Sharpening Chain Saws
Posted: Sat Jan 16, 2021 1:34 pm
I made my primary living for about 25 years as a logger. The old school chain saw toting type. I was approached by a few people over the years when they knew what I did saying that even though they had filed the teeth on their saws sharp the saw still wouldn't cut very good and would I look at it. It was always the same problem but I usually found one or two more problems as well. Since a chain saw is a woodworking tool, although only surpassed in crudeness by splitting mauls and axes, I thought it might make a useful topic.
Why it's not cutting
Saw chain teeth include a component called a raker or depth gauge as shown in the included illustration. It's function is to limit just how much bite the tooth can take. Too little and all you cut is sawdust and too much and it grabs too hard and pulls the saw into the cut, stalls it, and pulls you off balance. Router bits and saw blades also have this feature. Ideally a sharp chainsaw with the rakers filed to the correct height should produce a shaving that is about a 1/4" to 3/8" in length and width. This is usually what the primary problem is. Chain saw teeth taper in height so that the flat top of the tooth doesn't drag in the cut. As you file the tooth gets shorter. If you don't file the raker shorter as you wear the chain then it eventually gets so high in relation that the tooth stops cutting. The raker should be between .065 and .080 thousandths of an inch lower than the top of the teeth. My preference is .080". Filing the rakers is done best with a raker file. The difference between a raker file and standard file is that a raker file is smooth on it's edges. While I file saw teeth until they are sharp I file the rakers the same number of strokes each (usually 3 strokes when I decide they need filing).
There are filing gauges to show when rakers need filing and they usually come in the tool kit with the saw. My favorite is the one from Stihl which has a tang for cleaning the chain groove on the bar. When you lay the gauge on top the chain it shows what the chain attack angle is (should be either 30 or 35 degrees) and if the raker sticks up higher than the groove in the gauge then they need filing. Personally I prefer that they be as high as the bottom side of the gauge which is about .080 thousandths or a bit more. Rakers that are too high won't bite and you wind up having to force the saw into the cut. If filed too far down the saw tends to grab the wood and stalls in the cut and can be hard to control
The usual secondary problem
The other problem I usually see is that there hasn't been enough down pressure put on the file when filing the teeth. This affects the cutting angle on the teeth. When filing properly the file should be right on top of the side plates that join the chain parts together. This produces a deep gullet and the file creates a roughly 25 degree cutting angle same as for chisels and plane irons. If you aren't putting enough down pressure on the file when you file the gullet gets shallower and the angle that the file and tooth meet at is more of a scraping angle, probably around 10 to 15 degrees.If you have this problem the the first step to remedy that is to take a file and file straight down with no regard to the cutting edge of the teeth. The illustration shows the difference. Novice filers also have a tendency to file a radius on the teeth. This is caused by swing your elbow in an arc as you push the file forward. It's not a deal breaker but a properly filed tooth is angled at 30 to 35 degrees which produces a shearing cut which is more efficient. It may also help to rotate your wrist as you file.That can help to keep your elbow in line.
Saw won't cut in a straight line
This is caused by one of 3 reasons. First is that you may have hit something and dulled the teeth on one side of the saw. Second is that you have filed the teeth shorter on one side or at a radically different angle. And last is that the groove on the saw is worn too wide. If this is the case you can wobble the chain side to side excessively. In some cases you can go to a chain with thicker drivers, the part that fits in the groove. All my big (greater than 50cc) saws use 3/8" gauge chain (the distance from the center of one rivet to the center of the next rivet) and it comes in .050" .058", and .063" gauges. Stihl comes standard with .050 and Husqvarna comes standard with .058. Sometimes you can move up one gauge for a chain or two. Otherwise the bar needs replacing.
Filing tips
Always wear gloves when filing. Some of the worst finger cuts I've ever had is when I didn't. Expect to use 2 to 3 files per chain. Don't try to file with a really dull file file. Dull files tend to skate and since you're putting extra pressure on it trying to get it to work you are more likely to cut your finger. You also tend to not put enough down pressure on the file when they're dull which affects the cutting angle of the teeth.
Because saw teeth taper in height to reduce drag it means that as the tooth wears the file you started filing with may wind up too large a diameter to put the proper angle on the tooth edge. You can fix that by going to a smaller file.With 3/8 gauge chain you start with a 7/32" file so you can go down to a 3/16" near the end of the chain. With .325 gauge chain which is common on 50 cc saws and smaller you can start with a 3/16" and go down to 5/32". The 5/32" files don't tolerate much side pressure so be careful with them.
