R6 Joining techniques

Posted: October 10, 2010 in Resistant Materials

Initial knowledge – 1

Current working knowledge – 3

Prior to undertaking this course I had some knowledge of joining woods and plastics together and the various methods, although I had not actively practiced them for a long time.  One material I had no experience of joining however was metal.  Due to the nature of its design and the fact I was going to make my plate storage unit out of aluminium I needed to research various methods of joining metals, to fully understand the different characteristics, strengths and finishes I could get from the various techniques of joining the two together.  As part of my research I included a section on the different ways of joining metals:

There are many different ways of joining metals together, joints can be either permanent or temporary. Permanent joints can be made by burning, autogeneous welding, ordinary welding, brazing, silver soldering, soft soldering, folding or riveting. Temporary joints are screwing, bolting, wedging, taper fitting, force and shrink fitting. Although brazing, silver soldering and riveting are classed as permanent joints they can be taken apart again with care.

Burning

To burn a joint together, 2 pieces of metal are placed in position and fixed so that a stream of molten metal runs between the joint until the ends to be joined are the same heat. The molten metal that has been flowing to waste is then checked, the joint is filled up and left to cool. The joint is usually then placed in sand to cool and when it is any lumps are trimmed off leaving the metal in one piece without any sign of a joint. Almost all metals can be joined in this way.

Autogenous Welding

Autogenous welding, or thermit welding is done by placing aluminium and oxide of iron in a crucible which has a hole at the bottom fitted with a fusible plug. The crucible is place in position over the ends to be joined, and the ends are enclosed in a mould which has an outlet that can be closed after sufficient metal has passed through to heat the joint. When everything is ready the fusing action is started by lighting magnesium powder. The heat generated by the chemical action reduces the iron oxide to a fluid mass of iron which melts the fusible plug, allowing the metal to enter and fill the mould. This is called aluminothermy.

autogenous welding

Blowpipe Welding

Blowpipe Welding is carried out by means of various gases, such as acetylene, oxygen and hydrogen. The gases are under pressure and drawn from cylinders. The mixing of them takes place in the mixing chamber of the special blowpipes which are used. The edges of the metals to be joined are filed clean and bevelled, and while the fusion of the edges is taking place the actual joint is fed with a metal of the kind that is being welded, in the form of rod, to fill up the spaces in the joint. The advantages of this method are that the appliances are portable, so that welds can be made”in situ” and the heat is localized. It is quite a hard technique and the finish of the weld depends on the skill of the welder.

Electric Welding

Electric Welding is very suitable for repetition work, it is quick and dissimilar metals can be welded together, however, special plant and fittings are required for each individual job. The pieces that are to be welded are held one by each arm or guide, and are insulated from one another. The electric current is turned on and the ends are brought together but not quite touching. This causes an electric arc to form at the ends to be welded and they heat from the centre to the outside. When it is at the right temperature, the ends are forced together either by hand or hydraulic power.

electric welding

Ordinary welding

Ordinary welding is done by thickening up or up-setting the ends to be joined to allow for their reduction by hammering, then raising the pieces to a welding heat, laying one on top of the other and hammering them together. The work is heated in a coal or coke fire assisted by a blast of air. There are different types of welds such as scarf, tongue, split and jump.

Brazing and soldering

Brazing and soldering consists of joining metals by means of alloys, which are heated and fused together with the edges of the work, so that they alloy with the metals being joined. Important things to remember are that the joints must be cleaned, a flux must be used and the solder or alloy used must melt before the material that is meant to be joined. A coal gas blowpipe flame assisted by an air blast is usually, but a forge fire or spirit blow lamp can often serve the same purpose. When brazing or silver soldering, the joint should be clean and the edges fitting together. When fitted, the parts should be held together by ties of iron binding wire, when the joint is difficult to adjust one or more iron dowel pins should be used. These with the wire ties prevent any shifting of the joint during the progress of the operation. The work should now be carefully adjusted on the bed of the forge, care being taken to support it so that no strain is on the tied joint. When arranging the coke around joint to conserve the heat, place it so that all the parts are open to observation, otherwise while the solder is fusing, some adjoining part may be melted. The difference of the melting temperatures being very slight, spelter should be mixed with water and powdered borax in a small pot. When the work is in position the joint should be moistened with the borax water and the joint charged, i.e. just sufficient of the moistened spelter laid on the joint to fill it when fused. The work should now be heated in the vicinity of the joint, and when there are unequal thicknesses the heaviest parts should be heated first, gradually bringing the heat up to the joint watching that both sections receive the same amount of heat, otherwise it will not be a good joint. When a red heat is reached the borax will melt, any dry powdered borax should now be dusted on from time to time to prevent the solder perishing or being converted into oxide. As the heat increases the flame should be localized over the solder and a fiercer blast given until the spelter fuses and flows into and fills up the joint. When this point is reached the flame needs to be removed and the work left to cool. Soft Soldering This consists of cleaning the parts to be soldered applying a suitable flux, heating the metal, and applying just enough solder to make the joint. This is done using a soldering iron, spirit lamp, gas blowpipe or some kind of heat that is enough to melt the solder. When using a soldering iron, it must be well tinned, in other words the iron must be heated, filed clean rubbed in the flux and the point covered with solder. Sometimes it is advantageous to tin the parts to be soldered first.


