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How To Sharpen Square Chisel Chain With A File

We are frequently asked how to "file" square-ground chisel saw chain. Many people like the way square-ground chain performs and desire an inexpensive way to sharpen it. Most assume "square" filing is similar to round filing and with a few new skills, they can easily master it. The following information will help you learn those skills, but before you dive in, realize that square-filing  is much more difficult than round filing.

Square filing requires precise corner alignment and filing angles. From what we see, most who square-file, don't do it well. Many don't realize (or admit) the kind of cutting performance they would enjoy if their chains were sharpened better. So, don't get frustrated if your initial efforts don't perform as well as you had hoped. You are not alone. Give yourself time to work through a learning phase. Square-filing is a skill that takes time to develop.

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There are several shapes of files that can be used for sharpening square ground chisel chain. This one is a triangular chisel file. It actually has six sides, which produce three corners which can be used for sharpening. This is one of the least expensive chisel files and it fits most 3/8" cutter teeth well.

A Few Rules To Start With

With both square-filing and square-grinding, the rules for sharpening are the same: 1) The corner of the file (or grinding wheel) must stay aligned with the corner in the chain's cutter tooth. 2) At the same time, sharpening angles in the cutter tooth must be maintained.  3) You must repeat this accurately on every tooth, because all the cutter teeth need to be the same.

Pro users who sharpen square-chisel chain with a grinder have some built-in advantages. For one, most of step (2) is built into the grinder. By simply aligning the corner in the cutter tooth with the corner in the grinding wheel, they can produce well sharpened cutter teeth. Filing is more difficult. The challenge of positioning the file in the corner, holding the correct angles, and doing all of this while you are moving the file is difficult.

Step (3) is also easier with a grinder. Once a cutter tooth is sharpened, stops are set which limit travel or feed. These stops allow the operator to easily duplicate the results of the first sharp cutter tooth on all the rest.

These are reasons why many who square-grind produce top performing chains, while those who file sometimes don't. This is not meant to discourage you. It is to help you understand the skill you are working to develop is not an easy one. To do it well takes time and effort. 

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This flat chisel file has two corners that can be used for sharpening square chisel chain. A benefit of this type of file is its wide surfaces can also be used for depth gauge maintenance.

Square Filing The First Time

If you have a shop or garage with a good light and a work bench, this is a good place to learn. Hopefully your workbench has a vise, so your saw can be clamped firmly. Square filing can be done in the field, but it is easiest to learn in conditions with good light and stable cutter teeth.

Start with a square-ground chisel chain that still has its factory grind. If you start with a chain that has been resharpened, it may have gullet material that needs to be removed prior to your sharpening. The other benefit of having a factory grind is you have the factory's shape and angles to follow. Don't start with an old chain, especially one that has been filed before.

It is also helpful to have a second new square chisel chain to use for reference. If you have a question about what a sharp cutter tooth looks like, refer to the cutter teeth on the second unused chain.

Begin by placing your chisel file next to the face of a cutter tooth. Determine which corner in the file you will align with the corner in the cutter tooth. Once the corner is aligned, reposition the file on its other axis so the other surfaces match up. Study the GRAPHICS on this page. 

Once you have figured out the angles and corner alignment, take a few strokes. Remove the file and look at the cutter tooth. You will see some marks where the file has taken some steel. Make whatever file adjustment is necessary based on what you see. Proceed until the tooth you are filing looks like the sharp tooth on your reference chain.

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This graphic shows a view from the top of a flat chisel file in position in a square cutter tooth.

Learning What A Sharp Tooth Looks Like

When learning to sharpen cutter teeth, it is important to learn what a sharp tooth looks like. If you don't know this, your brain will not be able to tell your hand what to do. One telltale sign of a pro saw user's level of sharpening experience is how he determines if his chain is sharp. If  he "feels" the corner of his saw chain, he is probably a "new guy."  Experienced users don't do this. They know what a sharp cutter looks like. They can tell if a cutter tooth is sharp just by looking at it -- eyes work better than a thumb.

Duplicating Sharp Cutter Teeth

Now that you have produced one sharp tooth, it is necessary to duplicate what you have done on the rest of the chain's cutter teeth. Carefully sharpen the rest of the them. If you were using a grinder, you could set your stops which limit travel and feed. With a file, you must learn to do this by eye. Some people count file strokes to help them keep track of how much cutter tooth they have removed, but this is only a rough guide because each stroke does not remove the same amount of material.

