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This might be a silly question, but how do I accurately measure, mark and cut a board perfectly in half on a miter saw?

I know that if I want to cut a board of a specific length, I would mark that length, and then align the blade right on the other side of the line, so that the cut piece is exactly measured distance. But say I have a 8' board, and I want it in half (roughly 4' each, minus the kerf), how do I quickly measure/cut this? Ideally the result would have no waste.

Do I just align the blade with the line and cut directly on it, or do I need to measure the blade kerf and account for this somehow?

I realize a table saw would help make this more accurate and repeatable, but right now I'm limited to a miter saw.

  • It's hard to beat a miter saw or a radial arm saw for repeat cutting of long lengths - I'm considering 4 feet to be a long repeat. – Ast Pace May 6 '15 at 20:33
  • Measure from one side....exactly 3' 11''....then measure from the other side...3' 11'' again....then measure the gap between the two measurements, and half it...hey presto!!...exactly in the middle. – user547 May 6 '15 at 22:08
  • Please clarify, are you asking how to find the center of the board or how you should make a cut to ensure it is centered on that mark? Or how to get the most out of a board which needs cut in two equal parts? – Daniel B. May 7 '15 at 1:01
  • The golden rule. Measure twice, cut once. (well, 3 times. X, Y, and one more to be sure) – Touka May 7 '15 at 12:46
  • @DanielB. I am asking how to make the cut so that the results are two same length boards – Steven May 7 '15 at 13:09
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Typically if you need two or more equal-length parts from a single board, you'll select a board longer than what you need to cut all the parts, cut the board as close to "in half" as you care to, then gang all the pieces together or set up a stop block to cut them to the final equal length. Sometimes you may not cut to final length until after you've performed some other operations.

You don't necessarily need to find out the exact length of a board to find the exact middle. Note that you can get close measurements by marking with a sharp pencil, but if you need exact measurements you should use a knife to mark your lines instead.

  1. Take a tape measure and place the hook close to the edge of one face on the board.
  2. Extend the tape measure all the way to the other end. If it does not fall on an exact measurement that's easy to divide by 2, slide the tape measure sideways (so it slants across the face of the board) until the scale does hit the end of the board at a measurement that's easy to divide by 2.
  3. Divide the measurement by 2, find the resulting measurement on the scale, and mark it as the middle.
  4. Measure the width of your blade's tooth and divide it by 2 (i.e., multiply the denominator of the fractional measurement by 2)
  5. Measure the "half-kerf" distance to the left side of the first (center) mark. (If you measure to the right side, swap the words "left" and "right" in the following instructions.)
  6. If your second mark was to the left of the first mark, draw an X to the right side of your second mark to designate the right side as the "offcut." (Conversely, if your second mark was to the right of the first mark draw an X to the left side of your second mark.)
  7. Position the board on your saw so the left side of the nearest tooth exactly lines up with your second mark. (If you used a pencil instead of a knife to make the mark, try to account for the width of the pencil line.)
  8. Clamp your workpiece in place, double-check everything is still lined up (and readjust if necessary), then make the cut.
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7

You've hit on both options.

You measure the board to it's exact dimensions, (I have yet to see them exactly 8') then you mark the center.

At this point you can either attempt to cut the board centering the line with the miter or you measure the kerf, (usually 1/8") split that in half (1/16") make your line there and follow the new line for the miter.

My guess is it will be about 1/8" - 1/4" longer than 8' so the kerf will make it closer to 4' even.

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5

You could use the old geometry trick of bisecting a line:

http://www.mathopenref.com/constbisectline.html

It would require more space of course and something you could use as a larger compass, which could be nothing more than two 2x4's fitted perpendicularly with a pencil taped to the end of one of them and a nail sticking out of the other for a "compass" point.

Maybe not practical, but fun. :)

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  • I remember learning this in school! – Steven May 6 '15 at 15:46
  • I use this fairly regularly; however, it does require that you have an estimate of where the middle is already; I usually use it on the width of a cut since it's easier. – Daniel B. May 6 '15 at 18:54
  • Much easier to use a piece of string tied around your pencil as a compass! – David Richerby May 6 '15 at 20:16
  • Voice of experience here - the accuracy of the string compass depends on how taut you keep the string. It's length changes appreciatively with small tightening force. – Ast Pace May 6 '15 at 20:24
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    This doesn't work very well on long boards unless you add a few more steps. Also you aren't currently accounting for the saw kerf. – rob May 6 '15 at 23:25
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I don't want to be left out of this party, so here's my take. No matter how carefully you measure you're always going to have one half slightly larger than the other after the first cut. So you need to cut one slightly short and trim the other to match.

I think the biggest thing other answers are missing is the use of a stop block to guarantee a repeatable cut.

I would probably use a combination of bowlturner's and null's answers, since null's is a pretty good method of finding the center.

