Tools from Japan blog.

If you can cast your mind back about a week or so, where we mangled a few rectangular holes into some wood.

Some of those holes (also known as ‘mortises’) have spread all the way through to the other side, which means that they are now through mortises. A whole new ball game…

This is why I have been scarce. While a regular mortise and tenon joint is a pretty simple thing and fairly forgiving type of joint, a through mortise and tenon is nothing of the sort.

Less forgiving if your workpieces are part of a frame that must be square.

Less again if your workpieces are large.

And even less forgiving if they have to be very, very pretty and strong.

Guess what, I got all of them and got them in spades.

The order is for 3 doors. Two the same size, one larger. All have 1 3/4″ x 4″ rails and stiles, with a centre rail in each.

The panels to go in the middle are not raised panels, they will be flush and completely fill the spaces. They will also be glued in place and add strength to the door.

The doors are ^%$^%$^% enormous. I managed to get the two smaller doors framed up today (one frame still needs some tweaking) and I was shocked at how big they will be. Try 3 feet wide and 7 1/2 feet tall. Laying on it’s side, it’s not so impressive. Stand it up, and it really manages to be quite impressive.

The other door will be nearly 9 feet tall.

Help!

So if I am not here, I don’t think there needs to be too much imagination required to work out what I am doing.

(Did I mention that these things and a large bed need to be finished by the end of the month?)

Until I am out from under my rock again,

Stu.

First, I am better but still can’t kick the odd cough. Tom is good, although he is picking up bad habits from the kid who lives upstairs. Koko is not getting better, and is starting to worry us so she gets to see the doctor again tomorrow. It might not be anything too serious, but we don’t know so off she goes. We do know she is a very sick little girl right now, but she should be ok. I hope…

Anyway, what the title says is what I mean.

I am often reading about folks who are just starting out and realize that their first project might rightly be a workbench of some kind.

They can manage the bench top, just a bunch of sticks stuck together.

The base, that’s something else entirely. It’s going to need all kinds of real live joinery like mortise and tenons, and we aren’t talking pipsqueak joints here, big angry suckers to resist racking and twisting forces.

The problem is that most workbenches you might see being made are often made on a workbench!!! How the heck do you make a workbench without a workbench, especially when it comes to big, scary mortise and tenon joints, the likes of which few folks ever see, apart form in their workbench?

Well, here’s a way you can get slicker than snot (just to add colour there) mortises quick and simple. There are few fancy tools required and you may be able to limit this to the very simplest of tools if you are tool poor. There are a few rules that help a lot, and I will mention them along the way.

This is a pictorial essay boys and girls, but my usual camera was not available, so I got stuck using a shoe-phone to take the pictures.

First up, mark out where you want the mortise. I used a tape measure, some grade school mathematics and a honkin’ big square so I could transpose my measurements across four pieces. I only needed the centreline (as my mortising jig references off the centre) but also marked in the end points too. If you don’t have a router and edge guides, or even better, a simple mortising jig like this then you should also mark in the width of your mortise, and use a marking knife of some kind.

I do have a mortising jig, router and edge guides so I just mark the centre line, set up the contraption and off I go.

>I am not using a bench here, just the small saw ponies I made up a while ago. The boards are clamped down to a pony instead of in a workbench vise. I am working down low, and I prefer it for this. You could use regular height saw horses or even a workmate for this, heck even a stump would work well. You just need to be able to clamp the stick you are working on down so it won’t jump around.<

Before I use the router to get the show rolling, I always, ALWAYS take a very light test cut with the router spun up just to make sure the mortise is going in the right place and everything is as it should be.

Note the mark on the wood, the very shallow mortise shape a the reference lines on the jig there. The two lines are 2mm apart, and this gives me a little wiggle room within a known area of 2mm. This one was not right, it needed some adjustment. Because that shallow mortise is very shallow, it will plane out easily, but will also be covered up by the tenon shoulders.

And there it is. Only 35mm deep, and we need to go 70mm deep at least so we are only half way there now.

Using a router to start out makes life a lot easier later on as it gives you square sides to work from and a repeatable size to start hacking away from too. If you can’t use a router, then you will have to drill and pare or cut the thing completely by hand. Good luck!

