rbl2... Bill Hall:
Hook your tape on one edge of the board and pull it diagonally across the board until you have an even number on the edge.
Divide that by 2 and you have your center.
Note, that if the far edge of the end of your tape is on the boards edge, then set the far edge of the tape to the number you select.
In fact, you can easily divide the board into more than 2 pieces using this method. For example, if you wanted to divide a board into thirds, set your tape on the boards edge at 9 (nine) and then mark 3 and 6. POOF!!! You have three marks an equal distance apart.
Many times I have to find the center of a board and all I have handy is my trusty old combination square.
Here's how I do it.
1) Estimate as closely as you can, halfway across and set the square.
2) Draw a short line using your estimate using one side of the board. Leave the square setting alone.
3) Draw another line using the other side of the board.
The center will be half way between the two lines, irrespective of whether you were long or short on your estimate. I can usually get to within 1/8" or less. Split the difference and you are on the money.
When drilling on the Drill Press or cutting on the Table Saw, I usually don't have to draw any other lines. I just line the bit or sawblade up between the two that I already drew. That's usually just as accurate as if I hit dead center on the first estimate. Sometimes it's even easier to set the tool.
To me, this works just as well as drawing two parallel lines across the board and then trying to hit the diagonals on the money.
Back when we used pencils for drafting (Pencil Aided Design) you cranked your scale on an angle that put 0 (zero) on one edge and a nice whole number (2, 4, 6, 8, 10) on the other.
Then it was only a matter of simple math to find the center (1, 2, 3, 4, 5).
Unisaw A-100, who hasn't hand drawn a line in a very long time and ain't wanting to...
One piece of board longer than board to mark, (make a couple sizes they're easy) 3 holes - outside 2 farther apart than workpiece is wide, insert short piece of dowel or a nail in each. Center hole centered and sized to fit favorite pencil or scratch awl (or a nail).
Hold against (on top of) workpiece, twist till dowels touch sides, pull or push to mark through center hole.
Takes longer to type than to make or use.
Use a compass set wider than 1/2 the board width. Hold the point at one edge and draw an arc, hold the point on the opposite edge and draw an arc...
Where the two arcs meet is the "exact" center of the board... no need to measure anything.
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