How do I ensure a flat torsion box?

[CactusChip]

Hi all,
I've started my build of a Joe2006. I've already learned that MDF cuts just as easily sideways as it does forward and back on my benchtop table saw (coarse carbide tips gouge easily with a single sideways flinch... my 24" skins will now need to be 23.75". Oh well.)

My biggest concern is setting up so that my torsion box is totally flat. I am trying the "bridge across the sawhorses" method I saw on the Woodworks TV show. I'm working in the front yard, full of dirt and gravel. After several hours with a level, I believe that all directions are flat. My drywall T square also indicates flat. But my eyes tell me, when looking down the side, that there is a slight arch. I'll probably sand some tongue depressors to get the correct shim.

But when a level, and a T square, indicate flat when it really isn't, what can I depend on? Is there a method that people use to get the "final word"? In a worst case scenario, when I get things put together, I can use my router to shave off any high spots, assuming no twist in the rails. Eeek!

Thanks for any pointers.

[Geof]

If you stretch strings from corner to corner on a square or rectangular frame they will cross at the center. Before securing the top and bottom on the torsion box you need to make sure the frame has no twist. Secure one string tightly in place at both ends making sure it contacts the frame at each end. The easy way is to have a screw just down from the edge for this. Now secure the second string the same way at one end and pull it tight across going over the other string. The second string will either touch the first string or will be above it. If it is above that means the corners it is tied between are above the other corners, i.e. the frame is twisted. If it touches loosen it and pass it under the first string and pull it tight. Again it will touch or be below the first string. If it has touched both above and below your frame is not twisted; if it is now below and not touching the corners it is stretched between are below the other corners.

[CactusChip]

Simple and clean. That is a big help. Now its off to the back yard to continue on my project. Thanks again!

[BobPlus5]

When I use a plumb line to check the flatness of a wall, I always shim the line away from the wall, just in case the wall is convex toward the string.

I will add a level of complication to what Geof recommended. I would suggest running each string from the screw near the corner of the box up and over a thin shim located at the corner, across the torsion box to the opposite corner and passing over another shim as it goes over the edge and is secured on the side of the box. Do the same thing with the other string on the other diagonal. A gap between the strings shows that the box is twisted. You should be able to slide a fifth shim under the strings without deflecting the strings. If the string deflects, you have found a high spot. If the string does not touch the shim, you have found a low spot.

All five of the shims need to be the same thickness (1/2 inch or 1 cm is a good thickness).

[Madclicker]

http://www.diynetwork.com/diy/ww_mat...278181,00.html

[martinw]

Originally Posted by Geof If you stretch strings from corner to corner on a square or rectangular frame they will cross at the center. Before securing the top and bottom on the torsion box you need to make sure the frame has no twist. Secure one string tightly in place at both ends making sure it contacts the frame at each end. The easy way is to have a screw just down from the edge for this. Now secure the second string the same way at one end and pull it tight across going over the other string. The second string will either touch the first string or will be above it. If it is above that means the corners it is tied between are above the other corners, i.e. the frame is twisted. If it touches loosen it and pass it under the first string and pull it tight. Again it will touch or be below the first string. If it has touched both above and below your frame is not twisted; if it is now below and not touching the corners it is stretched between are below the other corners.

Dear Geof,

That method is blindingly obvious...... once it has been described to you!

Many, many thanks

Best wishes

Martin

[Geof]

Originally Posted by martinw Dear Geof, That method is blindingly obvious...... once it has been described to you! Many, many thanks Best wishes Martin

I will take that as a compliment .

I was trying to get a big welded frame square and untwisted and fighting with levels and straightedges when the light dawned on me. It is too simple really and the human mind always wants to find the complicated solution first.

[CactusChip]

Thanks everyone. I was really concerned because I had added an additional 12" to the X axis, but kept the original bed width. If I didn't get it flat from the beginning, I just *knew* that it would be next to impossible to straighten after the glue dried.

Now that my bed is flat, I have an area to lay up my Y torsion box. Thanks again!

[joecnc2006]

Originally Posted by CactusChip Thanks everyone. I was really concerned because I had added an additional 12" to the X axis, but kept the original bed width. If I didn't get it flat from the beginning, I just *knew* that it would be next to impossible to straighten after the glue dried. Now that my bed is flat, I have an area to lay up my Y torsion box. Thanks again!

got a build log?

[CactusChip]

Unfortunately, I have no build log. This project has been start and stop, simply because I keep getting called out of town to work for extended durations.

However, I have been taking pictures here and there. Maybe I'll start one. Since I'm not going out of town for a few weeks, I'll probably get it completed.

By the way, I really like the design!