A sharp tool is truly a thing of beauty. A dull tool is the cause of frustration and discouragement. I wish someone had taught me as a young man how to sharpen things. It took many years to learn. I was often frustrated. I did poor work, and broke many of the projects I was working on.

There is a unique feeling that comes from passing a hand plane over a board, producing a long thin shaving, or passing a sharp knife through a fish, and with a few strokes, have it ready to hang on the rack.

The idea behind sharpening an edge is simple. Reduce the surface area of the blade so it will penetrate the wood, meat, fish, ice etc. with as little effort as possible. A sharp tool penetrates easily. A dull tool has greater surface area on the edge, and resists penetrating.

The difference between sharp and dull is most noticeable when using hand tools. When using power tools, the motor does the work.

My wife’s grandma tested her knife by holding up a hair. If she could cut the dangling hair with one pass of her knife it was sharp enough for tanning and making rawhide. I have experimented for years trying to learn how to sharpen to that level.

There are three considerations in sharpening a tool:

• At what angle is the edge formed?
• How thick or thin is the actual edge?
• How rough or smooth is the edge and sides of the blade?

Angle

Many directions for sharpening say, “Sharpen the tool at 30° or 25°.” The material we are cutting and the toughness of the steel in the blade determine the best angle to sharpen the edge. I often wonder how the manufacturer could pretend to know what I am cutting.

Picture the Extremes

How thick or thin should I make the edge?

Imagine trying to chop a tree with a splitting maul. The blade is too thick. It will never penetrate the wood deeply enough to chop down the tree.

Imagine again trying to cut down the same tree with a razor blade. The blade can penetrate the wood fibers, but is so thin it will break on first impact.

Conclusion: if a blade is too thick, even if the edge is sharp, it will take too much energy to penetrate. If the edge is too thin, the blade will break.

Direction of Force

Consider the direction of the force you are using.

If the blade is shaped like #1 on the left, much of the force used to cut (penetrate) is used in pushing the material away. Little of the force is used in parting the material, which is what you want.

If the blade is shaped like #2, the blade will penetrate easily, but the slightest sidewards motion or hard obstacle will break the thin steel. This is the shape of a razor blade. It will cut hair (and skin) well enough, but couldn’t be considered for wood or bone.

What should the angle be for sharpening a blade? Once you know the quality of steel in the blade and the material you are cutting, then you can figure out the answer to this question.

The Rule

You want an edge thin enough to penetrate easily, and thick enough to last a while.

If you are sharpening the edge often, you need to thicken it a little. If you are working too hard, thin the edge a little.

Mixed Material

If we cut only soft wood or meat, it would be easy to figure how to sharpen an edge. However, wood has knots, meat has bones, and so do fish. If we sharpen a knife to cut fish and don’t think of the bones, our knife will soon be dull.

Width of the Edge

The actual width of the edge is very important. If you were to look at a dull edge under a magnifying glass, it would look like this:

If the surface area of the edge is reduced, the pressure required to penetrate the material is greatly reduced.

With a sharp edge like the one on the left, the surface area is almost zero, and all the force can be used in separating the material. This makes a tremendous difference if you are cutting fish all day.

If the edge is chipped, it obviously has a large surface area that resists penetrating the wood, meat or fish. One chip in a knife or axe can make a tiring difference.

New axes or other tools always come with an edge that is far too thick. You must thin the edge to your needs.

As you sharpen, there will be a hair of metal that clings to the edge of the blade. In some applications, you will want to remove it, but for cutting fish or meat, that “hair edge” helps sever the meat.

Digging Tools

A hoe, pick, or shovel should be sharpened to make digging easier. The thickness of the edge is determined by the kind of dirt you are working. If it is loose soil with no rocks, the edge can be thin to cut roots. If there are hard rocks in the soil, the edge must be thicker.

A. Fiber cutting tooth

B. Chisel tooth

Saw teeth "set"
to either side of
center line

Sharpened on One Side

Some tools are sharpened on one side only. Oldtimers used to sharpen their axes on one side for chopping and shaping boat and sled parts. Shovels, hand planes, drills, circular saw blades, etc., are all sharpened on only one side. Traditional tanning and skinning knives, including ulus, are sharpened on one side.

The flat surface of the blade on the right will follow straight down a skin or wood surface without deflecting as the one on the left might.

Rough or Smooth

If an edge is rough, it will have considerable friction with the surface it is penetrating. When cutting wood, a very smooth surface makes entrance of the blade much easier. When cutting fish or meat, a rougher edge helps to tear the flesh.

Swede Saw

Years ago, sharpening a Swede (or two-man saw) was an art that everyone knew. Since the advent of chainsaws, it is but a memory, but there are principles involved that apply in other blades. A combination blade for a circular saw has the same teeth as a Swede saw and cuts in an identical manner.

A Swede saw does two things in two directions:

1. The teeth shaped in figure A cut the fibers. They must have this shape as they cut in both directions, forward and backward.

2. The tooth in figure B chisels out the severed fibers. It is very important that the chisel teeth be slightly lower than the cutting teeth mentioned above.

The chisel teeth remove the severed wood fibers to make room for the cutting teeth to go deeper.

