What do a pencil sharpener, a screw, a scissor jack and an inclined plane have in common? Find yourself several different kinds of screws and take a look.
Common screws have one of two heads. The top one is a straight slot and takes a regular screwdriver. The bottom one has a cross slot and takes a Philips screwdriver.
Note: The raised metal going around a screw is called a thread. The top is called the head. Some screws have slots on their heads and take straight screwdrivers. Some have crossing slots and require a Philip’s screwdriver.
Question: How do screws work?
Material:
Several different screws, same diameter but different threads
Screwdrivers for the screws
Block of wood with drilled holes the size of the screws in it
Ruler
Procedure:
Examine one of the screws closely to see how the threads are arranged
Hold a screw between your finger and thumb turning it with the other hand
The screw turns into the wood a tiny bit then the head cuts into my fingers as I try to turn it a little more and can’t make it budge.
Put the end of the screw in a hole in the board and try to turn the screw several turns using your fingers then use a screwdriver.
Take that screw out of the hole
Find two screws with different threads, one with threads far apart and one with threads close together
Start these two screws in the board until they stand up by themselves
The head is shaped like a wood screw’s head so this screw is used for wood. Fine threads are often used for fine work such as furniture.
Measure the height of the two screws
Turn each screw two complete revolutions
This was the longest screw as well as the one with the finest threads. If you measure from one thread down two, this should be the same as the amount turning the screw twice will put it into the wood. For this screw that was 0.2 cm.
Measure the height of the two screws
Observations:
How are the threads arranged on the screw?
How does it feel to turn a screw with your fingers?
How does it feel to turn a screw into the wood?
How it feels to turn a screw with a screwdriver
This is a wood screw with definite threads not too far apart but not real close together either. The top will fit into the wood so it won’t catch on anything rubbed over the wood later.
Height of the screws:
Wide thread
beginning
ending
At first glance this screw went in the farthest but it was the shortest so it really only went in 0.4 cm in two full turns.
Narrow thread
beginning
ending
Conclusions:
If you could unwrap the threads on a screw, what simple machine would they become? Why do you think so?
Why do we use a screwdriver to put in a screw?
Compare how fast a screw with wide threads goes in to one with narrow threads.
Looking at the three screws it is easy to see the right one has fine threads and the left on has coarse threads. The middle one is in between the other two.
What I Found Out
I had three screws. One had coarse threads. One had fine threads. One was in between.
When I held a screw and turned it, it crawled up between my fingers. It felt like my fingers were sliding up the threads.
Of course I can’t really take the threads off. But if I could, the thread would become a slanted line and be like an inclined plane from the bottom to the top of the screw. I think that because the thread is a continuous line going up the shaft.
This is a deck screw. It has widely spaced threads to make it easy to put it into coarse wood. The top is angled to fit into the wood smoothly so the deck surface will be smooth.
Trying to put a screw into a hole in the wood using fingers does not work. The very tip will go in but then the fingers can’t turn it anymore. A screwdriver gives more power to my hand and makes the threads go into the wood.
After two full turns the coarsely threaded screw was only 1.5 cm high. It went in 0.6 cm.
My screw with fine threads started at 2.6 cm and ended at 2.3 cm so it went in .3 cm. The medium threads started at 1.7 cm and ended at 1.3 cm going in .4 cm. The coarse threads started at 2.1 cm and ended at 1.5 cm going in .6 cm.
Coarse threads go in much faster than fine threads.