Cutting an oak log

Cutting an oak log

Surely its' as simple as put it on and then cut out what you need, assuming the log is big enough? Cutting oak is an interesting and challenging problem at the same time. I have been to softwood mills, I have seen them programme a computer and tell the saw to cut on repeat and end up a lot consistant usable. Unfortunately, with Oak we cannot do that. There are lots of variables that we have to take into account when cutting oak logs. Knots, heart shake, shape, sap, rot and, in-bark all have an impact and not all are obvious from looking from the outside of a tree.
 The first rule for oak is the further you go up a tree, the larger and more common the knots are. For us the first 4-6m of a tree is relatively easy to deal with. This is the cleanest part of the tree with few to no knots and character. The tree at that height grows relatively straight, which makes it easy to handle. The majority of this section of the tree that we cut end up in two or four metre pieces and are cut to 35mm for one of our largest customers. They then resaw the planks to 35mmx5mm pieces and weave them to make panels which grace homes throughout the country. Because of the section they use the timber needs to be virtually perfect as any fault will result in the timber breaking when stress is put upon it. Cutting this is relatively straight forward. Its clean and so the main problem is minimising the waste from the heart of the tree. This is a crack that runs through the centre(although not dead straight). Generally the rule is the bigger the tree the bigger the heart. This means more waste. When cutting a beam, which we do with the trees after the initial  4m plus we box the heart in the middle of the beam cutting out the waste, but it is different cutting 35mm plank. When cutting the butt of the tree (the first length), the effect can be minimised by tapering the cut to remove some of the heart. The bottom of the tree is wider than the top and the crack is worse on the bottom as it has been exposed to the elements for longer and so the saw can be adjusted so that an angle is put on the tree going into the saw allowing us to taper out the heart. This allows us to get more useful timber.
  As we move up the tree it becomes more of an educated guess on what you are going to find when cutting a tree. There are a number of things to look for a tree that can effect its use. For waney edge cladding you don’t want any large knots sticking out of the bark. The bark needs to be intact and looking healthy and it needs to be straight otherwise the variation on width is going to make the cladding look odd and impossible to work out coverage. Cutting this is relatively easy once you find the right tree and involves cutting through and through the tree. This means a repeatitive cut one after another. Once the face becomes too large we turn turn the tree down on to the flat edge and repeat the process. As a rule we generally turn the tree down when we get to the heart as it is at its widest and also a heart crack will look unsightly. Normally this involves turning the tree down three times.

When cutting beams, the outside of the tree isn’t so important. Knots get smaller as you get closer to the centre of the tree and so you have to take an educated guess on whether the knots will shrink quickly enough to be acceptable on a beam face. Knots grow out from the heart of the tree and get smaller as you get closer to the heart. A ten by ten beam can have a bigger knot than a four by four beam and so that has an impact on the decision. The heart also plays a role. A big heart needs to go in a big beam otherwise there will be an unsightly crack along a face or two. If you are trying to cut multiple pieces out of a tree such as 4x4s you don’t want a big heart that may effect 4 or 5 of the pieces, whereas a tree with a little heart may only effect one or two or could be boxed.
Some trees have more sap than others. Sap is the first part of the beam to rot and so it needs to be minimised. Where you have a large amount of sap you get a lot of waste when cutting beams and so it may be better as cladding.

There are certain things that you have no control over. When cross cutting a tree to length you can discover rot in the tree. If it shows in the middle of the tree, it comes from a knot and it is important to discover where that knot is. The the area around the knot going to the rotten heart isn’t likely to be good for  much other than sleepers. This is especially prevalent in English oak and is normally where the rotten branches haven’t been cut back and have been allowed to rot further into the tree.
Dead knots can appear in a seemingly good tree. This where the knot is soft and the softness is contained just within the knot area, but it can go in several inches deep. The first couple cuts may show a normal knot and then one further cut shows the dead knot. We try to cut these out, but sometimes its not possible. They can make a unique feature in a beam and don’t effect the structural integrity the of the piece. The other thing that can surprise us is in-bark. This where the tree has grown over something, for example a branch that didn’t reach the outside. There is often no warning and it can create a weakness in a beam that can cause it to be rejected.
Another issue we have is the straightness. The saw cuts on a straight line and so any deviation in the tree will result in a lot of the front and back or the middle being wasted and the attempted piece we are trying to cut not being realised. A lot of time is spent ensuring that the tree we select is straight enough to get the beams we need. It is also the reason we do not cut trees over 4m at our mill aligning the trees on our saw can be tricky. For longer beams we work with our partner mills. A few of our timber framers require curved slabs that they then resaw to the beam shape themselves. This is tricky in itself. We have to find a clean tree with a deviation one way and then have to secure it safely to the saw for cutting. The framers want these pieces for braces as it will provide a more stable and even drying process for their curves within a frame.
In conclusion its a tricky process, but not without its enjoyment. When you select the right tree for a job there is a satisfaction and oak is such a beautiful timber the end results are wonderful and every piece is unique.
Sawmill manager

Posted by Alex Pickford-Waugh
2nd May 2023
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