Also, most people find it is easier to sharpen one side than the other. This is because most of us are either right or left handed. If you are not ambidextrous (the ability to use both hands with equal facility) you will have to compensate for this by eye, too.

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This graphic shows a view of a chisel file from the back of the cutter's tooth. 

A Good  Square Filing Strategy

Hand filers who have the most problems are those who never have their chains sharpened on a grinder. Without an occasional "restoration" grind, error starts to grow. By the third or fourth filing, there can be big differences in each cutter tooth. When this occurs, the chain may chatter and/or not cut straight. 

It is important for each tooth to be identical, because saw chain is a "team" of cutter teeth. When each tooth takes a different sized bite of wood, the chain will chatter and vibrate. In addition to vibration, sharpening inaccuracy leads to chains that don't cut straight. Here's why: If you add up all the chips taken by the left side cutters and compared it to those on the right, each side has to remove the same amount of wood for the chain to cut straight. If the teeth on one side of the chain remove more wood than the teeth on the other side, the cut will "pull" in the direction of the side that removes the most wood.

Consider having your chains ground every two or three sharpenings. A machine grind will restore all the angles, corner alignment, and tooth length -- so the chain never gets off very far. Make sure the grinder operator does not overheat the teeth. This not only damages the teeth, but makes them impossible to file afterwards.

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This cutter tooth has been sharpened with a square "chisel" file. Notice the line in the corner indicates this tooth has a slight side beak. When filing, always try to get perfect corner alignment, but if you are going to be a little off, its better to be low. You never want a top beak.


A few paragraphs ago you read how important corner alignment was. The following may seem like a contradiction, but read on and you will learn a filing "secret." The rule is: Align the corner of the file with the corner in the cutter tooth, (here's the secret) but if you are going to be off, be low. No human is perfect. This is especially true when trying to "square" file. Since it is nearly impossible to file a perfectly aligned corner, aim a little low. Your cutter tooth won't cut worth a darn if it has a high corner. On the other hand, if the corner ends up a little low, this creates a "side-beak." A chain sharpened with a slight side beak will still perform. 

The only down side of a small side-beak is some loss in the chain's stay-sharp ability. This explains why filed chain often has less stay-sharp ability than a ground one. Since it is easier to get corner alignment on a grinder, most grinder operators can produce a chain without a beak. 

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The corner of this "triangular" file is aligned properly with the corner in the cutter's tooth.  

Types Of  Files 

We sell several different files for sharpening square-ground chain. The most popular is the hexagonal or "triangular" chisel file as it is more commonly known. It does actually have six sides, but it has three "corners" or filing edges. This gives it one additional filing edge than the other chisel files. It is also the smallest file and fits into the throat of 3/8" pitch chain well for most people. 

Other choices include a double-bevel file and what's known as a "goofy" file. Both have two corners for sharpening, but offer the additional benefit of being able to lower depth gauges with the wide top and bottom surfaces. 

Filing Cutting Direction

We are often asked which direction the file should cut -- going "into" or "out-of" the cutter tooth. In practice, both are acceptable, but most filers file "into" the tooth. The experts at Oregon Cutting Systems agree this method cuts the cleanest edge on the cutter's chrome plating. There are a few people who prefer to file in the other direction. They do this so the hard chrome is "lifted" from the cutter tooth and not driven into the file. They argue this method helps preserve expensive files.   

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This file is cutting on its way "into" the cutter tooth. This is how most people file. It tends to be hard on chisel files, but it cuts the cleanest edge in the cutter's chrome plated surface.

Our Advice

Square filing is difficult, and chances are, once you decide to run square-ground chain on your pro saw, you will probably buy a grinder before long. One good thing that comes with filing experience is it you to learn what a sharp square-ground cutter tooth looks like. This is a big advantage when learning how to operate a square grinder.

If square-filing turns out to be too difficult to do for you, and you don't see yourself getting a square grinder, there is another option; file your square chain with a round file. While it won't perform quite as well as a square-ground chain, it will cut better than a poorly filed one -- and accurate round filing is much easier for most users. In addition, round files are less than half the price of square files. 

As always, we hope this information is helpful. Let us know if you have more questions. You ask, and we'll keep working on straight answers. That's why we're the Pro's Choice.

Got questions about filing square-chisel chain? Call or stop in.

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