Designing in Metal

In comparison with wood, metal is an expensive and difficult material in which to rectify cutting or machining errors. Therefore you should design any metalwork project very carefully before you begin. Designing requires a logical approach and a lot of practical knowledge relating to the project in mind; so learn to apply your mind to the problems before you begin to apply your hands.

Making a brief

A professional designer will be given a brief which sets out the needs of the customer, the cost limits and any other limitations. As a DIY designer you are in the fortunate position of being customer, designer and manufacturer; this means you can draw up your own brief, solve the design problems and make the project. In making a brief you should consider what it is you want, why you want to make it and any limiting factors — including cost, size, appearance and operation of the design. As a customer you may be attracted to a hi-fi stand made out of stainless steel, for example, but as a designer you may discover the cost is prohibitive.

In organizing your brief, you will probably begin to ask questions such as how the project is to work. How it will be used and where, and whether it is to be permanent or temporary. Other questions will include aspects of safety and technical and scientific problems, all requiring further information. The Course will supply most of the information you require; additional information can be obtained from the manufacturers and suppliers of materials and equipment you intend to use. Keep information you collect in a simple file so it is available when you need it and for reference in future projects.

Displays and exhibitions can supply ideas for projects. Some of the most ingenious solutions to design problems are produced by DIY enthusiasts who are often able to reach solutions which would not be commercially practical but which are ideal for the DIY ‘one-off’ design. Friends and colleagues can be helpful if they have completed DIY projects similar to your own.

Child’s swing

This example will highlight some of the design problems you may encounter when considering suitable projects, since it clearly illustrates that simple ideas can present major problems. Some of the designs illustrated would require heavy gauge tubing, the bending and welding of which could be a problem. You will have to determine the width of the uprights and the height of the swing, as well as consider how the structure is to be fixed to the ground to prevent the danger of it toppling over.

Organizing your design

Sketch your design ideas in a notebook and add notes of clarification. No great sketching skill is needed — the important factor is you should be able to understand sketches if you return to them. Often you will find it necessary to return to an idea which was previously rejected. This process is helped if you list your ideas about the design under five headings as follows: Frame or structure Most projects will require a frame; its size is determined by what it is to hold. The load it will have to carry will determine how strong it must be and you will have to think about how to make the structure stable. Its shape will depend upon how and where it is to be used and upon the type of casing or protection it requires.

A frame with a single screw or rivet in each joint is likely to sway or ‘lozenge’ and will need to be braced. Slotted holes instead of small round holes allow for adjustment and will help take up errors. A welded joint gives a flush finish to a frame, while screwed or riveted joints may mean the frame will have overlapping sections — thus affecting its appearance.

Casing and covering

The structure will determine the type of casing in many cases. Covers can be fixed or removable to allow access into the project for repairs or for fitting wires or pipes. You must think how to cover the project to avoid unsightl screw and rivet heads on the exterior. Consideration of such problems may cause you to go back and revise your ideas on the frame.

Assembly and fixing

Welding and brazing make strong and permanent joints; soldering is ideal for making permanent joints in articles of sheet metal. These techniques are covered in the Course. Difficult fixing problems can be overcome if you use adhesives based on epoxy resin.

If you decide to use nuts and bolts, screws or rivets, you will have to choose carefully because their sizes must match the proportions of the work; for example. It would be unwise to use 3mm (Win) screws through a 5mm (-A in) wide metal strip. With wood you can substitute different diameter screws without any serious effect; if you screw metal parts together using pre-drilled holes, any variation in the screw size may lead to the parts not lining up in the way they were intended.

Moving parts Some projects may require moving parts, such as a rotating spit for a barbecue. In this case you will have to consider using simple bearings to support the parts without friction. Basic movements can be combined and modified to produce the required movement; the actual mechanisms may vary but the principles remain the same.

Ensure moving parts are correctly aligned so they will not bind in operation. You should consider how the motion must be transmitted — by shaft, chain wheel, pulley or gear wheel for example. If the project is to be used frequently, consider the effects of wear and vibration on the moving parts. Energy supply Sometimes you may wish to supply energy to the design. This may involve an electric motor, heater or lamp. If designing a display tank for fish, for example, your main consideration will be one of safety — how the supply is to be connected and how the frame is to be earthed properly. Ensure trailing leads and overheating do not present a safety hazard.

Revising the design

Unless you are making a very simple job you may have to revise your design many times as you progress. It might seem better to get on with the job and deal with the problems as they arise in the course of construction. If you do this you will probably have to make just as many ‘changes, but instead of using up paper and pencils you will be likely to waste valuable time and materials.

Models

As you work your way through the design problems it will often be useful to make very simple models from scrap wood and card to help you solve the problems. For example, if you have to join two thin strips of metal without any untidy protrusion, the problem is illustrated well by pushing a pin through two strips of cardboard; this will tell you a screw will protrude in the project — a fact which may not be obvious from a sketch.

Accuracy

If your previous experience has been with wood, you will know it is possible to correct slight errors with, for example, a wood filler. Correcting errors in metal is much more difficult; a series of small errors in drilling bolt holes for a simple frame can make it impossible to assemble. You should check and recheck all your measurements very carefully on paper before beginning to cut or drill the metal.

Errors A good way of discovering faults or errors in your design is to put your sketches away for a few days when they are completed. Later, when you return to them for a final check, you will be more likely to pick up the mistakes.

