Making Blow-lamps
=A Paraffin Brazing Blow-lamp.=--The brazing blowlamp shown in Fig. 70
was made at a total cost of less than 4s. The illustration is printed to
a scale of about one-quarter full size. The lamp illustrated is not a
mere experiment, as the writer of this description had a similar one in
use for over two years, and during that time brazed hundreds of jobs
with its aid.
The container is of tinplate, and adapted from a workman’s tea can. When
purchased this will have a wire handle and two hinges, and these should
be removed and soldered up. The handle shown at H is made from a strip
of iron, 1/8-in. by 5/8-in. section, bent round to the shape shown and
riveted to the side of the container. These rivets should be well
soldered over inside to prevent leakage. The joint of the longer strip
is shown at X. The lid should next be taken in hand, a 5/16-in. hole
being drilled at one side close to the handle, to take an ordinary Lucas
cycle valve. A leather washer is fitted inside, and also one outside
under the lock nut, the latter being then tightly clamped up.
At E is shown the filling cap, the body part of which was taken from an
old paraffin oil-lamp container, and the screw cap made at the local
brass works; but this fitting can be obtained in a finished state from
many model-fittings manufacturers. This is soldered to the lid in the
position shown, a hole being afterwards drilled in the tin to suit it.
[Illustration: Fig. 70.--Paraffin Brazing Blow-lamp, the container being
shown in section]
The cock shown at D is an ordinary gas-cock, with a length of 3/8-in.
outside-diameter copper piping screwed and sweated in beneath. This is
passed through a hole drilled in the lid for its reception, and the base
of the cock is then sweated in position. Note that the length of this
pipe is such that when the lid is in place it clears the container
bottom by 1/8 in.
The coil of piping at P is 3/8 in. diameter copper tube coiled round to
the shape shown, the lower end being tightly screwed into the top of the
cock. The opposite end is screwed for a short length of 3/8-in. gas
thread, and very tightly fitted with a screw plug such as that used by
plumbers for shutting off a portion of gas piping. Before screwing this
on, a small hole about 1/32 in. in diameter should be drilled in the
middle of same; this is the nipple for the exit of gas to the burner.
The 3/8-in. copper tubing should not require filling with anything
before bending; this operation is best done round a mandrel of wood. No
heating is necessary. A certain amount of flattening of the tube will no
doubt occur, but this is immaterial. The end of the flame tube is, of
course, open, otherwise the flame could not emerge. The nipple end of
the flame tube is also quite open, with the nipple end of the coil just
projecting inside. The portion of tubing shown dotted in Fig. 70 takes
the vapour from the coils to the nipple; it does not pass through the
coils, but at the back of them, and bends round as shown.
The lid of the container should now be carefully soldered down all
round, and then the 3/16-in. brass stay rod shown at S must be fitted
and both ends sweated over. It is essential that this stay is not
omitted, as otherwise the pressure to which the container is subjected
would bulge out the ends.
The flame tube A should now be made of 1/32-in. sheet-iron, being held
in place by two or three clips riveted on and bent over the coil ends as
shown at C, only one of which is shown for clearness. The flame tube
does not taper, nor is the back end closed up. Its diameter should be
1-1/4 in. and length 3-1/2 in., but this depends on the size of the hole
in the nipple. No holes are required in this tube, as all the air is
drawn in at the end. The nipple has a gas thread cut inside it, and it
is screwed tightly on to the end of the copper coil. The length of the
pipe from tap to coil is not important; about 9 in. will be
satisfactory.
The best means of joining the nipple to the tube is to screw the end of
the tube before bending it round at the end, and then to screw the
nipple on tightly. The pipe can afterwards be bent as shown.
The jet hole in the nipple should be about 1/32 in. bare, and should be
so drilled that the issuing gas passes through the centre of the flame
tubes. The tube is open at both ends, to allow air to be drawn down and
complete the combustion of the paraffin vapour, the mixture igniting and
burning properly when the vapour passes out at the other end and comes
into contact with the atmosphere.
To use the lamp, it should be filled about two-thirds or one-half full
of paraffin oil, and the container cap then screwed hard down with a
leather cap as a washer. A cycle pump should then be connected to the
valve V, and a few strokes given, the cock D being meanwhile closed. The
copper coil should next be put into a gas flame or the fire for a few
minutes until nearly red hot, and then the cock D should be opened a
shade, which will allow the paraffin to rise up the pipe I and enter
the coils P, where it will become vaporised, and the gas will then issue
from the nozzle N, and burn at the mouth of the coils.
The lamp may then be applied to the job, and five or six more strokes
given to the pump, when the flame should burn with an intense heat and
give out a roaring noise.
The theory of action of lamps of this character is as follows: The
pressure of air in the container forces the paraffin up the copper tube
into the coil, where it is vaporised by the red-hot piping. The vapour
then issues at some pressure from the nipple hole and, passing down the
flame tube, an ejector action is caused which draws air in with it. This
air mingles with the paraffin vapour, and when the mixture issues at the
other end, and comes into contact with more air, combustion takes place.
The heat produced by the lamp should be quite sufficient to braze the
bottom bracket of a motor-cycle frame and other similar jobs. The size
of nipple with relation to the size of the flame tube is most important.
If the flame tube be too large in proportion, the result is an excess of
air, which cools the gas unduly and the flame dies out. A candle is put
out in a draught from the same cause. On the other hand, if the flame
tube is too small, or the nipple hole too large, the result is an excess
of gas, causing incomplete combustion and a partly luminous and smoky
flame, with less heating power. To ascertain if the nipple hole is too
small, take a strip of tin and, whilst the lamp is burning, hold the
tin partly over the end of the flame tube, so as to restrict the amount
of air entering, and note the result.
If the drawing (Fig. 70) be regarded as 4 in. to 1 ft., and the lamp
made accordingly, it will be of ample size to braze small and model
boilers. It must be understood, though, that the size of the lamp alone
does not govern the size of the flame, this depending entirely on,
firstly, the size of the nipple, and secondly, the pressure of air in
the container forcing the vapour out. The larger the container, the
longer the lamp will burn without refilling, and, incidentally, the
larger it is the weaker the container will be, and vice versa.
Should this lamp be required for brazing and silver-soldering articles
about 1/2 in. in diameter, it must be altered, as it is obviously much
too large. The same container will do, of course, although perhaps
rather unwieldy. If preferred, another container just half the size
could be made; this will be handier in use, although it will not allow
the lamp to burn for so long a period without recharging as the larger
pattern. For the tubing, 3/16-in. copper tube, preferably solid drawn,
will be suitable. This should be heated to a dull red and then quenched
in water to anneal it, this making the task of bending it much simpler.
If it kinks too much in the bent portions, pour some molten resin into
the tube, one end being plugged up for this purpose, and when set, bend
to the shape desired. The resin can be heated and run out afterwards.
The flame tube may be half the diameter and length of the one
illustrated, but no hard and fast rule can be given for the dimensions
of this part of the lamp, as the size of the nipple hole has everything
to do with this. Make the latter just big enough to admit a fine needle.
A simple method is to drill the cap almost through, and then punch the
rest through with the point of a needle. By experimenting, it will be
easy to find the right proportions of flame tube and nipple hole. The
screwed joints should be a metal-to-metal fit as it is termed, that is,
they should fit so tightly when screwed home that no leakage is
possible. This can be ensured by seeing that all male threads are a
tight fit in their respective holes.