Fair Woodworking

March 11, 2013

To divide into thirds

How does the saying go?

Given enough time, with an infinite number of monkeys on an infinite number of typewriters, somebody’s getting monkey poo in the face…

Or something like that?

I better start again.

Have you ever tried to divide material exactly into thirds without doing math? I try to avoid doing math without doing math, but anyways. I’m playing with Google Sketchup tonight, attempting to design some stuff. To try to get the full feel of the design, I felt that I needed to add the through mortice and tenons that I plan on using. A common rule of thumb is the two shoulders and the tenon should be of three equal thicknesses. By that I mean, if the tenon is 1″ thick the shoulders should also be 1″ for a total thickness of 3″.

Ok math over…

The dimension I was working with was a total of 5-1/4 thick to be divided into thirds. STOP! DON’T DO THE MATH! I know it’s not that hard but that’s not the point.

How would you lay that out? I’m sure there is a way to get sketchup to do it for you, but I don’t want to learn it, I just want my thirds.

I was sure that geometry was the answer, I just had to mess around with an infinite number of geometric options.

I figured circles were usually pretty smart, and they are fun to make on sketchup.

Before long I had enough circles drawn to baffle even Rory Tate

But no mater what I tried, three equal measurements were outside my grasp.

Then I moved to triangles. You have to draw your own triangles, but that gave me the opportunity to pretend I have a certain artistic flair. (I don’t have any but it’s fun to pretend anyway.)

Well a couple of times I got to the point that you would swear my creation came from the hands of Michael Angelo himself. No not the painter. The turtle.

But in the end I came up with this.

It’s ok, I wiped off the monkey poo.

divide to thirds

So what did I do?

Divide the width into quarters.

Draw diagonals from the quarters to the half.

Draw diagonals from the four outside corners.

Where the diagonals intersect (I circled them) those are the 1/3 points.

With two more diagonals between the outside corners and the quarter lines, and you can lay out fifths as well, but I’ll let you try that one on your own.

There you go math geeks!

I just made you obsolete.

Ha, ha, ha!


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