“Setting” a saw means bending the tips of the teeth slightly outward so the cut is wider than the thickness of the blade. If there is no set to a blade, friction with the sides of the cut will tire the loggers very quickly. If the set is too wide, the loggers work too hard removing more wood than is necessary.

The sides of the blade must be smooth and rust free. The friction of a rusty saw blade in the cut is tiring, especially when it is a pitchy spruce tree. Some people lubricate the blade with a bar of soap. If hard times come, this skill will be revived quickly.

Stones, Files, and Steels

There are three ways to shape a blade:

• With a file. Files work well on softer steels.

• With a stone. Hand stones do well on hard steels, but don’t work as fast as files.

• With a sharpening steel. Steels put a good finish cutting edge on a knife to be used for meat or fish. They don’t remove much material. They shape and texture the edge. Some sharpening steels are embedded with durable diamonds. Some sharpeners are made of porcelain.

Coarse or Fine?

How much of the blade must be removed?

Coarse. If there is much steel that needs to come off, a coarse file or stone is faster. However, caution must be taken with electric grinding wheels. Friction overheats the blade so that it loses its temper1, turning the edge soft and blue. It will then dull quite easily. Fine knifemakers grind the blades under water to keep the steel from overheating and to keep the grindstone from plugging with filings.

Fine. If there isn’t much steel to remove, a fine file or stone is in order to put the finishing touches on the edge.

Some people oil a hand stone to float the ground steel filings. Other people use saliva. Either method keeps the stone from becoming glazed and plugged. It can’t cut steel with the abrasive particles hidden under a layer of debris.

Care of Files

When filing, it is important to put pressure only on the forward stroke. The teeth are strong in a forward direction. If pressure is applied on the back stroke, the teeth are damaged.

Files must be protected from moisture. They rust easily. Also, they become dull when they impact other hard metals. Oldtimers often wrapped a file in an oily cloth to protect it from rust and contact with other tools.

Pressure on back stroke.
File teeth are bent over and damaged.

Pressure on forward stroke.
File teeth are strong.

File or Stone?

How hard is the steel you are sharpening? I prefer to use a file on softer steels. If the steel of the blade is as hard or harder than the file, the file will slip on the blades surface. The result is a damaged file (expensive).

For me, files work much faster than stones. I avoid buying knives and axes that are too hard. Granted, the harder steels keep an edge longer, but they are far more tedious to sharpen.

Hardened Steel

If an axe strikes a rock, the steel at the point of impact is hardened. When an individual goes to file the axe, the hardened spot will destroy the file within a few strokes. Careless people don’t understand the damage they do to an axe by driving it into the dirt.

Sharpening a Chainsaw

There is nothing mystical about sharpening a chainsaw. Like a Swede saw, it is cutting simultaneously in two directions.

• The side plate severs the fibers.
• The top plate chisels the fibers out of the cut.

Cutting blocks of wood

The side plate angle is determined by what you are cutting. If you are cutting rather dirty wood, like driftwood, you might want to sharpen the side plate thicker at about 25°. This puts more steel behind the cutting edge for strength. If you are cutting very clean wood, you might be able to sharpen the edge thinner, perhaps 35°. A thinner side plate cuts faster and more efficiently, but dulls easier.

File too small

File too big

The top plate angle is determined by the file size. A file too small will undercut the tooth, making it very thin. A file too big will make the top plate too thick. It will scrape rather than cut the wood fibers.

Ripping

Many Alaskans make lumber with a chainsaw. It is rough lumber and puts considerable wear on a saw, but in remote locations, there is no other lumber to be had.

The top plate angle is critical. Long shavings are being peeled out with the grain of the wood. If the file is too big and the resulting edge is too thick, ripping will be painfully slow.

The side plate angle isn’t as important for ripping because ripping goes with the grain of the wood, not across it.

If the log is very clean, I use an undersized file. It gives quick, clean, long shavings (but dulls quickly if any dirt is encountered).

I often peel the trees before I rip them to remove the dirt hidden in the bark from flood waters of the past. Oldtimers peeled the trees before they cut them down with Swede saws to extend the life of the sharpened blade.

Rakers

The rakers determine how deeply the chainsaw tooth cuts.

If the rakers are too high, the tooth cannot bite into the wood. The operator has to push very hard for the tooth to cut. The increased friction of this effort quickly wears the bar and chain.

If the rakers are filed to the proper length, the weight of the saw is enough to feed the saw into the wood.

If the rakers are too short, the chain will bite too deeply into the wood and frequently get stuck. Rakers that are too short produce very rough lumber and cause excessive clutch wear.

If the rakers are of even height, cutting is smooth.

If the rakers are of uneven height, some teeth will bite more than the others. Cutting is very erratic, putting great stress on the chain. The saw can easily kick back at the operator.

Getting proper raker height isn’t important if you are only cutting a few boards. If you are going to rip much lumber at all, it is critical to file rakers to the proper height. For cutting blocks, standard raker height is .025”. For ripping, I have filed them .030” to .035”.

Activities

Note: In the following activities you are asked to use tools and blades. There is obvious danger. Be careful!

