Three parts that must match
A routed dovetail is cut by a router guided by a template. The template's fingers set where each pin and tail go; a guide bush — a short metal collar fixed in the router's base, around the bit — runs along the edges of those fingers; and the bit, protruding below the bush, cuts the wood. Leigh puts it plainly: the bush runs along the guide finger, and the bit cuts only the wood, touching neither the bush nor the finger. The bit's shape decides the joint's slope, the bush's diameter decides how far the cut sits from the finger, and the template's geometry is drawn for both.
That is why a jig's bush, bit and template are one set. Change the bush diameter and every cut moves; change the bit and the slope and fit change. Leigh supplies its own bits with the D4R Pro and names the bushes that suit it; Freud sells dovetail bits made to suit popular jigs. Use the bush and bit the jig's manual names, and treat any substitute as a new set-up to test, not a swap.
Kinds of jig
| Type | What sets the spacing | Joints, as typically made | Limits |
|---|---|---|---|
| Fixed-template router jig | The template: evenly spaced, not adjustable | Half-blind drawer joints; some models also cut through dovetails with other templates | One spacing per template; capacity set by the template's length |
| Adjustable-finger router jig (e.g. Leigh D4R Pro) | Fingers you slide and lock | Through, variably spaced half-blind, single-pass half-blind, sliding dovetails and box joints (Leigh) | More set-up; Leigh's capacity is 24 in |
| Saw guide for hand-cut dovetails (e.g. Veritas) | Your layout; the guide holds the saw at the slope | Dovetails cut by hand | No router; slope fixed by the guide you choose |
Joint lists and capacities are each maker's for its own product; Leigh's are for the D4R Pro. Not a ranking.
- Half-blind and through. Half-blind dovetails hide the joint from the front, as on a drawer front; through dovetails show end grain on both faces. The joints are explained in dovetail joints.
- Single pass or two. Leigh's D4R Pro can rout half-blind dovetails in a single pass as well as through dovetails, whose pins and tails are routed separately.
- Box joints and sliding dovetails. Some adjustable jigs cut these too; Leigh does with its e7 bush, and needs that bush for box joints.
Guide bushes and why they are not interchangeable
A guide bush's outside diameter decides the offset between the template finger and the cut, so a bush of a different size cuts a different joint. Leigh's e7 bush shows how fine the margins are: it is slightly elliptical, about 7/16 in one way and .020 in less the other, so turning it a quarter turn in the router base changes the active diameter by .020 in — enough to tighten or loosen a box joint. A round bush of about 7/16 in with a barrel at least 1/4 in long will do Leigh's through, half-blind and sliding dovetails, but not its box joints.
- Centre the bit in the bush. Leigh notes that a bit not centred in its bush shifts the cut; in normal set-up this is adjusted out, but cutting pins with one router and tails with another, each with a different offset, misaligns the joint.
- Depth is measured below the finger. Leigh's depth of cut is the depth into the wood under the guide finger, and its bit projection is .450 in more than that.
- Router choice. The router must accept the jig's bush, or an adaptor for it, and hold the bit's shank; Leigh asks for the bit to go as far into the collet as possible. Router types are in the router guide and router bushings generally in router jigs and templates.
No universal dovetail angle
The slope of a routed dovetail is the bit's angle, and each maker names the bits for its own templates. Hand-cut dovetails take their slope from the layout or from a saw guide made to one ratio, as Veritas's guides are. A figure quoted as the right dovetail angle is a convention, not a rule; set the jig up with the bit its maker names and judge the joint by its fit.
The wood and the set-up
- Prepared stock. Leigh's manual says boards for its jig must be straight, flat and of even thickness and width, with square ends and edges; any jig registers from those surfaces. Milling them is covered in the jointer guide.
- Plywood. Leigh calls plywood generally unsuitable for routing on its jig because of tearout.
- Test boards. Rout a test joint in scrap of the same thickness after every change of bush, bit or depth.
- Using the jig. The order of passes, offsets and depth settings are in the jig's own manual; there is no general procedure that fits every jig. Choosing one is covered in how to choose a dovetail jig.
Safety— Power off for bits and bushes
Leigh's safety chapter says to disconnect the router from power whenever fitting bits or guide bushes or making adjustments, and, before connecting it, to check that the bit turns freely everywhere you plan to rout without touching the bush or the jig. Keep hands clear of the bit's path under the template, and follow the router's own manual; see router safety in the router guide and woodworking safety rules.
Hand-cut dovetails, laid out and sawn without a router, are taught in how to cut dovetails by hand. The other joinery tools are mapped in the pocket hole guide.
Before routing dovetails on a jig
- Bush and bit are the ones the jig's manual names
- Bit centred in the bush; one router for pins and tails
- Depth of cut set as the manual measures it
- Stock straight, flat, even in thickness, ends square
- Router unplugged while fitting the bit and bush; bit turns freely
- Test joint routed in scrap of the same thickness
Sources
Capacities, bushes, bits and settings are Leigh's for its D4R Pro and apply to that jig only. No fixed-template jig's manual was publicly available for review, so fixed-template jigs are described by how they work, not by any one model's figures. No universal dovetail angle, bush size or board capacity is given, because each jig's maker sets its own.
- 1.
D4R Pro Dovetail Jig User Guide. Leigh Industries (distributor-hosted copy).
Used for: The D4R Pro's 24 in capacity; through, variably spaced half-blind, single-pass half-blind, sliding dovetails and box joints on one jig; the e7 bush, elliptical with a ~7/16 in major axis and a minor axis .020 in less, so a quarter turn changes the active diameter by .020 in; a round ~7/16 in guidebush with at least a 1/4 in barrel usable for through, half-blind and sliding dovetails but not box joints; the bush runs on the guide finger while the bit cuts only wood; bit projection .450 in more than the depth of cut; two routers with different bit-to-bush offsets misaligning pins and tails; Leigh's own bits (140-8 straight, 80-8 and 120-8 dovetail) supplied; stock straight, flat, of even thickness and square, and plywood generally unsuitable because of tearout; power disconnected to fit bits or bushes, and the bit checked to turn freely without touching the bush or jig.
- 2.
22-102 Dovetail bit. Freud.
Used for: A dovetail router bit sold to suit popular dovetail jigs — the bit is part of a jig's set, not a free choice.
- 3.
Dovetail Saw Guides — instructions (INS-622_B). Veritas Tools Inc. (Lee Valley), 2018.
Used for: Saw guides for hand-cut dovetails, each made to one slope ratio, as the alternative to a router jig.
About this guide
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