Here we are, drilled down to just past 70mm deep. This mortise also happens to be 70mm long and 15mm wide. The drill bit is also 15mm wide, which makes drilling down easy as the sides will help keep the drill square.

Take note that I drilled 4 holes, leaving more wood in the middle than between the two holes on either side. I could have drilled 5 overlapping holes, but I found that drilling in this way gave me a better end result. You will see why soon.

Next step, take out the bits between the drilled holes. For this we need a few chisels.

A motley crew of tools if ever there was one, aye…

From left to right, 9mm, 15mm and 1 inch chisels. Two Japanese and a Marples blue chip. All very, very sharp and all at 30 degrees or so. No microbevels, they won’t help you here at all. If fact, you might curse your microbevels before you get done because we are going to chop and pry just a little, and a nice, flat bevel is going to help you out to this end. Trust me.

The hammer is a big ball pein thing. Biggest hammer I have without resorting to miniature sledge hammers. I usually use a small hammer with my chisels, in this case the BFH is the right tool for the job. Choke up on the handle and give small taps. From a hammer this big, the taps will be postive, and the tools won’t bounce. This will save you trouble and make for cleaner cuts.

You could use a smaller hammer. I wouldn’t…

Now we need to get those bits between the drill holes out of the way.

Start by taking the smaller chisel, and knock out bits off the end ones. Take small bites, and the chips will get out the way for you. If you need to hit the chisel harder than you would want to hit your hand with the hammer, then the bite is too big.

After getting down a little, you might find that your chisel is fighting for room. Make some more for it! Don’t pare flush just yet, just take a little off to let the chisel move freely.

On the right, we are down a fair way now and need to get the middle one out of the way. Take some off each side, alternating so you maintain a small ‘pitched roof’ at all times. You could go lower, but then you have to be careful about taking too big a bite. Work your way down carefully, and don’t worry about the chips until you can’t see what you are doing.

And there we are. Most of the bits in between the drilled holes are gone and without too much difficulty. This also shows why I avoided drilling 5 overlapping holes.

If you drill overlapping holes, then you are left with bits on the sides that are much bigger, and therefore stronger than these mangled peaks are. Paring them down flush becomes a multi step operation, and the more bites you take on the sides, the more chance for foul up and the rate of clogging is much, much higher. Taking small chips means that you can clear the mortise easily and often. Taking small bites that are still big in size means more trouble clearing things out.

And how do we clear out the chips without unclamping the board?

“Big deal, he’s using a vacuum cleaner, so what?” you say, not quite.

The vacuum nozzle is over one end, fingers cover up most of the rest except for a little at one end.

Doing this raises the air velocity in the mortise, making all those little chips jump about and get sucked up by the vacuum. Try it like this, you will be surprised and impressed.

(If you submit this to a magazine, I want half!)

BTW, the vacuum is a top of the line Makita. Worth every yen I paid for it and then some. If you don’t have a good vaccum in the workshop, scrimp and save and get one.

So, now we need to get those pesky peaks off the sides.

Easy!

Put the wide chisel on the top of the peak, flush with the side and give it a gentle tap. Then hold it flush against the side with your hand and tap it down.

The first tap locates the tip and lets you hold the chisel flush without it moving about. The next taps drive it down, and because you are holding it flush with the already square side and taking a small bite, you get a nice, clean paring cut all the way down in one shot.

And the chips clear out easily too.

Now we are at a point where you need to ask yourself a question…

Do you square the ends, or not?

I did. I didn’t have to, but I may make these tenons run through, and squared mortiseslook right, rounded sides look wrong. That’s my theory and I am sticking to it.

Take you chisel that matches the mortise width, and square off the ends.

Do not drive it down all the way. Your chisel will cause you no end of grief by getting stuck, cutting down on an angle and all kinds of strife you can’t even think of.

Instead, give the chisel a few taps, then take it out. Use the smaller chisel to take out the bulk of the corners, then drive the chisel down. Same as the bits between drilled holes, take off the bulk, then clean up with one shot.

