Most modelers have done soft soldering using a solderingiron or gun. Common alloys for soft soldering are composed of tin lead or silver and lead. They melt at temperatures from about 350 to 600 degrees F. Soft soldering does a lot of jobs well for small models, but is generally unsatisfactory where structural loads are high in Giant Scale Aircraft. The Shearing strength is only about 1500 pounds per square inch.

The next step up regarding strength and ease of fabrication is hard soldering, sometimes called silver soldering or brazing. This requires temperatures of 1200 to 1400 degrees F. and produces shearing strength in the range of 30,000 to 60,000 pounds per square inch. The hard soldering alloy for use on steel or brass contains silver, copper, zinc, and cadmium. A flux is required and is usually applied to the joint in the form of a paste typically made from borax, boracic acid and zinc chloride. An inexpensive propane torch may be used for most small jobs in model aircraft work. However, one of the newer high temperature designs incorporating a pressure regulator will prove more useful. These usually include a large and a small burner assembly. These torches use either propane or MAPP gas in 14 ounce bottles. The MAPP gas flame is about 400 degrees F. hotter than the propane flame and may be necessary on jobs involving more massive parts to be joined.

Extra caution must be used in hard soldering. The fire hazard from the torch flame, the hazardous chemical fumes from the heated soldering alloy and flux and the storage of bottled gas must be considered carefully. Hard soldering in a basement or other poorly ventilated space is absolutely unsafe. All equipment and supplier's rules and warnings should be followed.

Several combinations of dissimilar metals may also be hard soldered such as steel or brass. All joints must be closely fitted since the molten hard solder filler metal is distributed in the joint by capillary action. In this case, it means joint clearance in the range of 0.002 to 0.006 inch. This sounds difficult, but is not too hard with a little care. Additionally, it is very important that joints be clean. A rotary wire wheel, steel wool and emery cloth are very useful in getting a good clean finish on the joint after all dirt, oil and etc. are removed. Any contaminant left in the joint may result in an imperfect joint and difficult removal of the flux residue. Flux removal from a clean joint simply requires a good scrubbing under hot water. Flux removal from a dirty joint is just like chipping glass since the flux captures the dirt and makes a glass hard coating on the joint. All flux must be removed to prevent corrosion.

The tail wheel assembly shown in the sketch is from my TIGER II airplane completed in 1988. This design has proven quite adequate for an airplane of 10 pounds. A larger unit has been installed on POGO III for several years and has worked well. The TIGER II gear was made of 3/32 inch piano wire and 5/16 inch square steel stock obtained from a hardware store. A 5/16 inch cube was cut from the stock and two 3/32 inch holes drilled about 1/16 inch deep to receive the piano wire ends. The 3/32 wire was bent around a short piece of 1/8 inch wire fastened in a vise. The 3/32 inch wire was heated cherry red with a propane torch then bent around the 1/8 inch piece of wire using pliers to aid in the sharp bend. Reheating is O.K. to get the bend correct. Open air cooling is best to help prevent brittleness. An abrasive disc cutoff tool was used to trim both ends of the wire to proper length. The ends were deburred with a small file and emery cloth was used to clean the ends.

The holes drilled into the 5/16 inch steel were filled with paste flux and the 3/32 inch wires inserted, being careful to retain the displaced flux and add new flux at the joints as necessary. Any workable propane torch should supply adequate heat for this small mass to be hard soldered.

The 1/32 inch diameter hard solder alloy wire end was coated with paste flux prior to heating the tail wheel assembly. A couple of fire bricks or regular bricks make a handy work area. The cube-wire joint was heated with the torch. The first thing that happens is the water boils out of the flux leaving a white coating. Then, as the temperature increases the white coating will turn clear as the joint approaches a medium cherry red color. At this temperature the hard solder wire was held against the joint until the hard solder has melted enough to fill the joint by capillary attraction. The torch flame should not be used to melt the hard solder, the heat of the joint is sufficient. All cooling was in open air to minimize brittleness.

The cooled joint was then scrubbed with an old toothbrush under running hot water to remove the residual flux. The tail wheel assembly may be bent at the same time by clamping them in a vise. Heating should not be used for these bends since loss of temper and possible brittleness could result in these areas that flex.

The only other hard soldered part in the tail wheel assembly is the wire in the socket head screw used to secure the tailwheel and provide the tail wheel steering arm. The above procedure was used to hard solder the joint. A vise grip plier is used to tighten the set screw into the wheel collar in the final assembly.

Another relatively easy hard soldering project was the muffler for the Zenoah 23 engine installed in the TIGER II. A pressed steel muffler used on the Quadra 35 chain saw engine was modified for use on the Z 23 engine. The original Q 35 muffler is a two piece affair of 0.030 inch thick steel. The first step in modification was to use a rotary wire brush and emery cloth to remove the black finish around the louvers that were to be hard soldered shut.

The next step was to use a steel block for a bucking bar and a hammer to completely flatten the louvers and all other indentations on both muffler pieces. About 1/16 inch of unnecessary metal was ground off the outside lip of the mating area of the two halves. Rotary wire brush and emery cloth were used to clean all areas and joints to be soldered.

The original Q 35 exhaust inlet hole was left open at this time. The lip joining the two halves was hard soldered using the procedure described above for the tail wheel assembly. The appropriate hole for the Z 23 exhaust inlet was cut along with the bolt holes for mounting the modified muffler. Spacers were made to install inside the muffler for the mounting bolts to go through. These are essential to keep the bolts from crushing the muffler when they are tightened.

Next, a steel plate was hard soldered over the original Q 35 exhaust inlet hole. This area to be hard soldered was again cleaned before soldering to remove oxidation from the previous heat cycle. Hard soldering this plate is a little trickier since the torch flame must be played away from the covering plate to prevent it getting overheated in relation to the much greater mass of the muffler to be heated. Extra help is needed to keep the plate firmly in place against the muffler and maintain better heat distribution.

An old fashioned wood brace with 2 inch pipe reamer was used to get the final force fit of the 5/8 inch brass exhaust pipe in the muffler for hard soldering. The reamer taper provides an ideal joint and a force fit is nice to hold alignment for hard soldering. A telescoping brass tube 1 inch long was hard soldered inside the exhaust pipe because engine vibration produces a lot of stress on an exhaust pipe over 3 inches long. The inside tube was placed about 1/32 inch into the exhaust tube and this joint hard soldered before hard soldering the exhaust pipe to the muffler.

This muffler installation has worked without mechanical problems and is quite efficient since it has a lot of internal volume for its outside dimensions.