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Warning to prospective do-it-yourselfers.
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Investment (Precision) Casting.
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FABRICATION, Repairs, JOINING AND FINISHING..
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METALWORK METHODS AND TECHNIQUES.
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Hollowing or Blocking.
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An understanding of casting methods
and finishing processes is essential if the quality of cast items is to
be assessed. A good casting is a delight to look at. A poor one gives
a bad attack of the shudders!
Casting molten metal into a shaped
mould can be a relatively easy but : -
Examples of casting defects showing
poor foundry practice include: -
- Poor surface detail caused by
an inadequate pattern or moulding material.
- Shrinkage cracks.
- Evidence of mould parting lines
on surfaces that are intended for viewing.
- Evidence of poor surface
finishing, especially showing remains of feeders, coarse scratches
left after fettling and details removed by over-zealous polishing.
- Rough, crazed surface caused by
metal-mould reaction because the metal was poured too hot.
- Metal flow marks left on the
‘good’ surface during diecasting, possibly showing evidence of die
- Inclusions, usually of oxide
dross from the top of the liquid metal.
However, while an expensive art
casting should be nothing less than impeccable, do not expect perfection
in a casting intended to be a cheap and cheerful souvenir!
to prospective do-it-yourselfers.
Copper and brass melt at very
high temperatures not normally attainable in the home. Casting molten
metal can be very dangerous and should not be undertaken by the
inexperienced. Full safety precautions are needed and approved safety
equipment is needed. A special problem when melting brass is the zinc
oxide fume given off if the molten metal gets too hot. If breathed in,
this can give a severe headache and an attack of ‘The Zinc Staggers’.
While it is possible to set up to cast brass in a well equipped
workshop, full information and should be obtained and expert advice
sought and followed.
Casting involves the pouring of
liquid metal into a mould so that when the liquid solidifies, the metal
is in the shape required. The melting and casting procedures are only
part of a number of things to be done to produce castings successfully,
hence the art, craft and profession of the foundryman. Casting is also
essential before a metal can be fabricated but that is not covered here.
There are many casting processes now
available for the manufacture of items that are cast to shape. (Ref:-
‘Copper and Copper Alloy Castings’, E C Mantle & V Callcut,
CDA Publication TN42)
Selection of the casting process is
dependent both on technical considerations, on economics and of course
the available equipment. There are five main types of casting process,
divided into those using bonded refractory particles, usually sand, to
make moulds which are destroyed with each cast and those employing
permanent moulds of cast iron, steel or graphite. The permanent moulds
cool the metal faster than refractory moulds and this generally results
in castings having better mechanical properties.
Craftsmen from the Bronze Age
onwards until comparatively recently relied on only two methods, both
casting in to bonded refractory sand (or clay) moulds. The simplest
technique was to pour liquid metal in to a mould made by hand in the
required shape. This could be open at the top to give a shaped casting
with one flat surface, like a jelly mould, or formed of two parts
clamped shut in order to give a shape to all the item. An early
improvement used especially for small parts with intricate detail was
lost wax casting.
Most castings are made using sand
casting techniques, this still being the lowest cost method for small
scale production. Patterns of the shape required are made of wood and
placed in moulding boxes. Complex shapes may be made of two or more
sections to help the moulding with the parting line set to suit maximum
dimensions of the casting. The moulds are made from sand bonded with
clays or silicates or various organic mixes. This is poured over the
pattern and rammed home hard and evenly to take its shape. If half of a
component is moulded in one box, this is then turned over, the second
half of the pattern keyed in place over it and another moulding box
placed on top.
After moulding, the boxes are split
to allow removal of the pattern and replaced together for pouring. A
pouring basin and downspout are impressed in the sand to run a
non-turbulent flow of the metal in to the mould. Holes are also made
for vents and risers. The vents allow air to escape as the metal level
increases and risers let metal flow through the casting and upwards to
clear any dross that may otherwise be left to spoil the casting. Some
of the liquid metal in the risers sinks back in to the mould to feed the
casting by making up for shrinkage on solidification. Provided that the
running, gating and riser system has been well designed, the casting
will be sound and free from shrinkage cracks.
Cores are used to define the inside
shape of hollow castings and are also of sand usually bonded with
organic mixes or silicates.
After the casting has solidified it
is knocked out of the mould. The sand bond is more or less destroyed by
heat during casting and the sand may be recovered for recycling. It is
now necessary to cut off the runner, vents and risers and fettle the
casting ready for sale. Careful cleaning off of mould parting lines
without loss of detail and good attention to casting details and metal
finishing now marks the visible difference between castings made with
pride and those made only at lowest cost.
Longer runs of sand castings are now
automated with the use of a machine moulding line. Metal patterns and
core boxes are usual for longer runs of small to medium sized castings
with the patterns being fixed to metal plates for machine moulding. It
is often possible to mount replicate patterns on a single plate so
increasing output for a comparatively small increase in setting-up cost.
