Dear Kate and Ralph,
Here are more details:

Get a piece of cardboard or plywood, about 24x32". This needs to be sturdy enough to stay straight and flat when you lay the blocks on it and lift them up. I used regular corrugated cardboard, but I glued 2 layers together crosswise with Elmer's glue to make it stronger.
2) Determine the shape of the arch: for this, hold a piece of string in the air, I used about 62". When you hold the two ends about 20" apart, the string should hang down in a loose arc. This is the exact shape that you want!. (choose string that is not too stiff relative to its weight, so it can hang in the proper shape - in fact, a string of Mardi Gras beads would be ideal: it is heavy and flexible).
Hold the string in front of the board, with the ends even with the top of the board. You want to trace the shape of the hanging string onto the board. This is easier with two people, and some tape. Make sure the top of the board is horizontal when you do this.
After you trace the shape of the hanging string arc on the board, lay it flat. Now take a scrap of 2x4 (or a ruler) and draw lines on either side of the arc, 1 1/2" apart. This would outline the 2x4 arch if you had curved wood.
Divide the arch into pieces: It really does not matter how many pieces you divide it into. As you can see in the photograph , I made 9 segments, with the lower ones longer than the ones on top. I would start at the top, with a horizontal piece about 3 1/4" (make marks on the center line, where the string used to be, and measure the straight-line distance). This would be the 'keystone'. [Note that you don't necessarily have to have a keystone. If you divide the arch into 8 or 10 segments, you would have a crack right on top, and no keystone]. Go down the arch and mark the other breaks. At each break, draw a line perpendicular to the center arc.
With a ruler, you can now draw straight lines for each segment, so that you end up with a precise drawing of the block arch.
For cutting, use a table saw, and tilt the blade to the proper angle for each cut. [if all you have is a hand-held circular saw, you have to take the time to set up a working jig with a fence to ensure that all cuts are precise and perpendicular to the long axis of the 2x4. I don't think freehand cutting would be precise enough]. Use a protractor to read off the angles from the drawing on the board, and set the blade angle. Don't assume that the angles on the left and the right are identical, unless you're more meticulous than I was. After each piece is made, check it by laying it on the board and matching it to the other pieces. Label the pieces so you know which is which, and which way is the front.
How to raise the arch: Lay the (card)board on a table, raised up on scraps of wood or something, so that you can get your fingers under the edges. Lay all the pieces onto the board to make the arch. The bottom of the arch is even with the bottom of the board. You are going to tip the board up to a standing position, but you have to prevent the blocke from sliding down while you're doing this. Therefore, grab the thing such that your fingers grab the edge of the board, and your thumbs hold the bottom pieces of the arch in place. Now you can tilt the board up, and when you reach the final upright position, you can release your grip and take away the board. What is left is a free-standing arch.
Another way to prevent the sliding bit is by taping a thin board to the table (about as thick as the board with the arch on it, and not raising the board up. You start with the board plus arch up against this edge, so that when you lift up the board with the arch, the bottom pieces of the arch pivot on the edge of the taped-down piece, and the arch ends up standing on top of this taped-down piece.
Ta-daa!
                                 Don't hesitate to ask more questions if you have any. Also, I would really like to know how this turns out! Many of my emails disappear into never-never land and I never hear how these projects turned out.
                                                                                                                    cheers, Hubert

Ralph Harrison wrote:

Dear Mr. Science,

May I please have more detales on the 2x4 free standing arch. My class is studying arches and I will have my dad build it for my class.
Thank you,
    From, Kate Harrison at Whitter Elementry

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Deborah, Hubert, Saskia and/or Niels van Hecke
Email    : hubert@hubwest.com
Home page: http://users.hubwest.com/hubert/
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