Words and photos: Philip Ashley
Your woodworking machinery can be an expensive investment. Even low cost equipment can add up when you consider all your machining needs. A tablesaw, bandsaw, some planing and sanding equipment and you could be looking at the price of a small car. With this in mind, it’s important that you maximise the use of every machine you have.
This could mean doing more than the machine was intended for. Your tablesaw for example is quite capable of doing much more than sizing your material. With a sliding table it can straighten a piece of wood, lowering the blade can result in grooves or trenches, and tilting the blade will enable you to make mitred boxes. Here we'll look at using this machine to safely make a simple bridle or open mortise and tenon joint with the aid of easy-to-make jigs.
Mortise and tenon
A mortise and tenon is a traditional woodworking joint. Other types of joints that can be cut with a tablesaw are housing joints, where a slot is removed to fit the entire thickness of another piece, and the box-lock, pin or finger joint. This latter joint has some decorative possibilities if it is cut cleanly. The range of woodworking joints that can be made on a tablesaw is limited as the large-diameter sawblade needs to pass entirely through the workpiece, so stopped joints cannot be done. In addition, angular joints such as open dovetails look unsightly, especially in the socket, where the sawblade cannot produce the sharp corner required for a neat fit.
Above: Open mortise and tenon or simpe bridle joint.
Bridle joints can be cut as T-bridles and these are used in table and frame construction. An end or corner bridle is often referred to as an open mortise and tenon. It may need to be pinned to avoid movement, but it can be a neat, strong method of joining two frame members together. A dowel or biscuit joint may be used for the same purpose, but an open mortise and tenon is much stronger because there is no separate joining element such as wooden dowels. The joint uses the strength of both pieces of wood held together by a large gluing surface.
An open mortise and tenon will only be successful if the joint is cut precisely. This means that your machine must be in good order, your sawblade sharp and the material held with a well-constructed jig to prevent any movement of the piece. These factors will help to produce a joint with clean surfaces and sharp edges. More importantly, they will ensure that the cut is made safely. While some joints can be cut with the saw guard in place, this is not possible with open mortise and tenons. Because the joint must be cut across the ends of the pieces, the top guard will need to be removed.
Removing the top guard should only be done if absolutely necessary and only when the job has been well thought out. Australian standards for machine guarding are guidelines for manufacturers who almost always adopt them. Not doing so could lead to complicated explanations should an employee be injured in the workplace. These standards are not obligatory in the home workshop but are still a sound resource for safe woodworking. Austaralia standards state that if a machine guard is removed, you should provide alternative protection to at least the same level of safety as the original guard.
For our open mortise and tenon this alternative protection will be in the form of a well-constructed jig to securely hold the timber component. You will see that when we use the jig correctly there is no chance of an injury as during the cut the blade is completely covered by the jig and the component. At no time should you hold a piece of wood without a jig to do anything other than a normal straight saw cut on the tablesaw.
For the jig you will need two pieces of particleboard, MDF or plywood 600mm long x 300mm wide. You will also need some square dressed timber at least 90mm wide x 30mm thick x 600mm long. Some 50mm long screws, glue and two G-clamps complete the list. It is not possible to cut both members of the joint with the one jig. This is because the sawblade will cut quite a large slot in the jig and leave your component with no bottom support. You need to make two jigs.
First, start by determining where the component will sit on the jig. A good distance is 100mm in from the edge of the baseboard as it allows us to hold the work with G-clamps without the clamps touching the rip fence. Setting the piece to 100mm will also help us to work out the position of the rip fence in relation to our component and make accurate cuts.
One of the 90mm by 30mm pieces will be used as a fixed fence to hold our components. This will need to be glued and screwed to the baseboard. To achieve this accurately, I used a marking gauge to score a line 85mm from the edge of the baseboard and then drilled a series of holes 70mm apart — 85mm is the centre of the 30mm wooden fence located 100mm from the baseboard edge.
Use a drill bit larger than the screw threads so the screw fits into the hole loosely (photo 1), otherwise the screws may bite into both the baseboard and the wooden fence and not pull the two together. Countersink these holes on the underside of the baseboard so the screw heads don’t scratch your machine table.
