Brazing
Hard-soldering by brazing with spelter is used to a very great extent in
the metal industries, especially in the manufacture of cycles and
motor-cars. Although several mechanical joints have been tried in cycle
manufacture, the greatest number of joints are made by means of brazing
solder. A joint made in this manner is almost as strong as a weld, and
the steel tube itself will often break under a strain and the brazed
joint remain intact. Copper and brass tubes, when well brazed, will
stand a pressure of 40 lb. or more per square inch.
=Spelter.=--Hard brazing solder (spelter) is somewhat difficult to make.
The metals have to be melted in a crucible and cast at a proper heat,
and while in a certain condition have to be pounded or punned in a
mortar. This disintegrates the materials and forms crystals of various
sizes, some being as coarse as wheat grains, varying in fineness down to
that known as 0 0, which is very fine dust indeed, and used only on very
particular work, such as tubing 1/8 in. or 3/16 in. in diameter. The
quantity of 0 0 from 1 cwt. of solder is very small, the corresponding
quantity of coarse grains being much larger; so that unless a quantity
is required, it is cheaper to buy than to make. It is necessary to
employ the purest materials, and in purchasing hard solder it is
advisable to state the purposes for which it is to be used.
As a general rule, a solder should melt at a heat just under the melting
point of the metals to be united. Now, in ironwork, or with the steel
used in cycle work, this is impossible, for the melting points of these
two metals are too high to be effected by the bunsen blowpipe or brazing
hearth; but to join two metals in which the melting point approximates
very closely to that of the solder requires great care in order that the
metals may not be fused and the join spoiled. The reason why the melting
point of solder should be about the same as that of the metals being
joined is apparent when it is remembered that heat and cold, vibration
and concussion, tension and compression, have very considerable effect
on metals, and that if the expansion and contraction of these under
working conditions is not nearly alike, disruption or opening of the
joint will follow.
Hard solders or spelters are mainly composed of copper and zinc--that
is, they are brass alloy--the quality most extensively used consisting
of equal parts of copper and zinc. As the quantity of copper in the
solder is increased, so the fusing or melting point is raised.
Ordinary copper melts at about 2,000° F. and zinc at about 840° F., and
a solder composed of equal parts of each metal has therefore a high
melting-point.
A very hard solder consists of equal parts of silver and copper.
Generally, a spelter of different composition is required for iron,
copper, and brass work, that for the latter being required more readily
fusible than that for the former. A suitable spelter for ironwork is one
composed of 2 parts copper and 1 part zinc; a spelter for copper
consists of 3 parts copper and 2 parts zinc; while equal parts of copper
and zinc make a suitable spelter for ordinary brass work. If a very low
melting point be required, a little silver should be added to the
last-given spelter.
=The Flux.=--Borax is the best-known flux for brazing. It is beneficial,
however, to have the borax calcined (fused), as it settles down to its
work immediately when applied to the hot metal, whereas uncalcined borax
has a tendency to swell and fall off the work.
Spelter is in the form of filings, a thin stick, or wire. Filings are
apt to be blown from the work. Brazing is a very useful, and, if
properly done, reliable, method of joining two pieces of iron. A brazed
joint is considerably stronger than a soft-soldered one, and easily
resists temperatures that would cause ordinary solder to run.
The process is not at all difficult if there is sufficient heat, and,
for those who have no gas laid on, the purchase of a paraffin Ætna
brazing lamp can be recommended. With this lamp, of course, the bellows
is not used, and only an iron hearth with asbestos cubes is wanted; but
gas should be used if available.
=The Method.=--For satisfactory brazing, thoroughly clean the surfaces
to be joined, first with a file and then with emery-cloth, and, if
necessary, bind them together with thin iron wire. A flux of borax and
water mixed up into a thick paste is smeared round the joint, which
should then be warmed to get rid of the moisture. Heat the metal to a
white heat, dip the spelter into the borax paste, and apply to the part
to be joined, rubbing round the joint until the brass is seen to run,
when the heat can be cut off. The work should be almost covered in the
asbestos cubes, and the spelter applied all round and not only in one
place; failures to unite the parts mostly result from insufficient heat
or cleaning of the parts.