brazing

Riveting

The rivet is selected according to the thickness of the metal to be riveted, the holes of the rivet are then either punched or drilled, the rivet is placed in position and the metal set down with the rivet set. Sufficient length of rivet is left to project through the metal and form the other head. The end or tail of the rivet should be flat, the tail should be burred over with a light hammer and finished to shape, sometimes using a cup tool.

Screwing 

This is simply joining metal together with screws although for permanent joints it is not the best option as they may work loose. Screws can be made of iron, brass, bronze or steel and have many various shapes including half round heads, cheese heads and countersunk heads. All screws used for metal are known as tapped screws or metal thread screws to distinguish them from those used with wood.

Bolting and Wedging

This is used mostly for fixing together various parts of machines. Bolts are now usually made to the Whitworth standard. The size of the head bears a fixed relation to the diameter of the bolt. There is a large variety in use, such as hexagon, ball, cheese, coach, deck, square, countersunk, etc, with the name applying generally to the shape of the head. When an extra long bolt is required a bolt end is welded to a length of rod, as this is more convenient. Where there is a lot of vibration lock nuts have to be used because ordinary nuts work loose. Of these there are many forms. Wedging, keying, and taper fits are due to factional contact, and in the fixing of nearly all wheels and pulleys this property is utilized as well as in what are known as friction clutches. A Force Fit is where the pin is made very slightly larger than the hole and the two parts are forced together. An example of this is the lathe pulley

Bolting

I have included some rivets in my plate rack design.  However I have no experience of using heat methods on metal, this will be something I try to address in the forthcoming year.

Drilling the holes for my first attempt at rivets

I am aware of the various different types of wooden joints used in carpentry.  I have made simple lap and butt joints before, and also dovetails over a decade a go.  I appreciate the aesthetics of the dovetail joint and so to test myself I thought I would throw myself in at the deep end.  I began marking the joint out using a steel rule, a marking gauge and knife, a carpenter’s square and a pencil.  Once I had measured an even amount of dovetails from the centre point of the wood, I placed the piece in a vice and began sawing down the lines using a fine bladed dovetail saw. I then removed the excess waste with the aid of a coping saw, trying to ensure not go into my edge. Having removed the majority of the waste  I needed to chisel out some of the excess wood.  This is where I admittedly occurred some problems.  The chisel I used was not sharp enough and having made my joint in pine it began to take chunks out of the wood and did not give me the clean finish my fairly even dovetails had warranted.  Even though I used a thick piece of flat wood to control the straightness of my chisel I went over what was to be my edge and when I placed the two pieces of wood  together there were gaps in between the joint and they could be pulled apart easily, which is not what I wanted as dovetails are renowned for their strength as well as being pleasing to the eye.  I was shown how to sharpen a chisel by a classmate and went about remarking my joints this time using MDF.  I was slightly worried because I was aware that MDF may crumble like the pine, but I felt as a practise exercise and with a newly sharpened chisel it would be easier to remove any excess wood I encountered and if I had success I could make a complete box as practise and perhaps use it later down the line as a prototype for an automative toy.  I found the MDF a lot easier to remove and after some PVA gluing and quite a number of clamps I  finished my box gaining more experience making dovetail joints.  I now will look to use a different wood such as a nice hardwood to improve on my results and also use it as an opportunity to explore wood finishings.  An idea might be to attempt to make a box using four different types of joint and perhaps four different types of finish on each side to use as a visual aide I can refer too when teaching.

I used 3 different types of joint in my space saving unit.  The most commonly used was the mortise and tenon.  I did attempt to make the mortises by hand using a drill and chisel.  But the fact there was so many of them meant that it was more time efficient to use the mortiser.  This way I could keep all the mortises the same size and depth.

I chose to cut the tenons by hand using the tenon saw as I felt that i could get a more accurate result and size them perfectly to fit in the corresponding mortise.

 I used dovetails to fit the front of the drawers.  These turned out much better than previous attempts so I was pleased that I’d had all the practice before hand!

The final joint I used was a simple lap joint for the backs of the drawers cut using a tenon saw and chisel.

These are the other types of joint and there can be different variations of each type

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