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Torsion Box Assembly Table #1: Introduction

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Blog entry by Morton posted 12-06-2009 08:31 AM 4980 reads 3 times favorited 11 comments Add to Favorites Watch
no previous part Part 1 of Torsion Box Assembly Table series Part 2: Cutting the Parts »

I am ready to replace the top of my workbench with a dead-flat Torsion box to ease assembly of future projects.

Plywood Workbench with Sheet Good Top

I built my current workbench a couple of years ago from plans from Fine Woodworking. The base is made from laminated plywood and is very solid. I filled the inside of the base with 6 drawers which are full and work well. The top is 2 sheets of 3/4 material that has served its purpose, but is far from flat.

Part of the problem (non flatness) also stems from the fact that I have a heavy vice on the left side of the bench. This is mounted on the underside of the top, that is overhanging the base. So, the weight of that vice pulls the top down and bows the whole surface.

So – I am going to replace the current top with a dead-flat torsion box. I plan on re-mounting the vice to it, but will add support legs on the other side to help support the vice weight.

The plans for the box are from The Wood Whisperer. His instructional videos are awesome and it’s where I first learned of the idea.

I will post as I make progress and take pictures, and hopefully some video, along the way.

-- Morton - http://workshop.scottmorton.com



11 comments so far

View a1Jim's profile

a1Jim

115202 posts in 3038 days


#1 posted 12-06-2009 08:41 AM

Looks like a plan. Most bench tops hold up pretty well . perhaps the one you have now is thinner than most bench tops.

-- http://artisticwoodstudio.com Custom furniture

View Morton's profile

Morton

30 posts in 2561 days


#2 posted 12-06-2009 08:49 AM

Yeah – it’s only 2 sheets of 3/4 material, so it’s not a very thick top. I’m looking forward to something that will be dead-flat (initially) and hold up for a long time.

-- Morton - http://workshop.scottmorton.com

View SPalm's profile

SPalm

5257 posts in 3343 days


#3 posted 12-06-2009 03:01 PM

Hey Morton, nice to meet you.

Torsion boxes are dead flat, and that seems to be the main thing you are looking for here today. That is great for an assembly table which is a very nice thing to have, but not usually used for a workbench top. Bench tops are usually really strong so that you can bang on them and mount vises to them (and they also happen to be flat). Another problem with torsion boxes are that they are very thick by nature. This will make the whole bench taller, but maybe that is OK with you.

I think you should also ask around about classic workbench tops (think butcher block). A whole lot of us here have built benchs and could help you out.

Steve

-- -- I'm no rocket surgeon

View Morton's profile

Morton

30 posts in 2561 days


#4 posted 12-06-2009 03:51 PM

Steve—thanks for the comment, I appreciate the thoughts. I think for now, an assembly-table style top is what I’d like to have in the shop, but I am going to try mounting my vice to it and see how it all works out for me. In the (not too distant?) future, I hope to create a more traditional workbench and do more hand-tool work.

The height doesn’t bother me, as it’ll add a couple of inches (3” interior torsion box), but I have a few inches that I can cut off the workbench legs if I don’t like the new height.

At least, that’s the current plan, but I’ll definitely spend a little time thinking about other options.

-- Morton - http://workshop.scottmorton.com

View patron's profile

patron

13535 posts in 2802 days


#5 posted 12-06-2009 04:10 PM

sounds like a plan ,
flat is good .

i used to work with a guy ,
that was obsessed with this ,
he paid $300.00 to have a 4’ straightedge
made for him to .001” true !
spent more time showing it of than using it .
your idea sounds better .

welcome to LJ’s .

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View SPalm's profile

SPalm

5257 posts in 3343 days


#6 posted 12-06-2009 05:21 PM

Fair enough. A few comments if I may, this subject is dear to my heart.

I have built a lot of torsion boxes for this kind of situation. They were the favorite base and structure for homebuilt CNC machines. You can really crank them out quite quickly, don’t get hung up on .001 inches. They are just two skins separated by a webbing. I would use MDF rather than ply as it does not warp and is pretty dead flat to begin with (unless you want to use Baltic Birch $$$). You mentioned a three inch web, which is ok, don’t go any thicker, and maybe 2.5” would be just fine. If you use MDF, be sure to finish the outside when done.

(Side note: A very strong and easy to built torsion box is a slab of MDF skinned on both sides with plastic laminate. It is much stronger that you might think as to flex it you would have to stretch one of the laminates, not easy. This makes for a great router table top.)

Don’t get hung up on half lap (interlocking) web supports. In the real world we found that it did not add anything except more work. An air nailer really speeds things up here. You can cut holes in the skins without any depreciation in strength (like for your legs to enter inside for attachment). Glue every joint and glue the web to the skin. Glue is very important here. Nails and screw just are clamps until the glue dries.

