Cycle Brazing.
—In cycle brazing, the first consideration is the means of heating the heaviest joint to a brazing heat. This may be done in several ways, by a paraffin blow-lamp costing at least 35s. to 40s., or, what is better, a gas blowpipe 7⁄8 in. or 1 in. in diameter, with at least 1⁄2 in. gas supply pipe and a fan or bellows to supply the necessary air pressure. A small fan is far preferable to a bellows of any description, the flame being steady and constant, and the operator being able to devote his entire attention to the job. In the absence of power, obtain a small circular double-blast bellows and hearth, costing with blowpipe about £5.
The brazing materials are brass spelter. No. 3 size, or brass brazing wire and powdered borax; a tin to hold the mixture of spelter and borax, and one for the plain borax; a piece of iron wire about 1⁄4 in. by 18 in., flattened at one end to feed the spelter and borax to the joints; and a brazier’s brush, which is desirable, but not absolutely necessary, to brush the superfluous borax and brass from the outside of the joint as soon as it is removed from the hearth; this saves much work in filing up, and saves the files immensely. Do not purchase the borax ready powdered, but buy lump borax, as that purchased ready powdered is likely to be adulterated. In making the brazing mixture, use about equal parts, in bulk, of No. 3 spelter and borax.
In preparing the work for brazing, see that the surfaces are bright, clean, and free from scale. The joints should be a good tight fit, free from shake, and where a joint such as the back forks to the bridge lugs is being made, see that the tube edges fit close up to the shoulder of the lugs all round, and do not depend on the brass to fill up a badly fitted joint.
The chief things to observe are to make a sound joint the full depth of the lug, and not merely to get a thin film of brass round the outer edge. To do this, the flame should be directed on to the thickest part of the lug first before getting the tube too hot, and feeding the joint with borax before the metals get hot enough to scale. As soon as the lug and tube begin to get a dull red, feed with borax only, then with brass and borax, when it should flow almost like water and penetrate to the deepest part of the joint.
Another very important thing is not to “burn” the tube by getting it too hot, which will spoil it and cause an early fracture. If the above method of heating the lug first is observed, and the tube near the lug kept “wet” with borax to prevent it scaling, this should not happen.
Where the joint to be brazed lends itself to inside loading with the spelter, the work should be so placed on the hearth that the brass inside, when it melts, will tend to flow to the outside of the joint. Then if borax only is used on the outside until brass appears round the edges, it will be fairly certain that a sound joint will result. As soon as this comes through, feed a little brass-and-borax mixture to the joint, and, as soon as this melts, stop the flame and remove from the hearth. If the flame is kept on too long after this, there is a possibility of “soaking” all the brass out of the joint, especially so if the joint is not a very good fit.
Some braziers use a blacklead mixture for protecting thin tubes whilst brazing; but care must be taken to keep it out of the actual joint, as brass will not adhere to metal where this is present.
Cycle frame joints can be brazed on an ordinary smith’s hearth, but it is rather risky, and requires more skill than with a gas blowpipe. It also requires a good clear fire and a light blast. The job should be kept well fed with borax to prevent the tubes scaling and burning. The joint should be loaded from the inside with about a thimbleful or less of crushed borax and No. 3 spelter mixed in the proportion of about half of each in bulk. The heaviest part of the lug should be heated first, and the work must be turned frequently in the fire so as to avoid burning the tube. If the joint is fed from the outside with borax until the brass flows round it, a sound joint is ensured. This applies to a joint where the lug is fitted inside the tube. Where the tube is fitted inside the lug, a little brass and borax should be applied outside the joint just as the inside charge has melted, which can be seen with some joints by looking down the inside of the tube. When the tube is closed both ends, such as the last joint of a frame, the job is more difficult and requires careful judgment and skill to ensure a sound joint.