4: Methods of Joining Metals
Probably the oldest method of joining metals is by riveting. Over 4,000 years ago vessels were made by riveting together copper and bronze plates.
The craftsman of long ago must have worked under great difficulty using a very primitive drill to make his holes for riveting (fig. 1). Later when iron was used they found that joints could be made by forge or fire welding.
Joining metals by forge welding has been done for thousands of years but we are not sure when this method was first used. The swords used by the Vikings were made of plaited or twisted strips of metal which were heated to white heat and then hammered together, that is, they were forge welded (fig. 2).
The method of obtaining the heat was probably similar to that used today by the primitive Kikuyu people of Kenya. The furnace is a shallow hole in the ground lined with clay. A charcoal fire is started and kept going with plentiful supplies of charcoal. The blast comes from two bellows which consist of "baggy" goat skins tied over specially made earthenware pots. The loose skins are gathered up in the centre and bound on to stocks in such a way that they can be moved up and down thus increasing and decreasing the volume of air trapped inside.
At the bottom edge of each pot there is a tube which takes the air to a tapered fireclay funnel (tuyere) which leads into the heart of the fire. There are no valves in these bellows. As one bellow is pumped down forcing the air into the fire, the other is brought up and the air comes in at the place where the tube enters the tapered funnel (fig. 3).
Today work is much easier for a craftsman and we have more methods of joining metals than were known by the ancient metalworkers. For convenience wc can divide the methods of joining metals into two groups: I. using heat, 2. working cold.
In group 1 we have:
- soft soldering
- silver soldering
In group 2 we have:
- screws (this includes bolts, studs and nuts)
This method of joining metals does not require a lot of heat nor expensive equipment. The solder is an alloy mainly of tin and lead.
Lead melts at 327°C and pure tin melts at 232°C but when they are alloyed together they start to melt, i.e. they become pasty, at about 183°C. The range of temperature over which they are pasty depends on the proportion of tin to lead, e.g. plumber's solder (tin 30% and lead 70%) has a long pasty range—necessary for wiping a joint—whereas tinman's solder (tin 65% and lead 35%) passes quickly from a liquid to solid state (see fig. 4). Solders with lower melting points are made by adding bismuth to the tin/lead alloy. These are often termed Low Melting Point (L.M.P.) solders.
Preparation of Joint
Before two surfaces are joined together by solder they must be cleaned mechanically, e.g. by filing, wire brushing, scraping etc., but this is not enough. The remaining thin film of oxides must be removed chemically by using a flux.
SOLDERS AND FLUXES FOR SOFT SOLDERING VARIOUS METALS
|Work to be soldered||Flux||Type of solder|
|Brass, Gilding metal, Copper, Gun metal||Zinc chloride. Paste flux. Resin. Solder paint (contains its own flux).||Any solder can be used but the choice of solder will depend on the type of work being done.|
|Electrical work||Resin cored solder. Resin.||Tin 60%, lead 40%.|
|Galvanised iron, Zinc||Zinc chloride or very weak diluted hydrochloric acid.||Any solder with 40% or more tin.|
|Pewter, Britannia metal||Glycerine to which one or two drops of hydrochloric acid is added. Resin, tallow, resin cored solder.||65% tin, 35% lead or pewter wire. For amature use LMPsolder = tin, leadand bismuth.|
|Lead||Tallow.||Tin 30%, lead 70%.|
|Aluminium||Cannot be soldered in the usual manner but there are many kinds of special aluminium solders and fluxes available. The difficulty is in removing the oxides, which form rapidly, from the surfaces of the metal. This is often done by melting the solder on to the fluxed surface and then scratch brushing underneath the layer of molten solder so that the solder can "take" without the aluminium being exposed to the air.||Proprietary brands usually containing only tin (no lead) with some zinc. Typically 90% tin, 10% zinc.|
|Cast iron||Tinning is slow: afterwards any flux can be used.||50% tin, 50% lead.|
|Chromium steel||Difficult to "tin" except by methods similar to those used on aluminium.||Any solder with 45% tin or more if pre-tinning has been successful.|
Fluxes for Soft Soldering
The fluxes are divided into two groups: active and safe.
1. Zinc chloride, also known as "killed spirit" is popularly used in workshops. It is made by adding scraps of zinc to hydrochloric acid until all the bubbling has stopped. It should be made in an earthenware or lead vessel in a place where the fumes are readily taken away, such as on a window sill. Then it should be filtered and about 50% water added. If necessary it can be cleared by adding a few drops of concentrated hydrochloric acid.