Maybe think about drilling some holes in the top skin and attaching some T-nuts from inside. These will come in handy for mounting hold-downs or bench dogs later. Or gluing a block in some of the corners of the ribs so that you can drill dog holes later (remember where they are, maybe even take pictures).

Keep us posted, and I will get off of my soap box,
Steve

-- -- I'm no rocket surgeon

View bench_dogg's profile

bench_dogg

63 posts in 2598 days


#7 posted 12-06-2009 05:42 PM

I built the top in Marc’s video and really like it. I made a few changes; specifically I used melamine for the top and screwed it down so I can replace it if it gets too beat up. I like the melamine mainly for the ease of removing old glue.

-bd

View Jimi_C's profile

Jimi_C

507 posts in 2696 days


#8 posted 12-06-2009 07:28 PM

I agree with SPalm. I used 1/2” MDF for the skin and 1×4s for the web. I used two drywall screws at each butt joint for the outside square, and two more for the long cross members. For the shorter members, all I used to hold the pieces of wood in place was a single staple. Here’s my write up on my bench top: http://lumberjocks.com/Jimi_C/blog/10524.

I disagree that they don’t make good bench tops though. The top of mine is very solid, and you can see I doubled up the front piece in order to have more wood to screw into for the face vise (which I still haven’t attached to my bench…). You could do the same on one or both ends if you wanted to do a traditional tail vise.

Overall, I’m very happy with my cheapo bench, it’s been serving me well for a few months now, and if it ever gets beat to crap I can either just drop a new sheet of 1/4” ply on top or take the top off and build a new torsion box top.

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

View Morton's profile

Morton

30 posts in 2561 days


#9 posted 12-06-2009 08:29 PM

@SPalm – awesome info, thanks! Most of what you wrote I was planning on already, so that is great to get confirmation. Thanks for pointing out that 2.5” is fine, I might do that. I don’t plan on getting too hung up on flatness, I will do the best that I can. Love the idea of T-nuts on the inside, I was planning to double-up some areas with a small square of ply for more depth on punching through for benchdog holes. Really appreciate all the insight – great real-world info, thank you.

@Jimi_C – thanks for the link, I forward to reading through that. Glad to hear that you like it as a bench top – I hope to get the same use. Great idea doubling up the front edge – my plan was an internal 3/4 MDF with a 3/4 hardwood wrap all the way around – so a great 1.5” thick on all edges should be OK I think.

-- Morton - http://workshop.scottmorton.com

View SPalm's profile

SPalm

5257 posts in 3343 days


#10 posted 12-07-2009 04:00 PM

Hey, me again.

Just a couple of notes on torsion boxes. These are engineering structures, akin to a 3D I-beam. All the components (both skins and webbing) must be securely attached to each other. i.e. Lots of glue everywhere. This is more important than the thickness of the components. Two real world versions of this are the hollow core door and an airplane wing. The components for these are very thin, yet the result is amazingly strong against flex. The torsion wing is actually stronger than a solid one, as it does not have to support all the added weight, and it even has holes in the ribbing.

So this structure will resist flex because one skin will have to compress and one will have to stretch. Again, this is very hard to do. It will dent or puncture; think kicking in a door or walking on a wing. This is why it is not used a lot for bench tops (but hey…).

So I would recommend 1/2” MDF for the ribs and bottom, with 3/4” MDF for the top. Real wood still has that dimensional problem with changes in humidity and it does not glue well with endgrain to long grain, so it is probably not the best selection for the ribs. You can even cut holes in the ribs to reduce weight (ala the wing). Add internal blocks where you are going to attach vises, legs, etc. (this is what they do in aircraft wings). Then add a loosly attached, replaceable, additional sacrificial top to beat on or drip glue on (hardboard?).

Steve

-- -- I'm no rocket surgeon

View Morton's profile

Morton

30 posts in 2561 days


#11 posted 12-07-2009 04:10 PM

@Spalm – thanks again Steve, great info. I am planning on using 1/2” MDF throughout (ribs and skins), so that’s great to get confirmation on that. I am going to have a replaceable top, hardboard. I will “fill in” the areas that may get a vice or bench dogs. Prolly won’t bother punching holes in the ribs – not too concerned about weight. I may use a 3/4” MDF top, but currently am planning on 1/2” for both skins, and an additional 1/8” hardboard replaceable on top of that.

Lots of glue, check. Will use screws/nail-gun as clamps while glue dries. May even remove screws after glue sets (am I a Yankee or what?)

btw – love your idea of laminate for router top (laminate both sides)—cool, never thought of that as keeping the top flat in the same manner as a torsion box.

-- Morton - http://workshop.scottmorton.com

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