By Adam Cherubini
Frankly, I can do without dovetails quite nicely. You can nail two boards together and be left with something strong and serviceable.
But mortises are trickier to live without; you need to know how to cut them. Mortises join boards edge to end. They are super strong. Because of their strength, they are often used in structural applications such as timber framing, chair and table joints. Unlike dovetails, they either fit well and function – or they don’t and the finished product suffers in some way. And because they are essentially one-shot deals, it’s pretty important that you get them right.
How do Mortises Work?
Mortises are complicated pieces of engineering. Rightly so, I’ve heard a fair number of questions about them. “Do I need to peg my tenons?” “How large should my mortises be?” Understanding how mortises work will help you deliver the fit and features you need to make your projects function.
As you can see from the examples below, retention of the tenon in the mortise is an important contributor to the strength and life of your project. You can retain tenons in two ways: with glue or with pegs.
For glued joints, it’s essential to spread glue on the broad surfaces of the tenons. The tenon’s end grain is essentially worthless. And differential shrinkage may reduce the size of the tenon, causing the glue to fail. It’s also a very small area. I’m going to skip recommendations about glues, but according to the literature I’ve seen, PVA (yellow and white glues) requires pressure to develop bond strength. Because mortises are essentially unclampable, the pressure you get inside your mortises will be based solely on the tenon’s fit. So if you plan to use PVA, make your joint as tight as possible. I typically use hot hide glue, which I believe is a bit more forgiving of gappy joints – though you should strive to avoid them. A properly fitted sawn tenon mating to a cut mortise gives the glue enough surface to bond.
The other method of retaining joint integrity is to use pegs. The key to using pegs is producing contact at the shoulder. This is why I prefer to drawbore my pegs. I drill slightly offset holes and force them to align by inserting a peg with a tapered end. Some say clamping the parts together does the same thing.
Use Dictates Technique
Creating a good mortise requires an understanding of how the finished project will be used. A kitchen cabinet door may not need superior mortise quality. The load on the door is low. A chair or the stretchers joining your workbench legs require your best efforts. If you are planning to hold your project together with PVA glue, you’ll want nice straight, parallel mortise walls and ends. If you plan to use pegs – which I certainly recommend for a workbench – you may not need perfect mortises. But by all means make sure your tenon’s shoulder is tight against its mating surface before inserting the pegs or you’ll be left with a wobbly bench.
From the August 2012 issue #198
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