It sounds great in theory, but you will need to fiddle and fool a little to get it right. Practise makes perfect, or you could just leave them rounded. I don’t care, and neither does the wood. But you will have to round off your tenons to suit later on.

You might also need to clean up the bottom so it’s reasonably flat and clear and tidy up anywhere else the mortise isn’t really happy.

After all that…

You got a mortise!

This one isn’t perfect, it seems I got a little excited and took a little extra off that bottom right corner and the ends are not as clean as I would like.

And you know what?

I don’t care and neither does the wood.

This is a mortise and nowhere does it say that it needs to be picture perfect. You only need to make sure that the mortise and tenon fit together well enough for the glue to do it’s job.

And this one is, without question. That spot is less than a millimetre off, and isn’t very big. It won’t affect the joint enough to be a problem. These joints are oversized by a long way.

(And if it wasn’t all ok, I am using epoxy which covers a multitude of sins, especially sloppy joints… )

I hope that someone just starting out and needs to get some big joinery done without a bench finds some use in this. It works, doesn’t take too long, is cheap and has a very shallow learning curve.

Just what the doctor ordered.

Stu.

I guess that means I have a cold, which helps to explain why things are s-l-o-o-o-o-w around here right now. I am getting over it, but it’s still annoying and what’s worse is that I am not alone with my musical illness, both little folks here got it too.

I don’t think it is a cold, more like the flu. A little joint pain, but nothing too serious and not enough to sideline me.

But it is bad enough to keep me out of the workshop for nearly 2 weeks now. I have been there, and I have done some things, but nowhere near as much as I wanted or needed to get done.

What I did do was:

Made a router thicknesser type device. Not overly conventional, and I won’t need to make another one ever again. Worth looking at methinks.

Coughed.

Repaired and restored a #18 Stanley knuckle cap block plane. The repair was a little bit botched as the crack ran into the handy-grip part and I had serious trouble grinding the brazing back, but it’s back in service again and better than it was before. Strange, because it was a sweetheart (in more ways than one) before I attacked it.

Sneezed.

Made a 55 degree coffin shaped smoothing plane. Nothing special here, and the shape was more for comfort than anything else. This thing was made to compliment the router sled doohickey. Simple and effective little plane and comfortable to use.

Hacked up some green and yellow stuff.

Made a goodly chunk of the panels for those mysterious doors.

Coughed some more.

Milled up all the door frame material to go around those panels.

Took a tablet to stop my nose leaking.

Organised some things in the shop a little better.

And coughed again for good measure.

This cold/flu was nasty and capped off one of the worst few weeks healthwise I have ever had. Probably a goodly splotch of stress didn’t help much, and now little Koko has a pretty serious fever just to make things really fun and interesting…

If I don’t get back here for a little while, I don’t think you need to make too great a leap of the imagination to work out what’s going on here.

Thanks for your patience, and I’ll be back at it again as soon as I am reasonably able to.

Might throw some bits and pieces together just because I want to.

Stu.

>cough!<

and we finally stick on the top skin of the torsion box/assembly table.

There wasn’t much to it really, just so long as the core grid is flat and don’t spare the glue.

My top plywood sheet had a large bend in it, so I needed to hold it down so it could be properly attached to the core. I put in some screw blocks along the middle of the grid to help out and marked the top sheet of plywood so I could hit the core grid with small nails all along (not needed, but I did it anyway).

A nice, solid bead of glue along each and every rib where the top sheet would contact it. I think I used up all of my remaining glue on this thing, which would be nearly 2 kg of plain old PVA glue. That’s over 4 lbs of white glue, so when I say don’t spare the glue, I mean it. Just nails won’t work, but just glue will.

I was kinda hoping that the ‘weights’ I used to hold down the bottom skin would be enough…

(Recycling?)

… but they weren’t.

So I did the only thing I could think of when you need lots of weight and the glue won’t wait…

I jumped up on top and screwed the thing down, then ran nails along my marked lines.

And that in itself shows the strength of a torsion box.