=Brazing-lamps.=--These lamps are constructed to burn benzoline or
paraffin, and the more powerful types are fitted with pumps so that air
pressure may be exerted on the oil. The paraffin or benzoline is thus
forced into the burner, and by passing through the previously heated
tubular coil is converted into gas, which issues forth out of the gas
nipple, mixes with air, and then burns with a blue atmospheric flame of
high temperature. After filling the lamp, a torch should be held under
the burner to vaporise the oil, and thus ignite the lamp, after which
pressure is applied by means of the pump. Do not start pumping too
soon--that is, before the burner is sufficiently hot to vaporise the
oil--or the oil itself will be forced through the gas nipple. The flame
may be regulated at will, to suit the work in hand, after the burner has
become thoroughly hot to set the lamp fairly going. When brazing, lay
the article to be brazed on some broken coke, charcoal, or firebrick,
and if the article is comparatively heavy, cover it over with some small
pieces (to conserve the heat) except where the brazing is required.
After applying flux and spelter to the previously cleaned metal, direct
the flame of the lamp on it, gently at first, until the spelter fuses
and makes the joint.
=Gas Blowpipe for Brazing.=--Brazing blowpipes should be fitted with a
combination air and gas regulator, by means of which both the size and
nature of the flame can be adjusted to suit the work in hand. When
choosing a blowpipe it is always better to make one rather larger than
the work in hand necessitates--the flame can be reduced when required;
by this means a margin of reserve is provided which is useful in
emergencies. A blowpipe whose dimensions are 10 in. long, having a
5/8-in. gas supply reduced to 7/16 in. at the nozzle, and a 3/8-in. air
supply reduced to 3/16 in. at the nozzle, will braze ordinary latch keys
and other similar work if used with a No. 3 size blower. As already
stated, a paraffin, petrol or benzoline blowlamp is a cheap and
effective apparatus, especially where the quantity of work to be done
does not warrant the outlay of a foot blower, gas blowpipe, etc., but
gas has advantages over the liquid fuel, inasmuch as the blowpipe is
more under control. The quantities of gas and air can be readily and
more delicately adjusted during the actual brazing process, then as soon
as the spelter fuses and the gas is turned off, the stream of cold air
soon counteracts any excessive heat.
=Making a Gas Brazing Hearth.=--The brazing hearth can be bought
ready-made, or can be cheaply put together by the worker himself, the
necessary materials consisting of two bellows, some lead piping, and an
old packing case to support a large size frying-pan--about 1 ft. 4 in.
by 1 ft. The bellows are fixed one above the other, interconnected by a
fixed lead pipe, one pair of bellows worked by the foot pumping air into
the pair above it, from which the air is taken to a bunsen blast burner
by flexible tube. This bunsen burner can also be easily and cheaply
made, and as this works very well and will be found useful for both
soldering and brazing, the following hints on making it are given.
First, get two ordinary large house bellows about 1 ft. square, or
larger if possible; the larger the better. Next a good strong packing
case about 2 ft. long by 1 ft. 3 in. high and 10 in. wide is required.
One end of the box will form the base, and to make it steadier two 1-in.
boards should be screwed across to protrude about 6 in. on each side,
the front one being considerably wider, as can be seen at T (Fig. 62),
which shows the apparatus with one of the case sides removed. The
packing case proper is denoted by the letter A; the part A S A was
formerly the bottom of the case, but is now the back; and the top A V A
was the end of the packing case. Half-way between the top and bottom a
shelf B should be fixed, having a large hole cut out of the middle at G
to accommodate the union piece (seen in section), which holds the lead
air-pipe communicating from the lower bellows to the upper.
