This drafting table top will be 72 inches long X 45 inches wide X 2 inches high.
The grid, inch and a quarter square hem/fir in 9 inch squares; 8 long X 5 wide; 40 squares.
Top skin will be one piece of half inch BB plywood glued.
The bottom skin will be in 2 PIECES of quarter inch BB plywood butted and glued; I dont have a big enough piece for the bottom and this is not the US where you can trot down to the local store and get what you need, hence the 2 pieces and the query.
Will the 2 pieces on the bottom act like a hinge where they butt together and defeat the integrity of the torsion box??? Bottom will be in tension.
Thanks.
Replies
I think that if the butt joint on the bottom falls over a grid piece you will be just fine.
DR
That's what I was thinking too.
Thanks Ring.
I think I would have to disagree about the joint on the back being sufficient.
As a drafting table, will this top be movable or fixed in position? If it is tiltable then there is nothing keeping the top from flexing but the hinge, right?
The back of the table will be in tension so the joint will want to open as you press/lean down on the table. I think it will be a problem. Is it possible to "skin over" the entire back with something that won't stretch? Maybe a high pressure laminate (Formica).
Or perhaps adding a length of metal angle (iron or aluminum). Spanning the joint, across the width of the back.
Andy
"It seemed like a good idea at the time"
Andy,
The top will be raised/tilted most of the time; once you get a comfortable angle on the board you pretty much leave it there; about 10-15 degrees.
72 inch span supported on a metal rod at each end.
I'm sure there will be a slight deflection due to its own weight.
Have no formica or metal angles.
I do have one 4 X 8 sheet of T-111 which I hate to use as it seems like overkill for a lightweight torsion box.
The real risk of bending comes when you fall asleep over the table and your head 'hits the deck'. But then you are not drafting so the slight deflection will not matter.
Seriously, I doubt if flexing will be a problem as the loads imposed by 'elbow action' will be relatively light. By the way, you never mention if the joint will be along the width or length - big difference.
Jerry
I never considered impact loads like cement heads.
The butt joint direction is front to back; worst case scenario.
Plywood face grain direction for the top will be left to right.
Personally I feel that the 2 piece bottom skin could be omitted entirely; just adds weight and increases deflection; now if we added a 12 inch doubler strip below the joint this would provide some continuity and lots of glue surface.
Years ago the military made a couple of different wooden airplanes from plywood so the engineering has been long established and 40 foot airplane wings had joints; how did they do it??? Rosie the riveter wasn't much help.
Dont know what the military used, but, when making sailboat masts a 1 in 7 scarf joint was considered to be as strong as solid wood.
Maybe we are over-engineering this, my gut feeling is that a butt joint that falls over one of the torsion beams should be sufficient.
Its a drafting table, I would think the most load it will see is when he leans his elbows on it.
If you really want to get fancy, make an interlocking joint. One that, in cross section looks like taking your hands, curling your fingers and placing them end to end and palm to palm. Easy enough to do on the tablesaw, uses minimum material, and solves the tension concern.
Thanks mb,
I did read anatomy of ships; think they were Nelson's and remember the scarf joints in the 4 foot high keel using water pressure to compress the joint but we are talking 6 mm BB plywood.
This is not so much a structural problem as a deflection one; the board has to be very flat or you get a gap below the middle of the straightedge and it rides on the ends only.
The grid will be half lap joints.
I don't see any problem with the half sheet especially if the joint lands on a piece of the substructure. I've made a few like that and tested it with a load and didn't see any difference between the one piece skins. Torsion boxes over overkill for many applications.
Half laps! Totally unnecessary and more work and time. Ian Kirby's article of some twenty years ago just used long strips for the main substructure pieces with short little blocks for the rest of the grid. These pieces were simply butt joined and stapled to hold the form together untill the skins were glued on. You are best to glue both skins on at the same time.
!/4" skins would be more than enough got a drafting table. I made a torsion box for my table saw years ago and had it in storage in a damp unheated garage for 5 years. When I finally got around to setting up my shop again the torsion box was as flat as the day I made it. I used MDF for the core.
Ian Kirby was the first to present the torsion box and David Marks has some info on his website.
http://www.diynetwork.com/diy/ww_materials_products/article/0,2049,DIY_14442_2278182,00.htmlWoodworking Machinery Technician
Thanks Rick,
Really good website; explains everything well especially the beginning about setting up sawhorses and leveling everything with shims and winding sticks before you even think of starting.
