CIRCULAR CD SLIDE RULE

Hello Everyone!

Keeping it up with my math helping tools and after I finished my straight Cardboard Slide Rule, I decided to make another slide rule but of much easier and fast construction. I decided to go for a simple circular slide rule. Just as the previous slide rule, the intention is to have middle school students built their own and use it to get numeric ability. I know there are several apps that emulate a straight and a circular slide rule, but if you are using your phone it would be much easier to use the calculator instead of the slide rule. My dream is to have the students “thinking and reasoning” again instead of relying so strongly on technology.

The advantages of a circular slide rule is that the computing scales (C and D) of a 9.5 cm (3 3/4”) diameter tool are equivalent to a 30 cm (almost 12”) straight slide rule! So you can get a more accurate reading! The disadvantage is that as your scale is located closer to the center of the Slide Rule, the scale becomes very short and difficult to read.

Anyway, it is of a much easier and faster constriction and as it is built over a CD, it becomes very strong!

SUPPLIES
1 CD
Thin cardboard
Acrylic clear spray paint
1 transparency (very thin acrylic)
1 empty bottle of mustard/mayonnaise as the material is thicker than a bottle
Crazy glue
Rubber cement

TOOLS
Xacto knife
Cutting board
Metal ruler
Scissors
Thin saw
Heat gun
A piece of thick steel wire (hanger wire)

I found this scale on the Internet and it suit me well as the scales it has are very useful:
- C and D for multiplying and dividing
- CI for inverse
- A for squares and square root
- K for cube power and cubic root
- L for log numbers
You can download the scales I used at:
https://www.sliderulemuseum.com/REF/scales/Ying_Hum_VA3YH_Circular_SR-1.pdf

Once you print the scales (I used normal 180 paper), cut both circles and glue then on the support material. I used the CD for the larger circle and thin but sturdy cardboard for the inner circle. I used regular white glue for the small circle but I wanted the big circle to really stick to the CD so I used rubber cement.

To keep everything straight, I lay both pieces on the floor and put a lot of weight on top of them. This time I used the volumes of my Encyclopedia Encyclopedia... anything in the sake of knowledge!

Once both circles are dry, trim away any excess material. It is time to protect the printed scales with acrylic clear spray paint

Here I ran into trouble...
Once the clear acrylic paint was dry, the small disk would not rotate on top of the other one due to the roughness of the cardboard. To fix it, I cut the transparency to the size of the small disk and that fixed the problem. Now the smaller disc rotates freely on top of the large one.

I think that if you use some shiny smooth cardboard (like a detergent box) keeping the shinny side to be the contact with the larger disc, it would rotate easily and will not need the transparency circle.

Now it is time to assemble your circular slide rule for the first time. I used an old CD container to properly cut the hole into the small disc and I sanded as needed until it would fit easily in the container with no lateral movement.

Now you have to make the centerpiece so both discs spin freely but are firmly kept together laterally. Here you can use a lot of materials but to made it easy for me. I decided to use the center post of the CD container. I cut out a small piece... and it worked great!

From an empty bottle of Mayonnaise, cut out a FLAT, straight and clear piece of plastic. Trim it to an approximately size and trace the centerpiece to cut out the center hole. Cut it a tiny bit on the smaller size and sand it to the correct size. Be careful to have it to move only in an angular, it must not wobble around.

Once it is trimmed to size, trace a line in the middle and paint over it with a black marker. Wipe any excess and now you have your hairline on your cursor. Be careful that your hairline must pass over the center of the hole for the centerpiece.

Note:
I used the Mayonnaise bottle as the plastic is thicker than the one from a soft drink bottle.

Now comes the fun part!

Assemble your circular slide rule and using the heat gun, heat the steel wire. Once it is hot, place it on the edge of the CD and very gently bend and fold the plastic of te cursor over the edge of the CD. It may require several repetitions of the “heat the wire - bend the plastic” sequence to complete. Be patient.

Now it is time to perform your final assembly. You need to figure out hot to keep your circular slide rule assembled and operating. This is how I did it:

As stoppers around the centerpiece I used some foam spacers that use to hold the CDs in the container. I measure out the length of the centerpiece with the circular slide rule assembled and glue the bottom spacer into place with crazy glue. Cut te center pece to size and sand the bottom to make it smooth.

Now comes the tricky part as we do not want crazy glue slipping through the top. To avoid it, I cut out a thin transparent acrylic and insert it between the cursor and the foam spacer (cut out the inner circle a little small so the fit is tight). Place the top spacer and gently glue it in place with crazy glue. Let it dry and THAT IS ALL!

And you are... DONE!
Your Circular CD Slide Rule is complete!
Now, enjoy it as I am enjoying mine!
Final picture, my 2 homemade rules together!

Please let me know how did it go for you!
Happy calculating!