The top skin, being a piece of 1800 x 900 x 12mm (6′ x 3′ x 1/2″) plywood isn’t strong enough to take my weight. The lower skin being half the thickness at 5.5mm definitely isn’t strong enough to hold me up, and those long ribs aren’t strong enough either. But combine the ribs (being held in place securely so they won’t fall over) with the thin lower skin, and it held me up easily.

(Just quietly, I weigh about 90kgs/200lbs. The doctor says my height with my weight means I am obese. I think the doctor needs a head examination or should at least look beyond what his books tell him…)

So while letting the glue dry, I ran around and puttied up all the nail holes then sanded the whole upper surface smooth.

I found a piece of non descript hardwood (Ok, it’s beech. As if it was going to be anything else!) as a trim around the outside, glued and screwed it in place, trimmed it all flush then gave it a quick coat of urethane varnish.

And there it is.

The accounting for this thing was simple.

Grand total? $0.00

Well, not really because the plywood cost me some money when I bought it, as did the glue and screws. But I bought no new bits for this thing, it was all waiting in the shop for a purpose and I think I it is happier now that it has meaning rather than being a shop ornament.

Well, it still is a shop ornament albeit a really useful one.

What’s next you ask?

More panels! Yippee!!!

Not…

This time, I need to make some panels for some doors. They are not conventional panels, as making doors as large as these will be can’t have solid wood panels without causing major headaches. Solid wood panels can’t be structural, and with a weight of over 100lbs each, having floating panels is just asking for trouble. Besides, they don’t fit the style of door that is wanted.

So we cheat…

How we cheat is something for next time.

Thanks for being patient and reading,

Stu.

is finally done!

At long last, something to read that isn’t a blog post.

>linky dink!<

While we are here and I am bashing something into this here blog thingy, I won something this week and I probably shouldn’t have.

On Woodnet, A small informal contest was held for hand sawing. 3 different disciplines to see how good you are at wielding a handsaw.

1 was to put the maximum number of saw kerfs in a 3″ wide piece of wood. Andy (Andyman) won that with 76 cuts in 3″. Amazing number of cuts, and I know he can do better with more practise under his belt.

Next was to cut a mitred half lap, and this is the part I won with this joint…

That’s 3″ x 5/8″ hinoki, all done with a saw.

Not much room for improvement really, so I am happy with that even though it could be better.

But I am not happy that I won if only because it feels like I won by default. Only Andy up there ran against me, and while mine closed up better I think, I bet Andy did his is more difficult to work with wood (looked like walnut) so I’d call it even which isn’t a bad thing really, but I would have preferred to be have been slaughtered in this contest if the competition was there.

FWIW, if this happens again I will attempt to protect my cracked and lopsided crown. I should have a little more ability by then, and the weapon of choice may be a little superior too…

Oh yeah, the 3 part was to resaw a piece of wood. No takers.

Anyway, I got up what I wanted to get up and put something here in the blog too for good measure. I know I have been tardy these last few days (sorry!) but I hope the new essay/article/rhetoric is a small compensation.

Thanks as always,

Stu.

… to eat lunch around here.

Which is a sly way of saying that my torsion box will not be finished today.

I am currently sifitng out the final bugs on a small essay (20 pages or something) about comparing a few tool steels. A few means “all I could find, and then some”, so I am going to get that done tonight and tomorrow, which leaves little time for attending to our box here.

Might even get some more of the Japanese tool glossary done. This is really difficult as it has already been done several times by other folks, and it is difficult to do without it appearing to be a copy of pre-existing work. Luckily, I have photographs of most of the tools, and a few extras that are rarely, if ever, mentioned.

Should be worth a few precious moments to read it I hope.

After that, I’ll write something else. Not sure what yet, but it will be made of words and too many of them.

Next time, we will finish the torsion box completely. Well, as far as it is finished at the moment anyway.

Thanks for reading (and being patient!)

Stu.

Time to flip our monstrosity and see how badly we did.

There we go, about as ugly as it gets…

Well, not too badly, but this picture can shed some light on a few things, even if they are hard to see.

(Those strings come from each corner, propped up off the grid by small blocks, sized to lift the upper string above the lower string so that they don’t touch and have no gap when the box is twist free.)

