Parts and Materials The ATI-Hotchkiss wheelchair has been designed to use inexpensive materials readily available in most cities that have small- and medium-scale manufacturing. This chapter begins with an itemized price list of the materials that go into one chair. Before taking this list and dashing off in a rush of enthusiasm to buy one wheelchair's worth of materials, be sure to read the rest of this chapter. It will describe in more detail some of the various choices you will have to make as you set out to purchase the materials. Since the tubing sizes that are available vary from one country to another, we have included a table of tubing sizes to help you choose appropriate tubing. There are several different combinations of tubing sizes that can be used in making the Torbellino wheelchair.

This chapter also lists the prices of the basic supplies that will be used up while building wheelchairs, such as paint, flux, files, and cutting oil. The prices listed in the following table are typical prices for small purchases in the United States; they can vary considerably from one country to another.

* Tubing sizes can vary depending on sizes available. For a description of how to measure tubing, go here; for alternate tubing sizes, go here; and for methods of shringking and expanding tubing, go here.

** Other bearing sizes will work. See Chapter 11.

***Wheel size may vary. See Chapter 4 for details.

MILD STEEL TUBING

The frame, fenders, handrims, footrests, and x-brace of the Torbellino are all made from mild steel thin-wall tubing; they are not made from heavy water pipe. Finding the size of tubing that you need can be confusing at best. Tubing has three different dimensions that are important: the outside diameter (O.D.), the inside diameter (I.D.), and the thickness of the tubing walls (the gauge).

Diameter is another word for the distance across a circle. It is measured by the length of a straight line passing through the center of the circle. The two sections of tubing, shown in the diagram at the right, have an outside diameter of 1 inch; since they have different wall thicknesses (gauges), their inside diameters are different. If three dimensions are not already enough to keep track of, there are also at least three different systems that are used to measure and describe tubing sizes: 1) inch-size steel tubing 2) thin-wallconduit 3) metric-sizesteel tubing

The types of tubing that are available vary from one country to the next and may even vary within a small area. For example, one can find inch size steel tubing in Culiacan, Mexico, whereas only 50 miles away in Mazatlan, metric size tubing is the only type available.

The actual size of conduit and water pipe is usually much larger than the size that is given when you buy it. So-called one inch conduit, for example, is actually 1.160 inch, or nearly 1-3/16 inch, in outside diameter. In order to know exactly what tubing you are getting, you will need to measure it yourself. Don't buy tubing without measuring the outside diameter and the wall thickness with the vernier calipers and using the "Tubing Sizes" chart in this chapter.

Finally, plan ahead before you buy tubing. In our wheelchair design, the pieces of tubing used in the bottom of the x-brace and in the top of the footrest must fit snugly over the sideframe tubing. Use the following charts of tubing sizes to help you decide which sizes of tubing to buy. If tubing is not available in the sizes you need, it may be necessary to reduce or expand the short pieces of tubing until they fit properly.

In Paraguay, for example, ARIFA is making the ATI design using 1" O.D. tubing for the side frames. Unfortunately, 1-1/8" O.D. tubing is not available for the outer tubes. The wheelchair makers in Paraguay solve this problem by using 1-1/4" O.D. tubing and reducing it until it fits snugly. They reduce the outer tubing by cutting it by hand, squeezing it, and welding it back together. Techniques for reducing and enlarging tubing are described in Chapter 6. Combinations of tubing sizes that work well are as follows:

TUBING SIZES THAT PIVOT OVER THE WHEELCHAIR SIDEFRAME

* Mild (soft) steel tubing with a very thin wall of 0.035" (20 gauge or 0.9 mm) or a very thin wall of 1.0 mm (0.039") is sometimes available, but it should be used with great caution because it may be too weak. If it must be used for the pivoting tubes of an X- brace or footrest, the tubing joints should be reinforced with triangular braces and with wire or sheet metal wrapped around the ends of the pivoting tubes and brazed in place. We do not recommend tubing this light for sideframes; it is very difficult to bend and it may bend or crack in use. Extra thin wall tubing does make very good lightweight handrims.

AVAILABLE SIZES OF LIGHTWEIGHT STEEL TUBING

The chart which follows lists in more detail the thinwall tubing sizes that are available in each of the three systems: inch size, thin wall conduit, and metric size. Use this chart to help you choose tubing from the sizes available in your country.

