Torsion Box Assembly Table #3: Assembling the Grid

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Blog entry by Morton posted 12-10-2009 05:30 AM 4367 reads 1 time favorited 10 comments Add to Favorites Watch
« Part 2: Cutting the Parts Part 3 of Torsion Box Assembly Table series Part 4: Finishing the Box »

Today I assembled the internal grid and attached one of the skins.

I started by taking off the current workbench top, and leveling the top of the base. The top rests on two aprons that run end-to-end; one at the front and one at the back. I used hardwood shims (from a previous project) to adjust 3 of the legs and level the 2 apron pieces both left-to-right and to each other (front to back).

I then placed (2) 2×4s, which had been jointed and planed, on top of the aprons, the same as the torsion box will rest. I placed one of the 1/2” MDF skins on top of those to use as a surface to assembly the grid. I again checked that the skin was flat and level.

I later found that the middle of the MDF was sagging (only supported by the two 2×4 rails) and I then placed (4) 2×4 the other way to more fully support the MDF. It didn’t end up causing any problems (so far).

I started with the exterior 3/4” MDF pieces and joined those with glue and brad nails, keeping each joint square, to create the exterior rectangle of the torsion box. I then started gluing and nailing the (4) small 1/2” MDF pieces, and then the 1/2” MDF cross piece. I worked my way across the box in the same manner.

I used a speed square and some home-built squares to align each piece as I glued and nailed. However, the internal grid is not particularly square (each small square isn’t 90 at each corner). Luckily, that doesn’t seem to matter.

Finally the grid was complete. I tested each joint with my finger and used a block plane to knock down any height difference (just a couple of extremely light strokes). I then liberally spread glue on the whole thing, placed the 1/2” MDF skin on top and brad-nailed the top down along each row and outer edge.

The moment of truth: I brought out my Lee Valley straight edge and feeler gauges. The most dip I could find was about .005 inches. I’m extremely pleased with that!

-- Morton -

10 comments so far

View Scott Bryan's profile

Scott Bryan

27251 posts in 3246 days

#1 posted 12-10-2009 05:51 AM

Morton, it looks like you are making pretty good progress with the box.

-- Challenges are what make life interesting; overcoming them is what makes life meaningful- Joshua Marine

View charlton's profile


84 posts in 2832 days

#2 posted 12-10-2009 06:43 AM

Looking good. One thing I never understood was why Marc used this method for building the torsion box. I did the same thing but in hindsight, I wouldn’t bother with cutting the little pieces. I would just use a dado to cut slots in the pieces to make center lap joints instead. I bet this is much more accurate. I found toe-nailing to be a pain just as you found and it sometimes caused the already thin 1/2 MDF to split.

View jlsmith5963's profile


297 posts in 2772 days

#3 posted 12-10-2009 07:50 AM

Marc’s method comes straight from his mentor David Marks (as he demonstrated on his show Wood Works) and I never really understood it either. To each their own I suppose.

-- criticism: the art of analyzing and evaluating the quality of an artistic work...

View Derek Lyons's profile

Derek Lyons

584 posts in 2992 days

#4 posted 12-10-2009 09:17 AM

This method is preferred (by some) because cutting out all the little bits is a fairly straightforward task – while cutting that many joints and ensuring that they all line up precisely is a tedious, painstaking, and potentially error prone one.

-- Derek, Bremerton WA --

View charlton's profile


84 posts in 2832 days

#5 posted 12-10-2009 04:42 PM

Fair enough…though I still think that a dado set to 1/2” and a little setup similar to what you would use for making box joints on a table saw/router table would be fairly accurate. As jlsmith says, to each his/her own, I guess. :)

View Morton's profile


30 posts in 2523 days

#6 posted 12-10-2009 04:55 PM

I will add that cutting the small pieces was super easy at the chop saw, and very accurate. Assembly with toe-nailing and handling all the small pieces was a bit tedious, but in reality did not take long. The toe-nailing went poorly, hardly holding at all, but it was enough to get the next cross piece in where I could get more nails. The whole grid is quite rigid now. And as noted, it turned out to be very accurate.

So, I’m quite happy with this method – I would prefer it over doing dados myself – for me it was pretty quick and very accurate.

-- Morton -

View SteveMI's profile


949 posts in 2718 days

#7 posted 12-10-2009 07:02 PM

Thanks for the insight on adapting the grid design to the gun size. I would imagine you only have to vary the spacers distance and still keep the main supports at the same distance.


View Morton's profile


30 posts in 2523 days

#8 posted 12-10-2009 09:41 PM

@SteveMI – I haven’t done much torsion-box research as to why it’s so stable, but I’m not sure if you can offset the small pieces – my guess is that they have to align; to form straight rows. Some internet research might reveal an answer to that.

-- Morton -

View Jimi_C's profile


507 posts in 2658 days

#9 posted 12-10-2009 10:56 PM

That’s why I stapled my short pieces for my bench top. Toe nailing seemed like a pain, so I just used staples to hold them in place till I glued it.

-- The difference between being defeated and admitting defeat is what makes all the difference in the world - Upton Sinclair, "The Jungle"

View dlmckirdy's profile


196 posts in 2557 days

#10 posted 12-11-2009 12:24 PM

I don’t think the short pieces really need to be in all that straight of a line. If you have ever taken a hollow core dore apart, it is stiffened with strips of corrugated cardboard set on edge, which aren’t placed particularly neatly. These doors will stay straight and true for many years.

When I was an Airframe Repairman in the Army in Vietnam, the walkways on top of the Huey helicopters, all aircraft decks, and Jet Ranger bodies (fuselages) were two parallel aluminum sheets filled with an aluminum honecombed heavy foil. The biggest problem with these was adhesive failure between the skin and the honeycomb, creating voids, which compromised the panel’s strength.

It should not matter much how the internal web is assembled, so long as it is firmly adhered to the skins. I would use a good glue and use brads to hold the skin down while the glue dries.

I was also thinking that for a workbench top, a 1/2 inch work surface might be too fragile. I think I would use two 3/4 inch MDF or plywood sheets for the top of the box so that you have some flex strength in the skin so as not to break the adhesive bond. I like the torsion box idea for the bench top, especially if it is to double as an assembly table.

-- Doug, Bakersfield, CA - I measured twice, cut it twice, and it is still too short!

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