1: Those lines, in the middle, are perfect. What we can’t see is where along those lines the ribs rise and fall in relation to those strings. BTW, the stings are tight. I amused myself for 5 minutes by exploiting my impressive musical repetoire. (Mary had a little lamb is it…).

2: We have a lot of staples in the way of the critical upper skin. Those staples might make the upper skin wiggle and waggle, which is no good. They must be removed.

3: Some of the cross ribs don’t match the longitudonal ribs for finished height.

In short, this side needs some work to attain true ‘flatness’.

First things first, rip out all the staples. If you recall, I mentioned that staples are easy to pull out. They are if you have a technique down. I use a flat blade screw driver and a wood scrap. Put the tip under the staple, the scrap under the driver and pry it out. If it doesn’t come out completely, then grab it with pliers. If the staple snaps, drive the errant chunk down with a nail set.

Next, we need to work out how to get this wavy (but twist free!) grid levelled out for good glue bond and to help ensure a dead flat upper surface.

Now, to out this into perspective, we are talking a maximum of a half millimetre between long ribs, and about the same from long to cross ribs. About 0.020″ or so. Not much, but enough to add up if I let it go too far. I estimated that it went out of kilter maybe 2mm over the whole surface, and I am being very critical I know because it didn’t take much work to fix. But I am rambling, we need to correct this…

Because I used solid wood, and an easy to work one at that, the way to get this grid f-l-a-t is to go find a loooong handplane.

I happen to have a jointer I threw together a while ago, and the trusty #606C, so they got the job.

(No pics here folks, it was too embarrassing to show that I got it wrong, whaddya expect from warped, old plywood? Besides, I was too busy trying to fix the foul up to bother recording it for posterity…)

After taking most of the long ribs down to the level of the cross ribs, I then went after the high long ribs, as shown by the long straight  edge. Just a few passes at a time because the thing was essentially flat, it just needed some tweaking. Once the long ribs were good, back to the cross ribs, and vise-versa until the whole thing was flat by every measurement I could throw at it.

After chasing the thing for about 30 minutes, we got this…

Note the shavings in the grid cells.

The extra block there was to run around grid, checking against the taught lines running every which way. And it all checked out just fine. Maybe a few little hiccups here and there, but we are talking maybe 0.1mm or so. Effectively nothing.

At this point, I put in some extra pieces in the centre grid (across wise) because I knew the top plywood skin had a nice, consistent bend down it’s length, and I needed to be able to screw down the middle to something solid. I could have gone for the grid itself, but I thought it better to have a bigger target.

I also considered some expanding urethane foam to help beef it up some more, but dismissed because of how much I would need (about 4 cans of spray stuff!) and how much mess it would entail cleaning up the excess. Decided against it, and no regrets.

I also clearly marked out the top skin for rib placement in both directions, just so I could pin the skin in place ‘while the glue dried’.

Once all was right, correct and ready…

I had lunch.

And here, sitting in front of the computer, it seems like it might be a good time to get some shut eye too.

So for now, good night and thanks for reading.

(And don’t be afraid to ask questions, post comments or poke fun at me. Just don’t poke my stomach, it’s a little sensitive right now…)

Stu.

What it says is what I mean. I used a heck of a lot of staples assembling the core grid, hundreds of them. Anyway…

Our next job is assembling the pieces of the core, and this is where you get to find out how accurate you are.

Myself? There it is in all its ugliness…

Notice the narrower strip up the middle, that’s evidence of a slight error in calculation. I forgot about the thickness of the longitudonal ribs, and as such lost about 2 inches in the middle.

Maybe I shouldn’t have said that…

Uhh, it was a planned feature to make the centre more rigid.

What you will need to do now is start sticking the pieces together, and how you do this is not set in stone, but there are a few simple rules you really should stick to.

Glue is essential. Even the really poor end grain surface at the ends of real wood strips is fine. There won’t be too much strain on that iffy glue joint, but it needs to hold still.

Some kind of mechanical fastener is not required, but can be very helpful. I use staples out of a hand held staple gun.

