How does a saw cut wood?

1. Leo needs to cut a thick beam. He pushes a thin steel saw back and forth, and it sinks into the wood. How can a thin strip of steel do that?
2. Look closely at the bottom edge. It is a row of small, sharp teeth, all leaning the same way: forward, the way the saw is pushed.
3. As the saw is pushed, each tooth works like a tiny chisel. Its sharp front edge scrapes a thin curl off the wood. One tooth takes only a little, but there are lots of teeth.
4. Where do the curls go? Into the gaps between the teeth, called gullets. The gullets carry them along and drop them outside the wood as sawdust.
5. From the end you can see a trick. The teeth are bent a little, one left, one right. So the slot they cut is wider than the blade, and the blade slides without getting pinched.
6. Push after push, the teeth scoop out more wood and the cut gets deeper, until the piece drops off. All the wood from the slot has become a pile of sawdust.

Tap each part. The round lens shows the teeth close up, and the small picture shows the saw from its end.

• Handle. The handle fits your hand. Its shape lets you push straight along the blade, in line with the teeth.
• Blade. The blade is a thin sheet of springy steel. Thin means it only has to remove a narrow strip of wood.
• Teeth. Each tooth is a tiny blade with a steep, sharp front. On the push it scrapes off a small chip. On the pull it slides back over the wood.
• Gullet. A gullet is the gap between two teeth. Chips curl into it and ride along until they fall out of the cut as sawdust.
• Set. The teeth are bent a little to each side, first left, then right. This bend is called set.
• Kerf. The kerf is the slot the saw leaves behind. Thanks to the bent teeth it is a bit wider than the blade, so the saw does not get stuck.

Drag the saw back and forth, or press Saw. Watch the enlarged teeth, then try the other saw. What changes?

Experiment: Which saw gets through the beam in fewer strokes?

1. Saw with the big teeth and count the strokes.
2. Press New beam, choose the small teeth and saw again.
3. Compare the strokes and look at the cut edge each saw leaves.

What we found: Big teeth take big bites, so the saw gets through in fewer strokes, but they tear the wood and leave a rough edge. Small teeth take small bites: more strokes, but a smooth edge. Woodworkers pick the saw to suit the job.

Turn on JavaScript to try it yourself.

Two jobs. First cut the thick beam in 8 strokes or fewer. Then cut the thin strip and leave a smooth edge.

A knife cuts by pushing material apart. That works for soft things like bread or clay, but wood is too hard and too thick. A saw does something different: it takes the wood away, a tiny bit at a time, until there is a thin slot all the way through.

Each saw tooth is a tiny blade with a steep, sharp front. When you push the saw, every tooth scrapes a small chip off the bottom of the slot, like a line of tiny shovels digging a ditch. When you pull back, the sloping backs of the teeth slide over the wood, so the saw rests and gets ready for the next push.

The chips need somewhere to go. They curl into the gaps between the teeth, called gullets, and ride there until the teeth come out of the wood. Then they fall out as sawdust. The shovel idea is not perfect: a real shovel is lifted out to be emptied, but a saw tooth just carries its chip sideways out of the cut.

The teeth are also bent a little, one to the left and the next to the right. This is called set. It makes the slot, called the kerf, slightly wider than the blade, so the wood does not squeeze the blade and stop it. Saws with big teeth cut fast but leave a rough edge. Saws with small teeth cut slowly and leave a smooth one.

Key words

Tooth

One of the small sharp points along the edge of a saw. Each one cuts a tiny chip.

Gullet

The gap between two teeth, which carries the chips out of the cut.

Set

The small bend of the teeth to the left and right.

Kerf

The slot that a saw cuts. The wood that was in it becomes sawdust.

Teeth per inch

A way to say how big saw teeth are: a small number means big teeth.

Check your thinking

1. How does a saw get through a thick piece of wood?

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   Its teeth scrape out small chips, again and again. Each tooth takes a small chip. Hundreds of small chips add up to a cut right through.

2. What are the gaps between the teeth for?

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   They carry the chips out of the cut. Chips curl into the gullets and are carried out. Without them the cut would jam.

3. A saw's teeth are bent a little left and right. What would happen if they were perfectly straight?

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   The slot would be too tight and would pinch the blade. Bent teeth cut a slot wider than the blade. Without that room, the wood squeezes the blade and it sticks.

4. You want a very smooth edge on a thin strip of wood. Which saw should you pick?

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   A saw with many small teeth. Small teeth take small bites, so they tear the wood less and leave a smooth edge.

5. A shelf must be exactly 600 mm long. Why should you cut just beside the pencil line, on the waste side, and not down the middle of it?

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   The kerf removes a strip of wood, so cutting on the line makes the shelf too short. The kerf has width, often more than a millimeter. That wood turns to sawdust, so it has to come from the waste piece.