The chain on a chain saw must stay sharp to properly pull the saw through the cut. The need to push or force the chain down is a signal it's time to sharpen your saw's chain. A small, round hand file should be used to shave off dull chain edges. The size of the file must match the size of the chain to properly grind the metal down.
Finding Your Pitch
On most chains, a string of numbers is located on the side of the chain. These numbers signify the different measurements of the chain. For sharpening, the most important measurement is the chain's pitch. The pitch measures the distance between three consecutive rivets -- the tie straps holding the links together -- divided by two. The smallest pitch is 1/4 inch, and the most common chain saw pitch is 3/8 inch. Two other common sizes are 0.325 inch and 0.404 inch. The pitch of your chain determines the size of the file needed for sharpening.
Sizing Up Files
Files used for chain saw sharpening are round. These files are specially cut to fit precisely into the hook or arch of the chain. This hook has three unique angles; all need to be sharpened precisely for the chain to cut as intended. For this reason, only use round files designed for chain saw sharpening. Chains with a pitch of 1/4 inch and 3/8 inch generally take a file that's 5/32 inch, or 4 mm. Chains with a pitch of 0.325 and 0.404 take files that are 3/16 inch and 7/32 inch, respectively. However, these file sizes are not industry-wide standards. The package your chain came in will state clearly the necessary file size.
Specially designed chains need a specially cut file to properly sharpen all the chain's angles. For most standard homeowner chains, a round file will adequately sharpen all three angles the teeth form. However, some professional and commercials chains are square-ground. This means instead of a round hook, they have a flat, square-tooth point. These chains can only be sharpened with a tapered hand file. For the depth gauges in front of every tooth, a different file will be necessary.
Flat-Filing Depth Gauges
The metal nubs in front of the cutting teeth are called depth gauges. These gauges measure out how deeply the tooth will scoop into the wood. As the chain passes through the wood, the depth gauges score the wood. On the second pass, the tooth scoops out the wood. For the teeth to work properly, the depth gauges need to be flat-filed down as the teeth get smaller and smaller. Use a crosscut flat file, and move it parallel with the ground to file the gauges down.