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All Replies on Torsion box made with Cross Lap Joints?

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View PresidentsDad's profile

Torsion box made with Cross Lap Joints?

by PresidentsDad
posted 12-08-2018 06:37 PM


7 replies so far

View ksSlim's profile

ksSlim

1299 posts in 3279 days


#1 posted 12-08-2018 06:46 PM

Agree go for it.

-- Sawdust and shavings are therapeutic

View bondogaposis's profile

bondogaposis

5399 posts in 2740 days


#2 posted 12-08-2018 10:02 PM

If you are going to use 3/4” plywood for the skins, why not just glue 2 pieces of plywood together and skip the torsion box? How strong of an outfeed table do you need?

-- Bondo Gaposis

View clin's profile

clin

1030 posts in 1385 days


#3 posted 12-08-2018 11:00 PM

That’s exactly how I made my assembly table. In that case I used 1/2” MDF for everything.

The reason to use the half lap is it can be (though not necessarily) much faster than dealing with all the separate pieces. It does not make it stronger to any significant degree. In fact, there’s no reason to even glue the half-lap joints together. The webs in a torsion box are there to hold the skins a fixed distance apart.

Taking this to an extreme, the torsion box will work just as well if the webs are just random pieces that don’t even tough each other. Of course, a truly mission critical design, like an aircraft structural member would be designed such that every web is located in an optimal way to maximize strength and minimize weight.

I too would agree that this is overkill for an outfeed table, but it is a great way to build a very flat assembly table. While in many cases, a big advantage of a torsion box is reduced weight, the advantage to us woodworkers is it is a method to build a very flat table which can be helpful when assembly projects to ensure they come out square etc.

However, in order to build a flat torsion box, you need a flat surface to build on. When you glue the skins on, whatever shape the skins have is the shape the box will have. Things like your shop floor (garage floor for many), is not nearly flat enough. Creating this flat reference surface takes more time than building the torsion box. However, once you have a truly flat assembly table, you have a nice flat surface from then on.

Here’s a video on a torsion box build. I didn’t build my box this way, but his method of creating the flat build surface is similar to what I did.

https://www.thewoodwhisperer.com/videos/episode-18-assembly-table-torsion-box/

I also think 3/4” plywood skins is overkill. I used MDF because it is very flat and even more stable than plywood. Though certainly plywood will work. I used hardboard on the top as a wear surface. I don’t think there is anyway I’m likely to damage the MDF of my top short of deliberately hitting it with a sledge hammer. Even if I slipped using a more common hammer, I don’t think it would be likely to damage the top. And I’m sure a typical mallet would never be able to damage it.

Also, because the bottom skin will be in tension, that skin can be quite thin and you’ll lose little strength or rigidity. I think 3/8” ply would be plenty and I’d bet 1/4” would work well. Of course, if you want the table to be heavy, something we typically want in a woodworking bench, then nothing wrong with 3/4’ skins top and bottom.

Here’s a link to my LJ project using half-laps in the torsion box.

Click for details

-- Clin

View Fred Hargis's profile

Fred Hargis

5545 posts in 2882 days


#4 posted 12-09-2018 12:08 PM

I built my assembly table using 1/2 laps, and it’s held up just fine. The reason I did it was for easier assembly of the web.

-- Our village hasn't lost it's idiot, he was elected to congress.

View BlasterStumps's profile

BlasterStumps

1308 posts in 828 days


#5 posted 12-09-2018 12:39 PM

+ 1 for half laps. That is how I made a couple torsion box. One for my assembly table top and the other is the support under the mobile cart I have a drill press on. (big old Craftsman floor model). Work fine. 1/2” MDF.

-- "I build for function first, looks second. Most times I never get around to looks." - Mike, western Colorado

View PresidentsDad's profile

PresidentsDad

34 posts in 636 days


#6 posted 12-09-2018 02:16 PM

Hey all. Thanks for the replies. In really talking about cross lap joints. Are we taking about the same things? From Wikipedia… joint in which the two members are joined by removing material from each at the point of intersection so that they overlap.

View Fred Hargis's profile

Fred Hargis

5545 posts in 2882 days


#7 posted 12-09-2018 05:41 PM

We are, or at least I was. The term 1/2 lap is a little less appropriate for that joint, but more commonly used to describe it. We tend to be a little loose with precise language in woodworking.

-- Our village hasn't lost it's idiot, he was elected to congress.

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