Nothing is more frustrating than firing up the ' ye olde' chainsaw and barely being able to break the bark on the limb. A plastic picnic knife would be more efficient than that dull chainsaw. However, not to despair, I will set forth some simple steps on how to safely sharpen a chain saw and keep it operational.
Rule number one – a saw must be sharp to cut quickly and safely.
Before being your task of sharpening, you'll need the right tools to be successful.
A filing guide and round files – check your operator's manual or manufacturer's specifications sheet to determine the size of your chain which determines the diameter of your round file.
A depth-gauge guide.
If you are planning to sharpen your saw in the field, a stump vise (optional); however, this is really for an expert. The easiest method is to carry a spare chain in the field and do your sharpening at home.

There are several manufacturers that put out filing guides. I find the best guide is the one that clamps to the chain bar and regulates the travel of the round file over the cutters. Consistent filing strokes in the correct direction will ensure a sharp saw chain. Remember there are right cutters and left cutters.

The next most important tool is a sharp round file that fits the guide. However, you must make sure it fits the size of your chain's cutters. Check your owner's manual or chain saw instruction pamphlet that comes with a new chain. Wrap round file in cloth, after coating it in a lightweight machine oil. This will maintain top shape, ready for use.
TIP: replace the file every 5 to 6 sharpening s for optimum results and prevent your new chain from banging around in your tool box.

The next step in chain saw sharpening are the rounded parts of the chain that extend upward in front of every cutter. On close examination, you'll see that they reach almost to the top of each cutter edge. The distance that cutter should extend above the round part is 0.025". This gap regulates the cutting depth of the cutters into the wood. Naturally, as the cutters wear down, the narrower this gap becomes the less efficient the saw becomes. Therefore, after approximately 3 to 4 sharpenings, the round parts need to filed down. And the depth-gauge guide lets you do this easily. Consistency is vital. Forget trying to file without a guide.

Now that we have covered the basic tool requirements and their respective functions, let us get acquainted with the chain saw itself.

There are five base parts to a saw chain:
1. left hand cutters
2. right hand cutters
3. tie straps
4. drive links
5. rivets

There are two distinct features of a cutter, a ramp like depth-gauge or raker, at the front and a gouge-like cutting element at the rear. The gulf between them is called the gullet. The cutting element has a profile that looks like the number '7'. There are other angles involved for the proper function of chain, such as the bevel angle or the side-plate angle. For us DIY, we are only concerned about the cutting edge at 30 -35 degree angle and the gap of the raker.

The are several signals that tell you when it is time to sharpen your saw:

A biggy! When the chain no longer self feeds. A properly sharpen saw chain pulls itself down through the cut. If you are pushing down on the saw to make it cut, its time to SHARPEN.
When the saw's discharge is dusty. A sharp saw will spit out nice, square wood chips.

When you realize your chain is dull, stop. Continual use subjects the power head, chain, sprocket and guide bar to unnecessary wear and tear. Not only does the equipment suffer, it wears on the operator causing fatigue, frustration and impaired judgment; so, for safety reasons it is a good reason to stop cutting when a chain gets dull.

The best place to sharpen is on a workbench. It is stable condition allowing better control of the saw and sharpening tools.

Before you start make sure the chain has proper tension on the chain bar. It should be snug on the bar, yet, able to be pulled easily by hand. Loose chains simply create more problems. Because of the razor-sharp cutting edges, leather gloves for filing are a must.

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.

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.

Hey, we're not quite finished. There is the little matter of the depth gauge.

Examine the cutter top plate, noting it's downward slope to the rear. This slope or clearance angle, makes sure that the critical corner is always the highest point. As the cutter gets file back, the corner gets lower and lower. This condition will cause the chain not to bite and lose its self feeding characteristics.

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.

A final word, for best results always follow the manufacturer's recommendations. A properly maintained chain saw is one of the most efficient and satisfying tools you will every handle.