Which wood dents first?
Wood is not one thing. Some of it you can mark with a thumbnail and some of it you cannot, and somebody has to decide which one every single part of a toy gets made from.
- How long
- About fifteen minutes.
- You need
- Five wooden things that belong to you · Your own thumbnail
- Messy
- No.
Step 1. Find five wooden things that are yours.
A pencil, a ruler, a spoon, a block, the back of a chair you are allowed to touch. Yours, or ask first. Nothing here is worth a dent in something somebody loves.
Step 2. Press a thumbnail into each one.
Same push every time, on the underside where nobody looks. Some of them take the mark and hold it. Some of them let go of it while you watch. A pencil gives up almost at once, because a pencil is pine.
Your thumbnail is doing the whole test. It is pressing something hard into something softer and reading what is left behind.
Step 3. Line them up, softest first.
You now have an order. You do not have a number, which is the part that is missing, because your thumbnail pushes a bit harder on some days than others and nobody else has your thumb.
Step 4. Now the grown-up version of what you just did.
A steel ball, 11.28 millimeters across, pressed into the board until exactly half of it has disappeared. Then read the force off the machine. That is Janka hardness, and every number on the strip below was measured that way.
Same test as yours. A hard round thing, pressed in, and a look at what is left. The only difference is that the machine pushes the same on Tuesday as it did on Monday.
Step 5. Find our three woods on the strip.
The orange bars are the woods that are on our airplane. Hard maple is nearly all of it. Yellow birch is every dowel. Black walnut is the nose cap and the four engines, and black walnut is the softest of the three by a long way.
Step 6. Then work out what is wrong with that.
The nose is where a hand goes. It is the end that gets carried, and it is the end that hits the floor first when an airplane is flown off the edge of a table. We put the softest wood we own on it. The card under the strip says why, and what it has cost us so far.
Seven woods, hardest first.
- Hard maple6,450 N · 1,450 lb of push
The hardest one here, and most of our airplane is this. It is the wood a bowling alley floor is made of, because it takes the longest to dent.
On the airplane: Body, wings, tail, wheels, and the dowel through the nose joint
- White oak6,050 N · 1,360 lb of push
Not on our airplane. It is what a barrel is made of, because its pores are plugged shut and water cannot get through it.
- Yellow birch5,600 N · 1,259 lb of push
A dowel has to stay round while somebody taps it into a hole, so a dowel wants to be hard.
On the airplane: Every dowel except the big one in the nose
- Black walnut4,490 N · 1,009 lb of push
The soft one on our airplane, and the dark one. It is on the nose on purpose, and that purpose cost us five goes at one joint.
On the airplane: The nose cap, and the four engines
- Black cherry4,230 N · 951 lb of push
Not on our airplane. It comes out of the shop pale and goes redder every year it sits in the light.
- Basswood1,800 N · 405 lb of push
The carving wood. A knife goes through it without fighting, which is the same reason you would not make a wheel out of it.
- Eastern white pine1,700 N · 382 lb of push
The softest here, and the one already in your house. Press a fingernail into the flat of a pine board and the mark stays there.
Orange is a wood that is on our airplane
Clear wood at 12 percent moisture, out of the Wood Handbook, which is the book every number on our drawings gets checked against. A real board has knots in it and a knot is not in the table, so treat these as what the wood can do on its best day. The little squares are colors, not photographs of boards.
Why the soft wood is on the nose
Because of how it looks.
Dark wood on the nose and the four engines, pale wood everywhere else, and one crisp line where the two meet, just ahead of the flat part of the belly. That was the drawing before it was anything else. Nobody picked walnut for the nose because walnut is tough there. It is not.
Here is the bill for it. A glue joint between two woods is only as strong as the weaker one, and walnut is the weaker one twice over: slide it along the grain and it lets go at 9.4 megapascals where maple holds to 16.1, and pull it apart across the grain and it lets go at 4.8 where maple holds to 7.6. Every version of the nose joint has been governed by the walnut, never the maple.
- Rev F. Three small dowels through a flat joint. It was under the floor the whole time and nobody had noticed.
- Rev G. The dowels moved up. Not enough.
- Rev H. A square maple tenon in a walnut socket. It passed one way and failed the other way, which nobody had checked.
- Rev J. A round maple spigot, because a circle has no weak direction. Then the peel number went in and it fell under the floor too.
- Rev K. The joint moved 18 mm back down the airplane and became a loose maple dowel one inch across, driven through both. Released on 5 September.
Rev K stands at 2.50, where the rule says 2.0 is the least we accept, and it is built so the glue lets go before the dowel snaps. A broken stick can be pushed out and replaced. A snapped stub in a blind hole cannot.
And the part we are not allowed to skip: that 2.50 is arithmetic. Nobody has pulled on this joint yet. The two tests that would settle it are written down and have never been run, because they need a real airplane and there is not one. Until then the number is a prediction, and a prediction is not a measurement.
That is the end of this one
The word for what you just did is hardness.
Hardness is how hard it is to dent a wood, and it is a number, not an opinion. The test is called Janka and it is the same everywhere: push a steel ball 11.28 millimeters across into the board until it is buried halfway, and write down how much push it took. Once it is a number you can put two woods side by side and argue about the right one with evidence instead of with taste.
Where to go now