There it is, up the other end. Nothing special, just a $10 cheapie that throws staples, pins and nails. The thing has driven thousands of fasteners, and is one of my most used tools. To quote some bearded fella I don’t know, “just to hold things till the glue dries…”

Also present are a stack of short rib pieces, the longer ribs on the right, a bottle of glue (one of several) a couple F clamps, a corner clamp, a hammer and nail set, tape measure and some slightly under width short rib pieces.

If you look at the bottom of the pic, you can see the staples across the joins there.

Here’s a better picture.

Looks strange doesn’t it?

I do it this way for three reasons.

1: It’s fast and simple and I don’t need to get a nail inside a grid cell.

2: It is strong enough to hold things in place just fine until the glue sets up, but flexible enough that adjustment is easy.

3: If you make a mistake, pulling the staples is very, very easy. Try pulling apart a join that has been pinned from a two different directions, and the silly, simple staple starts looking really nice.

Throwing this together took me maybe an hour. Just a bead of glue on the end, a clamp (as above) to hold it together tightly and whack in a staple then move along.

A word of caution here too on how you go about this.

Start from one side, and get a few rows made up, but don’t go past the centre of the final size, width wise. Then start from the other side, so that any error ends up in the middle. I much prefer having an extra row in the middle to maybe make it a little stiffer there  than having it on one side. At the same time, only nail in one end piece for now. Leave one of the full width end pieces off for the time being, and do it after all the long ribs are completed.

Here’s part of the reason why…

You might need to trim things a little to get it all nice and pretty.

You might also find that leaving out the centremost row of short ribs might be a good idea, just in case you find that cumulative error has thrown the whole shebang out of square.

With the centre row absent, you can easily get things running right.

After stapling everything together, it might be a good time to do a quick check on how level things are. The luxury of solid wood is that you can easily adjust flat after the grid is finished. If you are using MDF or plywood, then check for level at least every 15 minutes just so the glue doesn’t start to lock things up out of flat.

What you will also find is that the staples will allow you to lift and flip the entire core grid with ease. Just make sure you keep stapled side in tension while you flip it over. If you let the stapled size come into compressions, the whole thing will probably come apart like a house of cards.

Once it is flipped, I can’t think of a better thing to do that throw in a bunch more staples.

Well, a better thing might be a get a skin on the bottom side while you have a shot at it.

A good bead of glue, a few pins to hold skin in place and a little weight to keep it all flat.

This is the bottom of the box, with 5.5mm plywood skin. This side remains permanently in tension, and doesn’t need too much puncture resistance so this stuff is fine. It will also conform to any slight bumps or hollows in your grid that may have crept in.

Now we need to let the glue cure so it’s better to find something better to do than watch glue dry.

Thanks for reading,

Stu.

No prizes for guessing what comes next, the core grid. And yes it really does look like a really big egg crate, especially because of the tighter than normal grid spacing I used.

At this point, we should have a base to work on that is in plane ie: flat!

Now take your proposed lower skin sheet and throw it up on the base. You need something to work on that has no gaps in it, unlike the current board base.

What are you using for core material?

If you are using plywood or MDF, then you will need to make sure that your powered sawing device is in very good working order (time for a tune up!) or you have some other suitable method to trim the material to size, like a router with a guide or in a table. Getting the pieces straight is most important for an assembly table, parallel is also important but not critical for a single sided torsion box like an assembly table or reference surface. If you need a box that is flat and parallel on both sides, then it is most important that you get the core material exactly on the money otherwise you will end up with something that you can’t even use for firewood.

If you are using solid wood, then things are not quite so critical when cutting it up because adjusting it for straight and parallel is quite easily done. Solid wood is also much more forgiving if you somehow manage to make your core less that dead flat. More on that later…

I used Hinoki, also known as Japanese cypress and is functionally similar to Port Orford cedar. This is a softwood (obviously) that is significantly harder, stiffer, more stable and more durable than most softwoods. It is know in Japan as the perfect wood for joinery as it is tough enough to take a joint well, but soft enough that it is easily worked. I won’t disagree with that statement, it is wonderful stuff to work with, and is the only softwood I will willingly use. I also have an allergy to softwood, as soon as I start working with it, my nose starts leaking. Hinoki doesn’t worry my breathing hole, which just adds to it’s good points.

