We have used five different sawmills since
we began the Lisnavagh Timber Project in 2001. These include mobile
& static saws and circular & band saws.
Mobile sawmills are literally mobile in the
sense that they can be hitched up to the back of a car and set up
almost anywhere vaguely level, including in woodland, whereas static
sawmills are usually housed in a building and are not mobile (although
they may have a greater capacity).
Circular saws have a the teeth mounted on
a round spinning disc, whereas band saws have teeth on one edge
of a metal band.
Mobile Band Saws
Over
the years, we have found the Wood Mizer sawmill very suitable for
most of what we saw here at Lisnavagh. We do not own a sawmill any
more, but even when we did this sort of sawmill was far quicker,
safer and more flexible than any of the other sawmills we used.
This is partly down to the contractor who (so far at least!) has
been able to work for us (for a week at a time) at a couple of weeks
notice.
On arrival, it takes about 15 minutes to
set up the Wood Mizer and start sawmilling. Depending on the type
of logs and cuts required, this sawmill is able to achieve over
200 cubic feet of sawn timber in a day. At the time of writing,
we have just finished a week of sawmilling at least 40 tonnes of
logs which produced 817 cu ft of boards (Average = 8+ tonnes of
logs, or 163 cu ft of timber per day).
The maximum diameter of a log that the Wood
Mizer can cope with is 30" (approx. 760mm). This is not usually
a problem, but sometimes some of the most prized logs are bigger
than this. Large logs (from old trees, which can contain amazing
features in the timber) can be too big for this sawmill. There are
two options: Find a bigger sawmill, or split the log into manageable
sections with a very large chainsaw. We have done both and find
the first option expensive and the second option is wasteful of
good timber - so neither is satisfactory! It is unusual, however,
but other options are being explored for these extreme cases.
From our point of view, this sawmill is simple
and yet very capable. This is not only down to it's design, but
also the experience of the operator. The Wood Mizer is our current
choice for a sawmill.
We
have also used a larger mobile band saw. This is a more expensive
machine to operate, and therefore to hire, but can handle logs of
a diameter up to about 40".
The blade's cut is usually only about 2 or
3mm, so there is little sawdust or wasted timber. With both of these
bandsaws, the blade is mounted and cuts horizontally. The blade
has to be sharp though. A blunt blade will tend to ride up over
any knots or other hard features and the boards will be uneven in
thickness.
Static Band
Saw
When
the Lisnavagh Timber Project was first started in 2001, we made
use of a Stenner 48 static bandsaw. This can cut logs up to 4 feet
in diameter, which is considerable.
Unlike the mobile sawmills, the Stenner's
blade cuts vertically. It has much wider band than the mobile sawmills,
which reduces the tendency for the blade (especially when getting
blunt) to wander.
The
operator of the sawmill is very experienced with hardwoods, and
was therefore a very useful source of advice to us in the early
days.
Mobile Circular
Saw
The
Lucas Mill was invented in Australia where it became very popular.
It is very mobile and can be set up pretty much anywhere, so long
as the ground is fairly level.
It has a circular blade which can be rotated
through 90 degrees to cut vertically or horizontally. The width
of the boards that can be cut is determined by the radius of the
blade.
The
Lucas Mill could be extremely useful for certain applications, and
particularly dissecting large hardwood logs.
Static Circular
Saw
This
is our own sawmill at Lisnavagh - or at least it was until 2003
when it was decommissioned and sold to an enthusiastic collector.
We believe that the sawmill was set up in
the very early 1900s. The 6hp Blackstone Engine, which powered the
sawmill and which was still working perfectly when it was dismantled,
was bought and installed in about 1930 and it was a wonderful sight
to see it in action. It was a single oil engine with a 6 ft fly
wheel.
This
was a very old sawmill but it worked extremely well. There were
two circular saws - one big and one small - for different scales
of sawing. Also within the building were a band saw, planer, mortiser
and various other items that were also powered by the Blackstone.
