Tuesday, May 27, 2014

Severed Tenon Repair: Part One, The Cone Splice

This is how the chair came to me. The entirety of the tenon remains in the mortise. Thanks, nail.

Here is an example of a typical problem I encounter in my studio: a loose chair comes in that has a round stretcher whose tenon sheared off at the shoulder. This happens sometimes with freak accidents but most often it is the result of wobbly chairs remaining in constant use. Every time the sitter twists, turns, or leans more force is applied to the few tight joints. Since the strength of a chair is found in the collaboration of all the parts, the remaining tenons receive all the strain that would normally be shared by the whole. There eventually comes a point when someone leans too hard and “SNAP!” there goes the tenon.

This repair will be posted in two installments. This first installment will show the grafting of a new tenon. The second will show my method for removing a nail that have been driven into the leg through the tenon but has been hammered flush to the surface making nail pulling a chore.


A Grafted Solution

There are a few different ways I’ve seen to remedy these situations. The first and unfortunately common way is to drill a hole and stick a dowel in it. This method leaves a weak spot where the bottom of the hole is. This is not an acceptable repair. 

A common splice joint

A common repair for restorers was a staple for me for years: the splice joint. Cut at a long angle and planed smooth, a new piece of wood is grafted on and shaped with chisels and then inpainted to blend into its surroundings. Strong enough but it requires too much inpainting. 

A clothespin scarf joint

The third is a variation on the theme: the clothespin scarf joint. This is a method that involves cutting a long V notch down the piece and grafting into that. The result is strong and requires less inpainting. The downside is that it still removes historic surface. 
 
Recently a friend and colleague, John Coffey from Locust Valley, New York, (check out his excellent portfolio at www.locustvalleyrestoration.com) forwarded me an article he wrote describing his method of repair. He calls it a cone splice because it involves boring a conical hole into the piece, turning a mating cone graft to fit, and shaping the other end of the graft into a new tenon. This is an incredibly strong repair, takes just as much time to perform as the other splice joints, and does not mar any visible surface. I have completely converted to this method and, thanks to John’s generosity; this is the one I will share here:

Before Treatment

Carved flush and center marked

The repair begins by carving any interfering fractured grain flush for a clear view for boring. Then by eye I mark out a center point with an awl.

Shop made cone bit. It is obviously a reground spade bit. It works wonderfully.



After drilling a small pilot hole to guide me I begin boring with the cone bit. Going slow and easy is important as you don’t want to break out the walls.

Prepped stock for the lathe

On the pole lathe

Now for the lathe work. The only lathe I have is a pole lathe and this works fine. After turning the tenon, I use the bit as a guide to form the cone.

Tenon first

Getting diameter of cone

Getting close to conical

Comparing the tapers. Still a little off.

Final fit is achieved

Once it was off the lathe I could smooth out the inconsistencies freehand on my Work Sharp sanding station. (I just gently turn the cone on the sanding surface emphasizing pressure where needed.)

Nice and snug

Glued in

Clamped

After the fit is nice and snug, I use hot hide glue and clamp it for at least an hour. I don’t final shape the stretcher’s tenon until the mortise is cleared. But that’s for the next installment… Part Two is here.

4 comments:

  1. Great job Joshua!!!!! Now I need to try it out. For these breaks are always coming into the shop.

    FR

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  2. Very interesting technique. I don't know much about restoration techniques but the preservationist in me always thought it was a shame that the splicing techniques which I have seen before required removing so much original material in order to effect a repair. Thanks for sharing this!

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  3. not to dredge up old news.... but I was cruising through lee valley and this seemed to fit with your repairs and maybe would help speed them up?

    http://www.leevalley.com/US/wood/page.aspx?cat=1,180,42240,53317&p=32280

    with the right taper you can use a pencil sharpener to create the taper. should be seconds compared to the time on the lathe. I imagine that your spade bit could be ground to the correct taper.

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    Replies
    1. Thanks, Jeremiah. Yes, that's the right idea but I would need a larger one. 3/8" is too small for any of my applications. Thanks for pointing that out!

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