2.Paste fluxes containing either zinc chloride or ammonium chloride in petroleum jelly.
3. Ammonium chloride (Sal ammoniac).
3. Oleic acid.
There are also many proprietary brands available, not all of them good.
Usually solders will "take" much more readily with active fluxes because they are slightly acid and tend to clean the joint by their etching action. The joint should be thoroughly washed after soldering to remove the corrosive flux residue. For this reason active fluxes should never be used on electrical work.
1. Soldering Iron Method
The soldering iron method is the most popular. Copper is used for the bit because it is a very good conductor of heat
A soldering stove is often used to provide the heat for the soldering iron (fig. 5).
The soldering iron is heated in the stove until a green flame shows that it is hot enough. It should now be removed from the flame if not required for immediate use and rested close to the flame but not actually touching it. Oxides form very readily on the heated copper bit so it must be tinned before use. Once it has been tinned properly it should not need re-tinning for a long time. The common fault in schools is overheating. This results in the bit being burned. Directly after each heating the iron should be fluxed and given a fresh coat of solder. It is important to heat the whole of the bit not merely the tip otherwise it will not hold the heat long enough to do a satisfactory job.
Tinning is done by heating the bit in the stove until a green flame shows. Remove from the stove and briskly file the faces with a file which should be kept for this purpose. Don't overdo the filing. Now put the tip into a hollow in a sal ammoniac block in which there are small pieces of solder. By this method the whole of the tip can be covered with an even layer of solder at once. It is now tinned.
When the parts to be soldered have been properly prepared and fluxed the tinned soldering iron is taken from the stove, fluxed and recharged with solder. The flux can be in a small tin lid and the solder in another. The charged iron should be held against the pieces to be soldered for a moment to allow the heat to be transferred from the bit to the work and then gently moved along the joint. Often it is best to use the edge of the bit as this allows more heat to get to the work. Surplus solder can be picked up with the tip of the soldering iron.
Figure 5A shows a small electric soldering iron. Electric soldering irons are usually used for electrical work but can also be used where the ordinary soldering iron is used.
2. Blowpipe Soldering
This is a reliable method, particularly on heavy gauge metal and on large work where a soldering iron would be inadequate. It is not to be recommended for tin plate work because the tin surface can be damaged by the flame.
The method often used is to cut small pieces or panels of solder from the strip which has been hammered and cut as shown (fig. 6).
Lay these small panels of solder along the edges of the prepared joint. Usually killed spirits is used as a flux. The flame, which should be small, must be kept moving. First the flux will bubble and usually in doing so it will displace some of the panels. If any of the panels move too far they can be gently pushed back into place with a thin pointed rod—a pointed rod will not conduct too much heat away from the work (fig- 6A).
When the melting point of the solder is reached, the solder will run into the joint. If the solder does not run into every part of the joint as required, it can be helped by quickly applying a little more flux to the parts where the solder has not run. This is best done with a small piece of cane which has been hammered to make the end fray (fig. 7).
Notice that flat work should never be laid flat on a fire brick or asbestos sheet, if it is it will take too long to heat up because the surface on which it is lying has to be brought up to the. same heat as the rest of the work.
It is a good plan to support it either on a bed of small pieces of broken fire brick, or asbestos cubes, or small pieces of coke. By this means the flame can get under the work. ' Panels of solder need not always be used. The strip of solder which has been suitably hammered at the end can be "fed" into the joint as the required heat is reached (fig. 7).
3. Solder Paint Method
Solder paint is available in small jars and contains its own flux. The parts to be joined should be clean and well fitting. The solder paint is then applied to both pieces before they are positioned. Heat is then applied either with a small flame from the blowpipe or if convenient by pressing the flat face of a soldering iron against them until enough heat is transferred to make the solder paint melt.
By this method the parts to be joined are cleaned and fluxed and each surface is then tinned. These tinned surfaces are then put together and gently heated either by a soldering iron or the blowpipe. Sweating usually works better if slight pressure is applied to the parts to be joined. When a stronger joint than we can make with soft solder is required we use silver solder.