One disadvantage of circular saws over band
saws is the amount of sawdust produced. The cut is about 1/4 inch
wide, so for every four 1" thick boards that are sawn, the
equivalent of one board has been turned into sawdust. This applies
to the Lucas Mill (see above) as well.
The production rate was quite slow - in fact
it was the slowest of the five sawmills that we have tried since
2001. So, even when we had our own sawmill, it was more economical
to hire in the Wood Mizer!
Measuring & Grading
When
the boards come off the sawmill, we measure and grade them - every
single one. We also give the boards a unique reference number which
is written onto the board.
Notes of each board's width, thickness, length,
grade, type of cut, the log/tree that it came from and any other
useful details are all listed against the reference number and later
entered onto the database.
Air Drying
Timber
needs to dry before it can be used for most purposes. This is because,
as timber dries out, it shrinks slightly and can twist or warp.
(Obviously it would be disastrous to use wet timber in a piece of
furniture, or in fact anything that uses any joints of timber, as
the timber will dry out, shrink and cause cracks to appear.) The
first stage is usually to air dry the boards in a well ventilated
shed, or covered area.
The sawn boards are stacked very carefully.
In between the boards, we lay smaller sticks of wood (called "stickers")
across the grain and about 12" to 18" apart, depending
on the nature of the timber. The stickers allow the air to circulate
through the stack of timber, all around each board, and allows the
boards to dry at a controlled pace. The stickers must be on top
of each other to avoid the boards at the bottom being bent by the
weight of the boards above.
Stickers are ideally made of the same species
as the timber being stacked, but otherwise we have used aspen, spruce,
pine, lime and others. If stickers are made of the wrong species,
it can cause "sticker stain" in the timber (dark or light
stripes in the timber where the sticker was placed). This is caused
by a chemical reaction between the woods, especially when wet. (It
is nothing to do with the colour of the woods involved). Stickers
must be of the correct thickness to avoid excessively fast or slow
drying.
When
stacking of boards is complete, the boards are tightly bound together
with strapping. This (or weighing the stack down with weights) helps
to prevent warping or twisting of the timber as it dries out.
The stack, or bale, is then moved into a
shed for air drying. (We use a pallet fork on a digger or a pallet
truck for this). The timber stays here until it goes into the kiln,
or it sold or used as air dried timber.
The rule of thumb is that it takes "an
inch a year" for timber to be air dried to the centre (It has
a moisture content of 18 to 22% at this stage.) In other words,
a one inch thick board will be air dried, to the centre, in one
year. A two inch thick board takes two years, and so on. This does
actually vary quite a lot depending on the weather, type of timber,
whether the timber is inside or outside, etc.
If the timber is destined for outside use,
air dried timber could be sufficiently dry for the purpose. Otherwise,
the timber needs to be dried further in a kiln.
Kiln drying
This the process of drying timber artificially
in a kiln. Kilns can be used to dry "green" timber (i.e.
freshly sawn timber) or air dried timber. The moisture content to
which the timber is dried depends on the purpose for which it is
required.
Generally,
a moisture content of 8 to 12% is necessary for any timber that
is to be used indoors in Ireland (e.g. furniture, flooring, kitchen
units, staircases, etc.). Air drying timber can get moisture contents
down to around 17 to 22%, but this is not dry enough for indoor
usage.
The reason for this is that the inside of
a house, especially with improvements in double glazing and central
heating, has a very dry atmosphere. If the moisture content of the
timber is too high, it will dry out until the moisture content reaches
an equilibrium with it's surrounding atmosphere (called the Equilibrium
Moisture Content - or EMC). While timber dries, it can shrink or
warp - leading to a potential disaster! In these situations, properly
dried timber is essential.
There are a few different types of kiln.
We have used dehumidifier type kilns and to date our timber has
been dried by contract. However, we completed building a kiln (pictured)
and started drying our timber in our own kiln from July 2004. The
kiln is, basically, an insulated box, which we built ourselves,
with a dehumidifier dryer, heater, fans and control unit provided
by Arrowsmiths in the UK. (See
www.arrowsmiths.co.uk). Our kiln in 16ft long.
Page Updated:
Sunday, 23 March, 2008
