2. TOOLS
Given access to a complete metal/machine shop and a carpentry shop, the required tools can
be made in almost any country. It is advisable, however, to have each of the machined tools
made from an example, so if the factory is to be in an area without examples, it may be
necessary to import the first of some of these tools. Second-hand equipment can often be
found in Europe for only a few hundred pounds sterling, and is completely serviceable.
2.1 PUGMILL
The Pugmill takes coarse aged clay, of correct moisture content and clay blend, and extrudes
finely blended, workable clay.
The Process
Before the clay enters the pugmill all sticks, roots and debris are removed. The clay blend
should be such that any lkg lump going into the mill is quite close to the correct proportion of
each clay type and the correct water content (around 22-27 per cent by weight). If the clay
has hard pea-sized lumps, the pugmill will not prepare it adequately (see Raw Clay Handling,
p 23). The feed clay must be well softened and broken down. Getting it into this state
requires time and weathering. A simple pugmill extrudes well blended clay without major air
pockets. While some minor bubbles may remain, these are eliminated in the pressing process.
More expensive 'de-airing' pugmills are widely available as well, but are not required. They
are more delicate in operation, much more expensive, and require more power.
A pugmill with an output of 80kg/min. is adequate to keep two presses busy for an output of
up to 7,000 tiles per day. This mill will require at least 15hp to drive it, which can be
provided by electric, diesel, or petrol engines. In Sri Lanka retired diesel tractors are often
used and can last for more than 10 years of continuous operation. The wheels are replaced by
belt drives going to turn the pugmill's gears and auger.
2.2 THE FLYWHEEL PRESS
The flywheel press is an invention of the 19th century which is still useful and appropriate.
Variations of the concept come in all sizes, from three person operations to electrically driven
powered presses capable of pressures up to 60 tons per square inch! The principle of the
flywheel press is straightforward; a massive wheel is horizontally attached on top of a coarsely
threaded shaft, which is attached to the top half of a die set. First the wheel is turned, thus
raising the wheel and top die. Then the bottom die is slipped under the press with more than
enough clay to produce the desired tile. The wheel is spun quickly down onto the clay. The
tremendous momentum of the flywheel is converted into downward thrust by the threaded
shaft, and the clay is quickly pressed into all points of the mould, with excess clay being
ejected through holes in the extremes of the mould, and from between the dies.