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Sunday, 3 December 2017

Getting the most from electric chainsaws


Here I explore some of the issues for electric and low-powered chainsaws, including the sensitive question of bar length

The benefits of electric chainsaws

We have a shed full of petrol chainsaws, which are a marvellous technology: powerful, fast, reliable and able to work anywhere. No other household technology can deliver such a big energy return on energy invested: delivering large amounts of wood fuel energy from a small amount of fossil fuel.
Compared to petrol saws, electric chainsaws are much less powerful and much slower for bigger cuts. They are also more fragile: their motors can be quickly wrecked by labouring them. But if you treat them right, electric chainsaws are an economical and helpful asset for the frugal household. They are cheap to buy and run, and last a long time if used carefully. They start instantly and easily – a boon to the house without bush mechanic skills. In our household electric chainsaws use free surplus solar electricity from our PV panels (we only use them when it’s sunny) to produce wood energy. They make no smoke and can be used comfortably in a shed. They are relatively quiet and don’t fill our valley with motor noise. They require much less time on maintenance – the motors go for years needing only cleaning with compressed air. When used for the right jobs, with care and understanding, they’re great.

Cordless electric chainsaws

A Stihl cordless electric chainsaw: I wouldn't have one
Battery-powered chainsaws are becoming prominent in the shops, but they look like a poor deal to me unless you’re doing a lot of something that really uses their cordless characteristics (perhaps like pruning trees in an old people’s home). Lithium batteries are remarkable energy carriers, but they are very expensive, use a lot of energy and valuable materials to produce, and they only last a few years. I love lithium batteries for electric bikes, where their power is a good match for the job, and they help to take a whole car off the road. But I think cordless chainsaws are pushing the battery thing past its effective zone: only just enough power for a very small chainsaw, a short charge life, requiring frequent recharging or multiple batteries, expensive saws and expensive batteries, and expensive replacement in a few years. 
I service a family cordless chainsaw (a Stihl), and I'm very unimpressed: the chains are extra narrow to reduce cutting load, but have tiny teeth with a very short life. The bars are also small, light, weak and short-lived. Over time, the cost (money and resources) of the saw, plus batteries, plus chains, bars and sprockets, promises to be far more than a larger saw. Really, the only plus is that they're quiet and don't need an extension cord. 
If your goal is frugality and energy efficiency and your product is household firewood, I'd leave cordless chainsaws alone. I expect far less cost and fossil fuel will be invested in using a petrol saw to cut wood into lengths in the forest, then recut back at the shed with a corded electric saw, than the embodied manufacturing energy in a cordless saw and batteries.

My experience with electric chainsaws

I’ve had good and bad experience with electric chainsaws, over 20+ years. My first was a Stihl E20, a marvellous 2kW machine made for professional use, comfortably pulling 3/8” chain on a 16” bar. It did a lot of work before its armature was cooked while a friend was using it. I was unable to get a replacement armature at the time, so it went into a sack in the shed. I was recently quoted $355 for an armature: this seems excessive to me so I don’t plan to repair it currently. Stihl no longer sells the bigger electric saws in Australia.
I then bought a Stihl E140 (1400W), which has worked well for over 10 years and cut many tons of wood. It’s mostly used in the shed for firewood: re-cutting short poles of smaller diameter wood to stove length. It cuts clean wood, up on a saw horse, so chains stay sharp for surprisingly long and have long lives.
Here's my Stihl E140, on the saw horse where it's cut many tons of wood
I’m very careful to maintain a high motor speed while using this saw, being conscious of the many power tools I’ve seen that have had their motors burnt by being worked too hard and run too slow.
My E140 was supplied with a 14” bar running 3/8 low profile (LP) x 1.3mm gauge chain. This has always been obviously too much bar for the saw: the motor can’t pull that many teeth through wood without being overloaded.
I now have a Makita 1800w electric chainsaw, reviewed in the next post. It's good, cheap and repairable at reasonable cost - I think Stihl has lost me as an electric saw customer. 
Makita and Stihl E140 side-by-side: basically the same layout

Who’s got the biggest bar?