Wasn't too sure about my half lap joints in the grid; seems you are cutting halfway through the structure then gluing it back together again; you just saved me a lot of time.
David Marks uses very heavy materials; OK for a fixed assembly table; I'll make it lighter for a movable top.
I've seen others talk about hlf lap joints in torsion boxes. I wonder how all this misinformation gets about. Torsion boxes can be really fast and simple and certainly strong. Marks uses a nail gun but Ian Kirby's original article in FWW 20 years ago used a simple hand stapler I recall. All you need to do is keep the grid together until you get it positioned with the glue and skin.
What was your source of info on making torsion boxes. I'm curious as to the half lap procedure. Seems the web spread more bad info sometimes than accurate info.Woodworking Machinery Technician
I just made a sample torsion box for a Fine Woodworking video article and used half lap joints because it was in fact as fast or faster than cutting and gluing up a dozen or more pieces of wood, it also allowed me to use 1/4' inch stock for the grid. Half lap joints on a table saw go very fast especially for work that will be hidden.
John White, Shop Manager, Fine Woodworking Magazine
Simpler and faster yet is using common builders Styrofoam as the core material. Use poly glue to adhere 1/4" skins and edging of choice and press in a vac sac. Presto! a great fast torsion box.
Using a foam core would also be my personal choice for a torsion box since it is about perfect from an engineering standpoint and probably the fastest way to build also.
John W.
Rick,
My original "clash" with torsion boxes was trying to trim?? like a foot off a hollow core door to fit a smaller opening; knee wall I think.
Cardboard and air inside the door.
Half laps came from my attempts at eggcrate drawer dividers which are way too skinny to nail. I am using a heavier grid now; bigger target for the hammer.
Answer YES to hinging.
And its not really a torsion box since its not reacting torsion. Its more like a bridge and it doesn't matter which direction the seam goes. On aircraft, you would lap that joint with a double row of fasteners at least to maintain axial capability. Your design has an axial discontinuity. The load has to go into that core piece then out again. So if you want to stiffen it up, do what we would do on a/c. Make a plywood doubler. You cold glue a strip to the 1/4" stuff to make a large panel, than screw a double row right thru the doubler.
I can see why folks would think this is overkill. But with a drafting board, strength really isn't the issue. Its stiffness.
That said, if it were me, I'd make a top out of solid wood. My experience with plywood is that its not flat and I have no tool to flatten it. With solid wood, I could plane it or sand it or scrape it. With a drafting board you need both flatness and stiffness.
Adam
Adam,
When I used to work for a living the board I had was laminated maple strips; dead flat with steel channels for breadboard ends so the wood could slide around; this was factory machine made to very close tolerances.
I dont have that skill level and all the wood I use is recycled; this top will be 72 X 45; the more joints you have on top the more they will telegraph through to your drawing and even with a board cover cushion you will still get occasional telegraphing unless you have it very smooth and dead flat as you said.
I was thinking of making the doubler strip to cover 2 grids front to back below the joint; glued and some brads.
The mother of invention is............lack of materials???????????
I'm trying to think all the way thru your job-- I don't see why you think its easier to flatten this substructure and why all those joints will be easier than flattening a glue up with a long hand plane. Understood about the recycled material, but you may find the tail wagging the dog a bit. You may be better off gluing up the hem fir and skipping the plywood altogether.
One challenge you will face is preload in the substructure. Not sure how you'll be making those connections. If you are planning to use screws, you may find after having done the asembling on a flat surface, once skinned, it is no longer flat.
Of course you'd have the ame problems with solid wood, but at least you could plane it flat to fix it.
Just a thought.
Adam
PS I fell in love with my board. It was small like yours and had an electrically telescoping pedestal. So much nicer to stand in the afternoons. Now computers have replaced our boards and we sit on our duffs all day. I'm one of the few in my generation who wore a tie to work and sat at a drafting board. It was a nice experience. Now I make a lot more money, I dress like a bum, and sit all day behind a filthy dusty computer (2 really) to do no faster what I could do then.
