Box Comb Joints archive

Tuesday 6 December 2011

John Bullar explores the next best thing to a dovetail

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Strong & reliable

Over the years I have collected a lot of wooden storage boxes in the workshop. Some are the cases tools came in, some are old cigar boxes, and others are home made for some need that came up decades ago. Although these are lightweight utility items with thin walls, all the corners are made with box joints, and that is why they have stayed together for so long.

Step 1

Box joints are also known as comb joints, or finger joints.

The joint can either have the same number of fingers each side like a hand, or they can have one finger less on one side than the other. The advantage of this second type is that each set of fingers is symmetrical, as in the example I will show you

Stacked cutter

Step 2

Special cutters are made for box joints with a series of blades. Some of them are made in one piece, while others consist of a stack of blades and spacers, often with a guide bearing beneath to limit the depth. The bearing is used for edge joints, not for the end joints shown here. The width of each cutting edge exactly matches the gap between edges, so the fingers and slots will fit together precisely

Step 3

Big cutters must be securely mounted in a large router fixed to a router table, or else a spindle-moulder, but never in a handheld router. Stacked cutters work much better if you stagger the angles of the cutting edges so they do not all hit the wood at the same time. This cuts down vibration and the chances of a snatch or kickback

Router table

Step 4

With the cutter stack securely mounted in the chuck, the fence is opened up and slid across the table to shroud the cutter. Fine-tune the fence position then clamp it so that just enough cutter is exposed to pass through the wood

Step 5

You can cut narrow box joints in a single pass, or else use wood that is up to twice the height of the cutter and make two passes, as I have done here. The height of the router needs to be adjusted so that the underside of the bottom cutting edge aligns with the underside of the wood

Step 6

The wood must be held securely at right angles to both the fence and the table while it is slid along the fence through the cutters. The router table I used was fitted with a sliding fence which is ideal for this kind of work. Alternatively, if you don't have a sliding fence you can use a large block of wood, or a sturdy box, and slide it along the fence to support the wood at right angles

Sacrificial timber

Step 7

Router cutters passing through end grain like this are certain to tearout the far side to some extent. Using new sharp cutting edges helps but even so, the pressure on the fibres pushes them out before they get sliced off. The solution is to always hold a piece of waste wood firmly against the back of the component you are cutting

Lower half

Step 8

The wood is turned over to cut the second side of the joint and this must be accurately aligned in height so that all the fingers have the same spacing. Press the cutter into a previously made socket while fine-tuning the cutter height

Step 9

The second pass of the cutter works in the same way as before, producing a series of fingers with an open socket at each side. If you are making a set of joints for a box, it makes sense to produce all the matching cuts before adjusting the height

Second setting

Step 10

The first piece of wood has been cut and it is used as a template to help adjust the cutter height, ready for the second piece forming the opposite half of the joint. The cutter height is raised so the cutting edge corresponds with the height of the existing fingers

Step 11

The router is fired up again. The second piece of wood is held against the waste and the router fence then passed past the cutter

Step 12

As before, the wood which is twice the width of the cutter height is turned over to make the second side of the joint. The cutter is raised by one finger's width and the edge is pressed into one of the previously cut sockets to help fine-tune the height

Step 13

The fourth and last pass through the cutter produces the second side of the second half of the joint. This second piece of wood ends up with a finger at each edge corresponding to the socket at each edge of the first piece

Fit together

Step 14

The two halves of the joint are pushed together to complete the assembly. As with any joint, regardless of whether you have made one before or not, it is worthwhile to practise on a piece of scrap beforehand

Step 15

The fingers are made slightly over-long for the depth of the wood, so the excess needs to be planed off to make them flush. Remember to always plane onto the joint to avoid tearout

Larger scale

Step 16

The joints we have looked at are great for small-scale boxes and the like, but heavier-duty joints need a bigger cutting system. For larger box joints, a straight router bit can be used together with the finger guides of a dovetail cutting jig. An adjustable jig allows you to play around with the width and spacing of the joint

Step 17

The wood is clamped vertically in the jig and the straight sided cutter passed across the top of it through the end grain. Again, there must be some support in the form of a piece of waste wood to stop fibres tearing out the far side of the joint

Step 18

The work is done in the same way as for dovetail joints but this time, the sockets and the pins are straight sided

Step 19

A variation on the box joint is the multiple mortise and tenon. This is mostly cut with the router and finger guide jig, and used in combination with a bit of chisel handwork. It is ideal for making T-shaped joints such as for strengthening partitions in large boxes. Box joints may not have quite the flair of their more exotic cousin - the dovetail - but they are equally reliable and effective for small boxes and once you're set up, they can be made very quickly


Woodworkers Institute

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Dovetail , John Bullar , box comb joints