do the size and shape of a saw’s teeth affect how well it can cut a piece of wood?
Based on the archaeological record, the fi rst true saws were used about 4,000 years ago. This is a time when people fi rst began fashioning tools from copper. drawings from ancient egypt dating back to about 1,500 B.C. show workers using a small saw to cut wood. The curved metal blade had a serrated edge and was attached to a wooden handle. These early saws were about the size of a large butcher knife, and they cut by ripping across the surface of the wood. in this activity, you will test to see why a saw blade can effectively cut through a piece of wood and see how the size of the teeth controls the way the blade cuts.
● crosscut hand saw for wood with large teeth
● hack saw or coping saw with small teeth
● metric ruler
● 12 in. (30 cm) piece of 2-in. x 4-in. (5-cm x 10-cm) wood or similar sized wooden board
● safety goggles
● work gloves
Safety Note This activity requires adult supervision. Make certain that
you and anyone near you are wearing goggles and work gloves during this
activity. Please review and follow the safety guidelines.
1. Put on the work gloves and goggles. Pick up the saw with the small teeth (hack saw) and examine the edge of the blade closely. Use the ruler to measure the length of the teeth. observe the shape of the teeth and record your description.
2. Take one piece of wood and lay it fl at on a sturdy surface (table or workbench). if possible, ask another person to assist you by holding the block of wood. Make sure that they put on work gloves and goggles, too. hold the saw by the handle and begin cutting across the top of the wood. observe in which direction the saw cuts and what happens to the sawdust as you cut. After making fi ve passes with the saw, remove it from the wood and examine the groove cut into the wood. compare the width of the groove with the width of the saw blade. Record your observations.
3. Pick up the saw with the large teeth (crosscut saw) and examine the edge of the blade closely. Use the ruler to measure the length of the teeth. observe the shape of the teeth and record your description.
4. hold the crosscut saw by the handle and place it on the wooden board so that it is about 3 in. (7.5 cm) away from the cut you made with the hacksaw. Begin cutting across the top of the wood with the crosscut saw.observe in which direction the saw cuts and what happens to the sawdust as you cut. After making fi ve passes with the saw, remove it from the wood and examine the groove cut into the wood. compare the width of the groove with the width of the saw blade. Then compare the width of the first groove you cut with the second groove. Record your observations.
1. Based on your observations, in which direction did the hack saw cut, when you pushed it or when you pulled it? in which direction did the crosscut saw cut?
2. how did the width of each groove compare with the width of the saw blade that cut it? Why do you think this was so?
3. how did the width of the two grooves compare with each other? Why do you think this is so?
4. how did the depth of the two grooves compare with each other? Why do you think this is so?
5. Which saw required less force to cut width? Why?
What’s Going On?
When a saw cuts into a piece of wood, it makes a groove called a kerf. in order to keep the saw blade from “binding,” or getting stuck in the groove as it cuts, the kerf has to be wider then the blade itself. To do this, the teeth of the blade have to be “set” in opposite directions along the length of the blade. if you examine the two blades, you will see that half the teeth are bent to the left, and the other half are bent to the right. As the saw cuts into the wood, it also has to remove the sawdust from the kerf. otherwise the wood fi bers will fi ll up the groove and make the blade stick. To do this, the teeth on most saw blades are curved slightly so that they rake the sawdust out of the groove as they cut.
Most saw blades cut in only one direction. in general, saw blades that are small and thin like hack saws and copping saws cut the wood on the pull stroke. Saws with large thick blades usually cut on the push stroke. This is because a thin blade will bend more than a thick blade. By cutting on a pull stroke, the thin blade will stay straight as it cuts. if a thin blade cut on the push stroke, it would bend, causing the cut to curve. in addition, because more force is used on the cut stroke, pushing on a thin blade could make it snap.
Generally speaking, the larger the teeth on a saw blade, the more force is required to cut with it. Larger teeth also will result in a wider kerf with a more jagged edge. Blades with small teeth are used for fine detailed cutting, while blades with large teeth are used for coarse cutting.