To get the most from small saws, especially electric, we do need to talk about bar length.
Marketing of domestic chainsaws seems to focus on exploiting blokes’ macho anxieties, and I reckon most small chainsaws are sold with bars that are simply too long for them. It’s as if people are being told they’re getting more saw if the bar is long – but they’re not.
Box photo designed to plug in to bar-length anxiety disorder
With all chainsaws, you get the best out of them with the shortest bar that will do the job, and this is much more so with low powered saws like electric ones. Short bars are safer: their noses are less likely to contact another piece of wood behind the cut and cause kickback, if they do kickback they have less leverage over the operator’s arms, and they are simply safer by being more compact. Short bars make a saw lighter to carry, quicker to sharpen, and lubricate bar and chain better with a given amount of oil.
On top of all this, a short bar makes your saw more powerful (in effect). There’s less chain dragging through less bar groove and wasting power on friction. Most importantly, a short bar sets a maximum number of teeth that can cut wood at the same time, which has real advantages.
With all types of saw, a key issue is power per tooth. When a saw is cutting, the motor power must be divided amongst the teeth that are actually cutting wood. The closer together the teeth are, the more teeth bite at once in a given cut, and the less power each tooth can have.
Up to a point, the more power each tooth gets, the more efficient the whole sawing process is: the tooth can cut deeper, lift chips instead of dust, and the tooth will stay sharp longer because it isn’t "rubbing" (rubbing is when a saw tooth rubs dust off the surface instead of biting under and lifting chips or a shaving. It happens to any type of wood saw if it's too blunt or underpowered).
A tooth that is biting deeply and cutting good chips will produce a high volume of chips, that are more likely to fill the space between the saw teeth, potentially overloading the cut with sawdust. Wider tooth spaces and shorter cuts can remedy this.
Lots of low-powered saws are sold with 16” bars (including the Makita reviewed in my next post). This is (I find in practice) too long for them. These saws don’t have enough power to pull 16” of teeth cutting at full depth, and if you try, their motors will overload and bog down, risking motor damage. To avoid overload, the operator can hold the saw back and make the teeth rub, which is slow and inefficient and also blunts the saw. He (or she) may rock the saw so that fewer teeth are cutting at any one time – clumsy and ineffective. If the operator pulls the saw back and uses the outer half of the bar, it will reduce the number of teeth cutting. However this seriously risks kickback, as the spikes are not engaged in the wood and the tip of the bar is pushed into the back of the cut, with a lot of leverage over the operator’s arms. These issues of power and safety are easily addressed by using shorter bars.
If you really need a long bar on a low-powered saw because you can’t do your cut any other way, you can increase the power per tooth by reducing the number of teeth along the chain. One way to do this is using “skip tooth” chain, which has 2 (or sometimes 3) blank links between teeth, instead of the usual 1. This is often done in chainsaw milling, where long bars are often used cutting in a difficult orientation (teeth ripping across end grain). Another option, also used in chainsaw milling, is to take a normal chain and grind away most of the top plates of a proportion of the teeth, following the style of Granberg ripping chain (have a google) - this doesn't have to be just for milling. 
Photo of modified chain
If you’re using a long bar so you can cut wood on the ground while standing up, there are often better ways. Squatting and kneeling are both good for your body! And you will reduce your cutting into the ground – the reliable way to blunt your saw and wear your bar.

Getting a shorter bar

My advice (if it isn’t already obvious) is to use a short bar on an electric chainsaw: 12” is a good size for most. You may be able to ask for a short bar on purchase, but most saws come in a box without options, so you may need to change bar and chain yourself.
If you want to buy a bar for a saw, you need to first know the chain pitch: this is a measure of how big the teeth and links are. Most consumer electric saws have a pitch of 3/8” Low Profile (also called 3/8LP or 3/8P). 3/8” means that the average length of a link in the chain is 3/8 of an inch (measure the distance between one rivet and the second one on, and halve it). A saw with a pitch of 3/8” LP will have a drive sprocket (on the motor) of 3/8” LP, and the bar will also have a nose sprocket of 3/8” LP.
3/8” Low Profile is different from and not compatible with standard 3/8” saw chain. It is specially made for small saws. Stihl calls it “picco” chain. 3/8LP has largely superceded the old ¼” chain for small saws, because it has 1/3 fewer teeth. In line with the principle explained above: on a lower powered saw, you want fewer teeth so that they each have more power. I’ve noticed that 3/8” LP bars are often sold as 3/8” (without mention of low profile), which is not helpful. This can make buying online a bit risky, but I’ve never heard of a 12” 3/8” not-low-profile bar, so that helps.
The chainsaw bar and chain have another critical measure: the pitch. This is a measure of the thickness of the chain drive links: the hook-triangle shaped bits that run in the groove. If the sprocket on the chainsaw is 3/8” LP it can drive any gauge of 3/8” LP chain. However the bar and chain need to be the same gauge, or the chain will be too tight or too loose side-to-side. If you are getting a new bar for your chainsaw, you don’t need to use the same gauge as your old bar, but you need to ensure you get chains of the same gauge as the new bar.
Chain of a particular pitch and gauge can be bought either in ready-made loops, measured by the number of drive links, or you can buy a roll (usually 25 or 100 feet long) and make your own chains. Riveting chain into loops is relatively easy, and doesn’t need any bought tools if you have a little metalworking ability.
Cheap Chinese bars are okay, if you are willing to give them some attention before using them. The ones I’ve been trying on my electric saws seem to mostly be made for pole saws: 12” bars, taking 3/8” LP 1.3mm gauge chains. A number of these I’ve tried seem to have too wide a gauge of bar groove for the chains supplied with them (like some Baumr chainsaws – see my review here). This will probably work fine when new, but will become a problem after a while. Hammering a new bar to fit the chain gauge is a reasonable solution, which I’ve used sometimes. Cheap bars sometimes also need dressing: filing, grinding or linishing the bar rails (the surface the chain runs on around the edge) to make them square. Here's a video on how to hammer and dress an old bar:

