always cut lengthwise with the grain to increase strength. Other
than choosing the proper dimension, there are few considerations to keep in mind when
buying dowels.
to over 11.2 in. Wood
dowels are 3 or 4 ft in length. Most dowels are made of high-quality hardwood, such as
birch or ash. The dowel is always cut lengthwise with the grain to increase strength. Other
than choosing the proper dimension, there are few considerations to keep in mind when
buying dowels.You should, however, inspect the dowel to make sure it is straight. At the store, roll the
dowel on the floor. It should lie flat and roll easily. Warpage is easy to spot. Dowels can be
used either to make the frame of the robot or as supports and uprights.
You've cut a piece of wood in two before, haven't you? Sure you have; everyone has. You
don't need any special tools or techniques to cut wood for a robot platform. The basic shop
cutting tools will suffice: a handsaw, a backsaw, a circular saw, a jigsaw (if the wood is thin
enough for the blade), a table saw, a radial arm saw, or—you name it.
Whatever cutting tool you use, make sure the blade is the right one for the wood. For
example, the combination blade that probably came with your power saw isn't the right
choice for plywood and hardwood. Outfit the saw with a cutoff blade or a plywood-paneling
blade. Both have many more teeth per inch. Handsaws generally come in two
versions: crosscut and ripsaw. You need the crosscut kind. If you are unfamiliar with the
proper use of the tools that you are planning on using, you should go to your local library
to look up books on woodworking, or find out when your local home improvement center
is having classes on tool usage. Using the right tools and blades along with learning a
few tips will make the work go much smoother and faster and result in a better finished
product.
You can use a hand or motor tools to cut and drill through wood. The choice isn't important
and is really a personal one. The most critical parameter, however, is to make sure that
you only use sharp cutting tools and drill bits. If your bits or saw teeth are dull, replace them
or have them sharpened.
Unless otherwise required, all cuts into wood are perpendicular (90 degrees) to its surface
and, in most cases, the edges are also perpendicular to one another. To ensure that the
cuts are at the proper angle, you should use either a miter box with your handsaw or a powered
miter saw. These tools will help ensure that the cuts you make are at the correct angles
and are as accurate as possible.
It's important that you drill straight holes, or your robot will not assemble properly. If
your drill press is large enough, you can use it to drill perfectly straight holes in plywood and
other large stock. Otherwise, use a portable drill stand. These attach to the drill or work in
a number of other ways to guarantee a hole perpendicular to the surface of the material.
Before cutting or drilling, remember the carpenter's adage: "Measure twice and cut
once."
You can easily shape wood using rasps and files. If the shaping you need to do is extensive—
like creating a circle in the middle of a large plank—you may want to consider getting a hand-held
rotary cutter. Be careful if you are thinking about buying a simple rotary rasp for your
drill; the bearings within a drill are designed for vertical loads, not side-to-side loads and by
using a cutter with your drill, you may end up ruining it. After cutting the wood down into
the desired shape, use increasingly finer grades of sandpaper followed by the painting
process outlined in the following.
The process outlined here will require just a few minutes to accomplish (over a few days,
allowing for the paint to dry) and will allow you to create finished pieces of material (wood,
metal, and plastic) that will have the functional benefits of:
1. Eliminating the dust on the surface of the material, allowing for effective two-sided tape
attachment to (and removal from). Eliminating the fibers in wood also will allow for more
effective glue bonds.
2. Smoothing the surface and reducing the lifted fibers that appear when wood gets moist
or wet over time.
3. Eliminating splinters and sharp edges when handling the robot and minimizing surface
splintering when drilling into wood.
4. Allowing for pencil and ink markings of the surface to be easily wiped off for corrections.
This is not possible with bare wood, metal, or plastic.
I tend to just use aerosol paint—when properly used there is very little mess and no brushes
to clean up. From an auto-body supply house, you should buy an aerosol can of primer (gray is always the first choice) and from a hardware store buy an aerosol can of indoor/
outdoor (or marine) acrylic paint in your favorite color. Red catches the eye, isn't overwhelming,
and will hide any blemishes in the wood or your work.