[Illustration: Fig. 62.--Home-made Brazing Hearth]
In the back, near the bottom, at S, cut a hole for the nozzle of one of
the bellows, and above the shelf cut another hole at S^{1} to take the
nozzle of the other bellows. The bottom bellows C should have the top
handle extended by a piece of stout bent iron N to act as a pedal, and
should be screwed to the base. A central hole about 3 in. in diameter
should be cut at P to allow the air to reach the valve. Now get a piece
of lead pipe 1/2 in. inside diameter, and, having cut down the nozzle to
just under that diameter, force the mouth of the pipe over it as at S,
and bend the pipe F as shown, to reach the centre of the shelf B and
enter the hole G until flush with the top. Remove the lead pipe and get
a block of wood about 3 in. square. Cut off the corners and bore a hole
in the middle, so that the lead pipe will just pass through it, and
countersink the hole. Broach out the mouth of the lead pipe until it
becomes bell-shaped and fits the block of wood, so that its edge is
flush with the wood when pressed hard against it. This is clearly shown
at G, where the dark shading represents the wood block. Cut a leather
washer the size of the block, with a 1/2-in. central hole, and lay this
over the hole in the bottom of the top bellows where the leather flap
valve is. Bore two holes in the wood block, and screw this down tightly
to the bottom of the bellows, so that the bell-mouth of the lead pipe
faces the hole and has the leather between it and the block. This should
make an air-tight joint for the fixing of the pipe to the bellows.
Next, push the nozzle of the bellows through the hole S^{1}, and screw
the bellows down tightly to the shelf B. Join the lead pipe to the
nozzle of the bottom bellows, and make an air-tight joint with glue and
tape bound round. Between the top of the upper bellows D and the top of
the packing case at V, a stout sofa spring E is fixed to keep the top
bellows shut down tight till air is pumped in from the bellows below.
From a screw near the top of the case stretch a house-bell coil spring
M, and attach its lower end to the foot-plate or pedal N. This spring
tends to keep the lower bellows open. When pressure is put on the
foot-plate N, air is pumped from the lower to the upper bellows, and
thence along the flexible pipe R to the bunsen blast nozzle described in
detail later. If it were not for the upper bellows, the air would come
to the nozzle in puffs, but the spring E keeps the pressure constant,
and a steady blast is secured. The rubber pipe R should be of sufficient
length to reach the hand conveniently, and allow room for movement.
The hearth K can be made from sheet-metal, with the edges bent upwards
to form a tray; or an old frying-pan will answer very well. Whichever is
employed, four iron stays or legs H must be used to raise it above the
top of the packing case as shown. These legs should be screwed at one
end to the sides of the case, and at the other to the pan, and if a
frying-pan is used, the handle may be cut and bent to form a hook as at
L; it then acts as a support for the blast nozzle.
[Illustration: Fig. 63.--Blowpipe or Blast Gas Nozzle for Brazing
Hearth]
For use with the brazing hearth, a blast gas nozzle on the bunsen
principle is required, and this is easily made from two pieces of
gas-piping, a right-angle joint, and two mouthpieces to take flexible
pipes. First, get 1 ft. of 3/4-in. brass (or iron) pipe and an elbow,
internally threaded at both ends. Cut 2 in. off the brass pipe, and cut
a thread at one end of the short piece and one end of the long piece, to
screw into the elbow as shown in Fig. 63. At the other end of the long
tube solder in a cock or mouthpiece to take a large diameter flexible
pipe from the gas bracket. Next get 1 ft. of 1/4-in. brass pipe, and
bend it to the shape shown at B, soldering a mouthpiece at F to take
the smaller diameter air-pipe R (Fig. 62) from the bellows. Bore a hole
through the elbow C, and push the pipe in, making a tight fit, and so
that it passes centrally through the larger brass tube until it nearly
reaches the end as shown at D. Run a shoulder of solder to hold the
small tube firmly in the larger one as at E, and unite with a drop of
solder the large tube A and the small tube B where they cross at G.
The gas passes up the large pipe, and out at D, and a blast of air is
forced through the centre of the flame through the small pipe B. The
shape of the small pipe allows of it being readily hung up on the hook L
(Fig. 62) when not in use. Instead of the mouthpiece shown at H (Fig.