SUITABLE SIZES OF THINWALL TUBING

TUBING AND BENDING DIE SIZE Different sizes of tubing require different size dies. Each different tubing size will will also require a die set with a specific bending radius. The Hossfeld dies are listed according to their Center Line Radius (C.L.R.) The Center Line Radius is the distance between the center of the circle and the center of the tubing as it is bent (see diagram). Before ordering die sets from Hossfeld, consult with the following charts and choose die sets that are compatible with the tubing you are using for sideframes and footrests.

DIE SIZES FOR SIDEFRAME TUBING

As a general rule, when bending the sideframe tubing, use the bending die set with the smallest Center Line Radius (bending radius) that will not wrinkle your tubing. We have found that Hossfeld bending dies with the following Center Line Radii will usually bend 18 gauge mild steel tubing (0.049" or 1.2 mm wall thickness). Note that our minimum recommended Center Line Radii are smaller than those recommended by Hossfeld for thinwall tubing; nevertheless we have used these dies with few problems. Other types of benders may have different minimum bending radii. Some types of tubing, especially tubing made of harder steel or tubing with a thinner wall such as 20 gauge (0.035" or 1 mm wall thickness) tubing will require die sets with larger Center Line Radii.

BENDING DIES FOR FOOTREST TUBING

The size of the Center Line Radius will influence the size of the footrest - the larger the radius the larger the footrest. Some people prefer larger footrests to support their feet, others prefer smaller footrests that take up less room.

SUPPLIES

In addition to the materials and parts that go into each wheelchair, basic supplies will be used up as you manufacture chairs. Each chair uses about a pound of bronze, a couple of sanding cloths, and one or more high quality hacksaw blades. You will also be using files that get dull and drill bits that can break. The following list of supplies will get you started. Count on purchasing more supplies before you begin serious production. If you purchase your supplies in relatively large quantities you should be able to negotiate a lower price.

WELDING ROD

While steel welding rod is far less expensive than bronze, we use bronze brazing rod with flux because it produces a stronger weld on thin walled tubing. The strength of a weld made with steel welding rod on thin walled steel tubing is not very predictable. We recommend that you use steel welding rod only on the handrims where the stress is less than it is on the frame of the chair.

Don't worry that the bright yellow bronze welds will make the chair less attractive. The bronze welds will be covered when the chair is painted.

TO PAINT OR TO PLATE

It is important to protect the chair from rust. In some climates rust is a much more serious problem than in others. A chair in these climates may fall apart in a year or two if it is not well painted or plated. Painting or plating can also help to sell a chair. The easiest chair to sell is one that looks good - and in the past that has been a chrome plated hospital-type chair. Unfortunately, that is also the most expensive chair. Recently brightly colored sports chairs have also become very popular, especially among young and active riders.

One of the most effective ways of preventing rust is to galvanize (zinc plate) the thinwall tubing. Zinc plating is less expensive than nickel or chrome, though more expensive than paint. It is most effective when applied after the chair frame is welded together, and is usually done by an outside supplier. Pre-galvanized tubing can also be used, but not without special precautions. Galvanized tubing gives off poisonous fumes when it is heated for welding. If it is used at all, it must be welded outdoors, positioned so that none of the fumes are inhaled.

Paint of any color can be used over plain or galvanized tubing. Paints and primers vary widely in how well they stick to the chair and how easily they can be scratched. Epoxy base primers stick to the chair the best. If they are not available, use an oil base primer, or a special primer for galvanized tubing. In either case the metal should be thoroughly sanded and then prepared with an acid etching solution to make sure the primer sticks to the metal.

Polyurethane base paints are more difficult to scatch than most. If they are not available, test and compare some of the paints that are available locally. Use any creative method to mutilate a paint job that you can come up with (scratch it with a pocket knife, bang it on the table, and so forth) to see which paint is best.

Nickel or chrome plating can add from $20 to $30 to the cost of the chair. Chrome often pits and peels unless a copper underplating is added, at additional cost. Since painting the chair is much less expensive, we recommend priming and painting the frame and only plating the footrests, brakes, and fenders.