A MISCELLANY OF IDEAS.
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Suggestions Made by Our Driver and Mechanic Readers.
MACE method of remetalling bearings _L is not always understood as clearly as it should be, and for this reason the contribution of "D.W.W.R.," of Trowbridge, is valuable. This week's prize of I5s. is awarded to him.
The old white metal should first be melted out of the bearings, which can best be accomplished by applying a blowpipe to the back of the bearing shells, previously thoroughly cleansed from oil and dirt, and the inner surface covered with best-quality tininan'S solder. The tinning should be carried well over the edges of the shells and beyond the points to be reached by the white metal. Having coated the surface of the shells, they should be wiped with a piece of clean cloth while they are hot, to remove the superfluous metal and any traces of flux. The surface should then be gone over again and given a thin coat of solder.
Flux may be removed from the surface by washing in hot water. The best material for wiping the bearing shell is closely woven cotton fabric that is free from nap. Cotton waste is unsatisfactory, as it may leave strands upon the surface of the tinnieg, which burn and form a scum that is afterwards difficult to remove,
For warming the shells a gas blowpipe.. is the best method of furnishing heat, but should it be unavailable, a spirit lamp may be used, as spirit gives a much cleaner flame than paraffin, and is, therefore, preferable. If paraffin has to be used the flame must be carefully adjusted and kept away from the tinned surface.
When a number of bearings is to be relined, it is best to place them in a censtic-soda solution, approximately three per cent, soda, for about 30 minutes, keeping the solution near boiling point. Afterwards the bearings should be washed in clear running water. An alternative method of heating and tinning shells is as follows :—After thoroughly cleansing in caustic solution, as described, hold the bearing shell with a pair of pliers in a pot of molten tin or best solder, allowing sufficient time to heat it. then go over the inner surface as quickly as possible with the flux, and again immerse it, at the same time imparting a rocking motion. The bearing should finally be withdrawn and rubbed with a clean cloth.
When old white metal is melted out for relining, it is sometimes found that, on tinning, the solder applied to the shells with the object of giving it a heavier coating will not adhere, although the original tinning may appear perfectly clean. In this case flux should be used to unite the old tinning to the new and the surface carefully watched to ensure that the new tinning is effectively uniting and is not merely covering the surface, with a tendency to run off as if the surface were slightly greasy. It is advisable not to trust to old tinning, and as much as possible of this should be wiped off while hot.
Dealing now with the white metal itself, care should be exercised not to overheat it in the melting pot, the intensity of the heat is most important as it affects the structure of the metal when it is poured. The correct temperature for the latter process is from 570 degrees to 625 degrees Fahrenheit. Should a thermometer not be available, a rough test is to dip a piece of ordinary dry, white writing paper into the molten metal ; if it enters easily and colours to a medium brown, the metal is hot enough, but if it should become charred or catch alight, the metal should be cooled at once.
When the metal in the pot has become molten, the heat should be reduced sufficiently to maintain the metal at the correct temperature. A bluish-coloured ash forming on the surface of the metal is oxide, and indicates that it is being overheated. It is most important that the metal be well and frequently stirred.
The bearing shell should be mounted vertically in a jig ; the jig, bearing and mandrel are then heated until the tinning on the inner surface just commences to run. The flame from the blow-pipe should not be allowed to play ' on, the tinned portion, as this will cause a film to form on the surface and prevent the white metal from adhering.
The metal should be poured slowly at first to avoid wastage should there be any leaks round the sides of the shell, also to enable the air to be driven out to prevent blow-holes, pouring being carried out at one point only.
In order to obtain smooth castings, so soon as the bearing is full, dip a piece of clean steel wire into and out of the metal, at the same time preventing it setting at the top by means of a blow-pipe. The metal at the bottom should set first and the cooling take place gradually in an upward direction. It may be neceseery to add a little more metal at the top owing to shrinkage during cooling.
In eases where white metal does not adhere to the shell after careful tinning, the reason is one or more of the following :--(a) The flame has reached the tinning and oxidized it ; (b) the shell has been overheated and the tinning thus oxidized; (c) the shell has not been heated sufficiently to make the tinning run; (d) the shell has been lined too thickly or filled solid, in which case the metal has shrunk away from the shell in cooling.
Lined bearings should ring true when tapped with a hammer. It is an advantage occasionally to polish the jig and mandrel with emery cloth and smoke them to prevent the white metal from sticking.
TWO useful tips in connection with screws are given by "FIE.," of Bath. The first deals with the conversion of an ordinary screw into a thumbscrew, as it is not always convenient to wait while one of the latter is obtained or to make it from the solid.
An ordinary cheese-headed screw is taken and the slot filed out ; into this is sweated a piece of metal plate cut to a, suitable shape for gripping with the fingers.
For tightening a screw, either temporarily or permanently, it is advantageous to split the threaded portion vertically for a short distance and to expand it with a wedge so as to increase the diameter and thus make a tighter fit in the threaded hole. The result may be lasting in some eases, whilst in others the elasticity of the split will act as a lock by reason of the friction it imparts, thus preventing the screw from working back under vibration. In certain circumstances it may be advisable to drive the wedge home permanently into the split in order to prevent closing.