You could use nearly anything, but I recommend using a wood that has a good stiffness to weight ratio, especially if the box is to be moved in the future. Softwoods tend to punch ‘above their weight’ when it comes to stiffness, simply because they weigh less (on average) than hardwoods. Less weight means a bigger chunk of tree, so it stands to reason that a bigger bit of wood will be stiffer.

My own personal favourite for tosion box cores is not a softwood. If I was able to, I would use Kiri, also known as ‘Paulownia’. The reason being that Kiri is very light in weight, it is very stable, quite soft but tough (if that makes sense) and is very, very stiff. Apparently, it has the highest stiffness to weight ratio of any wood and having use it, I believe it. Amazing stuff, but not exactly low cost or easy to work with handtools. Better make sure your tools are seriously sharp…

Whatever your material, it is essential that it is straight and at least very close to parallel. Heck, make sure the stuff is right, otherwise you may be losing some strength and stiffness simply by using core material that isn’t quite right.

Now you should have your core material sliced into strips according to the thickness you decided that your torsion box needed to be. In my case, 80mm.

You will also need to know how big each grid section will be. Based on how much material I had and how much area I needed to cover, my grid size was about 120mm square, give or take a few dozen millimetres.

Hopefully, your grid strips will be long enough to stretch along the longest dimension with length to spare. If so, trim them to length and put the offcuts somewhere safe.

If not, you will need to join strips to get the length you need…

This sounds like a disaster waiting to happen, but it isn’t. Sure, full lengths are best, but joined pieces are fine if you have to go that way.

You will need to make sure that the ends match each other very well, and if possible some re-inforcement of the join is good too, but not essential…

The reason why a joined strip is fine is quite obvious really.

A strip that has a slot cut in it to accomodate a strip that will cross it (half lapped core) is quite alright. I don’t do it, but many do and it works just fine because at that point, the core grid isn’t doing very much work. Most of the load goes through the skin pieces. We know that the skins are not terribly stiff, but a very small piece, less that a millimetre long is quite stiff. As such, core strips that are not in one piece are not going to compromise your torsion box by any significant amount.

So don’t worry about it.

You should be able to get full length strips if you are using only sheet goods though. Having strips that will go the distance are much easier, so if you have the means, please use them.

My strips were not long enough, not by a long way. All I did was glue the ends together, along with a clinched staple (staple all the way through, the protruding ends turned back on themselves into the wood) or three. Just made sure that the strips were straight and that I would alternate the joins through the box. Not too much trouble at all.

Next is to cut short pieces to go between the longer strips/ribs in the core. The length of these is the grid size less the thickness of one strip. For my box, 120mm less the 12mm stock means 108mm long.

Again, make sure these pieces are accurately cut for length. Pretty important because if they are all over the place, then it makes assembling the core more difficult, and you may end up with a core grid that is less stiff than it might be.

So now you should have a few long strips and a big pile of shorter ones. Hopefully enough to make the grid without having to make some extras…

For now, take a deep breath and a rest. The next job is to assemble the core, and I hope you like playing robot because it’s a bunch of identical operations, and the more pieces you have the worse it is going to be.

See what I mean, a big egg crate assembled by a robot…

Goodnight,

Stu.

In the previous blog post, I went through a whole bunch of ‘other’ torsion boxes, and perhaps I might have been a little harsh on them.

Nothing could be further from the truth.

For an assembly table, any or all of those torsion box designs will work very, very well. They are certainly stiff enough to hold up almost anything you might wish to throw up on them (such is the strength of the basic torsion box design) and will remain as flat as you might wish.

For my own torsion box, I need something that will stand up to much, much greater pressure. Essentially, it will be the table for a veneer press of sorts, and as such it needs to be much stiffer. Several orders of magnitude stiffer in fact, which it just happens to be. I also wanted it to be light enough to be man-handled without too much trouble, and I needed a minimum size. I got everything I needed, and more than I wanted.

These are the stair treads I made up to go with the big panel.