Not sure how good a scarf joint you'll get with a piece of ply like that. I think, if you cannot get a single piece for the bottom, get a second piece and make it overlap a significant distance on each side of the seam, say 12" on each side. You'll be gluing long grain to long grain, so should work well. You should put this reinforcement on the bottom and hide with an edge strip, as gluing on the top side of the bottom is going to tremendously complicate correctly dimensioning your grid pieces.
EDIT: If you can play with that 2" thickness, you can tremendously increase the stiffness of a torsion box top. The reference below explains it very clearly.
You could also use FOUR pieces for the bottom, make sure the seams are well offset from each other, and face-glue it all together. Increasing the thickness of the skin increases the stiffness of the box by the cube of the increase; twice as thick is eight times as stiff.
For a torsion box to get its benefit, you must withstand compression and tension on both skins, and I think you're going to compromise it with anything that doesn't keep that joint dead tight.
It's true that you can work a thick board to be flatter than a piece of ply sitting in a rack. But if you assemble the torsion box dead flat, it will stay dead flat. Think a 2" thick plank of ANY wood will stay as flat through the seasons? Heh, not a chance on earth.
The best writeup I've ever seen on torsion box construction, and what makes it strong, is in Ken Horner's new book "More Woodworkers' Essential Facts, Formulas & Short-Cuts." Its ISBN is 1892836211. The chapter on torsion boxes will help you understand the implications of different skin thicknesses, cell sizes, and material choices. I've not seen as useful a presentation on this topic anywhere. (No affiliation, only bought it and found it excellent)
Hope this helps.
Edited 6/11/2006 9:15 pm by John_D
John,
It will be a while before I visit the States and get a copy of Horner's book; meanwhile I am starting to get a handle on this torsion box design.
There seems to be 2 trends in the forum........................dont do anything and stiffen it up. Living in the tropics I was leaning towards the former with a cold one in hand.
I do have lots of 6 mm BB plywood AC but it is all in less than full sheets; the landfill can be less than accomodating; but as you suggested, offsetting joints and doubling thickness will increase the stiffness. Originally I was thinking of just using a doubler at the one joint.
Ever hear of pre-stressing a torsion box; putting an ever so slight crown; read tension; on the top surface that gets cancelled out under load; no compression; no deflection?? just a thought; I know you can do it with concrete spans to get rid of hammocking; wood is probably too flexible to deliberately induce man made stresses.
The trees will revolt.
There are formulas in the Horner book for calculating deflection, so you could see if it was going to be acceptable for your load. Worst case of course is finishing it, and discovering that when you actually work on it, it flexes unacceptably.
I bought the Horner book at a local bookstore, but you could get it from Amazon. Don't they ship internationally? If not I bet you could find another vendor, or someone here to send it to you.My goal is for my work to outlast me. Expect my joinery to get simpler as time goes by.
http://www.amazon.co.uk/exec/obidos/ASIN/1892836211/ref=sr_11_1/202-2901327-3984656?%5Fencoding=UTF8&v=glance
There's an amazon.co.uk in BritainWoodworking Machinery Technician
John,
I wont be applying any finish until the board is up and running smoothly then it will get removed, finished and re-installed.
Shipping to Bonaire is a no-no except by Fedex and that is for emergencies.
You need something here, you find somebody going to the US or Holland and give them your list; they bring it back in their suitcase. You reciprocate. I know it is tough to bring back sheets of plywood.
I shop online, use a US mailing address and visit once a year with MT bags.
Wow that's something. I take easy transportation so for granted.My goal is for my work to outlast me. Expect my joinery to get simpler as time goes by.
With sufficient glue along that butted seam, there will be no hinging effect. If you were making something that was structural...... that had to support a really good load, then yea, I'd say you shouldn't compromise the bottom "stressed skin." But hey, this is a drafting board. The 1/2" ply backed up by a grid of lumber is more then sufficient.
And let's just suppose that it doesn't work. All you gotta do is get a piece of 1/8" ply or anything, and glue it to the back over the entire surface. Easy peasy.
Easy peasy.................. nice and easy; I do like your approach to woodworking; you'd fit right into the island culture here.
The top is not really a structural problem in the load carrying sense; you are right but there can be no deflection on the top or drawing gets a bit erratic so we need stiffness and flatness, rigidity and????????????
You can certainly add more skins to the bottom if plan A doesnt work; be nice though if I only have to wrestle with this beast once.
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