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With the rapid growth of the use of thin wall tubing in many fields, manipulation problems have become of major importance and the production of perfect bends has been a matter of increasing difficulty.
Many materials have been used as filler in tube bending with a certain measure of success. Resin, pitch, sand, lead, spiral springs and internal mandrels have been satisfactory up to a point in preventing buckling, flattening or rupture of the tube wall, but frequently fail when the bend required has been light. Sand is difficult to pack tightly enough to give sufficient support to the tube wall. Lead has a setting shrinkage which results in comparatively poor support and further, its high melting point precludes its use with light alloys. Resin and pitch have given fairly good results, but fail on the sharper bends, and in addition are messy and dangerous to handle. Spiral springs and internal mandrels are, in general, limited in use to the gentler bends.
A very serious objection to all these fillers, other than the spring and mandrel, is the difficulty experienced in completely removing all traces of the filler after bending. Small particles left in the tube and subsequently becoming dislodged may cause serious troubles in service, or if left in the tube give rise to regions of marked weakness after processes involving heat treatment.
Cerrobend is an alloy of bismuth, lead, tin and cadmium. It has peculiar properties which makes it an ideal filler for tube bending. It has a melting point of 158º F. that is much less than the temperature of boiling water. Cerrobend has been used successfully in the bending of tubing with walls as thin as 0.007 in. to small radii. Cerrobend conforms snugly to the inside of the tube that the tube can be bent as though it were a solid bar. This quality is due to the fact that Cerrobend expands slightly instead of contracting on solidification.
Molten Cerrobend normally cools and crystallizes slowly, resulting in a coarse crystal structure and is quite brittle in this condition. However, if it is rapidly chilled, it acquires a fine-grained structure that renders it very ductile.
In using Cerrobend for bending metal tubing, the alloy should be placed in a clean stainless steel pot and put into boiling water. A regular kitchen double-boiler is ideal for small quantities. The water is heated to boiling which completely melts the Cerrobend. Prolonged heating should be avoided. There are electric and natural gas pots available with controlled thermostats. Heating over an open flame is not advised but can be done satisfactory if closely watched and the use of a thermometer is a must. One end of the tube is closed tightly with a cork or other stopper. The inside of the tube is lightly oiled with a cooking oil, such as olive oil, or any oil that does NOT contain a detergent. This can be done by filling the tube with oil and then pouring it out leaving a small amount in the bottom. If the tubes are oiled before loading there is no possibility of the alloy coming into contact with the tube wall since the chance of the oil film being broken at the operating temperature of 212º F. is extremely remote.
The molten Cerrobend is now poured into the tube displacing any residual oil. It is better to hold the tube at a slight angle when filling and allow the metal to flow down the inside wall of the tube. This will help prevent air gaps. Once the tube is full of molten alloy, it is plunged quickly, corked end first, into a tank of cold water about 50º F. This insures a fine crystalline structure in the alloy, which makes it ductile and easy to bend.
As it has a low thermal conductivity, Cerrobend cools very slowly, even when water-cooled. After chilling, the tube and its filler should be allowed to attain room temperature before bending. Almost any type of bending equipment can be used if the bends are made slowly at uniform speed and uniform loading. Too much speed or uneven application of power may cause failure. In the case of thin wall aluminum or duralumin tubing bent to a small radius, the bending should be done in multiple stages rather than all at one time.
Cerrobend may easily be removed from the tubing by heating with steam, boiling water or hot air at temperatures approximating the boiling point of water (212º F.). The use of a bare flame is not advised because the tube wall may be overheated or even burned. The metal can be drained out of the tubing in a molten condition and used again and again. It is also advised that the tubes, after unloading, be cleaned by blowing with steam or by a pull-through to sweep out any small beads of Cerrobend which may be retained in the oil film and possibly cause interference during subsequent treatment.
When the tubing is of too small diameter to permit pouring the molten metal into it, the alloy can be drawn into it by suction without trouble. This practice is generally followed with tubes that are ¼" in diameter or less.
The Cerrobend method has been applied successfully to the bending of copper, brass, duralumin, plain steel and stainless steel tubes. Tubes plated with chrome or nickel can be bent without danger of the plate flaking off.
It is said that the alloy has found special favor in airplane factories, for fuel, oil and hydraulic tubes and pipes lines. Tubes used for constructing airplane frames are also formed by the use of Cerrobend. Tubes of irregular section can be bent as readily as round tubes.
A most valuable development of the use of Cerrobend is in the forming of rolled or extruded sections. The need of complicated rolling machines for this work has been obviated. The procedure is to cast the alloy in a suitable mold completely to embedded the section, and then bend the block of alloy round a former of dimensions allowing for the thickness of alloy surrounding the section. Under these conditions (particularly if the former is grooved exactly to accommodated the cross-section of the alloy block and prevent any cross-sectional distortion), it is impossible for the section to ripple or spread in any direction and perfect bends are achieved.
How to Bend Tubes and Sections
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