If you look at the very bottom of the picture, you will see something covered in craft paper. Under there lurks my torsion box sitting on 3 saw ponies.

I measured at several points to the floor while 20 of these 15kg each stair treads were on there, and wrote down the measurements. After they were taken off for delivery, I measured again. To the best of my ability (lots of error possible here folks) I couldn’t find any flex anywhere. 300kg and no flexing is not too bad I don’t think.

The minimum width I need is 900mm, which it is. I also needed a minimum of 1200mm in length, and got 1800mm so I got more than I wanted there.

As for manhandling, that’s easy. The box weighs 33kg. A full sized sheet of 3/4″ MDF weighs about 36kg. It is very easy to move around, and the smaller than big sheet size adds to it’s portability.

So that’s it. I got what I needed, and that won’t necessarily be what you need or want.

Snap shooting? Huh?

Well, we went through some off the hip mathematics the other day, and it might have appeared that my wonderful little torsion box was thoroughly thought out and engineered to be what it is.

Uh… no…

The fact is I threw it together based on what looked right and what I had. After I started it and gone beyond the point of no return did I run some sly calculations and came away satisfied that I was going the right way.

I either over engineer or under engineer my stuff, but in this case I got it right on the money. I hope I can keep it up.

(I really hope so because I need to make up some furniture grade boxes soon…)

Now we get to the interesting part, building the thing!

To start with, we need a flat base on which to assemble the core. If you stick it together on a wonky surface, you are going to get a wonky box. So it pays to spend a few moments to get a solid, flat foundation.

I borrowed ideas from Mr. Marks and Mr. Whisperer here, so I can’t claim too much credit here.

To start, you will need some sticks of wood that are long enough to cover the full length of your box, at least one per foot of width. They will need to be dead straight and have parallel edges. I just jointed then thickness planed some spare beech sticks I made up for those stair treads but didn’t need or couldn’t use.

You will need another stick of wood to use as a straight edge. Make sure you can afford to lose some of the wood as you should joint it straight before you use it very time. Wood moves, so be ready to allow for it. Again, I use a spare stick from the treads. I also thickness planed both edges parallel, and for a very good reason because…

A nice, long level is useful if not essential. I don’t have one of these, but I do have a small torpedo level.

Two birds, one stone. You know the drill…

(If you have to pull this manoeuvre, you will need to check it. This is easy, simply check for level in one direction, then swap the ends of your new long level and check it again. If both directions are the same, you are good to go. It also helps if you know how to read a level too I guess. Me, I rarely need one any more. You gain an eye for level with enough practise.)

Now we need something to sit our sticks upon, and saw horses are just about perfect. First thing to do is joint the tops of them flat and smooth. I just take the whole darn horse and run it over the jointer. Takes about 5 seconds and it’s done. You will also need some way to adjust each of the saw horse legs independently. You can use spacers, I have a mess of oak wedges that I use.

So we have horses, sticks, adjusters and a level/straight edge.

(At this point, Mr. Whisperer has a video, go have a look!)

Adjust one horse to level. Then adjust the other horse to level. Check between horses for level and adjust. Keep jumping between each horse and over both of them until you have both horses in the same plane as each other.

Lay your jointed, parallel sticks on top and check again, this time going from corner to corner to check for wind (pronounced waind, and a fancy term for twist, ok?).

If it all checks out, you should be good to go.

I also check with my w(a)inding sticks, just to make sure…

Looks pretty good to me.

(These winding sticks are getting on now, and have thoroghly settled down and don’t move any more. They are cherry with a strip of maple and a nick of blackwood on the close one, cherry with a strip of ebonized white oak and a nick of maple on the other for centre markers. Finished with a couple coats of blonde shellac. I wouldn’t be without them, and you should have some too. Great little shop project and they can be as simple or as elaborate as you want. The basics are stability and contrast in colour.)

So, right now we have a nice flat foundation upon which to build or core, and eventually our torsion box.

If you don’t get this right, then there is little point continuing. It really is that important to get this first step on the money.

Next time, we start assembling our core grid and this is where I buck the norm in a big way…

Thanks for reading,

Stu.