How to Make Finger Joints
Tuesday 23 March 2010
Each part of the finger joint - also known as a box-lock or corner-lock joint - is a regular 90 degree geometric opposite of the other part. There is no angular mechanical lock as there is in a dovetail. It relies on the long-grain gluing surfaces. Historically it is a very good joint for mass-production. Machines can be easily set up for the repeat cutting and vast quantities of these joints were used in furniture and small objects between the 1850s and the 1950s. Nowadays it is more likely to be used in small-box making and as a decorative feature when there is insufficient time for dovetails.
In the USA tablesaws can be used in a different way from the way we use them in the UK where crown guards must be used, thereby preventing the use of tablesaws to cut this joint, see reference to a book by Tag Frid in the sidebar, right.
The joint can also be cut on a router table or in a proprietary device such as the WoodRat. It can of course also be cut by hand and Ernest Joyce shows an interesting method of marking for the saw kerf, cramping together and sawing one cut then easing the components left and right to allow for the kerf on each side of the fingers and sawing again.
It is an underused joint and it allows the possibility of selecting contrasting timber inserts for the fingers that could be used to spice up an otherwise bland timber.
Because I do not have room for a big tablesaw in my workshop I tend to rely on a well set-up bandsaw with a sharp blade. The new M42 blades available from Axminster or Workshop Heaven are a fantastic revelation for bandsaw users. I can now expect to have an M42 blade on the saw for four to six months. This would have been unheard of a few years ago.
My preferred method for finger joints is to lay them out, cut the slots on the bandsaw, perhaps with the pieces paired up if they are large enough to get a cramp on, cut the bottom of the slots with a fretsaw and then clean up with a sharp chisel much like you do for a dovetail.
The finger joint does, however, lend itself to machine production, and I am relying on hand tools because I am familiar with them, do much restoration work and do not have much space in my workshop. As the by hand method is so obvious I do not show it in the pictures.
I would like you to assume that timber is prepared all square and that, unless I suggest otherwise, the proportions for layout are symmetrical. Note that in my photographs I am, for the sake of clarity, showing a pencil line whereas in real life I always use a knife.
Mark the layout on one component. Once you become experienced at making this joint you will not need to mark out, but in the first instance I think it is a good idea.
The width of the component is dependant upon the cutter. Choose a cutter that gives a pleasing number of whole fingers. If the design is paramount you will need to buy in a cutter that will give you whole fingers.
You should ensure that the last finger is entire and not diminished because the board was not wide enough. A thin finger presents a weakness and might break off in use, but equally a part-finger messes up the machining process because the second board to be machined has to be oriented one whole recess to the left or right of the first board to be machined, and the same pin and jig used. A part-finger or part-recess will throw out the lining-up process.
I am referring to a 4-sided box as an example but these principles apply to all shapes. Design the long sides of the box such that the fingers are symmetrical at each end of the side. The top and base of the side should have long fingers not recesses. The side will look more visually complete if designed this way.
Router expert Anthony Bailey likes to use a purpose-made jig.
A locating peg is fitted in the sub-fence, the same distance from the cutter as the diameter of the cutter. The sub-fence is adjustable left and right so that it is easy to locate the peg the precise distance from the cutter. This setting is crucial.
The component is placed against the peg and the first slot is cut. The fence and component are retracted from the cutter and the component is placed with the recess that you have just cut over the peg.
Hold the component tightly, or clamp it, and move the fence forward again to cut a second recess. Retract, locate over the second recess, clamp and move forward to cut the third recess. Repeat as necessary.
It is a very good idea to try a couple of practice pieces first, but once you have got the hang of it attractive tight-fitting joints can be produced with ease. Do not forget that for the second component you need to offset the recesses by one width.
This system requires a router with a fine-height adjuster. Without the adjuster you will be struggling to get the settings exact.
Using joint-cutting jigs
I am using the WoodRat but other joint-cutting jigs could also be adopted. The WoodRat offers the opportunity of cutting repeat joints from a prepared template. You can have a universal template or the template can be the first component to be cut.
The problem of breakout is solved by using WoodRat special cutters as described in the manual. With a little ingenuity pairs of components with visually and physically harmonious finger joints can be cut in one go. Contrasting splines can also be cut, and the Rat can be used for many other joints too.
First, prepare two boards. Set one up in the vice below the cutter and cut the first recess as marked out, then remove this board and set it up in the far left-hand vice.
Place the second board in the vice below the cutter and line up the board with the layout marks for the mating recess to be cut. Before cutting, draw with a pencil, on the frame of the Rat, the edges of the recess in the first board. This will give you reference marks for the left and right adjustment to cut each finger and recess. It sounds complicated but once you have done it on a practice piece it makes perfect sense.
This is an underused joint. I have looked through the last 50 or so issues of F&C and have not found any examples yet. I suspect that the wow factor of dovetails makes people choose that joint over the simpler finger. Dovetails do have a mechanical advantage but with the adhesives of today a finger joint that relies on glue might be just as effective and efficient. It is also easier to make.