The K-Zone: The real man's guide gardening tools: the chainsaw

A chainsaw is a very useful tool for pruning, felling, and cutting up trees. In my experience, petrol-engined machines are overwhelmingly more useful than electric ones, and probably aren't any more dangerous in themselves. However, petrol-engined chainsaws are generally heavier, larger, and therefore more difficult to handle than electric ones, and usually more expensive.

Disclaimer: it goes without saying that anyone who uses a chainsaw ought to have proper training from a competent person. Nevertheless, nobody (with the possible exception of your spouse) will stop you buying a chainsaw from a DIY shop and setting about yourself with abandon, and most chainsaws are supplied with only the most rudimentary instructions.
I do not claim to be any kind of expert on chainsaw operations. What follows is based on my own experiences over the last few years, and might help somebody just starting out with a chainsaw, and daft enough not to have proper instruction.
It goes without saying that this article is in no way a substitute for formal training, and in no way do I condone the use of chainsaws by incompetent, untrained operators. As a minimum, you should read the instructions and safety information supplied with the machine: in particular, there will be important information about chain tension, idle speed adjustment, etc. Bear in mind that there are (in the UK) legal restrictions on the use of chainsaws in the course of business.

Primary dangers

The chainsaw consists of a chain of very sharp blades, propelled around a bar usually 14-18 inches long (sometimes longer for specialist work) at high speed by a powerful motor. Most injuries are caused when flesh comes into contact with the moving blade. This may happen because the operator slips and falls on the blade, or simply swings the saw around without waiting for the blade to come to a standstill. A particular concern is the vicious kickback -- the blade is thrown rapidly backwards when the chain comes into contact with something it cannot easily cut. Most kickbacks are caused when the far end of the chain comes into contact with something, and modern chainsaws are usually supplied with a chain guard that fits over the end of the bar to reduce the likelihood of such occurrences. Injuries to the back of the left hand (with a right-handed saw) are particularly common.

Secondary dangers

Because chainsaws are typically used for cutting large pieces of wood, there is a risk of heavy objects falling or rolling onto the operator. Chainsaws are very noisy, and hearing damage can result from prolonged use. Some operators experience vibration injuries (e.g., `white finger') but these are not so common with modern machines. Because of the direction of motion of the chain, injuries caused by objects being thrown into the operators eye are relatively uncommon -- but that's no reason to push your luck.

Safety tips

Use protective clothing, etc
It's obvious really, but not so obvious that I haven't seen people using chainsaws with no protective equipment at all. In all circumstances you should wear stout gloves -- ideally specialist chainsaw gloves with a reinforced panel on the back of the left hand. If you're using a chainsaw for more than a minute or so, hearing protection is a good idea. If you're felling or pruning trees, you need a helmet to protect you from objects falling on your head, and steel-toecapped boots to protect you from objects (including the saw) falling on your feet. Leg protection (chaps or full trousers) is a good idea if you're likely to be cutting downwards. Like many chainsaw operators, I prefer not to wear upper-body protective clothing (except as above). I feel that the very real risk of overheating and dehydration with such clothing outweighs the relatively small risk of upper-body injuries. But your mileage may vary. The Health and Safety Executive recommends that all chainsaw operators carry or wear a first aid kit, rather than relying on a first aid post some distance away. Your first aid kit should include a dressing suitable for large wounds -- chainsaw injuries aren't likely to be fixed by an Elastoplast.

Don't put your body in the kickback arc
If the saw kicks back -- and it will at some point -- the tip of the chain bar will describe an arc whose centre is approximately at the centre of the mass of the saw -- usually near your left hand. If you hold the saw so that you are cutting downwards, with the blade in line with the centre of your body -- which is what most people naturally do -- the tip of the blade will fly towards your face if it kicks back. The way to use a chainsaw that leads to the lowest risk of a kickback injury is to put the workpiece between yourself and the blade. If you can't do that, stand well to the left of the kickback arc. Then, if it does kick back, the blade tip will end up to the right of your right ear -- still frightening, but not catastrophic.

A well-maintained chainsaw, operating within its design limits, is very easy to use
A well-maintained chainsaw, used properly, cuts by itself. If you are cutting downwards, the weight of the saw alone should normally be enough to push through the cut -- you don't need to force it. If you need to push on the blade, or the blade tends to get stuck in the cut, something is wrong -- most likely the chain is blunt, or the cut is closing up around the chain (see below).

Before cutting, think about where the cut piece will fall or roll
Naturally you aren't going to put any part of your body underneath something that will fall when you cut it, but you might be surprised how quickly a heavy log rolls away from the trunk it was cut from. If possible, position yourself uphill from any place where falling wood will land.

A properly-adjusted chainsaw does not turn the chain at idle
For safety reasons, a petrol chainsaw should be capable of being left ticking over between cuts. A properly-adjusted machine will not idle fast enough to engage the clutch and start the chain turning. The engine might idle too fast if the choke is open. This is why the instructions supplied with most chainsaws state that, when starting the machine cold, you should close the choke as soon as it seems that the engine will fire. A chainsaw whose chain moves when idling, with the choke fully closed, might need its idle screw adjusted. However, a common cause of this problem is an incorrect fuel-oil mix in two-stroke engines, so it's worth checking this before tinkering with the carburettor.

Be aware that trees don't always fall where you want them
Even if you cut the requisite hinge, there is always a chance -- a good chance, in my experience -- that the tree will fall other than where you planned. Of course, a skilled operator stands a better chance of getting the fall right than an unpractised one, but it is very difficult to get a leaning tree to fall other than in the direction it is leaning. Generally you need ropes and pulleys for this.

