A lot of quality old chairs feature a tenon and mortise construction. The tenon is a narrow rectangular shaped protrusion at the end of a chair leg or other part, and it fits into a mortise or channel on the other part of the chair. Sometimes, these are held in place by pegs.
If you are restoring an old chair, dealing with a broken tenon can be an issue. Luckily, there are lots of solutions from grabbing the wood filler to hiring a CNC machinist. Let these tips guide you in the right direction:
1. Try Wood Filler
If the tenon just has a small crack or even several cracks, you may be able to just fill it with wood filler. Smooth the filler into the cracks, use a small clamp to hold everything together and let the filler dry. Then sand down the excess filler to make the surface even.
2. Make a Wedge
When tenons have large cracks, wood filler won't necessarily provide the structural stability you need to hold the chair together. Remember, once the tenon fits into the mortise, it needs to be able to withstand a lot of pressure. That's what keeps the chair from falling apart.
Instead, in these cases, try grabbing your chisel and making the crack into a wedge shape. Then, hand cut or hire a CNC machinist to make a wedge of the size you need. Glue the wedge in place, and sand it down to make it level with the rest of the tenon.
3. Use Hide Glue
When gluing your wedge in place, consider using hide glue. It is one of the oldest woodworking glues, and it offers a lot of advantages including drying clear. It also holds the joint extremely effectively so you may want to use it when you attach the tenon to the mortise, regardless of which method you use to repair the tenon.
4. Turn to CNC Machining
In some cases, the tenon may simply be past the point of repair. In those cases, you may just want to have a new piece made. The new piece should include the chair leg, rung or whatever part you are dealing with as well as the tenon on the end.
Having all of this available in a continuous piece makes the chair stronger. CNC machinists can look at your broken piece, and input the design into their computers. Then, they can cut the piece of wood that you need.
To learn more, contact a CNC machining professional who is experienced in restoration and repairs.