Introduction: CARDBOARD SLIDE RULE

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About: Hello Everyone! I am a GrandParent that loves to build “things” and I am also an engineer!

HELLO EVERYONE!

As an “old school engineer”, I studied the first 3 years of my engineering degree with a slide rule. I witness how the first electronic calculator introduced in 1972 (HP35) almost immediately caused the sudden death of my calculating tool. Even so, I used my slide rule until 1975 and have always been amazed by it and how it works... It has a genius design! I still have my original slide rule and even use it from time to time... (see picture)

Later on, I became a college professor and was always amazed by the difficulties my student had for “feeling” the numbers. As an example I could ask them estimate the volume of a glass of water and will get answers in the order of several cubic meters! When challenged they will immediately reply showing me their calculator “Look! It is the answer I get with my calculator.”

I am now convinced of how useful is a slide rule and started to look around if any is available and found out they do not make them anymore. Simple electronic calculators have such a low price that nobody seems to be interested in a slide rule that cannot add or subtract. ... by the way, adding and subtracting are the only operations a slide rule cannot do. For learning these operations, an abacus is the best!


A slide rule would help to have better “thinking students”, I decided to built one! Instructables has at least 2 similar projects posted. I study them them and decided to built a simple, cheap and challenging slide rule that would be of use for students of middle and high schools. Even better if it is a project they could do with their teacher!


Many thanks to the authors of these instructables!
https://www.instructables.com/id/Custom-Slide-Rule...
https://www.instructables.com/id/Making-Your-Own-S...

Supplies

The supplies are simple and easily available. I list the ones I used but it was only because I already had them at home.
- Cardboard (thick and thin)
- 180-220 paper
- White Glue
- Acrylic transparent paint spray

The tools you need with this Instructable are:
- Cutting board
- Xacto knife
- Metal ruler

Step 1: SLIDE RULE SCALES

The real secret of a slide rule are the scales! They are the ones that “perform” all the calculations. Fortunately, the internet is full of sites that provide good quality pdf scales we can print.

As my calculating tool is designed for middle and high school students, I selected the following scales
- basic scales (C and D)
- square value scale (A And B)
- cubic value scale (K)
- inverse scale (CI)
- trigonometry scales: Sin/Cos (S), Tan/Cot (T)
- logarithm scale (L)

I found all these scales in the instructable posted by Phil B.

https://www.instructables.com/id/Making-Your-Own-Slide-Rule/

The template is for a “paper slide rule” but I was unable to make it work properly, probably because my paper was not thick enough.

I printed my template on 180 paper

Note:
You can customize your own slide rule with the scales you will need and even do it using both sides. In this case, we only use the top side.

Step 2: CUT OUT THE PARTS OF THE SLIDE RULE

Using your Xacto Lnife and metal ruler, cut out the 3 parts of your slide rule and glue them to a thick and stiff piece or cardboard. to prevent it from wtapping, set it to dry under heavy weight. Due to the nature of the project, I considered that using my 2 books of Calculus as weight was appropiate.

Once dry, trim out any excess.

Step 3: MAKING THE SLIDER AND SIDES

1. Take a piece or thin cardboard and glue the slider over it. Be sure to have enough thin cardboard on top and bottom of the slider to later glue to your other parts. Do not forget to put some weight over it to keep it from wrapping.
2. Locate the top and bottom parts and separate them from the slider with a thin piece of cardboard or paper so it will move easily.
3. Trace. The outside of both pieces on the thin cardboard and cut it to size
4. Trace a parallel line on both sides of the slider and they will mark your slider edge. Cut it out and you will have the pieces that go under your top and bottom parts.
5. Glue the top and bottom parts. Please, note that my final picture shows the top part bellow and the bottom part above (my apologies)

Step 4: MAKING a SPACER

Glue a strip of 180 paper under the top and bottom parts to create a spacer that will ease the travel of the slider

Step 5: ASSEMBLING THE SLIDE RULE

1. Assemble your slide rule making sure that all number “1” of scales C,D,A,B line up
2. Put glue ONLY on the top and bottom sides
3. Set the assembled slide rule over thick and stiff cardboard
4. Check the alignment again
5. Slide out the slider
6. Let it dry under weight

Step 6: CHECK IT OUT!

Once the glue is dry, trim the excess cardboard and you can slide the slider into position. By now, you have a perfectly working slide rule (minus the cursor). It may be necessary to work out the slider to make it slide easily from side to side.
As the slide rule has only one working side, I used the back side to put up some useful conversion factors.

To preserve the printing of the scales, I sprayed several thin coats of transparent acrylic paint. Do it on a well ventilated place.

Step 7: MAKING THE CURSOR

This final step is crucial for the accurate performance of the slide rule.
1. Measure the distance between the left most mark on any scale and the edge of the slide rule. In this case, it is 12 mm
2. This means that the maximum width of the cursos is the double of the above distance: 24mm
3. Is selected to do a cardboard cursos so I made a frame 24 mm width and the length corresponds to the width of the slide rule PLUS 2 cardboard thickness
4. Originally I made a frame for the bottom part of the cursor but then decided for a full piece of cardboard
5. For doing the hairline, cut out a piece of transparent plastic and trace a line over the number 1 of the top and bottom parts of the slide rule. Draw a line wth a marker over the trace an clean the plastic. Now you have a double hairline.
6. Glue the cursor frame locating the hairline right in the middle and trim the excess transparent plastic
7. Locate the bottom part of the cursor right on the edge and very careful align the hairline with the number “1” of te top an bottom parts and glue it into place.

Step 8: FINAL ADJUSTMENTS

Now it is time to adjust your slide rule...

In my case, the width of my slide rule was not constant and the cursos fitted right in one end but was loose on the other end. To correct this, I glue a trim made of thin cardboard and then sand it to the correct size so the cursor travels freely but in a tight matter, it should not wobble as we move it.

Step 9: FINISHED AND TIME TO ENJOY IT!

The slide rule is complete!
Time to brush up on the ability of using it!
I placed a simple table of useful conversion factors on the back of my slide rule! They always come in handy!

Final picture... my 2 slide rules!

Note:
I am considering glueing additional small cardboard pieces at the edges of the slide rule to work as stoppers of the cursor