How To Sharpen a Chain Saw

Do you remember how your chain saw cut when it was new or last equipped with a new cutting chain? It should cut like that--or better--every time you use it. If it's not cutting like new, you're wasting time, damaging your equipment and putting yourself at risk.

Manufacturers design chain-saw cutting chains for easy maintenance. However, like most service tasks, to do the job right you must first understand the basic principles involved.

How saw chains work

A saw chain comprises five basic parts:

  1. left-hand cutters
  2. right-hand cutters
  3. tie-straps
  4. drive links
  5. rivets

Because the cutters do the work of slicing and removing wood fiber, they are the primary focus of our attention here.

Cutters have two distinct features: a ramp-like depth gauge, or raker, at the front and a gouge-like cutting element at the rear. The gulf between them is the gullet.

The cutting element has a profile that looks like the number "7". Its two surfaces (the top plate and side plate) contain three different angles. The top-plate angle is the easiest to recognize. It's the familiar 30- to 35-degree rake you see when you look down on the cutter. The bevel beneath the top plate is the top-plate cutting angle. Like a chisel, its optimum setting is 60 degrees. The third angle is the toughest to visualize. It's the side-plate angle or the arc formed in the side plate. This angle typically is 85 degrees or less, depending on the brand and style.

Here's how these angles combine to cut wood. The sharp corner, where top-plate and side-plate angles converge, slices across the wood grain. Alternating left- and right-hand cutters work on each side of the cut. The chisel-like angle beneath the top plate scoops out the chip. Cutters take turns biting as they "porpoise" through the cut at speeds approaching 60 miles per hour.

The depth gauge controls the depth of each "bite," or thickness of each chip. Depth gauges typically are 0.025 to 0.035 inch lower than the critical corner. This difference is what gives the corner its ability to cut.

When to sharpen

Cutters are plated with a thin but tough coating of industrial chrome. They would stay sharp almost indefinitely if you always used them on clean wood. However, in the real world, wood often is dirty or laying near the ground where contact with dirt, rocks, embedded grit and other debris is difficult to avoid. At the high speeds of a chainsaw, it doesn't take much to dull a sharp chain. That's why prudent saw operators often brush, wash or chop off dirty areas before cutting. It can save a lot of time in the long run.

Several signals tell you when it's time to sharpen:

  • When the chain no longer self-feeds. This is the most obvious signal to re-sharpen. A properly sharpened saw chain pulls itself down through the cut. If you find yourself pushing on the saw to make it cut, or using the bucking spikes to apply heavy leverage, it's time to sharpen the chain.
  • When the saw's discharge is dusty. A properly sharpened saw chain expels nice, square wood chips. If your chain saw is producing wood dust instead of chips, it's time to sharpen.
  • When the chain looks shiny. Look at the top plate and side plate. If the chrome plating has worn away, it will expose the steel underneath, and the cutting edge will be shiny. To restore the cutting edge, you must file the steel away until a thin overhang of chrome returns.

It's important to stop cutting when you realize your chain is dull. Forcing a dull chain to cut subjects the powerhead, chain, sprocket and guide bar to unnecessary wear and tear. Studies show that dull or improperly maintained saw chains are the true source of most bar-related failures.

Dull chains also wear on operators, causing fatigue, frustration and impaired judgment. This is a safety hazard and another good reason to stop cutting when the chain gets dull.

Preliminary considerations

The best place to sharpen, if possible, is on a workbench. It's more difficult and time consuming to do a good job in the field. Most professional operators carry several sharp chains into the field. When one gets dull, they simply replace it with another. This is faster than field sharpening, and it confines chain-maintenance chores to the workshop.

Before you start, make sure the chain saw is steady and the work area is well lit. If the bar teeters up and down, place a wood block under it for support. Check to make sure the chain is properly tensioned. If you're unsure of the correct procedure, consult your owner's manual. The chain should be snug against the bar but still pull easily around by hand. Loose chains wobble during filing, and they cause bar, chain and sprocket damage during operation.

Because you'll be working around razor-sharp cutting edges, leather gloves are a filing necessity.

Use the right tools

If you're holding it correctly, a proper-diameter round file simultaneously restores the three different angles of a saw-chain cutter. Different-sized chains and different cutter styles call for different file diameters. It is vital to use the right size file.

