## Thursday, December 23, 2010

### Kindergarden science - Paper Cogs

Cogs are a corner stone of mechanics. Although they are rather simple to make they tend to have a low tolerance to faults. The key to getting something that works in the end is to measure them as precise as possible at the start.

You can look below to construct the cog pattern from scratch or you can print out 2 patterns and paste it on to the cardboard.

Stage 1) Create some interest
Ask your kid about a clock(one with the cogs showing is best) how it turns and works and show her pictures of the insides. Ask her if she wants to make some cogs.

Stage 2) Gather materials
Get the cardboard with the cog patterns on it. Get scissors tape and a sharp pencil or Japanese chopsticks.

Stage 3) Cut it out.
Cut the cogs out. Don't worry to much about being too precise, just get them to follow the lines. My daughter was able to cut it quite neatly when I showed her to cut straight lines, even despite the cardboard thickness. Also clip the points of the teeth so they are flat (the points are weak and will easily get jammed). Cut a long strip of cardboard as the backing to the cogs. Then the adult should cut 2 cardboard pins(shown on the pattern) from the left over cardboard.

Stage 4) Assemble.
Place the two cogs on the backing board. The teeth should inter-mesh with about 1-2mm of gap between the two cogs. Then with Japanese chopsticks or a sharp pencil punch a hole in the middle of the cog and though the backing board. Push in the cardboard pins and fold them over on the backside and tap them to the backing board.

Stage 5) Test
Spin the cogs and check that they work correctly. Draw arrows to show (and discuss) how the cogs turn around.

What they learn
• Basics of mechanical devices
• Basics of rotational dynamics

Appendix
How to construct the Cog pattern from scratch.

0.1) First create a compass. Tape 3 sharpened pencils together in a triangle with of the 2 leads about 5cm apart.
0.2) Get some thick cardboard, thicker is better.
0.3) Draw a circle with the compass.
0.4) Then rule a straight line through the center of the circle, using the edge of the ruler to measure the 90 degrees draw a second line through the center to form a +.
0.5) Where each of lines touch the circle place the compass point and draw a lines marking the circle both above and below. These marks are exactly +/- 60degrees from the line. Once finish you will have marks on the circle at every 30 degree(including the + lines). There should be a total of 8 points marked and 4 points where the lines cut for a total of 12.
0.6) Now take the compass again an draw a circle around to of the neighboring marks. The two new circles will cross at 2 points, 1 the center of you main circle and another point outside of the circle. Draw a straight line through the 2 points where they cross. This forms the 15degree mark on the circle.
0.7) With the ruler place the corner of it on the 15 degree line facing into the main circles middle, and with the sides touching the marks on the outer circle. Trace its edge.
0.8) repeat for other 11 marks.
0.9) repeat for another cog.

Sounds complex but its quite simple to do.