Brass stock Brass stock Brass stock
This is useful for some of the smaller parts of mechanisms. It is obtainable from model-making stores. Brass is available in most of the same sections as aluminium stock, but in smaller dimensions. It is often sold in 200 mm lengths. Brass is more expensive than aluminium but fortunately we do not need a lot of it.
Brass is easily worked with drill and hacksaw. The thinner stocks can be bent by hand.
The photo of the gripper on p. 311 shows how it can be bent to form jaws.
Its distinguishing feature is that, it is reasonably rigid but has a degree of springyness that aluminium does not have. This is why it or similar alloys are used for electrical contacts, and various kinds of spring clip.
Plastic Plastic Plastic Plastic
Model shops stock a wide range of plastic rod, tubing, angle, channelling and sheets.
These are are in small sizes, being intended for scale models, but can be useful. Usually they are high impact polystyrene and special adhesive is used when building up boxes and frames.
Another source of plastic parts is the DIY store. The plumbing department stocks a range of tubing and other plumbing parts that can be used in robot building.
Examples of plumbing parts are the plastic pipe caps used as light shields for the IR sensors of the Quester (p. 273).
The gardening department of the DIY store may provide handy plastic tubing used in garden reticulation systems. The spacers that separate the decks of the Quester are cut from long PVC riser tubing.
One of the more generally useful materials for robot construction is 3 mm expanded PVC board. It is often used by signwriters and a visit to a local signwriter may provide some offcuts. If this fails, try a local plastics company. The board comes in sheets about 1 m by 2 m.
Unlike expanded polystyrene, which is soft and crumbly, expanded PVC is firm. Yet it has a certain amount of compressibility which means that nuts and bolt-heads sink a fraction of a millimetre into the surface when tightened. This makes them less likely to be loosened by vibration. The sheet is easy to drill and cut. A steel rule and sharp craft knife are all that is needed for cutting straight-edged pieces.
The sheet is manufactured in a range of attractive colours, The Quester, for example, is bright tomato-red.
Foam board is a similar material. It consists of a 5 mm thick sheet of solid plastic foam coated on both sides with a plastic film. It is white on one side and coloured on the other.
The board is not quite as strong as expanded PVC, but is just as easily worked and suitable for small lightweight robots, such as the Scooter and Android. Robot bodywork and other structures can be assembled by using craft glue, as explained on pp. 213-215.
There is plenty of scope for givng the robot a really unique appearance.
Foam board is sold at office materials stores. The brand (Elmers) used for the Android is supplied in sheets that measure 568 mm × 762 mm, which is a convenient size.
Wood Wood Wood Wood
Wood is rarely thought of as a robot-building material but, at times, it can be just what we need. It is strong for its weight and easily cut, drilled, painted, carved and glued.
Balsa. Because of its low density and easy workability it is a firm favourite with flying model aeroplane constructors. For the same reasons it is useful for robot building too.
The Android shows one way of using it.
Fixings
These hold the parts of the robot together — mostly nuts and bolts.
Buy in a stock of nuts and bolts in the sizes most suitable for small structures. The most generally useful size of bolts is M3 (3 mm diameter) and you need nuts to fit.
Occasionally a smaller size is required. For instance, a motor may have mounting holes with M2.5 or M2 threads. Small parts such as microswitches may have 2 mm unthreaded mounting holes.
You need an assortment of the different lengths. The 10 mm and 15 mm sizes cover most needs, but sometimes longer bolts such as 25 mm are wanted, and a few of the 6 mm size.
Washers have several different functions. Plain washers, placed next to the head of the bolt, help to spread the load at that point. They are useful when bolting a relatively massive item, such as a motor, to a relatively flexible panel. Shake-proof and spring washers help prevent the nuts from loosening. Use them for bolting metal parts to other metal parts. They are not needed when bolting to expanded PVC sheet and the material itself is suitably springy.
Nylon nuts and bolts, from electronic parts suppliers, are necessary if there is a risk of the bolts causing a short-circuit. This could happen if a circuit-board is bolted to a metal panel. In such cases use nylon bolts and nuts or plastic stand-offs.
Spacers are short tubes, length 6 mm to 38 mm, made of metal or nylon. They are intended for holding a circuit board clear of the panel on which it is mounted but have several other uses. We sometimes refer to the shorter ones as collars.
There are dozens of kinds of adhesive, of which we employ just three. For routine fixing, general adhesives such as UHU®,, Bostick®, or similar products are our standby.
Another general glue, which sticks expanded PVC sheet and Foam Board is a variety of craft glue called Sticky Craft Glue, made by CraftSmart. It is milky when applied but dries clear. Clamp the pieces under slight pressure while the glue sets. Examine it from time to time at first to check that the pieces have not slipped.
Super glues are quick setting and strong. We use a variety of this known as Fix-Lock anaerobic adhesive. A drop applied to a nut and bolt runs into the narrow space between them and sets hard. This locks the nut on to the bolt, preventing it from working loose. A locking adhesive such as this is invaluable when building robots from metals parts.
Although it holds the nut secure, a little force with a spanner will loosen it if necessary.
It should really be classed as a tool but it seems more sensible to describe it along with the adhesives. The tool is the glue gun, which melts glue sticks and has a nozzle for applying the molten glue to the workpiece. A glue gun is a handy tool to have on the workbench for all kinds of gluing jobs.
Velcro would seem to have little to do with robots but in fact it can be very good at fixing things that can not be fixed by nuts, bolts or adhesives. Velcro Sticky Back tape consists of the usual ‘hook’ and ‘eye’ tapes with strongly self-adhesive backs. Typical AAA and AA battery holders have no mounting holes, and there is nowhere they can be drilled to take a bolt. We use this tape for fixing battery holders and similar items.
Last but by no means least, be sure to have a pack of Blu-Tack to hand, as well as a packet or roll of double-sided self-adhesive tape.
The glue sticks are melted by the electric heating coil in the gun. Press
the trigger to extrude molten glue from the nozzle.
The tools you need for constructing robots partly depend on the materials you use. For Foam Board the main tools are a steel ruler, a craft knife and a plastic chopping board (use one discarded from the kitchen) or cutting mat. You need a few other tools for mounting the motor and circuit boards. For building an aluminium framed robot such as the Gantry, a drill press is almost essential and so is a hacksaw. When you have decided what materials are to be used, select your tools from those described below.