Mortise and Tenon Joints
Thursday 22 April 2010
John Bullar takes a look at this stalwart of joinery
A classic square peg in a square hole, the mortise & tenon joint has been around
as long as people have been working in wood - or stone, for that matter. Apparently the lintels on top of Stonehenge were secured by simple mortises & tenons, and they seem to have lasted ok!
I am going to take you step-by-step through a dependable way to make mortise & tenon joints with hand tools. You can also watch a video of this on my website: www.furnituremaking.org
Then we will look at some simple workshop machines to speed up the process when you need a number of matching mortise & tenons.
Choose a chisel
1. A decent chisel is an essential tool for hand cutting mortises. Ideally it should have a thick shaft with square sides and it must have a sturdy handle so you can knock it with a mallet. If there is a metal ring or ferrule on top, you can use a metal mallet, otherwise use a wooden one.
2. The width of the chisel is going to form the width of the joint, so try to choose one that is just under half the width of the wood. The edge must be razor sharp with an angle of 30 degrees for hardwood, slightly less for softwood.
3. Position the chisel on the middle of the wood and adjust a marking gauge so it lines up with one side of the chisel. Then mark around the sides and end of the tenon and also mark the side of the mortise. Re-adjust the marking gauge for the far sides and end of the joint, but press the stock of the gauge against the same face of the wood each time.
4. This type of joint is sometimes called a blind or a stub mortise & tenon because the mortise socket does not pass all the way through the wood. Typically the socket will be two thirds of the depth and it is best to make the tenon a couple of millimetres shorter so it will not foul the rough base of the mortise.
5. Use a try-square and a knife to mark four joined-up shoulder lines around the tenon. Place the knife tip in the end of each line then slide the square up against it, ready to mark the next line.
6. It is a useful habit to pencil mark each waste piece with a cross before you start any cutting. That way you cannot accidentally cut away the wrong piece, something everyone does at some time in their woodworking career.
7. Saws designed for cutting joints have backs to stiffen them and add weight. The plastic handled, steel backed saw in the foreground has hardpoint teeth which are induction tempered for a long lasting edge but cannot be re-sharpened or adjusted, unlike the finer but more expensive brass-backed saw behind.
8. The wood must be tightly clamped upright in a vice for rip-sawing the cheeks of the tenon. Hold the saw handle with a three-fingered grip, while resting the blade against the thumb on the other hand, ready to position the start of the cut.
9. To crosscut saw the shoulders of a tenon, the wood is normally held on a wooden board known as a bench-hook. This has a block on top to brace the wood against and another beneath to hook over the edge of the bench.
Hand cut mortise
10. The tenon has four shoulders, so its width is less than the full width of the wood. The tenon is used as a template to mark its width against the length of the mortise. The sides of the mortise socket were marked at the start of the job, along with the thickness of the tenon.
11. The mortise is cut with a chisel between the gauge lines using a series of closely spaced chops. Before starting to chop the mortise, the wood must be firmly clamped down on a sturdy bench. This prevents bounce, making the chisel action more positive as well as safer.
12. Once the chisel chops reach the far end of the mortise, the blade is turned around to make a clean vertical end. Then the blade is turned over and used to scoop or pare out all the chippings, before repeating to make the socket deeper.
Fit the joint
When the mortise is a couple of millimetres deeper than the length of the tenon, it is time to trial fit the joint. Patience is needed to chop out a mortise socket this way. However, it is worth remembering that the total time taken to hand cut a joint is often less than the time needed to set up a machine.
13. With the tenon fitted in its mortise there should be little or no gap around the shoulder line. Also the faces of the two pieces of wood should, if possible, be flush with each other, although these can be planed together after the joint has been glued up.
Hollow chisel mortisers
14. The most common machine used to assist with this joint is the hollow chisel mortiser. This has a quiet but powerful induction motor and it works like a robust drill-press, lowered by a sprung lever arm.
15. In the mortiser is a drill chuck that holds a spiral auger bit inside a rigid hollow tube or chisel. This is round bored inside to house the auger and square sectioned outside to shape the mortise. The chisel has a sharp prong at each corner shaped to funnel the waste into the mouth of the auger.
Routed mortise & tenons
16. Routers, especially the larger half-inch versions, are excellent for cutting mortises. With the aid of a jig, the same router and bit can be used to cut a matching tenon. Set-up time is a bit longer but the repeat joints are quick and the results are good.
17. Because the router can only cut mortises with rounded ends, the jig uses a built in template to round the tenons in the same way. One big advantage of jigs like this is that you can make compound angled joints with them, but they are expensive.
18. Every woodworker has a drill of some sort that can be used to bore mortises. Ideally the drill is clamped in a stand, or comes as a drill press so it moves vertically without wobble, but hand drilling can be used with care. Forstner bits are ideal because they cut clean overlapping holes. The sides of the mortise are then flattened with a chisel.
19. It is easier to pare off the corners of a tenon and round them with a chisel than to square up the ends of a drilled mortise socket. Drilling is a quick and reliable method of producing rounded mortise & tenon joints using standard workshop tools.