1. 1. Collect as many blades as you can. Identify tools for cutting wood, dirt, metal and food. Are they sharpened on one side or both?
   Look at the edges with a strong magnifying glass. Draw several of them.
2. How thick are the edges? Can you find a relationship between the materials they cut and the thickness?
3. Try to sharpen all of the above tools with a file. Are some of the edges harder than others? What do people use to shape and sharpen the harder steel tools?
4. Carefully test the blade of a hand plane with a file. Are both sides equally hard?
5. Tap the above blades with a piece of hard steel. Do any of them ring? (Small blades are hard to test.) What can you say about the steel that rings?
6. Carefully test a dull knife in cutting wood. Bring that same knife to someone who knows how to sharpen. Ask that person to sharpen the knife for you. Try it again. Is the difference noticeable?
7. Try cutting wood with a steak knife. What happens and why?
8. Try shaving a piece of wood with a razor designed for a man’s face. Use gloves. What happens and why?
9. Try digging with a dull shovel, particularly in a place with grass or small roots. Sharpen the shovel. Do you notice any difference?
10. Look at a dull edge under a magnifying glass. How is it different from what you expected? Can you understand why pushing such a rough surface into your work is difficult? Now look at a sharp edge with the glass. Even this looks crude. Compare both blades in ten words or less.
11. Scissors don’t cut the same way as a knife. Study scissors and describe how they cut.
12. Cut fish with a dull knife. Sharpen it and cut fish again. Estimate what percent of effort was saved by sharpening it.
13. Cut a block of wood with a dull chainsaw, timing the cut. Sharpen the chain and cut the block, again timing the cut. What is the difference?
14. Ask in the village if anyone knows how to sharpen a Swede saw. Ask the person for instructions.
15. Collect different files. What are the differences other than size? Put a piece of paper over the file and with a crayon or lead pencil do a “rubbing” of the file. Compare the imprints from the different files.
16. With an old file, file aluminum (like an old prop). What problem arises?
17. Compare the two sides of a sharpening stone. Which one is for faster and which one for finer sharpening?
18. Sharpen a knife for meat and finish the edge with a butcher’s steel. Cut a little meat. Now try the same edge on wood. What do you notice? Strop the edge back and forth on a piece of leather for a while and then try again on both meat and wood. What do you notice? Which edge is better for meat, the rough or smooth one? Which is better for wood?
19. Students should each share a story of a time they cut themselves being careless with a tool.
20. Draw or trace a Swede saw blade identifying the two kinds of teeth. Describe to someone else how each of these relates to a modern chainsaw tooth.
21. Picture in your mind what would happen if a Swede saw blade had only this kind of teeth:

22. Picture what would happen if it had only this kind of teeth:

23. If you can get some beaver or muskrat teeth, test the front and back for hardness. Explain how they are self sharpening.
24. Picture in your mind what is happening when the rakers on a chainsaw chain aren’t filed evenly. Can you imagine the jerking of the chain as some teeth bite deeper into the wood than others?
25. Put pressure on a bathroom scale with a fish cutting knife. Record how many pounds you can assert. Now put pressure with an ulu. How many times more pressure is possible with the ulu? Test the whole class. First let students estimate, the test. How many times more pressure can the ulu put than the knife? Average the results. How does the knife serve as a lever?

Student Response

1. What happens when an edge is too thick?
2. What happens when an edge is too thin?
3. What are the three considerations in sharpening a tool?
4. What two things determine how thick or thin an edge can be?
5. Draw a shovel blade sharpened for rocky ground. Draw one sharpened for ground with no rocks, but lots of roots.
6. What is the difference between cutting meat and wood in terms of the friction of the blade?
7. Draw a Swede saw blade. Which teeth are for cutting fibers and which for chiseling the severed fibers out of the cut?
8. Which is harder: a file or a sharpening stone?
9. An axe of fairly soft steel has a hard spot. What might cause this?
10. Why do people put oil or saliva on a sharpening stone?
11. Why do we put pressure only on the forward stroke of a file?
12. Draw a chainsaw tooth. Label which part severs fibers. Which part chisels the fibers from the cut?
13. What do the rakers do on a chainsaw tooth? What happens if they are too high? Too low?
14. Why should the height of the rakers be the same on all teeth?

Math

1. When Sal cuts fish she puts about 15 lbs of pressure on her knife. She finds that sharpening reduces the surface area of her cutting edge by 30%. How much pressure will she apply to do the same work?
2. Hank can cut a block of wood in 30 seconds when his chainsaw is sharp. It takes 1.5 minutes when it is dull. If he can cut a tree into blocks in 25 minutes with a sharp saw, how long will it take with a dull saw?
3. Hank also discovered that he could increase the speed of ripping 35% by filing the rakers on his saw from .025 to .040. If he could cut 350 board feet per day before, how much lumber can he cut with the rakers filed properly?
4. Hank estimated that it would take him 5 days to cut the lumber he needed. Once he filed the rakers, he cut 35% faster than expected. How long will it take him to cut the lumber now?
   Let “x” equal the amount he could cut before filing the rakers.
   [(x v .35x) z 5]