  1. Use a tape measure (or any other rigid tool with a length slightly more than the length of the board and a known center). I wouldn't use a string because it would stretch. Place one end of the tape measure at the end of the board, use a friend to hold it if you have to. Place the final marker at a point which is easily divided and mark. As mentioned, it's practically impossible to position for a single cut that will guarantee two boards of the exact same length. instead we get close and make two cuts.

  2. Your miter may or may not have a stop. If it does not, you can use a board clamped to your miter table as the stop. Position the board so that the blade is on the inside of the measured line(This guarantees that the "unfinished" piece will be slightly longer than the "finished" piece).

  3. Secure your stop so that the next cut will be of the exact same length. Make sure it's secure so that positioning the next cut will not shift your stop.
  4. Make your cut. You now have two pieces, the one that was against the stop is now your "finished" piece. Set it aside and place the unfinished piece against the stop. Cut off the sliver left on the end and your two boards will be exactly the same length and only a very small amount of board will have been lost, approximately 2x the kerf.

Bear in mind that this applies to a board which is already squared on both ends

Another option, if you are guaranteed that your board is rectangular (parallel sides with right angles at every corner): you use another technique to find the center which would be more accurate with no measuring at all:

  1. Use a straight edge along the top left corner diagonally to the bottom right corner. Mark a line at approximately the center of the board.
  2. Move the straight edge so that it is now from the top right corner to the bottom left corner and again mark a line at approximately the center of the board.
  3. The intersection of these points is the center of the rectangle. Use a square to make a line through this intersection; this is your center line.
  4. Follow steps 2 through 4 from the first procedure to make your cut.
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4

The quickest way to measure is not to measure at all.

The board is a rectangle, therefore opposing sides of it are parallel to each other.

Now say you lay your measuring tape across the board like so enter image description here

wherever the middle of the tape is, it will also be the middle of the board (or half of the board width) You can place the tape however you want. You are basically measuring from one edge to the other at an angle. Now angle the tape in a way that is shows a value that's easy to divide by 2, but longer than the board width, say 10 for example. You can now mark the middle of the tape at 5 and can be sure that this is the middle 0f the board, too.

If you do this two times, you have two points that connect to a line, that is exactly in the middle of the board.

The interesting thing is that this holds true for any fraction and does the math geometrically, which always works, no matter how ugly the numbers are.

Let's say your 8 is a rough cut and is actually something like 8.345 (I made that number up, the unit doesn't even matter) And you want to divide that evenly by 3.

Ok 8.345 / 3 = ?? phew, not that easy. You could use a calculator...or do what I stated above.

Place the 0 of the tape at one edge of the board and angle it so that 9 is right at the other edge.

To divide the board in thirds, mark 3 and 6.

By choosing a total tape length from edge to edge of the board that you can conveniently divide into the desired fraction, the geometry will do the math for you and probably be done with it a lot quicker.

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  • Interesting technique. I'm pretty sure the question is about the length of the board, not the width. I guess this can also be used on the length of the board. Although assuming that the board is perfectly parallel on its full length is quite an assumption; not sure if it's ever true with lumber... – Maxime Morin May 7 '15 at 1:40
  • Yes I was talking about the length not the width. – Steven May 7 '15 at 1:58
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    I actually don't think this will work well as most tape measures have a width. Without punching a hole dead centre through your tape measure your measurement will be affected by the width of your tape measure. The greater the angle to the board the greater the effect. – Not loved May 7 '15 at 3:33
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    @Luke McGregor use only one side of the tape – null May 7 '15 at 6:18
  • Interesting approach. But not, in my experience, one that will get used very often. Generally you will cut 'on the line' when using a bench saw. – Ian Lewis May 7 '15 at 9:28
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The answer is simple. Know the width of your blade's cut. Your average circular saw with carbide teeth will leave about a 1 eighth groove. Other blades may be thinner or thicker. Make your measurements accordingly. In short, do the math. (I feel my reply should have been made with a deep Bangor brawl and a corn cob pipe)

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    and if you don't know the width of your kerf, cut a few mm(*) into a scrap block and measure it. (*: Yes, I'm European) – Martin Bonner supports Monica Oct 13 '17 at 10:57
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Assuming a consistent weight, balance the board on a thin edge (like a knife blade). The point at which the board is balanced is, by definition, the center.

Mark and cut.

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    This technique does not guarantee that the line upon which you balance the board is at right angles to the long side. Also, the premise is incorrect; your technique finds a line which contains the center of mass, which as you note is only the center of the board if the board is of uniform density. To say that the center of a board is "by definition" the center of mass seems suspect; I would say that the center is by definition the point at which the board may be divided into two boards of equal size. – Eric Lippert May 7 '15 at 14:01
  • This also assumes that the board is a perfect rectangle, or at least has mirror symmetry about the center axis. – Daniel B. May 7 '15 at 19:48
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    "Assuming a consistent weight" is a big assumption. In pine or another softwood a knot toward one end or the other will be significantly heavier than the surrounding wood. That's an extreme example but wood is inherently variable, so it cannot be assumed that woods generally are so uniform that this would work as we would like it to. – Graphus May 8 '15 at 8:25

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