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Re: Box-beam strongback
By:Mike Spence
Date: 9/15/1998, 4:46 am
In Response To: Re: Box-beam strongback (Mark Kanzler)

> Must be a special font. It looks like a little 1/2 on my screen (like the
> special keys on old typewriters) Did you see the umlaut (two dots) above
> the "O" in his name?

Everything came across fine on my system: regular ol' Win95.

> I was thinking of making a strongback from two layers of 3/8" plywood
> so I don't have to scarf to join to length. I may make a separable slip
> joint between three 7ft sections held togethr w/ wood screws (reinforced
> near the joints). I have some old scrap plywood available for this, all I
> need is some time...

I used 1/2" ply for my box beam - 13 feet: just a hair under 2"x4" (I stuck it through the planer). I did it this way because it was far easier for me to make the strongback slightly undersized than to make the 2"x4" cutouts (in the forms) for the strongback slightly oversized. The narrow faces were 2", and the wide faces were 3". I used 1/4" wide 1/8" Masonite splines to hold everything together, and made the spline dados with the tablesaw. It worked out _really_ nicely, quite stiff, and very straight. I was very pleased.

As you may remember I actually went through with my tapered-end insert idea. The inserts were made from four layers of 1/2" ply, cut to the appropriate tapers (for the smaller forms _and_ clearance), with a 1"x3"x12" long tenon that plugged into each end of the box beam strongback. When you plug them in, clamp the insert in place by pinching the walls of the strongback somewhere over the tennon (so that the insert doesn't move when you shim it - this would become obvious for anyone who takes this route), shim the inserts parallel to the strongback, then screw through the strongback walls into the insert's tennon and remove clamp. I was also very impressed with how well this arrangement worked out. Nice 'n clean.

After that, just start threading on the forms onto the strongback and fasten them any way you choose. I used four 2"x4" - 1/2" ply plates to pinch each form in place: two in front of the form, left and right, and two behind, left and right, screwed to the strongback walls. This approach allows me to knock the whole assembly down into an 8" high stack of forms (16 of them), a 13' long 2x4, and two end-inserts under 3' in length, not to mention a bag full of 2"x4" plates. It took a couple of hours to do all 64 plates, but the forms are as rigidly fixed as I could expect them to be. If I had to set up the assembly somewhere else, I doubt it would take more than half an hour to put the whole thing back together, as the screw holes in the plates were all pre-drilled, ie. all of the fastening plates are identical/interchangeable.

If I were to go into the forms making business, I wouldn't change my methods one bit. It may seem excessive and unnecessarily complex to some, however, it resulted in an extremely accurate and stable foundation on which to work.

And for someone like yourself, Mark:

>I like the way a 2x4 plywood box down the center of the forms looks.
>It appeals to my engineering aesthetic eye from a perpective of the way
>airplanes are sometimes built. It seems elegant and practical.

I suspect that you would be attracted to this type of approach, if only because it's different! ;-)

Mike Spence

Messages In This Thread

Box-beam strongback
Mikael Östlund -- 9/14/1998, 4:44 pm
Re: Box-beam strongback
Mikael Östlund -- 9/14/1998, 5:34 pm
Re: Box-beam strongback
Nick Schade -- 9/14/1998, 4:49 pm
Re: Box-beam strongback
Mark Kanzler -- 9/14/1998, 6:53 pm
Re: Box-beam strongback
Mike Spence -- 9/15/1998, 4:46 am
Re: Box-beam strongback
Mark Kanzler -- 9/15/1998, 10:39 am