Portable digital scales

A portable digital scale in your toolbox at a flying session or a contest is one of those items that has become as essential as a winder or torque meter. Being able to weigh motors, half motor sticks, or check a model’s weight is nice because you have feedback right at the flying site.

For the past couple of years, I have been using the Gemini 20 scale from American Weigh Scales, Inc. (AWS). This scale has a 20-gram capacity and a 1-milligram resolution (.001 gram). It costs approximately $25 shipped from Amazon and runs on two AAA batteries. The nice thing is that it comes with a reusable box for transport, so it is easy to pack along with the rest of your modeling equipment. Its small size and low cost make this my dedicated flying-session scale, so it never leaves my toolbox.

These portable jewelry scales tend to have small weighing pans, which can make it difficult or impossible to weigh motors and airplanes, but there are some easy modifications that allow you to make a bigger weighing pan or even convert it to a hanging-type scale.

In this column, I will discuss how to make a raised, removable weighing pan. A larger, raised pan makes weighing motors much easier because the rubber motor can hang off the sides of the pan, yet still be high enough above the scale to prevent it from touching anything else, which could result in an inaccurate reading.

The modification requires some balsa and a few short pieces of aluminum tubing. I used K & S Precision Metals 3/16-inch outside diameter (OD) and 5/32-inch OD aluminum tubing because that is what I had readily available. Different sizes can be used if they telescope into each other. You will need two pieces of 3/16-inch tubing cut to approximately 5/16 inch long and one piece of the 5/32-inch tubing cut to approximately 2 inches long.

The short pieces of tubing are glued to a ⅜-inch disk that I made from some medium-weight balsa wood. A sharpened brass tube works nicely for making these round disks. The disks are then glued to the aluminum tubing with 5-minute epoxy.

The pan is also made from the same medium-weight 1/16-inch sheet balsa wood. The pan I made is 2 ¾ × 4⅝ inches, but you can go even larger. Just make sure that it fits in the travel box.

I added some ½ × 1/16-inch balsa strips around the edges to make the pan stiffer across the grain. The strips create a depressed area in the center, so small items such as O-rings do not roll out of the pan.

In the center of the bottom of the pan, epoxy one of the short tube assemblies. Make sure to get it as straight as possible. Sliding the 2-inch standoff tube into the short tube helps with visual alignment until the epoxy starts to set.

Next, drill a ¼-inch hole into the top of the clear cover, directly over the center of the existing pan scale. The second short tube assembly is then glued to the existing pan scale. Position it beneath the hole you drilled in the clear cover. This time, install the 2-inch standoff and the new balsa pan and visually align it so that the pan is flat while the 5-minute epoxy starts to set up.

After the epoxy is set, the scale is ready to use. Turn it on and it will automatically zero out the weight of the pan. Once powered on, if you remove the pan and standoff tube, you will see a negative weight. This is the weight of the tube and pan. The pan and tubing should weigh approximately 4 or 5 grams.

Remember, the scale has a 20-gram capacity, so do not go overboard building the pan. The scale, pan, and the standoff all fit back into the box, which has a magnetic closing flap to keep it all inside. If you prefer the pan slightly higher, cut a longer piece of tubing to raise it up.

Until next time, keep the weights down and the times up!

SOURCES:

AWS, Inc.

(800) 706-7253

www.awscales.com

K & S Precision Metals

(773) 586-8503

www.ksmetals.com

National Free Flight Society (NFFS)

www.freeflight.org