Woodworking jigs: what they are and how they work

A woodworking jig is a device that locates, guides or supports a tool or workpiece for a particular operation. A straight fence can be a jig. So can a drilling block, a router template, a tapering sled or a former used to clamp a lamination. The form changes; the logic does not. Each one creates a known reference and makes the next cut or hole depend on that reference rather than on a fresh pencil line.

This matters most when a relationship repeats. Four shelf-pin holes need to share a line and a pitch. Several legs need the same taper. A circular top needs one centre and one radius. A guide bush needs a template that includes the cutter offset. In each case, the jig turns the relationship into physical geometry. It can reduce marking work and variation, but it can also repeat a mistake perfectly. Proving the first setup is therefore part of making the jig, not a separate afterthought.

§1 AnatomyThe parts of a useful jig

Start with the datum: the face, edge, centre or corner from which every other position is measured. Add a locator such as a fence, pin, stop or shoulder that puts the work against that datum. Decide how the work and jig stay in contact during the operation. Finally, define the tool path: a guide edge, bush, bearing, hole, rail, slot or pivot.

Those jobs should be visible in the design. If the work can seat against two different faces, the jig has two possible answers. If dust or a burr can hold the work off a stop, the reference is not repeatable. If clamping force bends the jig, the tool path changes after the setup was measured. A good jig does not merely fit the tool; it makes the intended seating obvious and leaves room to see or feel that the work is home.

A template is usually the shape or edge the tool follows. A jigcan include that template plus the locating and holding system around it. The words overlap in normal workshop use, but the distinction helps with planning: template geometry controls the cut, while jig geometry controls where that cut lands on the workpiece.

§2 Router workTemplate edges, guide bushes and pivots

Router jigs expose two common sources of wrong-size work. The first is treating a guide bush and cutter as though they share an edge. They do not: the guide touches the template while the smaller cutter runs at an offset. The template must therefore be larger than a finished opening or smaller than a finished outside piece. That direction matters just as much as the numerical offset.

The second is setting a circle jig directly to the finished radius. A router bit cuts on both sides of its centreline. When keeping the inside disc, the cutter centre must run outside the finished edge by one bit radius. When making an opening, it must run inside by the same amount. Write down which material survives before drilling the pivot hole; “200 mm circle” alone does not say which centreline path is correct.

Both cases benefit from a witness test. Use the same guide, cutter, template edge and direction intended for the job, make a shallow trial in scrap, then measure the resulting feature. That test catches an incorrectly measured guide, cutter runout, a rough template edge and a keep-side mistake before those errors reach the workpiece.

§3 Bench workDog holes are a layout jig built into the bench

A dog-hole grid turns the bench top into a repeatable set of locating points. Its usefulness comes from the relationship between holes: a known pitch, rows that suit the front edge and vices, and enough clear structure under the top for the chosen hardware. The grid should be designed around workholding, not copied because another bench used a familiar number.

Keep the pitch exact and put leftover space into margins. Stretching the intervals to make a row fill the top produces a pitch that no longer matches accessories or a drilling template. Mark every centre from one datum corner instead of stepping from the previous hole, because a small marking error otherwise accumulates along the row. Where a vice, screw, rail or fixing sits below the top, treat it as a no-drill zone rather than hoping the grid misses it.

§4 Design sequencePlan the operation backwards from the finished edge

1. Name the result. Write the finished edge, hole centre, angle or repeated distance the operation must create.
2. Choose one datum. State which workpiece face or jig point owns every measurement.
3. Identify the kept side. Mark the finished material and waste before applying cutter or saw compensation.
4. Separate locating from holding. A clamp should keep the work against a stop, not pull it into a new position.
5. Check the whole tool envelope. The cutter path is only part of the moving assembly; the router base, handles, saw body or drill chuck also need space.
6. Make the first proof disposable. Test the seating, sequence and measured output in scrap before treating the jig as production-ready.

For a one-off straight cut with an accessible edge, a marked line and a normal guide may be clearer than a dedicated jig. Build a jig when repeatability, awkward geometry or an otherwise hidden offset earns the setup time. Do not use one merely to add complexity: every stop, fastener and moving interface is another place where the reference can drift.

§5 ChecksBefore the first real cut

• The intended datum is labelled and the work can seat against it without rocking.
• Stops cannot be confused with clearance blocks or the opposite end of the setup.
• The kept side and tool-travel side are marked on both jig and scrap.
• The actual cutter, bush, bearing, blade or drill used in the proof matches the planned setup.
• Clamps do not distort the jig or obstruct the tool path.
• The measured proof result — not the jig's nominal dimension — matches the finished requirement.

A jig does not certify that an operation is safe or suitable. It only controls the geometry it was designed to control. Tool condition, workholding, guarding, feed direction, material and operator control remain separate decisions.