Although the methods mentioned above can be applied to many fabricating activities, mechanical cutting techniques are used by most welding workshops as being the most cost-effective and versatile method. Cutting and machining equipment is freely available in most fabrication shops and is frequently less capital intensive than the sophisticated laser or plasma cutting systems discussed above. Furthermore, the systems described in Sections 4.2,4.3 and 4.4 are capable of straight or simple bevel cuts only - if double bevel preparations are required then two or more cuts are necessary and J-preparations are not feasible. Edge preparations can be produced in a number of ways such as high-speed milling machines, edge planers, routers and various types of saws. Where air-powered equipment is used care needs to be taken to ensure that the air supply is clean, dry and oil-free to prevent contamination of the surfaces, which would give rise to porosity during welding.
Routers, planers and edge millers are capable of producing J - and U - preparations when fitted with the correct shape of tools. The equipment for these tasks can be hand-held and similar to that used for wood working, the only requirement being the need for slightly greater power or floor mounts for greater capacity. High cutting speeds can be used without the need for lubricants or coolants, although this does not remove the need for thorough cleaning. Hand-held rotary cutting machines are ideally suited to back-gouging and for removing excess weld metal. The depth of cut can be adjusted and various cutter forms are available, including V-blades for bevelling and flat blades for weld cap removal.
The guillotine can be used to shear sheets of up to 6 mm thickness without the need for further preparation work. Over this thickness some dressing of the sheared edges is necessary if the best weld quality is to be achieved. Shearing of the edges of alloys containing more than 3.5% Mg is not recommended if the edges are to enter service ‘as sheared’ because of the risk of the work-hardened edges suffering from stress corrosion cracking. Edges that are welded after shearing do not suffer from this problem.
Sawing is a very effective method of cutting and bevelling aluminium using either portable or floor-mounted equipment. To achieve a good quality cut high cutting speeds are necessary, around 2500 metres per minute (mpm) peripheral surface speed for high-speed steel circular saw
blades, 3500 mpm for tungsten-tipped blades and 1800 mpm for band saw blades. It is usual for band saw blades to have wider spacing on the teeth than for steel to prevent snagging, 8 to 16 teeth per centimetre being recommended. Band and circular saws can produce straight cuts and, when equipped with a tilting table, bevelled cuts. The saw cut surface tends to be rough and readily traps grease and dirt, making cleaning and degreasing difficult. It is recommended that the sawn faces are milled or filed to produce a smooth surface when the best quality of weld is required.
Grinding is best performed with high-speed, semi-flexible aluminium oxide grinding discs. Care needs to be taken to ensure that the grinding is controlled and is not heavy handed. Over-enthusiastic grinding can give a torn and rough surface which will be difficult to clean. Material may also be smeared over the surface, physically trapping dirt and grease and giving rise to porosity on welding. Rotational speed of the discs needs to be in the region of 8500 rpm. Care should be taken that the grinding machines are capable of maintaining these speeds when under load - some machines are incapable of attaining or maintaining these speeds in operation. Grit sizes range from 24 to 120 and the discs selected should be of the nonloading type. Under these conditions the discs should not become clogged and the speed of metal removal should not be affected. Grinding can be used to clean the weld preparation prior to welding, to blend the weld into the parent metal, to remove excess weld metal and to back-grind a partially penetrated weld to sound metal. To achieve the best results this requires suitable and well-maintained equipment operated by trained personnel.
Hand-held abrasive belt sanders are readily available and enable finishing operations to be carried out without too great a risk of damage owing to incorrect manipulation of the sander. Belt widths of 3-100mm can be purchased; the narrow belts in conjunction with sander arms of up to 500 mm in length enable dressing operations to be carried out when access is very restricted.
Most machining and grinding operations can be carried out without lubrication. Dust may therefore be a problem and operators may need to be equipped with dust masks or respirators and the equipment with dust collectors. Noise can also be a problem and ear defenders will be needed for some of the machining and grinding tasks.
One last but very important point to be made before ending this section on the cutting and machining of weld preparations is that the equipment must not be used on aluminium if it has been used on other metals. Crosscontamination of aluminium with copper, iron, etc., may result in welding or service problems. Wire brushes, grinding discs, cutters and milling heads must only be used on aluminium and should be identified as such if there is any possibility of cross-contamination. Machining equipment should be
4.9 Typical pickling shop. Courtesy of R. Andrews.
thoroughly cleaned of any foreign metals before being used on aluminium alloys.