To accurately attach the wooden fence (photo 2), cut a 70mm wide piece of material and use this to locate the wooden fence while you glue and screw it to the baseboard. Our simple jig is now complete. The component will be held between the fixed wooden fence and another clamped tightly with two G-clamps. A filler piece the same thickness as the component completes the sandwich.
You may find that your machine will not cut more than 70mm in height. This will limit your component to 50mm or so, as your jig baseboard will probably be 16–18mm thick. A thinner baseboard will give you more saw height but I would not go below 10mm and at this thickness only use plywood. With a component width of 50mm you will get good support from the 90mm high wooden fence.
As the sawblade will only be cutting 50mm into the end of the component, there will be 40mm of the fence to attach the G-clamp to. You will need to sit the cramp nearest the front of the jig high enough not to come loose when the saw takes a cut out of the component. The clamp may also squeeze the blade as it cuts causing the blade to grab the wood and push it towards you during the cut (kickback).
When you assemble the ‘sandwich’ make sure there are no gaps between the component and the filler piece. This ‘filler’ will support the component as the sawblade cuts across the endgrain. There must be no sideways movement of the component during the cut. The filler must be the same thickness as the component to allow the cramp to secure the component. The use of a G-clamp is essential as quick-release cramps often come loose with machine vibration. Again, the clamp nearest the saw cut should be high on the sandwich and the second cramp in the middle for maximum hold.
For this article I used pine workpieces 50 x 30mm. They were about 900mm long but can be any length. A mortise and tenon joint should have proportions of thirds. The mortise is the female part of the joint and in this case should be 10mm wide and in the centre of the piece. The tenon is also in the centre and should be 10mm thick. I never allow for glue; any wooden joint I make is tight enough so that I can hold one piece vertically and the other will not fall out.
Now let us consider the safe operation of the cut. A well-constructed jig will hold the component firmly in a snug sandwich. The use of G-clamps will ensure maximum holding power. The clamps will also make handy handles when you push the jig against the machine fence and over the sawblade. If you use the clamps as handles your hands will never come near the sawblade (photo 3). Never hold the component. Do not remove the riving knife, if set correctly it will not affect the cut.
The photos show the cut being made with a high degree of safety as my hands grip the clamps well behind the cut. Once the saw starts its cut, the wood acts as a guard, covering the sawblade. The riving knife covers the rear of the blade. You should not let go of the clamps until you pull the jig back, well clear of the blade. Stop the sawblade while you make adjustments to the machine fence.
To set up for the mortise, set the machine fence to 110mm plus on the scale (photo 4). The ‘plus’ should be about 0.5mm to allow a test cut that will permit further adjustment for an accurate cut. Once you have a cut that leaves 10mm, pull the jig well clear of the sawblade, take out the component and turn it around to make the second cut.
After this, you can move the machine fence back by the thickness of the blade until the entire mortise has been removed (photo 5).
To set up for the tenon, use your second identical jig. Set the machine fence to 110mm LESS the thickness of the blade. For a blade with a kerf of 3.2mm, set the machine fence to a little more than 107mm and after a trial cut, fine-adjust the fence to get a 10mm shoulder on the tenon (photo 6).
Once you have a cut that leaves a 10mm shoulder, pull the jig well clear of the sawblade, take out the component and turn it around to make the second cut (photo 7).
After this, stop the machine, replace the top guard and use the crosscut fence to cut the shoulders of the tenon. Don’t cut the shoulders with the jig because you need a neat, sharp shoulder for the best result (photo 8).
This operation can also be done on the spindle moulder and in this case the same safety considerations apply. On the spindle you have a sawblade or cutterhead protruding from a fence and you need to provide some way of transporting the component past the saw or cutter with the maximum safety and control. You would use a jig exactly the same way as we have done on the tablesaw.
Using a tablesaw to make an open mortise and tenon will result in extremely accurate, snug fitting joints that will
enhance your construction. Your safety is assured with a well-constructed jig, strong G-clamps and safe operating
Philip Ashley has written innumerable articles for Wood Review magazine on machinery, tooling and safe work practices.