Chris Schwarz's Blog

Diagonal Wedges: The How & Why

When wedging through-tenons, I prefer to orient the wedge diagonally across the tenon. This is a somewhat atypical way to work, so an explanation is in order.

A diagonal wedge has the advantage of closing up any gaps on all four edges of a rectangular mortise. That’s because it pushes the tenon against all four walls of the mortise. The more typical wedge, on the other hand, will push against only the end-grain walls of the mortise.

The diagonal wedge has the disadvantage of splitting the mortise and ruining the work. Pushing a tenon against the long-grain walls of a mortise is asking for trouble.

A Sharp Eye & Wedge
I haven’t split a mortise with a diagonal wedge in many years. Here are the steps I take to reduce the risk of trashing the joint.

  1. I saw a kerf in the tenon. For large-scale work (such as a workbench), I’ll use a handsaw with a thick kerf. For typical tenons, I use my tenon saw. For delicate stuff, I use my dovetail saw. Some people bore a hole at the bottom of the tenon to prevent the tenon from splitting. I haven’t found that necessary in a well-made joint.
  2. I use a thin wedge with a 4° included angle. Thin wedges with a sharp point are a good place to start. They can get into tight kerfs. You can blunt them, if necessary, by cutting off the tip. This will make them less likely to get crushed during insertion.
  3. After assembly but before wedging, I open the kerf with a chisel. Right before I drive the wedge I take a wide chisel and mallet it into the kerf. This deforms the mouth of the kerf, making it easier to insert the wedge.
  4. When driving the wedge I use consistent, sharp raps with a hammer. The trick is to watch the wedge carefully when you hit it. Each strike should make the wedge move a little into the kerf. When you hit the wedge hard and the wedge doesn’t move, it’s time to stop hitting the wedge.

The reward for these extra steps is a joint that is strong and is much less likely to have any gaps. Give it a try – on a test joint first – and I think you’ll like the results.

— Christopher Schwarz

6 thoughts on “Diagonal Wedges: The How & Why

  1. SawdustWylie

    OK, I’ll admit I’m confused. When you say you saw a kerf, you just mean the kerf for the wedge, correct? I thought that was pretty much required, unless some people just try to slam a think wedge right down into the end of the tenon?
    The comment about some people bore a hole in the bottom of the tenon to prevent splitting – I’m not following that. Where do they bore the hole?
    Thanks, just not getting my head around your comments.

    1. Sean N

      Yes, the kerf is for the wedge. The concept of drilling the hole comes from metalworking; a hole can be drilled at the end of a crack to distribute stress and prevent the crack from propagating. Here is an image to illustrate: http://content.aviation-safety-bureau.com/allmembers/faa-h-8083-31-amt-airframe-vol-1/images/Figure%207-93.jpg
      In this case, the “crack” is your saw kerf. You would drill the hole across the tenon, close to the shoulder, and parallel to the kerf. In theory, the splitting effect will terminate at the hole (inside the mortise) and not creep out to the visible portion of the work.

    2. Christopher SchwarzChristopher Schwarz Post author

      Apologies for the confusion. In traditional work you do one of two things to insert the wedge without sawing the tenon:
      1. Apply the wedge between the tenon and mortise wall. This usually requires two wedges and is found in door work, especially.
      2. Split the tenon with a chisel instead of sawing it. So assemble the joint, split the joint and then drive the wedge.

      Hope this helps.

      1. SawdustWylie

        Wow – I had no idea that’s how it was done in “traditional work”. Makes perfect sense now. I was truly stumped how you could insert a wedge without a kerf. 🙂

        Great respect for your knowledge and love of woodworking technique history, and your balance of using what’s best of new and old techniques and tools.

        @Sean – thanks for your note as well. I was stuck on “the bottom of the tenon” versus bottom of the kerf. 🙂 But yes, quite familiar with crack stop holes. In fact after reading your description, it reminded me I did exactly that on some double-wedged double tenons on a coffee table about 15 years ago! https://photos.app.goo.gl/L3Ykk7sbeWNG7Waz1

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