Know your limits
And know the limits of your equipment. For my part, I'm reasonably happy to use a chainsaw to fell trees of up to about 16-inch diameter, with a clear line of fall, on flat, dry ground. I won't use a chainsaw on a slope, when it is slippery, or above chest height. I certainly won't get up into a tree with a chainsaw, even though I do own the necessary safety equipment. If I have to cut anything more than about five feet above the ground, I use a hand bowsaw. It takes a bit longer, but I want to live. I don't own the equipment necessary to fell a tree that doesn't have a clear fall line in a safe direction, so I wouldn't attempt this. Finally, if it's getting towards the day and my chainsaw runs out of petrol, I don't fill it up -- I stop work. This is superstitious behaviour, I suppose, but it does limit my opportunities for using a chainsaw when I'm tired.

Getting good results

When you cut down with the underside of the bar, the saw will pull towards the workpiece
If the work is fixed -- a stout tree branch, for example -- it will pull your arms towards the work. If the work is not securely mounted, the work will be pulled towards you. Even heavy logs can be pulled surprisingly forcefully towards the saw. The amount of pull will depend on the power of the saw and the throttle setting, among other things, but could be more than 50 pounds. You can avoid this effect by starting downward cuts with the body of the saw touching the work, and then cutting by rotating the bar downwards, rather than pushing the whole saw downwards. Of course, if the work is thick, you'll have to push downwards as well as rotating downwards. In any case, you should usually avoid cutting with the furthest third of the bar, because kickbacks are mostly likely if the tip of the bar snags on something.
If you cut upwards with the top side of the bar, you'll get the opposite effect -- the saw is pushed away from the work.
These push and pull effects are not necessarily a problem, but they can be surprisingly forceful, so it's as well to be aware of them.

Petrol chainsaws are designed to cut at full throttle
At full throttle a powerful chainsaw is extemely noisy and smokey, and you are likely to be tempted to regulate the speed (and the noise) with the throttle. Unfortunately a petrol chainsaw is not designed to operate this way, and it won't be efficient or safe. The blade needs to be spinning at full speed before it even touches the workpiece. If it isn't, it will either not cut at all -- because the clutch will disengage -- or it will snag.

When replacing the chain, put it on the bar the right way round
This seems obvious, but there's nothing to prevent you putting it on back-to-front, in which case it won't cut very well. The correct orientation will usually be marked pictorially on the machine somewhere near the bar.

Watch the chain tension
When cutting continuously, the chain will get hotter than the chain bar, and expand more. This means that the chain tension will reduce. If the chain becomes too slack, it can jump off its sprocket, which often means that you'll need a new one. In any event, there will be a certain amount of downtime. If you adjust the chain tension when the chain is hot, you'll reduce the chain slack, but when the chain cools down it will bind on the bar, and not cut at all. Most manufacturers recommend adjusting the tension when cold, but this means that you'll need to keep an eye on the chain slack when the chain warms up, and either tweak it or let it cool down if the chain starts to hang off the bar. Needless to say, a chainsaw that offers tool-free tension adjustment will provide for easier tension control than one that requires a screwdriver. A new chain will stretch a bit, and therefore lose tension, in the first few minutes of use.

Before cutting, consider whether pressure will develop on the bar during the cut
Chainsaws are provided with a clutch, to prevent damage to the engine if the chain cannot cut. In general, if the chain becomes crushed between two pieces of wood, you won't be able to power out of it, nor will you be able to pull the saw out of the cut. Instead, you'll have to stop work while you insert wedges with a sledgehammer to free the blade. The problem is much worse on resinous woods, particular softwoods cut in the summer.
All this means that, before you cut, you'll need to think about how the wood will move during the cut. For example, if you are cross-cutting (that is, sawing up) the trunk of a felled tree, and the trunk is propped at both ends on its branches, then if you cut downwards the weight of the tree will cause the cut to close up around the saw blade, and it will get trapped. Instead, if you cut upwards from below the trunk, the weight will tend to open up the cut, and it is less likely to trap the blade. On the other hand, if you are lopping a stout branch from a tree, the weight of the branch will open up the cut if you cut down from above. This creates another problem, of course -- as the cut progresses the weight of the branch will put tension on the remaining, uncut wood, which might tear. The solution is to make a small cut from below, and then cut down from above.

If you don't cut a hinge when felling a tree, try to cut in the direction of the tree's natural angle of fall
It is standard practice to cut a hinge (a wedge-shaped cut-out) in the side of the trunk to control the direction of fall. Any chainsaw will be supplied with instructions showing how to do this. Where the trunk is less than about three inches in diameter at the base, it is not really practical to cut a hinge. It you don't cut a hinge, but instead cut across the trunk in one movement, then one of two things will happen. If you are cutting towards the tree's natural direction of fall, the tree will start to fall as soon as it's own weight is too heavy for the remaining wood to support. You might be left with a jagged stump, which can be avoided by making a shallow cut in the other side of the trunk before starting the main cut.
If you cut against the tree's natural direction of fall, the tree will settle on the blade, and the cut will close up and jam the blade. If you're lucky, and strong, and the tree is not too heavy, you'll be able to put your shoulder against the trunk and push hard enough to open up the cut and pull the blade out. If not, you'll probably need winching gear to free the blade, which is a drag.
The trickiest situation is where you have a sapling or coppice pole growing straight upwards, and there is no natural fall line. In that case, if you cut straight across, you'll either get the blade snagged, or the tree will fall in an unpredictable direction -- Sod's law says that it will usually fall on you. Even a three-inch diameter coppice pole is quite heavy if it's thirty feet high. You need to be sensitive to the pressure on the blade, and be prepared to change the direction of cut if the tree starts to settle on the blade.