63), a cock will be found more convenient; but it should not be too
small, or it will restrict the flow of gas, which should be as great as
possible. No gas-cock should have less than 1/4-in hole.
The “fuel” (heat conservers) consists of chunks or cubes of asbestos,
and when these are blown upon with the gas flame, the heat is quite
sufficient for moderately heavy brazing. Be careful not to get any kinks
in the flexible tubes, or the air and gas will be reduced in quantity,
if not stopped altogether.
A square of thick asbestos (sheet) is useful for laying on small
articles whilst brazing, and a piece or two of charcoal will be handy
for silver-soldering.
=Brazing Iron and Steel.=--Before attempting to braze either iron or
steel the surfaces should be thoroughly cleaned by filing or grinding,
etc. Brass or copper may be cleaned by dipping in a solution of 1 part
nitric acid and 2 parts of sulphuric acid. This same solution can be
used to remove the scale after brazing. The parts should be fastened
together in the position they are to occupy when joined. The fastening
may be effected by the use of wires, screws, bolts, clamps, etc. If
practicable, the parts should be held in such a way that they can be
turned over during the brazing process without disturbing the relation
of the parts, thus affording a better chance to apply the flux and
brazing material.
In brazing sheet metal, if the seams are not required to stand much
working after soldering, they may be joined edge to edge. When seams are
formed in this way, little nicks, about 1/2 in. apart, should be filed
out along the edges, so that the solder flowing through the nicks will
render the joint sound. If the seam is to be worked after soldering, a
small lap is necessary to ensure adequate strength. To form seams of
this type, first thin the edge of the metal along the ends that are to
form the seams, about 1/8 in. in from the edge, so that when the two
edges are lapped over each other the combined thickness at the seams
will be the same as the single thickness of the metal at other parts.
Cut a small cramp at the top and bottom of the seam, and fit the
opposite edge in these cramps. After preparing the seams by either of
the above methods, fasten binding wire round the articles so as to hold
the seams securely in position. Now powder some borax flux, mix equal
parts of the borax paste and grains of spelter, and along the seams
place sufficient of the mixture to solder them when melted. Some dry
borax should also be kept ready at hand, so that a little may be taken
and thrown on the solder at any point where the material does not appear
to be flowing freely. Gently heat the article by some suitable means,
such as foot bellows and blowpipe, so that it will expand equally, and
not disarrange the seam; increase the temperature until the metal is a
dull red, and the spelter runs. If necessary, with a piece of wire
flattened at one end gently rub the solder along the seam until every
part is joined.
=Brazing Copper Rod.=--For uniting two pieces of copper rod, 1/4 in. or
3/8 in. in diameter, first prepare the joint as at A in Fig. 64, and
file the surface of the copper clean in the immediate vicinity of the
joint. A mixture of borax and water and spelter should now be applied to
the joint, which should rest on a small heap of broken coke, the coke
being also built round it. The flame of the blow-lamp should be directed
at first on the coke surrounding the joint, and then gradually brought
to bear on the joint itself. If necessary, add a little more spelter
before any of it fuses, and when the copper begins to get red hot,
throw just a pinch of dry borax on the joint to facilitate matters.
[Illustration: Fig. 64.--Dovetailed Joint in Copper Rod]
[Illustration: Fig. 65.--Dovetailed Joint in Key Stem]
=Brazing Key Stems.=--In brazing together the broken parts of a key
stem, first it is necessary to file the fractured ends quite true; this
may entail the shortening of the key by 1/4 in. or 1/2 in., and as
another 1/4 in. will be lost in making the joint, it may be advisable to
use another key bow having a longer piece of stem than the one that was
broken off. With a warding file cut a dovetail on each of the ends to be
joined, as shown by Fig. 65. A small, half-round file will assist in
making the edges true and square. The pieces must interlock perfectly,
and when this is the case, very lightly hammer the joint, round which
then bind seven or eight turns of brass wire to act as spelter. Wet the
joint, sprinkle powdered borax on it (this is to serve as the flux),
and, holding the key in a pair of tongs, place it in a clear part of a
forge fire made with charcoal, small coke, or coal cinders, and
commence to blow steadily the forge bellows or blower. Failing a forge
fire, use a blowpipe, the key being placed on a piece of charcoal or
pumice-stone whilst the heat is being applied. If the forge fire is used
it is as well to support the key on a guard of thick iron plate having a
hole in its centre over which is the joint to be brazed. By this means
the necessary local heating is obtained, and much labour in cleaning the
key afterwards is avoided. On being heated, the borax swells and boils
up, and should be pressed down with a spatula, previously dipped in cold
water to prevent the hot borax adhering to it; a suitable spatula is
made by flattening one end of a 1-ft. length of a 1/4-in. round rod,
having at its other end an eye by which it may be hung when not in use.
