Dent the Wood for Tighter Joints - Popular Woodworking Magazine

Dent the Wood for Tighter Joints

 In Chris Schwarz Blog, Woodworking Blogs

We’ve all dented some important part of a project and hand to remove the dent with heat and steam. It’s a routine and and annoying fix. But what if we could use the same process to make our joinery tighter?

That’s exactly what I do with many joints in my chairs. I slightly compress the fibers (denting them) and then cause them to expand at assembly with hot hide glue.

This idea first occurred to me years ago when I wrote an article on biscuit joiners for Popular Woodworking Magazine. Some of the manufacturers of beech biscuits would compress them during manufacture. The biscuit would fit loosely in its slot until it encountered the water in the glue (modern PVAs and liquid hide glues are about 44 percent water). Then the biscuit would swell, locking it in place.

So Here’s What I Do
First I size my tenon to the mortise so it’s a little tighter than I like. The tenon should go in, but with hammer taps.

Then I fetch the secret weapon: Non-scratch pliers. These have adjustable jaws that are 1/2” wide. With this tool I can gently – gently – compress the fibers without tearing them, like serrated metal-jaw pliers are wont to do.

How much do I compress the joints? At this point the tenon should go in with hand pressure – like a typical well-made tenon.

Add the Glue
You can use any glue to assemble the joint. I prefer hot hide glue because the results are instant. I put glue in the mortise, paint a little on the tenon and immediately knock the tenon home. The heat and water swell up the tenon and lock it. Hard.

I have never split the work using this method. The only result is joints that are unusually good.

Give it a try before you take a potshot at the method.

— Christopher Schwarz

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Showing 4 comments
  • gunnarvt

    Typo correction: kigoroshi

  • gunnarvt

    That matches the Japanese technique kogoroshi.
    https://www.youtube.com/watch?v=f7HaAZ1ugcA

  • Loxmyth

    I’ve heard of folks using their bench vises for the same pre-compression. That probably works better on rectangular tenons… though a simple jig would solve the shape issue. (Drill mortise-sized hole in block of scrap, saw it in half to get two semicircles; the saw kerf may reduce the radius enough in one direction to do the job, or trim a bit more until it does.) The no-scratch pliers are a good quick solution if you have ’em on hand, though.

  • tms

    I’ve used a similar method on the edge to edge bottom planking of a flat bottom skiff. If you use a screw eye on a stick to make a running dent on the edge of the planks, then plane them to shape, the dent will swell when the boat is is wet and ensure the seal.

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