What did i do wrong?

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Forum topic by IceCactus posted 02-25-2013 07:06 PM 1331 views 0 times favorited 6 replies Add to Favorites Watch
View IceCactus's profile


45 posts in 2688 days

02-25-2013 07:06 PM

Hey guys, newb question

I decided to build a couple torsion box beams to use as a workbench. I used 3/4 MDF and made the faces of the beams 12” wide. I ran 2 dados on each face, 3/8” thick for the length wise spans to sit in. I glued and clamped the lengthwise spans in place and let dry. I then cut the short spacer peices that fit inbetween the lengthwise spans. I found out though, that my 2 lengthwise spans are not perfectly perpendicular to the face, so none of my small web pieces sit correctly and i now have a beam that’s not flat on the last 2’ of it :(

What did I do wrong? The dados were tight fitting. I used a 3/4” router bit.

6 replies so far

View nwbusa's profile


1021 posts in 2281 days

#1 posted 02-25-2013 07:11 PM

I’ve never seen a torsion box design that uses dado joints. Usually the inner ribs are just butt jointed to the panels. Maybe the dado joint didn’t seat properly?

-- John, BC, Canada

View gawthrrw's profile


207 posts in 2442 days

#2 posted 02-26-2013 12:05 AM

I agree with nwbusa, I have never seen torsion box’s with dado’s. butt joints are plenty strong.

-- Rob, Dallas TX

View IceCactus's profile


45 posts in 2688 days

#3 posted 02-26-2013 04:42 AM

These are the plans i got the idea from…I have seen it a couple other places too.

I didnt screw the panels on though since i didnt have the fastcap system and i figured clamping it all down would be good enough.

View RogerInColorado's profile


321 posts in 1949 days

#4 posted 02-28-2013 01:16 AM

Be grateful that MDF is cheap. If you can find either the Norm Abrams video on assembly benches or the David Marks video on assembly benches, watch them. They focus on the need for the platform that you use to make a torsion box to be dead flat and level. If it isn’t neither will your torsion box be flat when it’s finished. If you can’t find either of those two videos, watch every other torsion box video you can find and take the sum of the best practices as you own for the next build. I think in the good ones, you will see the intermediate pieces being checked for square at lots and lots of points before the glue and the top go on. There are other tricks to pay attention to, so take notes.

View waho6o9's profile


8187 posts in 2571 days

#5 posted 02-28-2013 01:21 AM

+1 for David Marks video, it’s very good.


View Shark64's profile


12 posts in 2063 days

#6 posted 02-28-2013 06:45 AM

These torsion boxes remind me more of box beams rather than torsion boxes. Whatever the name is, I would suggest that you use the short pieces during assembly to keep the long pieces perpendicular to the faces. You could also make and use some assembly squares for the same purpose. As indicated in the other comments, a flat and level surface to build upon is important.

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