Set up a painting area in a garage or some other well-ventilated area by laying down
newspaper both on a flat surface as well as a vertical surface. Next, lay down bottle caps
to be used as supports for the materials that you are going to be painting. You don't have
to use bottle caps—scraps of wood or other detritus can be just as effective—just make
sure that when you are painting something you care about that the supports are smaller
than the perimeter of the object being finished. You will want to finish the ends of
the plywood and don't want to end up with paint flowing between the plywood and the
support.
Lightly sand the surfaces of the material you are going to paint. You may want to sand
the edges of the strips more aggressively to take off any loose wood that could become
splinters. Once you have finished, moisten a rag and wipe it over the surface you have
sanded to pick up any loose dust.
Shake the can of primer using the instructions printed on the can. Usually there is a small
metal ball inside the can and you will be instructed to shake it until the ball rattles easily
inside. Start with the two plywood strips that have been left over. Place one end on the bottle
caps, supporting the surface to be painted and spray about 6 in of the strip, starting at
the supported end. Most primers take 30 minutes or so to dry. Check the instructions on
the can before going on to the next step of sanding and putting on new coats of paint or
primer.
After the first application of primer, you will probably find that the surface of the wood
is very rough. This is due to the cut fibers in the wood standing on end after being moistened
from the primer. Repeat the sanding step (along with sanding the ends of the wood
and wiping down with the damp rag) before applying another coat of primer. After the second
coat is put down, let it dry, sand very lightly, and wipe down again.
Now you are ready to apply the paint. Shake according to instructions on the can and
spray the plywood strips, putting on a thin, even coat. You will probably find that the paint
will seem to be sucked into the wood and the surface will not be that shiny. This is normal.
Once the paint has dried, lightly sand again, wipe down with a wet cloth, and apply a thicker
coat of paint. When this coat has dried, you'll find that the surface of the plywood is very
smooth and shiny. You do not have to sand the paint again. The plywood is now ready to
be used in a robot.
Figs. 9.2 and 9.3 show approaches for constructing a basic square and round motorized
wooden platform, respectively. See Table 9-1 for a list of the parts you'll need.
Base |
10″ by 10″  ″ to  ″ thick plywood |
2 |
DC gear motors |
2 |
5″ to 7″ rubber wheels |
1 |
1  ″ caster wheel |
2 |
2″ by 4″ lumber (see text) |
1 |
4× ″D″ cell battery holder |
Misc. |
1” by  stove bolts, 3″ by stove bolts, nuts and flat washers, 1″ by  stove bolts, nuts, flat washers, and lock washers |
To make the square plywood platform shown in Fig. 9-2, cut a piece of - or -in plywood
to 10 by 10 in (the thinner -in material is acceptable if the plywood is the heavy-duty
hardwood variety, such as aircraft-quality plywood). Make sure the cut is square. Notch the
wood as shown to make room for the robot's wheels. The notch should be large enough to accommodate the width and diameter of the wheels, with a little breathing room to spare.
For example, if the wheels are 6 in diameter and 1.5 in wide, the notch should be about 6.5
by 1.75 in.
- or -in plywood
to 10 by 10 in (the thinner -in material is acceptable if the plywood is the heavy-duty
hardwood variety, such as aircraft-quality plywood). Make sure the cut is square. Notch the
wood as shown to make room for the robot's wheels. The notch should be large enough to accommodate the width and diameter of the wheels, with a little breathing room to spare.
For example, if the wheels are 6 in diameter and 1.5 in wide, the notch should be about 6.5
by 1.75 in.To make a motor control switch for this platform, see the parts list in Table 8-4 in
Chapter 8.
Fig. 9-3 shows the same 10-in-square piece of - or -in plywood cut into a circle. Use
a scroll saw and circle attachment for cutting. As you did for the square platform, make a
notch in the center beam of the circle to allow room for the wheels—the larger the wheel,
the larger the notch.