With this spatula, also, powdered spelter may be added to the joint if
required. When the brass wire begins to run, assist the flow by adding
powdered borax, and when all the brass has run into the joint, rub off
superfluous molten metal from underneath and allow the joint to cool
gradually. When cold, file up and clean the stem of the key until only a
thin bright line of brass can be seen.
=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.
=Brazing Cast-iron.=--Although it has been dogmatically asserted both
that cast-iron can and cannot be brazed, it may be stated that the
general results of attempting this process are so indifferent as to
warrant the conclusion that this process cannot be recommended. In
brazing, one of the conditions essential to success is that the metal to
be brazed and the spelter should unite to form an alloy just where the
brazing occurs, and that this should take place spontaneously. This
actually happens when brazing copper, brass, wrought-iron, etc., but not
in the case of cast-iron. If, however, the reader desires to experiment
in this direction, the following hints may be useful. First of all,
remove all dirt and grease from the cast-iron, and then chemically clean
it by immersion in hydrochloric acid, afterwards well rinsing it in
clean cold water. A mixture of borax and water and spelter should now be
applied where the brazing is required, and gentle heat then brought to
bear on it until the water is evaporated. The heat should now be
increased until the casting is red hot in the neighbourhood of the
brazing; and some workers claim that at this juncture the best results
are obtained by dusting the red hot cast-iron liberally with boric or
boracic acid powder. A hard spelter should be used in preference to a
readily fusible one, otherwise the spelter would be fused much too soon,
and before the casting is raised to a sufficiently high temperature.
An experienced worker who believes that it is possible to make a sound
joint in cast-iron by brazing, states that he has brazed articles with
equal parts of borax and boracic acid. The chief difficulty is the flux.
He has tried one called “Ferroment,” which seems to give good results.
The first casting brazed with it was 3 in. wide and 5/8 in. thick, and
this casting at the time of writing had been in work six months for
fifteen hours per day. This same worker prefers to roast borax before
use, as it stops on the work better. Also, when a deep, wide joint is
being dealt with, he finds it an advantage to smear on a little clay
underneath and the side, as should the joint get slightly hotter in one
part the spelter will run through and make an unsound joint. The heat
required to braze cast-iron varies somewhat with the spelter used. If
brazing by means of a smithy fire, the spelter will show a blue flame
when it starts to run, and the article must then be removed from the
fire. In using a blowpipe or blowlamp, the blue flame does not show, and
one has to look for the spelter melting, and see that it flows well
along the joint before removing the flame. Spelter which has been kept
in stock a good time may not flow well. The worker in question prefers
brazing wire to grain spelter, as by means of warming the end of the
wire and dipping in the flux (which will adhere to the hot wire), it may
be put just where desired. He collects all the soft brass turnings from
his lathe and uses them for brazing.
Another worker has stated that those who have a forge of any kind will
find the following an effective method of repairing an iron casting. A
flux may be made of chlorate of potash 4 oz., boracic acid 1 lb., and
carbonate of iron 3 oz. These should be mixed well together and pounded.
The parts to be brazed together should be carefully cleaned by scraping
them, and brought to a bright-red heat. Then apply the flux and spelter
and increase the heat.
Still another worker says that in brazing cast-iron, if powdered soda is
used instead of borax, the result will be a perfect joint.