> Nick After building my box beam I ran a string down the center and found
> that in the last 39 inches of the beam it has a curve in it of about 1/8
> to 3/16 of an inch the rest of the beam is straight.The beam is made of
> 3/4 in ply and is filled in the middle. Is this a problem I can fix, or is
> it expensive firewood? THANK YOU J.D.

Just in case Nick is busy, let me jump in with my two cents (that would be 3 cents Canadian).

Live with it.

Since you can determine a center line using a string, ignore the sides or physical center of the beam and use that string line as the origin for all your measurements.

I saw a picture of a boat-building strongback that used a tightly stretched wire instead of a string. The wire stayed in place during the building process.

Soem people use a chalkline and 'snap' it to mark a straight line. I have heard that this may cause minor variations near the middle. A trick I've tried, and which worked very nicely for me, was to stretch the string tight, and then go over it with a gentle mist of spray enamel (I used red, by the way). The string does not move, so it stays perfectly straight, and it blocks the paint from reaching the strongback. After a few minutes for the enamel to dry you lift off the string and have a sharp, straight, light line, surrounded by the paint mist. It is musch more durable than chalk, real easy to see and measure from -- even when there is a thin layer of sawdust, and unlike the practice of leaving the string in place, this can not be dislodged or moved by errant woodchips sneaking under the string, or pushed aside by a splinter.

With a box beam you can do this to both the top and a side, giving you straight lines to measure from. When you do the side, before you spray paint it, flip the beam over so the string rests on it, and can not sag in the center. later, when you set up the beam you can use a simple water level, made from a 15 to 20 foot(5 m to 6.5 m) piece of vinyl tubing with some water in it, to see how much propping you'll need, and where that propping should be, to support the beam so it does not sag.

As for the actual building forms which are placed on this beam, the cut out for the beam should be slightly oversized so these can be shifted and aligned as necessary. Shim them into place with wedges or scraps of wood, and then screw them in securely as yor plans show.

If you insist on destroying this thing, don't use it as firewood. There is a good chance the foam inside will either send out toxic fumes as it burns, or it will melt all over your fireplace. In either case it will ruin your day. Instead consider recycling it into a balance beam for the neighborhood kids by mounting it on posts, a few inches above the ground.

Hope these ideas help.

Paul Jacobson