Don't worry about filing down into the side plates on the chain if you need to to get the right cutting angle on the teeth. In several hundred chains I only ever remember breaking links around the rivets. By the time you file back far enough to be close to a rivet the chain is worn out anyway.
Why it's not cutting
Saw chain teeth include a component called a raker or depth gauge as shown in the included illustration. It's function is to limit just how much bite the tooth can take. Too little and all you cut is sawdust and too much and it grabs too hard and pulls the saw into the cut, stalls it, and pulls you off balance. Router bits and saw blades also have this feature. Ideally a sharp chainsaw with the rakers filed to the correct height should produce a shaving that is about a 1/4" to 3/8" in length and width. This is usually what the primary problem is. Chain saw teeth taper in height so that the flat top of the tooth doesn't drag in the cut. As you file the tooth gets shorter. If you don't file the raker shorter as you wear the chain then it eventually gets so high in relation that the tooth stops cutting. The raker should be between .065 and .080 thousandths of an inch lower than the top of the teeth. My preference is .080". Filing the rakers is done best with a raker file. The difference between a raker file and standard file is that a raker file is smooth on it's edges. While I file saw teeth until they are sharp I file the rakers the same number of strokes each (usually 3 strokes when I decide they need filing).
There are filing gauges to show when rakers need filing and they usually come in the tool kit with the saw. My favorite is the one from Stihl which has a tang for cleaning the chain groove on the bar. When you lay the gauge on top the chain it shows what the chain attack angle is (should be either 30 or 35 degrees) and if the raker sticks up higher than the groove in the gauge then they need filing. Personally I prefer that they be as high as the bottom side of the gauge which is about .080 thousandths or a bit more. Rakers that are too high won't bite and you wind up having to force the saw into the cut. If filed too far down the saw tends to grab the wood and stalls in the cut and can be hard to control
The usual secondary problem
The other problem I usually see is that there hasn't been enough down pressure put on the file when filing the teeth. This affects the cutting angle on the teeth. When filing properly the file should be right on top of the side plates that join the chain parts together. This produces a deep gullet and the file creates a roughly 25 degree cutting angle same as for chisels and plane irons. If you aren't putting enough down pressure on the file when you file the gullet gets shallower and the angle that the file and tooth meet at is more of a scraping angle, probably around 10 to 15 degrees.If you have this problem the the first step to remedy that is to take a file and file straight down with no regard to the cutting edge of the teeth. The illustration shows the difference. Novice filers also have a tendency to file a radius on the teeth. This is caused by swing your elbow in an arc as you push the file forward. It's not a deal breaker but a properly filed tooth is angled at 30 to 35 degrees which produces a shearing cut which is more efficient. It may also help to rotate your wrist as you file.That can help to keep your elbow in line.
Saw won't cut in a straight line
This is caused by one of 3 reasons. First is that you may have hit something and dulled the teeth on one side of the saw. Second is that you have filed the teeth shorter on one side or at a radically different angle. And last is that the groove on the saw is worn too wide. If this is the case you can wobble the chain side to side excessively. In some cases you can go to a chain with thicker drivers, the part that fits in the groove. All my big (greater than 50cc) saws use 3/8" gauge chain (the distance from the center of one rivet to the center of the next rivet) and it comes in .050" .058", and .063" gauges. Stihl comes standard with .050 and Husqvarna comes standard with .058. Sometimes you can move up one gauge for a chain or two. Otherwise the bar needs replacing.
Filing tips
Always wear gloves when filing. Some of the worst finger cuts I've ever had is when I didn't. Expect to use 2 to 3 files per chain. Don't try to file with a really dull file file. Dull files tend to skate and since you're putting extra pressure on it trying to get it to work you are more likely to cut your finger. You also tend to not put enough down pressure on the file when they're dull which affects the cutting angle of the teeth.
Because saw teeth taper in height to reduce drag it means that as the tooth wears the file you started filing with may wind up too large a diameter to put the proper angle on the tooth edge. You can fix that by going to a smaller file.With 3/8 gauge chain you start with a 7/32" file so you can go down to a 3/16" near the end of the chain. With .325 gauge chain which is common on 50 cc saws and smaller you can start with a 3/16" and go down to 5/32". The 5/32" files don't tolerate much side pressure so be careful with them.
Don't worry about filing down into the side plates on the chain if you need to to get the right cutting angle on the teeth. In several hundred chains I only ever remember breaking links around the rivets. By the time you file back far enough to be close to a rivet the chain is worn out anyway.
