Page 75 - Welding of Aluminium and its Alloys

Preparation for welding 65

blades, 3500mpm for tungsten-tipped blades and 1800mpm 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 rec-
ommended. 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 8500rpm. 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 non-
loading 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 finish-
ing 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
500mm in length enable dressing operations to be carried out when access
is very restricted.
Most machining and grinding operations can be carried out without lubri-
cation. 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. Cross-
contamination 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