SILVER SOLDERING OR HARD SOLDERING
Silver solder is available in different grades. The temperature at which these melt can vary between 630°C and 830°C. Most silver solders are made from alloying silver copper and zinc. Here are some from the wide range available:
Easy-Flo melts at approx. 630°C
Easy Solder melts at approx. 723°C
Medium melts at approx. 765°C
Hard melts at approx. 778°C
Enamelling melts at approx. 800°C
Fluxes for Silver Solders
Most silver soldering can be done by using borax as a flux. Borax may be bought in lump form (borax cone) (fig. 8) or in powder. If the borax cone is used it is usual to use it in conjunction with the borax palette and a little water. The cone is gently rubbed in the palette into which a few drops of water have been placed. Soon the water will become cloudy and then after a little more rubbing it will have the consistency of thin cream. It is now ready for use as a flux. If powdered borax is used it can be made into a thin paste and then applied.
Other proprietary fluxes in powder form are available. In many cases these are more suitable than borax particularly when the makers of the solder make a flux to be used specifically with the solder.
Silver Soldering Method
1. The parts to be joined must be well fitting and clean.
2.These parts must be fluxed properly particularly where you want the solder to run.
3.The parts must be secured.
4.Heat the work with the torch and when hot enough apply the solder strip. When the work is hot enough the solder will run right along the joint. If panels or filings of solder arc being used, these must be applied before heating commences. Both parts of the work to be joined must be at the same temperature, i.e. they must both be at the melting point of the solder.
5.Do not quench a soldered joint if it can be avoided because this causes undue stresses owing to the different rates of cooling of the parts.
6.Remove binding wire or clips etc. after cooling.
7.Pickle to remove oxides and flux residue. This is done by warming the article in a solution of 1 part sulphuric acid to 10 parts water, i.e. the pickle, in a copper pan. Large articles can be heated to black heat, i.e. not red hot, and then immersed in the pickle which is usually kept in an earthenware vat. Do not leave soldered articles too long in the pickle because it attacks the zinc in the solder and makes the joint porous.
Sometimes it is necessary to silver solder a small part on to a much larger one. Owing to the length of time required to bring the larger part to the right temperature the flux is burnt away at the edges of the joint. This prevents the solder from running. One way to overcome this is to prepare the parts by drilling holes as shown in figure 9.
When the panels of solder and flux are in the hole the boss can be secured in its place with clips. Take care to properly flux the place where you want the solder to run. Now heat the work until the solder melts and you see the fine silver line of molten solder all round the joint. Allow to cool and clean in usual manner.
How Solder Joins Metals
When sufficient heat is applied to the solder it melts and combines at the point of contact with the metals it is joining. This of course applies only to well-fitting, properly cleaned and fluxed metal (fig. 10).
This combining of the solder and the metals to be joined happens not only in soft soldering but also in silver soldering and brazing. The intermetallic compound is usually only a few thousandths of an inch deep.
This is similar to silver soldering: in fact silver soldering is sometimes known as silver brazing. To be precise brazing is the joining of metals with brass. Brass is an alloy of copper and zinc.
A higher temperature is required and a stronger joint is made by brazing than by silver soldering.
The brazing alloy is called brazing spelter. Spelter is obtainable in rod, powder, granulated or in ribbon form. If used in powder or granulated form it is usually mixed with flux and water to make a paste.
There are several grades but these are three popularly used and known as:
Soft 50% copper 50% zinc melts at approx. 870°C
Medium 54% copper 46% zinc melts at approx. 880°C
Hard 60% copper 40% zinc melts at approx. 900 °C
Flux for Brazing
Borax can be used for most purposes but there are several made up powders available.
Prepare as for silver solder. In the school workshop the biggest enemy to brazing is heat loss. This causes the operator to take too long on the work and this in turn causes excessive oxidation. Often this is owing to insufficient care- being taken in packing firebricks or asbestos round the parts to be joined. Also brazing is reserved for the larger pieces which conduct the heat from the joint very readily. Figure 11 shows a well packed joint to be made on a gate frame.
If a brazing rod is being used it helps if the tip of the rod is heated and then dipped in the tin of dry flux. This causes the flux to adhere to the end of the rod. When the parts to be brazed are hot enough, this flux on the end of the rod will help the flow of molten brass when it is touched on to the joint. Remember the heat to melt the rod must come from the work
and not the flame. Important too is the fact that brazing should be done at one heat, i.e. the torch should not be taken off the work once having started until the brazing operation is completed; otherwise excessive oxidation takes place.
For circular work a rotary table is very useful. This can be used for soldering as well as brazing (fig. 11 A).
Cleaning the Joint After Brazing
The makers of the flux will usually give their recommendations. Hot water is often mentioned for the removal of flux residue.
