Servo - July 2009

ably rigid. Therefore, hold off on andwiching layers of the material together until you’ve cut it to the size you want. If the small pieces still seem too thin and lightweight for the job, cut an extra layer and use glue to hold the layers together.

Attaching Parts to Your Prototype Bases

Most of us use traditional fastening techniques when building permanent designs, such as metal or plastic nuts and screws. You can use these fasteners in a rapid prototype base, of course, but there’s no reason to always fall back on this design pattern. There are other methods that are faster, and yet allow re-use of motors and other components.

First, let’s discuss what not to use. Avoid gluing things like motors and other parts to the base. This makes it much harder to reposition things if you need to make adjustments. Plus gluing restricts you from easily reusing items. For similar reasons, I don’t recommend using double-sided tape. It’s fine for bonding sheets of cardboard together, but not so good for motors and other parts because it can leave an unsightly residue when you take things apart. It also tends to be rather sensitive to oil, moisture, and dirt, giving a poor bond unless everything is sparkling clean. I’m also not keen on duct tape, though it certainly has its uses.

It’s best to keep away from motors and other parts that are too big and heavy for the design. If your robot needs a standard size servo motor, avoid using a larger motor just because you have it on hand. Same with batteries. Leave the lead-acid battery from your car in your car.

The lightweight nature of the typical fast prototyping material prevents you from mismatching parts. If you don’t have exactly what you need to experiment, either wait until you can get the right part or use more sturdy construction methods.

On my list of “okay to use with caution” is Velcro. It’s a nifty product, but not always ideal for robot

projects. Using Velcro to attach a servo motor to a base is a quick and easy construction technique, but bear in mind that the Velcro interface isn’t dimensionally stable. The motor will likely twist and move — and even come off — during use. You can get heavy-duty Velcro where the bond is much tighter, but these also tend to use a more permanent adhesive to stick the hook and loop fabrics to their respective parts. You may have a hard time peeling the Velcro off the motor (or other component) when you’re done with your prototype.

Among the cheapest and easiest fastening methods I’ve found is the cable tie, also known as a tie-wrap (after the original name Ty-Rap) or zip strip. These come in many different widths and lengths, and are under 10 cents each even if you don’t buy them in bulk. The typical cable tie is a “use once” fastener: Slip the end of the wrap through its ratchet gear rack and cinch up. The ratchet holds the plastic of the tie in place. You cannot remove the tie other than cutting it off.

There are other ties that use a two-way ratchet, so you can remove the tie and re-use it. These cost most, but are very handy in robot prototypes

because they allow you to reposition and move parts on your bases without cutting off the tie and using a new one. A typical use of a cable tie is mounting a motor to the base. After marking the location for the motor, cut or drill two holes on either side of the motor casing for the tie to thread through. Hold the motor in place, insert the tie into the holes and around the motor, and pull tight. Make sure you use a tie that’s long enough.

This method works with flat-case motors like R/C servo motors and round case motors (metal or round). Use enough of ties to keep the motor securely in position.

To work effectively, the case of the motor should not be asymmetrical. Otherwise, the shaft of the motor may not protrude at a perpendicular angle from the base. If this happens, use shims to level the motor. You can make the shims using small pieces of cardboard or even stiff paperboard (from index cards, for example). Also handy is compressible foam used to pack static-sensitive integrated circuits, or even florists foam (get it at a craft store) or modeling clay.