This is a form of sand casting in
which the moulding medium is coated with a thermosetting resin for
better bonding. It requires special equipment. The coated sand is
allowed to fall onto heated metal pattern plates. After a short dwell
time these are inverted, allowing excess sand to fall away and leave a
shell of sand adhering to the pattern. Baking to harden the resin
results in a strong ‘biscuit’, easily handled and stored, from which
moulds are assembled. Shell cores, made by essentially the same process,
are frequently used with ordinary sand castings. Mould making is
virtually automatic, requiring low skill levels and the moulds are
highly reproducible. The process is used for the economic manufacture
of long runs of brass components used in the electrical and general
engineering industries, some of which may end up in the home.
Investment casting by the "lost wax"
or ‘Cirè perdue’ process has been used for centuries for casting statues
and other art castings and this is still its main outlet in the bronze
foundry industry. It is also a process used widely where complex, high
precision components are required as it offers the opportunity to make
by a single casting an object that might otherwise entail difficult and
expensive machining or a welded or brazed assembly.
The process uses a pattern of wax or
other low melting point material which has itself to be 'cast' in a
mould, although it may be built up from several parts. The pattern is
then dip-coated (invested) with a refractory slurry, several coatings
being applied to build up a shell of adequate strength. The wax is then
melted out of this shell which is baked to strengthen the bonding of the
refractory, remove moisture and burn out the last of the wax. It is
then ready for casting. It is also now possible to make up the patterns
in foam plastic that does not have to be burnt out before pouring.
The casting then has no joint line
as the pattern is melted out rather than being withdrawn from the mould
and there is little restriction on shape or complexity of the casting.
Dimensional accuracy is high and the surface finish is as fine as that
of the pattern. Very accurate coring is possible to give precise
location of inserts or holes.
'One off' castings such as statuary
are best made by this process but it is also well suited to the
production of multiples of small engineering castings all joined to a
single casting sprue. In the latter case it is often good practice to
spin the mould (in a safe enclosure) as the metal is poured so that
centrifugal force drives the liquid metal into every nook and cranny.
Besides artists, modellers benefit
greatly from this process as they can obtain finely detailed components
to help complete intricate brass models. The manufacture of brass and
woodwind instruments also benefits because one casting for a valve
actuator can replace an assembly that might have to be hard soldered
from many individual fabricated pieces.
Statuary castings are made in
conveniently sized sections that are then welded together and carefully
smoothed so that the joints do not show. The casting is then subject to
an expensive patination process to give the expected and much-admired
For long production runs it is
frequently economic to make permanent moulds from which many castings
can be made. The moulds are usually of metal and the liquid metal may
either be poured by chill casting in to simple moulds or gravity
diecasting in to shaped moulds, or forced in under pressure.
In this process the metal for the
castings is pored into in metal moulds. The dies, usually made of
steel, are constructed in two parts which open to allow extraction of
the casting. Gravity die casting is most readily applied to solid
shapes, but many quite complex hollow components are manufactured using
either withdrawable metal cores or sand or shell cores. Good
reproducibility of dimensions with generally closer tolerances than for
sand casting. Good surface finish although there may sometimes be
obvious flow lines. Castings have higher tensile properties and
hardness than sand casting. Most suitable for large quantity production
from 1,000 castings upward. Taps and other plumbing fittings are
frequently made in this way. If used to produce decorative items, care
should be taken that metal flow lines are not obvious on the finished
In this process molten metal is
injected in to steel dies under high pressure. Machines with locking
forces of up to 500 tonnes are used to create the casting. Multi-cavity
dies are used to give high rates of production which in turn reduces
costs. The process can produce complex shapes with very thin sections,
thus reducing the need for machining. Automated production gives a
consistent product and a thin section of 1.5mm can be cast to close
tolerances and excellent surface finish, saving metal costs and weight.
Near net shape can be achieved needing little machining which reduces
costs. A full range of brasses can be cast, including the high copper
alloys. This is only suited to long run production but may be the
cheapest method for items such as attractive cast brass switchplates for
domestic electrical equipment.
Diecastings are ejected from the
mould by hydraulically operated pins. On the rear of diecastings the
location marks for these are usually visible giving a very clear
indication that the component has not been sand cast many years ago.
Chill casting is a name given to a
gravity diecasting process used for producing simple regular shapes,
usually solid or hollow rods or thick walled tubes. It is mainly used
for casting phosphor bronzes, leaded bronzes and gunmetals, the end
products being bearings, valve spindles and the like. To some extent
the process has been superseded by continuous casting.
Centrifugal casting is used
extensively for producing discs or rings for the manufacture of gears
and worm wheels, for inserts for valve seats and for flanges for pipe
fitting. Thick walled cylinders may also be cast and these can be used
as pipes, but are often divided into shorter lengths for bearings, valve
seats, electric motor slip rings and so on. These are the chief
applications, but the process can be applied to other more or less
symmetrical shapes. Lugs or bosses may be included and flanges can be
The process consists of pouring
metal into a mould or die usually of steel rotating at fairly high
speed. For unusual shapes or sizes, sand moulds may be used and to
improve the mechanical properties chills are often inserted round the
periphery. Rings or discs are produced with the die spinning on a
vertical axis, while it rotates on a horizontal axis for cylindrical
shapes. While mainly used for heavier industrial products, advantage is
taken of the centrifugal process on the small scale for precision
casting as mentioned.
