Monday 9 July 2018
I once heard that there are more than 1000 different subdivisions of the mortice and tenon. This joint is a natural progression from the simple lap joint, through the bridle joint to a full mortice joint. Tenons can either be blind, not going all the way through, or through where the tenon is visible on the outside of the joint. Through tenons can be made into decorative features by using contrasting wedges or having them extra long and shaped.
The subtleties occur when the components being joined are non-rectangular as in a door frame which has a rebate to take a panel or a pane of glass.
If a rail and stile are rebated then the shoulders have to be adjusted in length accordingly. If there is a glazing rebate and a moulding on the face side then the mouldings have to be mitred or scribed.
As the joint must consist of two outer walls and an inner tenon the morticed component is divided into thirds.
You may, though, have a stile that is 38mm thick. Thirty eight does not easily divide into three. In that instance select the nearest chisel width that is convenient and use that width for the mortice. In this instance, see photos, I would use a 12mm chisel, although it would be perfectly acceptable to use the nearest imperial equivalent instead, and as long as the rule of marking from the face edge is adhered to there will be no problem.
Specialist mortice chisels can be purchased. They are thicker, front to back, than normal chisels. This is to give them more strength when levering out waste. Technically it is now possible to remove the waste by other methods such as drilling or routing.
Much of the need for specialist mortice chisels has disappeared unless you intend to cut lots of mortices by hand, in which case these specialist tools are ideal. They were ground at a more obtuse angle and also had bigger handles, often with hoops, to make them last longer because they are constantly being struck. The success of a chisel cut depends upon the relationship between sharpness and force. This, of course, has a bearing upon accuracy. A more obtuse grind angle will last longer but will need more force. There are also other variables such as the timber concerned and the steel of the chisel.
Photo 1: The timber marked up ready for cutting the mortice. In normal use there would be two stiles and two rails which would be clamped together and marked at the same time in order to ensure identical dimensions
Photo 2: Start to remove the waste with a drill. Any drill can be used. A drill press is easiest, but an electric drill in a small plunge stand will do, as will a brace and bit. Of course you can also use a router. Do not forget to mark the drill with tape for the correct depth setting. This picture also shows the Leigh clamp from Axminster Tools which is really easy to use with old benches like mine
Photo 3: Set in the mortice outline with a chisel. Do the ends first, across the grain. This will stop any tendency for the long grain side cuts to run on beyond the mortice ends. It does not matter if the setting in cut overlaps the sides on the face that the tenon is coming in from. However, with a through tenon you do not want to see the setting in lines on the exposed outside face
Photo 4: Once the majority of the waste is removed the mortice can be cleaned up with a chisel. You will notice is that the chisel you intended to use is too tight if you attempt to start cutting right at the end of the mortice. Clean out the sides first using the widest chisel that will fit, then revert to your correct width chisel and work back towards the ends of the mortice. It goes without saying that the chisels must be razor sharp. Sneak up on the end of the mortice. If you try and chop it all out in one go the bevel will push the chisel over your scribe line. Work through to half the depth from one face and then turn the timber over and work through from the other side
Photo 5: Tip. A clamped on guide block is very useful when cutting angles, either 90 degrees or any slope for a mitre or other purpose. The principle is the same whether using a 90 degree guide or a 45 degree as shown here
Photo 6: Cut the tenon in the same way as for a bridle joint, but notice that for a rebated frame the tenon will need a haunch, see drawing. This photo shows the tenon part cut. For those not wishing to use a bandsaw a simple jig can be made, as shown here, to hold the tenon between two square blocks, allowing it to be surfaced to the height of those blocks
Photo 7: When cutting your first haunched tenon with a moulding come at it slowly. It is a good idea to do a full size drawing on a piece of hardboard. Set out and chop the mortices first, then run the mouldings on the morticed components. Transfer all the main measurements using a Vernier or dial calliper to the tenon component. Do not cut the tenon full length to start with, offer it up and you will then see which parts need to be longer and which shoulders need to be cut back. Finally do the scribe or mitre cuts for any mouldings. Notice the mistake when the router dug in due to insufficient support as I finished the moulding cut. This would be trimmed out when the shoulders are cut, but I would have preferred it not to have happened
Photo 8: Finished joint
Photo 9: The main joint in a chair, the side rail to back leg joint. In this instance the maker has thickened up the rear leg to give an extra long tenon with large shoulders in order to resist the most common form of chair abuse, leaning back and balancing it on the rear legs