Small work can be boiled in a solution of water and alum.
Figure 12 shows a blowpipe in section. The soft flame shown is used for obtaining an "all over" heat on a piece of work. It is made more fierce (hard flame) by either increasing the air blast or cutting down the gas for the purpose of concentrating the heat on a smaller area.
At the bottom of figure 12 is shown a Davi-jet burner. This is useful for soft soldering particularly on pewter ware. The intensity of the flame is adjusted merely by turning the gas tap.
Electric Arc and Resistance Welding
Arc welding is done by using a coated metallic electrode, which makes an electric arc with the job being welded, or two carbon electrodes. The parts are joined by fusion. The metallic electrode is actually a metal filler rod coated with a fluxing agent. This coating also contains a material which gives off a gas as it burns therefore excluding the atmosphere from the joint being made.
Resistance welding is not unlike the old process of forge welding. A heavy electric current is passed (by means of two electrodes) between the parts to be joined until the fusion temperature is reached. These parts are then held under pressure for a brief moment until fusion at that point is complete.
In this process, unlike silver soldering and brazing, the parent metals are melted. The oxy-acetylene flame melts the edges of the parent metal into pools to which a filler rod of the same material is melted.
Forge welding, or fire welding, uses the blacksmith's hearth. Before a fire-welded joint can be made it is essential to have a clean clinker-free fire and the parts to be joined must be upset and scarfed (fig. 13). The scarf must have rounded faces so that they touch in the centre; thus the molten scale is driven out during hammering. It is usual to have an assistant for fire welding. The parts to be joined are heated to a creamy white heat and removed from the fire and tapped on the edge of the anvil to shake off the dirt. Care must be taken to ensure that the scarfs are in proper relationship to each other before the first blow is struck in the centre of the work. It must now be repeatedly hammered, working from the centre, to drive out the molten scale. It may be necessary to reheat for the weld to be completed.
A properly fire-welded joint cannot be detected from the parent metals. A beginner will do well to start by making a faggot weld as an exercise. This does not require an assistant nor a pair of tongs (fig. 13).
Some skilled blacksmiths can fire weld* without using any flux but others always use a flux. Silver sand, burnt borax, laffite welding plate or a powder sold by The Amalgam Co. of Sheffield can be used.
This is a broad term which includes: bolts, studs, set screws, grub screws, thumb screws, socket head cap screws, Phillip screws etc.
Usually these are used to secure two or more pieces together except where a bolt or screw is used as a hinge pin.
These are as shown. They always have a nut and usually a washer too (fig. 14).
These have no nut (see top of fig. 15).
Grub Screws or set Screws
Often used to secure pulleys to shafts (bottom of fig. 15).
These are used where easy removal is desired (bottom of fig. 15)
They have no head and no nut. They are sometimes used in place of bolts. It is easier to replace a worn stud than to re-drill and tap a hole after the thread is worn or stripped. A good example of the use of studs can be seen on the cylinder block of a car (fig. 16).
They include wing nuts, hexagon-headed nuts and castellated nuts (fig. 16). Lock nuts are often used where vibration might loosen a single nut. A castle or slotted nut secured by a cotter pin is similarly used to prevent loosening by vibration.
Metal parts or plates can be joined by rivets to make a permanent joint or, if a single rivet is used it can be as a pivot or hinge.
Those commonly used are shown in figure 17.
Rivets are classified by the shape of the head, their length, diameter and the metal from which they are made.
The diameter of the rivet used must be related to the thickness of the plates being riveted. In school we often use our own judgement when selecting rivets for a piece of work, but it is common practice when riveting plates between 1/16 " and 1/2 " thick to use a rivet the diameter of which equals twice the thickness of the plates being riveted. Plates thicker than 1/2 " should have a rivet 1 1/2 times the thickness of the plate. When riveting plates of unequal thickness the calculation is based on the thinner plate.
The distance between the centres of two rivets in the same row (pitch) should not be less than 2 X diameter of rivet and
the centre of the rivet should not be closer to the edge of the metal than 1/2 X diameter of rivet. It is usual to have the rivets and the parts to be joined of the same material.
On large work the rivets are often made red hot before being inserted into the hole. This makes a better joint because as the rivet cools it contracts and pulls the plates together. Also the rivet is easier to work as it is more plastic when red hot.
Rivets are often made as spigots on the ends of rods which are hammered over as shown (fig. 17).