Using materials

Metals are not bought in the same way as wood. Most timber is on display in the timber yard but metal stockists do not have their metal on display because the variety of shapes and sizes available as standard is very large. Metal is sold according to the shape of the cross section. Most suppliers have a catalogue which lists the sections and sizes available; many catalogues also give a diagram of the section and the weight per metre run (foot run) for each size of section. Some give conversion tables and tables of areas. The standard engineering sections are round, square, hexagon, strip, equal angle, unequal angle, channel, tee, ‘I’ section, half-round, tubular round, tubular square, tubular rectangular, sheet, coiled strip and wire.

It is essential to choose your materials from a catalogue because you cannot afford, in terms of money, to choose oversize material and cut it down to the required sizes. A well designed project should involve the minimum amount of metal removal by cutting, filing or machining. Metal is costed by weight and you will be able to calculate, from the catalogue, the approximate weight of metal in the project. The prices will be higher if you buy nonstandard sections.

The metals you choose should depend on cost, strength, resistance to corrosion and appearance. Aluminium, for example, is a versatile metal, light and easy to work; it has a good appearance but does not have the strength of mild steel. Brass and copper are more expensive, but useful for projects which are intended to be ornamental.

Strength Detailed calculation of stresses in metal structures is too complicated in most DIY projects but fortunately most projects are unlikely to involve very heavy loads. In general, good sense and experience will lead to a suitable choice of metal sections for most jobs, using the basic principles of stress and working out simple calculations. Failure of a structure could lead to damage

to people or property: a badly designed boat trailer or a metal beam intended to support lifting gear in a garage could cause serious damage if they collapse. In such circumstances you should seek professional advice before going ahead.

Shelves and racks made to carry books, records or DIY materials can impose heavy loads on supports and fixing bolts; for example, 100 average sized hardbacks can weigh well over 20kg (441b). A simple and conservative way of calculating a safe and economical choice of metal section is to work to a load of 2kg per square millimetre (2800lb per sq in) of metal supporting the load, if the metal is aluminium. Double this figure for brass and copper; for steel use four times the figure for aluminium.

Sagging

This often occurs when building shelves; use thicker wood for your shelf or build in supports along the span of the shelf. If you double the depth of the section, you can reduce the sag to one eighth of the original. However, when making welded or riveted structures it is not always practical to change the design when you find the shelf sags with the load you place upon it. For any material, sagging depends on four factors: the depth of the material spanning the supports, how securely the span is secured to the supports, the distance between the supports and whether the load on the span is concentrated or spread along the span. All these factors should be considered at the design stage because of the difficulty of making changes later.

Buckling

Long, unsupported struts, particularly in softer materials, are inclined to buckle when subjected to heavy loads. To reduce the risk of buckling, the ends should be securely fixed and horizontal supports should be fixed at intervals. The distance between the supports should be about 20-25 times the diameter of the strut.

Fastenings

Holes drilled for screws or rivets can weaken a metal structure. This is unimportant if it is not to carry a heavy load; but, for example, holes drilled in the cross bar of a child’s swing could lead to the metal cracking and failing around the holes. Where there is any doubt about possible weakness, it is best to weld or braze the joint. If this is not possible, you may have to redesign the joint.

When drilling holes it is usual to stick to some simple rules which help to ensure the joint does not fail. Where the metal is going to be subjected to bending loads (when you bend it in order to make the project), the holes should be drilled, if possible, along the ‘neutral axis’ of the bar or tube. For symmetrically shaped bars and tubes, this is in fact the centre line; along this line the stress in the metal is at its lowest. The spacing of bolt and rivet holes should always be arranged so the distance between hole centres — or hole centre and edge of the metal — is not less than twice the diameter of the hole being drilled.

Corrosion

The most serious consequence of corrosion is that metals lose their strength. The design problem is to decide how complete protection is to be obtained. If the job is assembled before painting, it becomes impossible to protect all the joints from rusting. Sealing compounds may have to be used to prevent moisture entering cavities in the design; rubber or plastic seals and grommets serve the same purpose. These may affect the dimensions, since allowance will have to be made for the seal or grommet. Thus corrosion protection will have to be carefully considered in the initial stages of the design.

Any job made completely from aluminium is more resistant to corrosion than iron or steel, because aluminium combines with oxygen in the air to form a thin oxide coating which tends to prevent further corrosion. But if the aluminium is in contact with iron, steel, copper or brass in a moist atmosphere, there is a very strong tendency for the aluminium to corrode rapidly. This can be a difficult problem since most fasteners are made from steel or brass. If corrosion is likely to be a problem, the aluminium should be painted with a zinc-based primer to prevent contact between the dissimilar metals. Don’t use a lead-based primer since this tends to accelerate aluminium corrosion.

Finishing the design

You will have to decide upon the required finish at an early stage in the design. The final appearance will depend upon the materials chosen and the way in which they are assembled. You must also consider how the finish is to be applied because this may affect the process of assembly; it may even cause you to change your assembly methods. If a part is painted before assembly, it cannot be welded or soldered and some other fastening method is needed. This problem of finishing any but the most simple projects illustrate one of the major differences between working with wood and metal: most designs in wood are painted, varnished or waxed after assembly; if you examine almost any metal object, you will see the finishing processes are carried out at different stages.

Painting parts

Metal parts often have complicated shapes which cannot be easily painted by brushing. Most painting of metal is carried out by dipping small parts or spraying larger parts. This is most easily done by suspending the work on wires during the painting and drying periods. It is often useful to incorporate small holes into a design so the part may be positioned in the best way for painting. When a lot of parts are to be painted or treated against corrosion, you should plan the sequence of jobs so there are no holdups as a result of waiting for parts to dry or harden.

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