In general, 1/4- and 3/8-inch-pitch, low-profile chains call for a 5/32-inch file; 0.325-inch-pitch chains require a 3/16-inch file; and standard 3/8-inch-pitch and 0.404-inch-pitch chains demand a 7/32-inch file. Beware, however, because many exceptions exist. For example, some brands of standard 3/8-inch-pitch chain require a 3/16-inch or 4.5-mm file. Check your chain-saw owner's manual or saw-chain instruction sheet to determine the correct file diameter. If you're not sure, ask your dealer. He also may have literature showing the proper filing angles and special procedures for your chain.

Getting the angles exactly right is not as important as removing all damage from the side plate and top plate and making sure the critical top corner is really sharp. However, for good results, you must consistently hold the file at the correct height and orientation within each cutter. This is difficult to do without some type of file guide.

Like most tools, simpler is better when it comes to file guides. The best guides are plates that drop over the chain. These keep the file at the right height and attitude and have witness marks to show proper top-plate angle alignment. The best thing about this type of guide is that you can see what you're doing. Guide plates that clamp to the file and rest on the cutter top plate also are good, but they tend to obscure your view of the cutter. Whichever guide you select, make sure you get the right model for your chain.

Perfect your sharpening technique

Place the file guide over the cutter, lay the file on the guide and align it with the witness marks. Now you're ready to stroke the file. If you're using a clamp-on-the-file guide, lay the guide plate on top of the cutter and align its witness marks with the guide-bar plane.

Now you're ready to actually sharpen the chain. Hold the file with both hands. File from inside the cutter to outside using full strokes. Apply light pressure and let up on the return stroke. Remember, a file only cuts in one direction. Your file guide is working properly if about 20 percent of the file diameter is above the top plate. This is very important. File all the cutters on one side of the chain first (for example, all of the right-hand cutters), then file the other side.

If the file is too high, it will put a back slope on the cutter, as will a file that is too large. Back-sloped cutters do not feed properly. This is the kind of chain that you must force to cut. A different problem can result from files held too low or files that are too small: they create "hook." Hooked cutters are dangerously aggressive at first but dull quickly. Using the proper size file and file guide eliminates these common sharpening errors.

File each cutter until you've removed all signs of damage. Remember, the uppermost corner must be truly sharp and defined by a thin chrome edge. Make sure right- and left-hand cutters are equally sharp and that their top plates are similar in angle and length. If not, the chain is likely to pull to one side when cutting.

Don't forget the depth gauges

Look at the cutter top plate. Notice how it slopes down to the rear. This slope, or clearance angle, assures that the critical corner is always the highest point. As the cutter is filed back, the corner gets lower and lower.

To offset this gradual reduction in cutter height, you must file the depth gauge periodically. Otherwise, the clearance between depth gauge and corner becomes too small, and the chain will not "bite." This is another way chains lose their self-feeding characteristics.

After several sharpenings, it's time to check the depth-gauge setting. Standard depth-gauge jointing tools are available for this purpose. However, some file guides double as a depth-gauge tool and already are pre-set for your chain's recommended depth-gauge clearance.

If you're using a standard depth-gauge jointing tool, you will need to know your chain's recommended depth-gauge clearance. Let's say it's 0.025 inch. Now, find the corresponding slot on the depth-gauge jointing tool. Place the tool over the chain, making sure its top surface rests squarely on the top plates. Slide it so that a depth gauge appears in the slot labeled ".025".

If the depth gauge protrudes above the slot, it's too high. Lower it using a 6-inch flat file. Stroke from inside the cutter to outside. Keep filing until the depth gauge is flush with the top of the slot. Repeat this sequence until all the depth gauges are the correct height. Remove the tool and complete the job by rounding the leading edge of each raker.

Make sure you don't file the depth gauges too low. This will create an overly aggressive, rough-cutting chain that's apt to slip the clutch. It also increases kickback risk and causes premature wear on the chain, bar and sprocket.

For best results, always follow the manufacturer's recommendations. Though many chain saw operators routinely neglect proper chain-sharpening techniques, it's all in a day's work for those who understand the principles and use the right tools. A properly maintained chain saw is one of the most efficient and satisfying tools you will ever handle.

Bill Combs is director of product service for John Deere Consumer & Professional Products (Charlotte, N.C.).

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