- or -in plywood cut into a circle. Use
a scroll saw and circle attachment for cutting. As you did for the square platform, make a
notch in the center beam of the circle to allow room for the wheels—the larger the wheel,
the larger the notch.The wooden platform you have constructed so far is perfect for a fairly sturdy robot, so the
motor you choose can be powerful. Use heavy-duty motors, geared down to a top speed of
no more than about 75 r/min; 30 to 40 r/min is even better. Anything faster than 75 r/min
will cause the robot to dash about at speeds exceeding a few miles per hour, which is unacceptable
unless you plan on entering your creation in the sprint category of the Robot
Olympics.
Note: you can use electronic controls to reduce the speed of the gear motor by 15 or 20
percent without losing much torque, but you should not slow the motor too much or you'll
lose power. The closer the motor operates at its rated speed, the better results you'll have.
Throttling motors will be discussed later in the book.
If the motors have mounting flanges and holes on them, attach them using corner brackets.
Some motors do not have mounting holes or hardware, so you must fashion a hold-down
plate for them. You can make an effective hold-down plate, as shown in Fig. 9-4, out
of 2 by 4 lumber. Round out the plate to match the cylindrical body of the motor casing.
Then secure the plate to the platform. Last, attach the wheels to the motor shafts. You may
need to thread the shafts with a die so you can secure the wheels. Use the proper size nuts
and washers on either side of the hub to keep the wheel in place. You'll make your life much
easier if you install wheels that have a setscrew. Once they are attached to the shaft, tighten
the setscrew to screw the wheels in place.
Figure 9-4 One way to secure the motors to the base is to use a wood block hollowed
out to match the shape of the motor casing. a. Side view; b. Top view.
Two motors and a centered caster, attached to the robots base as depicted in Fig. 9-5,
allows you to have full control over the direction your robot travels. You can make the robot turn by stopping or reversing one motor while the other continues turning. Attach the
caster using four 
-by-1-in bolts. Secure the caster with tooth lock washers and nuts.
-by-1-in bolts. Secure the caster with tooth lock washers and nuts.
Figure 9-5 Attaching the caster to the platform. a. Top and bottom view; b. caster hardware assembly
detail.
The caster should be mounted so that the robot base is level and it can swivel with a minimum
of resistance. You may, if necessary, use spacers to increase the distance from the
base plate of the caster to the bottom of the platform. If the caster end of the robot is much
higher than the wheels, then the motor mounting bolts or the rear of the robot might rub
on the ground. If this is the case, you should be looking at using a smaller caster, larger
wheels, or mounting the motors on the bottom of the robot base.
You can purchase battery holders that contain from one to six dry cells in any of the popular
battery sizes. When using 6-V motors, you can use a four-cell D battery holder. You can also use a single 6-V lantern or rechargeable battery. Motors that require 12 V will need two
battery holders, two 6-V batteries, or one 12-V battery (motorcycle batteries are often
excellent power sources for robots, which require 12 V and a fair amount of current). For
the prototype, 6-V motors and a four-cell D battery holder were used.
Secure the battery holder(s) to the base with nuts and bolts. Drill holes to accommodate
the hardware. Be sure the nuts and bolts don't extend too far below the base or they
may drag when the robot moves. Likewise, be sure the hardware doesn't interfere with the
batteries.
nuts and bolts. Drill holes to accommodate
the hardware. Be sure the nuts and bolts don't extend too far below the base or they
may drag when the robot moves. Likewise, be sure the hardware doesn't interfere with the
batteries.Wire the batteries and wheels to the DPDT through control switches, as shown in the
Minibot project described at the end of Chapter 8, "Plastic Platforms." One switch controls
the left motor; the other switch controls the right motor.
To learn more about . . . |
Read |
|
Plastic robots |
Chapter 8, “Plastic Platforms”; |
|
Metal robots |
Chapter 10, “Metal Platforms” |
|
Using batteries |
Chapter 17, “All about Batteries and Robot
Power Supplies” |
|
Selecting the right motor |
Chapters 19, “Choosing the Right Motor for the
Job” |
|
Using a computer or microcontroller |
Chapter 12,“An Overview of Robot ‘Brains’ ” |