What Is Runout, and Why Should I Care?

Today I got some good work done on the vitrine.  Most of the nail holes on the carcase got filled with putty, we tested some paint colors on the back, and I glued the feet on. 

As a quick and dirty jig for laying out the position of the feet, I grabbed a scrap stick I had rolling around.  The stick was 5/8" ash that followed me home from Denmark last fall during the chair build.  We cut up a bunch of really nice, straight ash for sticks.  The nature of sawed lumber, as opposed to split wood, is that not all of it was perfect. 

This particular stick had really straight grain on one side, and the other side had really bad runout.

This photo shows pretty straight grain.  Not much runout at all.

The runout on this stick is pretty bad.  You can see the grain "runs out" in less than an inch!

When I first started woodworking, I predominantly used machines.  Nothing against machine woodworking, but during this phase of my woodworking I paid no attention at all to runout, I didn't really understand the problem.

This little stick has all kinds of lessons regarding runout.  The first thing I noticed was that this stick is no longer straight.  Even though the wood was kiln dried, sawing the 8/4 roughstock into 5/8" strips opened up the wood so the remaining moisture inside could now dry out. 

It is hard to see in the photo, but this stick is not by any means straight anymore.  The big bend in it happens to be exactly where the runout in the above photo is.

The near side has straight grain, the far side has runout.

Just so you can see the nature of this stick, here are some more closeups:

As you can see here, the runout goes on both faces.

If this stick were to fail, I predict it would fail in exactly this spot.

Why is runout so weak and unstable?  Well, the strength of wood is exactly along the grain. 

Think about chopping firewood.  You use an axe to split wood along the grain.  You would never put a log lengthways on a chopping block and strike the bark.  It takes a lot of strokes to cut a log with an axe this way.  But, it only takes one swing to split the log along the grain. 

I was so certain that I was right about the weak part of this stick being where my finger is, that I decided to break it over my knee.

I grabbed both ends of the stick, and put my knee in the center.  The spot that I am pointing to above is about 2/3 the way down the stick. 

Let's see where it breaks when I stress it.  Photo courtesy, The Frau.

Sure enough, the stick doesn't break where my knee is, but it breaks exactly where the weak runout is.

Note the position of my knee.

More closeups:

Right along the grain.

I think it is astonishing how well this experiment worked.  The split happened right exactly along the grain line.

Another view.

Yet another.

One thing of note is that this took hardly any force to break.  Perhaps I should try this on another stick with no runout, just to see what I have to do to get it to break.

The moral of the story?

Chair makers know what they are talking about when they use parts with no runout.  Chairs take a lot of stress, and if there is a weak part, it will break.  Runout on a part like this one is weak, and if I had used it on my chair, that is where the weak spot would be.

Of couse, this lesson could be transferred to any piece of furniture.  Use straight grained wood on parts that take stress.