The Glasgow rose - An Arts & Crafts hallmark, Adopted by Gregory Paolini Design

 Woodworking Tips from Gregory Paolini
Information, Tips,  and Know-how for woodworking Enthusiasts

 

Considering joinery in a project design

Text & Photos by Gregory Paolini

A case study of the "D" handle book rack
Photo of bookrack In issue #197 of Fine Woodworking magazine, I show readers how I make an updated version of Gustav Stickley's #74 bookrack.  My more contemporary design doubles the v shelf space, eliminates the tusk tennons, and tapers the sides.  The article has been very popular, and many readers have sent me photos of their version of the finished project.
close up of thru tennon But, I've also received a fair amount of e-mail regarding the joinery on the book rack.  Most often, I'm asked how come I "broke the rules" of joinery by relying on end grain to secure the through tennons? 
So who's right?  Is my design unfaultable, or did the readers catch me in the act of committing a cardinal joinery sin?  Or could the answer be everyone is right, and everyone is wrong?
  
Taking a look at gluing surfaces
two boards with glue Generally, we break gluing surfaces down in to the category of either either long grain  gluing or end grain gluing.  Long grain to long grain glue surfaces are without a doubt the strongest gluing surfaces we can take advantage of. An example of this would be two boards, jointed, and glued edge to edge.  This type of glue joint almost always is stronger than the actual wood that surrounds it.   
two boards with glue End grain to end grain joints are the other end of the spectrum.  They are the weakest glue joint in woodworking, and that's why we never glue two boards end to end with a butt joint.
Of course, there are many joints that utilize these gluing surfaces in many different ways.  A traditional leg to apron joint, for example, is a variation of long grain to long grain gluing, except the grains are oriented at 90 degrees to one another.  This introduces seasonal expansion and contraction across the joint surface, which is another stress that the joint must overcome.    In addition to glue securing it, this type of joint also gets some mechanical help from the actual fit of the mortise to the tennon.
  
Are there any other types of gluing surfaces?
Well, we know that long grain joints run parallel to the grain, and are the strongest joints.  And we know that end grain joints are perpendicular to the grain, and are the weakest.   So what if we take a little of both?  If we create a miter joint, we're actually creating a glue surface that's half long grain, and half end grain, or what I like to call "Hybrid grain".  This is similar to the type of surface half blind dove tails rely on for their glue strength, as well as the glue surface employed by my book rack design.  But how strong is this type of gluing surface?
  
Just how strong is that joint?
Show a joint In Fine Woodworking issue #203, Douglas Moore and Thomas McKenna collaborated on a fantastic article comparing 18 various types of joints, and how they fail under a racking stress.  The article also states the load, in pounds,  at which the joint fails, and it ranks those joints from strongest to weakest..
The article was a big eye opener for a lot of woodworkers I know, and even threw a couple of curve balls at me.  I was really surprised at how well some of the joints faired in this torture test! Of particular note was the miter joint, which ranked as the 6th strongest joint, and kept its strength to well over 1300 pounds!   And that joint is nothing but hybrid grain, and glue!
While the miter joint was very strong after glue up, I wouldn't expect it to retain it's strength over time, as several seasons of expansion and contraction exert their own stress on this joint.  But the joint that's really in question here is the through tennons on the book rack.  Those joints are not miter joints, because only one component is hybrid grain - the end panels.  The shelf tennons are long grain, the best type of glue surface,  and they have a full shoulder area as well that helps add strength to the joint.  Plus, the whole project is made from quarter sawn lumber, which cuts our expansion and contraction concerns in half over standard lumber.   
  
In conclusion
Pic of windsor chair Style will always be a huge determining factor in our designs.  For example, a Windsor chair could be made much stronger if we increase the size of the individual components, and utilize tusk tennons - The result would have a bit of a negative impact on the graceful nature of the piece though.  So we compromise.  We have to take into consideration all of the stresses that will be imparted on the chair, and which joinery techniques will provide a satisfactory solution for the situation. 
And now we come full circle to my interpretation of Stickley's #74 Bookrack.  I wanted a more contemporary feel for this Arts & Crafts classic.  I purposely eliminated the tusk tennons, replacing them with through tennons, which I realize took away some of the strength from the original.  But I also looked at what all of my stresses were on the piece, and how much strength I needed to satisfy those stresses.. 
Picture of bookcase, loaded, being slid along floor A bookrack just sits there.  People place books on the shelf, and remove them.  This presents a sheering stress on the through tennons, which for practical purposes is un affected by the presence or absence of a tusk.  A tusk would help considerably if we encountered racking forces, but the only time we'll ever see that is if we try to slide the bookrack along the floor length wise.  And since we have two big hand holds to lift and move the bookrack by, we most likely will not be sliding it.
  
So I'll wrap things up by pointing out that beside my desk, I have one of my "D" handle book racks, which I made a decade ago, and it's still as sturdy as it was the day I glued it up.  I'd like to think that I didn't really break any rules here at all - I just kind of "bent" the rules a little, to achieve the design style I was after.  But I will say with out a doubt, that if you want to bend the rules, you need to learn what they are and how they work, or you'll certainly break one of them 
Now let's make sawdust!

 

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