1. Most hacksaws cut on the pull stroke, while most large cross cut saws cut on the push stroke.
2. I n both cases, the width of the groove cut by the saw blade was wider than the blade itself. This is because the teeth of the saw are bent slightly to the right and left along the edge of the blade. This keeps the blade from getting stuck in the groove as the blade cuts down.
3. The groove cut by the saw with the larger teeth was wider because the teeth were longer and more bent.
4. The groove cut by the saw with the larger teeth was deeper because the teeth were longer and removed more fibers as they moved cross the block of wood.
5. The saw with the smaller teeth requires less force to use because the small teeth did not make as large a bite in the wood.
Rock Saws and Cutt ing Concrete
As it turns out, not all saws have wedge-shaped teeth. Saws used to cut rocks (lapidary saws) and concrete use a different approach. Instead of having little wedges cut and chop at the surface, these blades use an abrasive material to grind their way through the rock. To do this, the abrasive on the blade has to be harder than the rock itself. Two of the more common abrasive materials used today are carbide steel and industrial diamonds. Yet, the rock saw is not a modern invention. In ancient Egypt, people were using a type of abrasive saw to cut through stone, too. Archaeologists believe that the rock saws they used had no teeth. Instead, these devices simply slid back and forth over the surface of the rock. The secret to their cutting power was a layer of wet quartz sand that was placed under the saw blade. Quartz is harder than many of the other minerals that the Egyptians were cutting. Examples of these primitive rock saws have yet to be found, but scientists do have several lines of evidence to show that they were used. Ancient drawings show workers cutting stone with sawlike instruments, and many of the stone coffins found in burial sites have saw marks on their lids.
Machines Made Simple
People usually don’t think of something simple, such as a hand axe or a saw, as a machine. In our modern world, the word “machine” is usually reserved for complex mechanical devices driven by engines or motors. It the strictest sense, though, all simple tools are machines: They all help to convert mechanical energy into useful work. To a scientist, the word work means moving an object over a distance. Over the years, scientists have come to recognize six basic machine types, all of which are classified as simple machines. We’ve already discussed one of them: the wedge. The other five are the screw, the inclined plane, the lever, the wheel and axle, and the pulley. As the term simple machine suggests, these devices are basic. They have few or no moving parts. They are important because they are used in many ways and are often components in much larger, compound machines. Simple machines work by trading force for distance. If you recall from Experiment 3: How a Saw’s Teeth Impact Cutting, a saw with small teeth requires less force to push, but you have to move it many more times, compared with a saw with large teeth. In the next two sections we’ll take a look at how all of the simple machines work. First, we’ll examine a second type of device put to work by humans. It’s the simple machine called the lever.
Looking at Levers
Like the wedge, a lever is a device that is used in many places. Levers are at work on a teeter-totter (or seesaw) on a playground, and on the handle that flushes a toilet. Early on, humans discovered that a lever could help them accomplish tasks that could not be done with a wedge alone. Take hunting, for instance. Although a simple hand axe was great for cutting and skinning an animal after it had been killed, it wasn’t useful for actually hunting game. That’s because in order to use a hand axe, you had to be very close to the animal. This didn’t work well for fast animals, such as deer, which could simply run away. A hunter could throw a hand axe at a large animal, but even if his aim was accurate, the blade would just bounce off the animal’s hide.
The solution turned out to be a simple one. By attaching the blade to the end of a long stick, the spear was born! Scientists aren’t sure exactly how long humans have been making and using spears, but based on the evidence, it’s believed to be about 100,000 years. Before people started using spears for hunting, they were probably using pointed sticks for digging up roots. A spear is such an effective weapon because it gives a person a type of mechanical advantage called leverage. In Experiment 4: Lifting with Levers, we’ll dissect how a lever works, so you can see for yourself where leverage comes from.