Electric motors

A key thing about electric chainsaws is that they have electric motors (!), and that these are very different from petrol motors. What kills petrol chainsaw motors is (roughly speaking) going too fast or too dry (too little oil or too lean a fuel mixture). What kills electric motors is going too slow.
The faster an electric motor goes, the less power it draws. The slower it goes, the more current it draws and the more heat it generates in the motor.
When you first switch on an electric motor, it draws a huge current for a short time until it spins up. Next (before it gets a job to do) it uses minimum current when spinning at top speed with no load. Then, as the motor is loaded up and slows down, it draws more current and becomes more powerful. However if the motor is further loaded and slowed even more, the current increases but the output power decreases. When it’s bogged down, a high proportion of the electric energy stays in the motor as heat (instead of leaving as shaft power), and the copper windings heat up. If the copper windings get hot enough, their insulation will start to fail, and electricity can then find short circuit paths through the motor, making even more heat and “burning the motor out”. 
Brush motors, as used in mains powered chainsaws, drills, circular saws etc., often show this sort of winding failure as arcing around the brushes. This is usually a symptom of winding failure in the armature: the spinning rotor in the centre of the motor. Burnt armatures can’t generally be repaired, but they can often be replaced if reasonably priced. I recommend checking the price of a replacement armature when considering a new electric chainsaw (or other large brush-motor power tool).
Some electric chainsaws, including my Stihl E140, are made with a thermal cutout switch, to help protect the motor. This switch doesn’t sense the temperature of the motor windings, instead it is designed to heat internally in a similar way to the motor, and to switch off when the current has been high enough for long enough.

Electric chainsaw consumables

If you’re using an electric chainsaw to produce firewood, or some other repeating task (not just pruning the garden twice a year), it’s worth being conscious of what parts you can expect to consume over time.
Chains and sprockets are the main consumables of any chainsaw. These parts wear and are replaced together, like the chain and sprockets of a bicycle.
It’s most economical to rotate 2 or 3 chains together with one sprocket, then replace chains and sprocket when the chains are sharpened away. As most chainsaws are sold with a single chain and a new sprocket, it’s good to decide on what bar you intend to use soon after you get your saw, and get 2 or 3 chains to suit. This could be from the seller, or from another supplier, especially if you are chainging bars. I change chains and turn over my bar (to wear both edges evenly) every few sharpenings, swapping to the chain with the longest teeth.
I usually use chains until teeth start breaking off, because the teeth have been sharpened so far back. By the time they are this worn, they cut a narrower kerf and are more difficult to use.
The sprocket will take a long time to wear – you’ll have to wear out 2 or 3 chains first. However I like to have these things in stock well ahead of time: living out of town it saves a lot of cost and trouble to be well stocked.
The bar will last for a few sprockets – which is years of cutting. Unless they get bent by a big log falling on them, well maintained bars wear out by the groove becoming too shallow for the chain – so the drive links reach the bottom.
Electric chainsaws, used with care, will eventually wear out their motor brushes: the carbon blocks that conduct electricity into the spinning armature. Some brushes are made so that they stop working before they wear down to their springs and damage the commutator on the armature (the ring of copper bars on the rotor). Sometimes a power tool suddenly stops working, appears to be dead, but simply needs a pair of new brushes worth a few dollars.

Summary

Electric chainsaws can be a useful and economical part of a firewood cutting system. If you want a long and productive life from your electric chainsaw, keep them spinning fast and running cool and avoid frequent starts. As a key to working your saw within its capabilities, I recommend looking sceptically at the supplied bar length and considering how you can give the fragile electric motor an easy life with a short bar. Running 2 or 3 chains in rotation will reduce maintenance costs.







1 comment:

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