Saturday, January 4, 2014

Complex Curve Repair Simplified

Another Day Another Chair Repair
I work very hard at being humble and modest.  People who know me will laugh at that statement, but I rarely, in fact, say that I am the "best" at anything I do.  That is because I know others who are my inspiration, who are dramatically better at what they do than I am.

However, I often walk home from work thinking to myself, "I am the best chair repair man in this city."

I don't say it out loud, since, these days, other pedestrians will think I am talking to someone on the phone while I walk.  (In the past you had to be mentally unstable to walk around talking to yourself; these days it is the norm.)

Yesterday I was thinking, with a bit of satisfaction, how well this particular repair had gone.  I visualize in my mind all the elements of the repair and the way it will survive the stress of use and, if every little element of the project went well, I know I earned my pay.  Perhaps I'm obsessive.  No, to be honest, I AM obsessive!

This project was a pair of Louis XV revival 19th century armchairs which had a brass leaf gilt finish.  They had been poorly repaired over the years, including many small brad nails sunk into each of the joints and missing areas filled with epoxy filler.  Using nails to repair furniture is a crime.  First of all, it just doesn't work.  Second, it prevents the actual repair, since the joint cannot be opened easily.  Third, it requires some amount of damage to remove the nails, either by punching them out, or digging them out with tools.

All the nails do, in fact, is prevent the chair from falling apart, while at the same time allowing enough movement that the chair becomes a rocker.

One of the chairs had its crest rail broken and repaired several times.  All the wood around the mortise was damaged, and the tenon in the style was messed up.  There were several elements of the wood which were fractured and areas which were built up with epoxy.  In effect, there was nothing left structurally of the joints.

A Single Clamp Pulls Joint Together
I cleaned each of the small fragments and, over several days, rebuilt the cheeks of the mortise.  Still there was not enough wood to hold properly.  Therefore, I drilled two 3/8" holes by eye into the ends of the tenons and also into the sockets of the damaged mortises.  You need to do this carefully, since there is no mechanical way to accurately align the holes.  I put some light tack masking tape on the wood, each side of the joint, to provide at least a line of sight.  The rest is experience.

Note that the crest and back of these chairs are curved in two directions.  Therefore, this is a perfect example of using vector forces to create proper clamping cauls.  Look at the joint and visualize the single vector which bisects the surface of the joint, in the center, at 90 degrees.  That is where the clamp force needs to be applied.

Search this site for other posts on Vector Clamping.

Now take soft wood, like poplar or pine, and make wood cauls that you can clamp to the frame on each side of the joint.  These cauls need to have a fairly large surface area, so they don't slide and can be clamped securely.  Also these cauls need to have a "purchase" spot where the final clamp will grab.

Perfect Alignment

Note the two cauls on either side of the joint are held by two pony clamps each.  Then the longer single pony clamp is applied which pulls the joint together.  Finally, a second pony clamp is used to provide alignment top to bottom and a "C" clamp is used, with some plexiglass, to align the faces front to back.

The plastic allows visual inspection as all the clamps are tightened, in sequence, to pull everything together.

There is also a weight used on the front of the seat to keep this chair from falling backwards and spoiling my day.

Of course, I only use Old Brown Glue for chair repairs, since it is the best glue "in the universe" for this type of work.  I don't brag about my talents, but I do brag about my glue.

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