The process is limited to the
production of rods, sections and hollows from which a wide range of
bearings, thrust washers and gears may be machined. The bar is also
used for valve spindles and similar articles. Most copper casting
alloys can be continuously cast but the preference is in phosphor
bronzes, leaded bronzes and gunmetals that are needed for bearings but
cannot be hot worked to size by extrusion or hot rolling. In the
process, metal flows into a graphite die of the required shape which is
attached to a holding furnace. The die is sleeved with a water cooled
jacket. As the metal passes through the die it solidifies and is
withdrawn as a solid product usually by rollers. It is then cut to
convenient lengths for handling, typically 3 or 4 metres
‘Many persons hold the opinion
that the metal industries are fortuitous and that the occupation is one
or sordid toil and altogether a kind of business requiring not so much
skill as labour. But as for myself, when I reflect carefully upon its
special points one by one, it appears to be far otherwise.’
(Georgius Agricola, ‘ De Re Metallica’, 1550)
The appreciation of the quality of
collected items is assisted by an understanding of the craftsmanship
that has been put in to the making of them. This applies whether they
are made by hand or produced in quantity. This section gives details of
some of the basic manufacturing techniques. The information may also be
of use to those contemplating repairs or other copperwork.
Fabrication is a term used to
describe the working of metal to the required form or shape needed.
Semi-fabricated shapes (or ‘semis) are used as the basis for the final
machining, joining and other operations needed to finish a product.
‘Semis’ may be produced by hot or cold working or a mixture of both.
The way that they are made naturally affects the properties of the
finished article. Anyone making products needs to ensure that the metal
that they start with will give satisfactory properties in the finished
product so that it is fit for purpose.
Metal is usually more malleable when
hot because the crystals of which it is made reform easily while it is
being worked. Hot working is used to break up the coarse grain
structure present in cast billets, cakes, slabs or ingots and to get
metal cheaply towards near-finished size. Metal oxidises when hot, so
the surface finish of hot worked metal is frequently dark brown or black
with oxide. Quenching metal in water after hot working is useful in
loosening oxide and making subsequent cleaning easier.
working is used to fabricate metal to final shape. The metal hardens as
it is worked and this leaves it in a strong, springy condition ideal for
light, durable products. If metal become too hard to work, it can be
softened again by annealing at a temperature that will allow
recrystalisation, usually a dull red heat for copper and a little lower
for brass. Quenching can again be used to help oxide removal but does
not usually affect the properties since most common coppers and copper
alloys are not heat treatable.
Metal may be deformed heavily during
final shaping, as occurs in the production of hollowware. With brass in
particular, this strain hardening will still be present in the final
product and may lead to ‘stress cracking’ (or ‘stress corrosion’ or
‘season cracking’) in service in conditions that may be only very mildly
aggressive. This can usually be prevented by a stress-relief anneal.
This involves heating the shapes to a moderate temperature, less than
the recrystalisation temperature, so that internal stresses in the
crystals are released but the metal is not noticeably softened.
Below are the most important
techniques used to produce hand hammered metalwork, generally starting
with an annealed blank of metal.
Repousée is a form of decoration
where a design is made by marking from the back of the blank with tools.
The technique was used in the school with copper, brass, silver and
steel. The blank, with the exception of steel, is first softened using
heat. It is then held on a bed of warmed, sticky pitch. The pitch
supports the metal and yet is soft enough to receive the impressions
formed by the tools. The three main tools used are the round raising
punch for the main design, the tracing punch for `chasing' outlines from
the front of the piece, and the finishing punch for smoothing down the
background of the design.
This is a simple method for making
shallow dishes or bowls. The metal is beaten with a mallet or hammer
over a bowl-shaped depression in a wooden block or sand pad. The metal
is stretched and thinned, so this method is limited to making shallow
items. As soon as the bowl approaches the required shape it can be
checked with a template made from thin card or tin-plate. When finished,
the bowl is annealed, pickled and scoured thoroughly in readiness for
Sinking is the process by which a
plate or tray with a flat rim is made from a blank. The blank is held
firmly and the metal struck with a hammer inside a line which is marked
on the rim.
This method allows all sorts of
shapes to be made from the blank. Raising is preferred to other
techniques because it avoids the thinning or stretching of the metal. In
this method the surface of the metal blank is `pushed' using a hammer
along the guide lines. In this way, the edge of blank is progressively
reduced during each course of hammering, the bowl or pot being literally
`raised' from the disc.
When a piece of work is finished it
is ready for planishing. This process of gentle hammering corrects
small irregularities, and hardens the surface of the metal.
main processes are
Hot rolling. Power for turning the rolls originally came from water
mills in the early 18th century, then steam engines and
is now all electric.
Extrusion (forcing hot metal through a die), a process invented by
Alexander Dick in the late 19th century.
Forging (hammering between open dies to produce simple shapes such
as blocks, discs, shafts and rings - hollow forgings can be produced
with the use of loose tooling/formers)
Stamping (a near-net shape process involving forging between shaped,
Piercing, a process used to make holes through round billets so that
they can then be drawn to tubes.