Making Your Own Slide Rule

I have thought about making my own slide rule, and finally did.  This Instructable is partially a description of how I did it and partially an evaluation of the effort.

Back in 2008 Instructables member legionlabs published "How to use a slide rule."  In the comment section Instructables member Mr ross made reference to a May 2006 article in Scientific American magazine titled "When Slide Rules Ruled."  Once that article was available on-line in PDF, but has since been removed.  Scientific American still makes it available, but at a cost of about $7 US.  The article included a downloadable PDF file pictured in the graphic for this step.  It is a printout for a fold-up paper slide rule.  I decided I wanted to make one of these just for fun, but mount it on wood and Plexiglass.  After printing the PDF file, I enlarged it with a copy machine to about 150%.  The idea was that the scales would be larger, easier to read, and more precise. 

I experienced a couple of disappointments with this project, and will discuss them later.  For that reason, I will not give a lot of detailed steps on the construction of my slide rule.  But, I will share a couple of things others may want to use.  

I used Plexiglass screwed to 1/2 inch plywood.  In order to hold the slider (center movable piece) in place and still allow it lateral movement, I used a dovetail router bit to shape the edges of the Plexiglass pieces.  This slight bevel holds the slider down, but allows it movement.  

I covered the Plexiglass pieces with double stick cellophane tape.  Then I pressed the paper scales to the tape.  I covered the paper scales with cellophane packing tape about 2 inches wide.  I trimmed the edges with a very sharp knife.  

The screws near the D, L, & S scales fit into holes no larger than the screw diameter.  The screws near the T, K, & A scales fit into holes that are over-sized so the top slide rule stator can be moved a little left-to-right so the left indices of all scales align properly.  The over-sized holes near these scales also allow the top stator to adjust in and out toward the slider for the desired smoothness of movement.  

The selection of scales available is basically those found on a "trig" rule.  These are the scales found on the Nestler 23 rule favored by Albert Einstein, Werner von Braun, and Sergei Korolev (chief designer for the Russian space program).  This link includes a photo of von Braun's Nestler 23 rule.  Most of the scales were not labeled on the Nestler 23.

I soon noticed that the paper scale and the double stick tape began to separate from the Plexiglass at the ends of the slider.  Although not easy to see, I folded a piece of cellophane tape over the ends of the slider and trimmed it.  There is a slight discoloration and a faint line.  Soon I removed the stator pieces and did the same with them.

I have not made a cursor for this slide rule, yet.  Most calculations I have ever done on a slide rule involve basic multiplication on the C and D scales.  The D scale is directly below the C scale visible in the photo.  It is not difficult to set the factors and to read the answer on these scales without a cursor.  When using the A and B or S, T, & K scales, a cursor becomes more necessary because the answer needs to be read on the C or C1 scales.   See the second photo for this step.  I used a note card aligned with the top edge of the rule as a cursor.  It aligns with 27 on the second scale from the top, which is the K scale.  Follow the edge of the card downward and you see it also aligns with 3 on the D scale.  The cube root of 27 is 3.  Below the K scale is the A scale.  The cursor aligns with 9 on the left half of the A scale.  The square root of 9 is 3.   

Notice one inherent problem with the PDF file that serves as a pattern for this slide rule.  See the first photo again.  The left index for the B scale is aligned with 4 on the left half of the A scale.  The left index on the C scale below it should very accurately indicate 2 because the C scale gives the square roots of numbers on the A or B scales, but the left index of the C scale is a full line width to the left of the 2 indicating 1.99 as the square root of 2.  Calculations done on the C and D scales of this rule are quite accurate, but I found several problems on the A and B scales that are not due to my printer or my construction methods.      

The A and B scales are copies of the C and D scales, but in half-size so they are doubled.  Multiplication and division problems that may be solved on the C and D scales may also be worked on the A and B scales.  Because the A and B scales are smaller, the answers are more difficult to read with accuracy.  Notice the yellow arrow in the photo.  The 1 on the B scale is aligned with the 2 on the A scale.  Move to the right on the B scale and notice the green arrow.  The 3 aligns nicely with the 6 on the A scale.  2 x 3 = 6.  Although not marked with an arrow, the 4 aligns nicely with the 8, too.  But, when you look at the 5 on the B scale, it is at least the width of a line off of the 1 (or 10) on the A scale.  It would appear that 2 x 5 now equals 9.97, or thereabouts.  The PDF file for this slide rule has some problems that make it unfit for anything but practice by a beginner.  

If you merely want to practice learning to use a slide rule, there is a nice virtual slide rule on-line.  It is a replica of a PIckett 10 inch aluminum slide rule.  You can click and drag any part of the rule.  Some readings may be slightly inaccurate, depending on quirks in your Internet browser.  That was my experience.  I learned to use a slide rule with a Pickett 120 Trainer.  It is a printed plastic rule, but quite accurate.  The original manual for it can be downloaded here.  Use your browser's "Find" function to search for "M52."  It is a simple manual, but very adequate and covers the scales on both the virtual slide rule linked above and the PDF slide rule I built.  

Right now I am talking with a friend who studied electronics just a couple of years after electronic calculators unceremoniously drove slide rules from the engineering scene.  He wants to learn to use a slide rule out of personal interest and curiosity.  I may give him the slide rule I built so he can practice.  Otherwise, for someone like him, I would suggest watching the auctions at eBay and buying a favorably priced rule.  An aluminum Pickett 10 inch rule similar to the one used for the virtual rule linked above can often be had for $25 or less plus shipping, even on a "Buy It Now" sale.  Recently I posted an Instructable on how I bought a nice old Dietzgen slide rule for $2.50 plus shipping and replaced the defective/missing cursor glasses myself.  If you have not seen that Instructable, there are some useful links there.  If you saw it when I first posted it, I have since added some links and information.  The photo for this step is my new old rule before I repaired the cursor glasses.   

There are other sites on the Internet with information for those who wish to build their own slide rules.  If you want to be very precise and do it the hard way, this site tells you how to find the logarithms of numbers and etch them onto a scale.

If you want to download copies of just about every slide rule scale ever devised so you can make your own custom rule, this site is what you need.  

If you would like to download PDFs of various classic slide rules and transfer them to your own base construction materials, this is the site you want.  There are even some circular slide rules you can build.  Scroll down about 2/3 of the page to see a nice wooden slide rule built by a man from Spain.  (See the photo with this step.)

Here is a basic circular slide rule design you can build.

In summary, my experience is that you can spend a couple of hours making a slide rule that may not be accurate or work as smoothly as a vintage slide rule you can pick up at an on-line auction.  If your time is worth anything, the slide rule you purchase at an auction site may actually cost less than the one you attempted to build.