Wednesday 11 July 2018
Dovetail jigs can be divided into two broad categories: simple inexpensive fixed-comb jigs and sophisticated expensive adjustable jigs. The simple jigs can cut only one type of dovetail – front of drawer – in one size, one spacing, and one depth of cut. They can't cut through dovetails such as would be used at the back of a drawer. You can buy additional templates and cutters to make different size and type dovetails, but you might spend more than if you bought a more expensive model with greater capability. You are at the mercy of your board width, too. Choose your boards in widths that give you symmetrical pins and tails.
Sophisticated jigs have templates with adjustable fingers, which enable various kinds of dovetails and joints to be cut with their number, size and spacing determined by the user, and no constraint on board width. The best known of these are the Leigh jigs and the Trend DC 400.
Two other sophisticated dovetailing devices merit comment: the WoodRat and the Incra Jig. The WoodRat is not a jig since it has no templates or guide bushes, and can perform many other woodworking operations. The Incra Jig is a precision router table fence sufficiently accurate to make a series of cuts across a board.
Routers for dovetailing
A dovetail cut is always a heavy cut. You cannot take several light passes – you have to cut at full depth, therefore you need reasonable power. For most jigs, cutters come on 1/4in or 8mm shanks, so a medium-power router with 1/4in and 8mm collets is a sensible choice, unless you are cutting large dovetails, on 1/2in shank cutters, when you are obviously going to need a 1/2in router.
Another essential item is a fine height adjuster. Not only is it invaluable for setting precise depth of cut but without it there is a danger that you will release the plunge and let the cutter try to come up through the guide bush and template, with disastrous results.
Finally, you have to ensure that whatever jig you buy will work with your router. Most dovetail jigs work with a guide bush to steer the cutter around the template, but several e.g. the Axminster, Screwfix and SIP, use bearing-guided cutters. If yours is a guide-bush based jig you have to make sure that you can fit the guide bush to your router. Most basic jigs come with a Trend-compatible bush, which can be fitted to a wide range of router models. If your router won't take Trend-compatible bushes directly, you might be able to fit a Trend Unibase or Circular Sub-base to it.
With Leigh jigs, there are two types of bush: a screw-in one that fits directly to Trend-compatible routers and an American style two-part bush that requires an adaptor plate. Leigh offers a range of adaptor plates for different routers, which take both types of bush.
The De Walt DE 6212 jig comes with two-part American bushes and three adaptor plates to fit the De Walt range of routers. One of these is for the DW 615 model, so it can be fitted to all those routers that are Trend compatible (Pic 6).
Your first job is to mount the jig on a board that can be clamped to your workbench. I make my mounting boards to look tailor-made for the jig. Apart from looking good, it enables me to carry the jig, or hang it up.
Once on its board, the jig then has to be properly set up to get evetything in motion. I'll begin with the simplest, exemplified by the Trend CDJ 300 and the CMT 300. Both of these have fixed stops to position the boards and offset the vertical board relative to the horizontal one (Pic 7).
The template comb controls the shape and size of the pins and tails and the depth of the pin sockets. With the Trend, two knurled wheels are screwed in or out to line up the back of the fingers with a line drawn on the horizontal board (Pic 8).
With the CMT, the comb has an alignment mark 'S' at each end. Two boards are clamped into the jig – one horizontal and one vertical – and the comb adjusted backwards and forwards until the alignment marks are exactly over the junction between the boards. The adjustment is made by turning locking nuts on the comb-carrying rods (Pic 9).
Fine-tuning is done by making a test cut and correcting according to whether the pin sockets are too deep or too shallow.
To make the test cut, put the guide bush – if using one – in the router, plunge the collet nearly to the guide bush, thread the cutter shank through the guide bush and tighten the collet nut – you might need a bit of experimentation to find the best amount of plunge and cutter shank to insert in the collet. If your dovetail cutter is bearing-guided this stage is very much easier since you can install the cutter before plunging the router. The same applies if you have a Trend CDJ or CMT jig, where the guide bush is just big enough to let the cutter through.
Now set the depth of cut according to the User Manual and make the first test cut. Clamp two boards in the jig, with their edges flush under the comb, take your router and make the first cut across the vertical board from right to left i.e. in the 'wrong' direction of cut. This puts a razor-sharp edge on the inside of the corner (Pic 10).
Then cut from left to right, going in and out of the comb, feeling your way with the guide bush or bearing tight against the fingers. Inspect the cut and repeat as necessary until the cut is completed (Pic 11).
Fitting the joint
Fit the boards together. If the tails are too tight to go into the pin sockets, reduce the depth of cut by about 0.5mm and try again. If the tails are too loose, increase the depth of cut by 0.5mm and try again. When you have established the correct depth, make a simple depth gauge for future use (Pic 12).
Note – the fit of the joint is determined solely by the depth of cut, and cannot be varied for different thickness boards. This comes as a surprise to many beginners. With sophisticated jigs, you have a range of different size cutters, which you match to the thickness of your boards.
When your dovetail fits together, you can consider the depth of the pin sockets. If they are too deep, bring the template comb a bit forward, if too shallow set the comb back a bit, and make another test cut. When you are satisfied note the position for future reference. Take your time, once you've got its done for good.
Setting up differences
With the, Screwfix, Axminster, SIP and several similar jigs, there are three differences in setting up, compared with the CMT and Trend:
the stops are adjustable and have to be positioned by the user,
the front of the comb has to be set back about 3mm from the front edge of the vertical board, the depth of the pin sockets is controlled by a stop bar, whose position varies with the size of your router base.
Starting with the left-hand end, draw a pencil line down the centre of the first finger in and clamp a board horizontally with its edge immediately under the line. Move the horizontal stop up to the board edge and tighten it (Pic 13).
Clamp a vertical board in the jig offset to the right of the horizontal board by the pitch of the comb, typically 1/2in. The easiest way to do this is to draw a line on the horizontal board 1/2in in from the left-hand edge, and clamp it under the horizontal board so that its left-hand edge is lined up with the pencil mark. Now push the vertical stop up against the vertical board and tighten it (Pic 14). Repeat the operation at the right-hand end of the jig.
The rounded fingers of the comb have to be set about 3mm back from the front edge of the vertical board. This is easily done with the aid of a simple gauge made from two pieces of 3mm hardboard glued together (Pic 15).
The instructions that come with these jigs are usually incomprehensible. I have simplified the procedure:
Subtract 3mm from the thickness of the vertical board. Double the answer.
With the cutter and guide bush in the router, measure the distance from the edge of the router base to the edge of the guide bush, or the edge of the bearing if you are using a bearing-guided cutter (Pic 16). Measure from the edge that will bear against the stop bar.
Add the results of steps 1 and 2 and set the stop bar this distance back from the front of the fingers. A calliper gauge is very useful for this (Pic 17).
Make a test cut and move the stop bar back or forward according to whether the sockets are too shallow or too deep. When the joint is flush, note the measurement. I write it on a sticky label stuck behind the front of the stop bar. Now you can start making those drawers and boxes.