To find the centerline of a board, in order to rip the board into two boards of equal widths, I like the following method, shown in the following Sketch:
Notes for the Sketch above:
- Note 1 — Board to be divided into two equal widths.
- Note 2 — A ruler with inch increments 4″ thru 8″ shown.
- Note 3 — This line represents the centerline of the board. Notice that the centerline matches up with ………………the 6″ graduation on the ruler. Which is the midpoint between the 4 & 8 inch graduations ………………on the ruler. The end graduations do not have to be 4 & 8 inches. I just want end ………………graduations which can be easily divided by two. In this case (8 – 4 = 4) and 4 divided by ………………2 equals 2. Therefore 4 + 2 = 6, and the centerline lines up with the 6″ graduation.
Another method I use for finding the centerline of a board, is a steel tape. For example, if a steel tape shows the width of a board to be say 7-11/16″. I would go thru the following process in my head:
- Would mentally divide the whole number (7 in this example) by 2, and 7 / 2 = 3-1/2. I like to think of the 3-1/2″ as 3-8/16″ for this process.
- Next I take the fractional part of the board width, and divide the numerator (That’s right, not the denominator) value by 2. So half of 11/16, I would think of as 5.5/16″. Showing a fraction in the numerator, in this manner, is not an orthodox method, but it works for me.Also note that I like keeping the fractions in sixteenth values.
- Therefore half of the board width will be (3-8/16″ + 5.5/16″ = 3-13.5/16″). As you can see, I prefer to work with 16 and not 32 or 64 increments per inch.
Here’s what I consider a no math req’d method for finding the centerline of a board. Using a combination square, adjust it to mark with a line, what you approximate by eye to be the centerline of your board. Perform the above process with a combination square, from both sides of the board, as shown in the Sketch below.
Notes for the Sketch above:
- Note 1 — Board to be divided into two even parts.
- Note 2 — Position combination square as shown, & draw Line “A” at the end of the combination ………………square blade.
- Note 3 — Reposition combination square (Do not change combination square setting) to other side of ………………the board as shown, & draw Line “B” at end of combination square blade.
- Note 4 — The dotted line represents the centerline of the board. This line is determined by ………………approximating by eye, the half way point between Lines “A” & “B”. I feel that the closer ………………Lines “A” & “B” are to one another, the better my eyeball approximation will be. I like ………………for Lines “A” & “B” to be 1/8″ or closer to one another, when using this method.
So in closing, if you need to find the centerline of a board, you now have three options to choose from. And having options to choose from, is always a good thing.
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