Saturday, February 14, 2009

Fixing a Mortise and Tenon Joint

The mortise and tenon joint is a strong and reliable way to keep two pieces of wood together. The Stokke Tripp Trapp chair uses them to keep the angled side pieces connected to the foot pieces.  All of our kids have used Tripp Trapps, or similar, chairs from the time they could sit up right long enough to eat (and were interested in solid food).  From toddler through third grade or so, the chairs brought every one to the table without high chair trays and other barriers.

However, as the kids got older, that mortise and tenon joint would loosen on each chair. Inevitably, I would have to disassemble the chair, glue up that joint and clamp it together.

Well, now we don't need most of those chairs and so we're passing them along. One of the chairs has a pretty loose joint so I bring it down to my shop.  I pull the foot plate off and take a look. Well, now, I had already glued up this puppy quite a few years ago and remembered it well.  

It was the first time that I had seen a tenon that broke off such that part of the tenon remained inside the mortise.  I don't find it a problem to fix a split piece of wood.  All those funny cracks and angles are extra surface area with which to make good contact between the two pieces. Frankly, I was rather surprised that it had failed again.  All that extra wear from folks much older than 8 years old using the chair took its toll and the chair needed to be re-glued.  I rasped off (most) of the old glue, filled it up and jammed the pieces back together.

After the glue set, I put the chair together and noticed that it wobbled pretty badly.  That glue job stank.  The foot board, which has the tenon, wasn't even close to fitting to the vertical with the mortise.  The whole thing needed to be pulled apart and redone.  Back down to the shop.

It took some time and care to chip away the glue and carefully loosen the joint.  

Now I took a close look at that joint.  I fit the pieces together and measured a 3/16" gap.  Oh boy, that's a big gap.

I scraped off more old glue, rasped off a bit of wood that had swollen and tried again.  Nope.  I repeated the process for quite a while.  I got the gap down to 1/16", but no matter what I tried, I wasn't getting a flush joint.  This time, though, I was determined to get a flush joint.

Somehow I needed to figure out why the tenon wasn't going flush into the mortise.  There were doubtlessly high spots on both mortise and tenon that were keeping things apart.

I remembered something my dad, a dentist, did when he was fitting a new bridge or cap onto a patient.  He had these nifty pieces of red and blue paper and he would have the patient bite down of the paper.  Where red or blue was left was where the new tooth was too high and needed to be ground down.  I needed something similar; some way of marking the mortise so that I could see where material needed to be removed.

I took out my shop pencil and generously colored the tenon.  Then, I pushed it back into the mortise and tapped it into place.  The picture below shows the tenon covered with graphite where I first thought the problem may have been.  I later covered the entire tenon with graphite.

Sure enough, when I pulled the pieces apart, inside the mortise and rubbed away from the tenon were the areas that were making contact.  In the picture to the left, you can see the dark marks where the graphite was rubbed off the tenon onto the mortise.  I got out my dremel and rifling files and got to work.

It took a few repeats to find all the offending surfaces and then so smooth everything down to nice finishes.  I would take out the tenon, cover it with graphite from the pencil, tap it into place, pull it out and look for the marks.  Then, using dremel, rasps and sand paper, work down the offending area.  I worked the both the mortise and the tenon in alternating turns so that I wouldn't introduce too much of a gap.  I would rather repeat the process a few times than have a joint with too much space.  No matter how much glue I would then use, the joint would never be satisfactory.

After I repeated the process a half dozen times or so, I was satisfied with the way the two pieces fit together.  It looks like the way it belonged and should have been in the first place.  The final joint is on the right.  Nice and tight.  I cleaned off all the graphite and used Gorilla Glue to fasten everything permenantly and bound the foot board to the riser with to band clamps, one along the bottom of the foot and the other making a right angle with the riser.

Now that will be a nice joint that will take full advantage of the structural strength of the tight mortise and tenon joint and will let the next users of the chair raise their kids on it.

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