Working Edges: Part One.

Post number three in this quick look at common sharpening problems takes a closer look at the working edge of the cutter. This is the crucial bit that slices through the wood and so it must be sharpened correctly. I've seen a number of common errors when filing the cutters and getting the working edge correct is probably the most difficult. Click on the jump to find out a bit more...

There are a number of things that need to come together to end up with a good sharp chain that's capable of cutting effectively and efficiently. When we sharpen a chain we need to know:

  1. the file size;

  2. the filing angle;

  3. the depth gauge setting.

We've already seen in the previous post that selecting the wrong file size will affect the side plate angle, or the amount of hook, given to the cutter; and this time we'll have a look at the issues surrounding the filing angle.

The filing angle, given by the manufacturers, lets you know the best angle for the type of work that the chain is to be put to. For example, a crosscut chain will have a filing angle between 25o and 35o, whereas a ripping chain is much shallower - the ideal is 0o but values in the range of 5o to 15o is more common. This is because of the different way the cutter has to remove the wood when cutting across, or with, the grain. Cutting across the grain (cross-cut) the cutters must be set up to sever the fibres, when cutting along the grain (rip-cut) the cutter must essentially chip the wood away. We'll take a closer look at rip cutting and cross-cutting in a later post and see how it actually works.

Back to our filing angle; there are several ways of filing the cutter - from using just the file on its own, to using a Dremel with an attachment, through to the professional grinding wheels for accurate setting. Each has it's own merits and drawbacks (perhaps that could form another post in the future!). Here, we'll stick to using the good ol' file with a file guide.

The potential problem with using just the file is that you have no control over the height that you are filing at, making it all too easy to set the incorrect side plate angle. The file guide, used properly, should reduce the chance of this - notice that it doesn't completely alleviate it as you can still place it incorrectly or put too much pressure down on it. But it helps, and it's also got the lines etched in to it to make lining it up to the correct angle a simple task.

Stihl recommend a filing angle of 30o with their Rapid-Micro and Rapid-Super chains which makes it easy to remember, but you need to be a bit more careful with Oregon chains, even those with the same identifier on them. An Oregon #21 or #22 chain could have it's filing angle set to either 25o or 30o depending on whether it's a 'round ground chisel' chain or a 'micro chisel' chain.

Once you know the angle, the problem then is that it can be hard to achieve if your filing stroke is not even. The picture at the top of this post shows what happens if you change the angle right at the end of the stroke; you can see that the working edge of the cutter is no longer straight. To correct this, make sure that you keep your filing stroke completely straight from the beginning to the end, and maintain an even pressure.

In the next post we'll continue with the working edge and the difference in shape between a chisel cutter and a semi-chisel cutter. Chisel cutters tend to be very easy to file with a straight edge all the way across, but time and again I see semi-chisel cutters not done to their best, so we'll pick up on this and have a look why...

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