A simple jig for cutting perfect mortise and tenon joints

A mortise and tenon joint can look intimidating when the goal is a tight, square fit. In practice, accuracy comes from controlling the workpiece rather than relying on advanced machinery. A basic jig gives the router, chisel or saw a stable reference, making repeatable joinery possible in a home workshop.

The design described here suits furniture repairs, small cabinets, tables and frame construction. It uses inexpensive sheet material, a few screws and a straight guide. Once adjusted, the jig can help align mortises and tenons across several matching components.

Australian makers often work with dense timbers such as spotted gum, jarrah and blackbutt, which can expose small errors quickly. Cutting in measured millimetres and testing on scrap is especially useful when timber prices are high in local hardware stores and specialty yards.

Jig approach	Best suited to	Main advantage	Limitation
Router mortise jig	Repeated frame joints	Fast and consistent	Needs careful depth control
Chisel guide jig	Small projects and repairs	Low cost and quiet	Slower material removal
Table saw tenon sled	Straight, repeatable shoulders	Excellent alignment	Requires a stable saw setup
Hand-cut reference jig	Traditional furniture work	Flexible and portable	Depends more on technique

Materials and dimensions

Make the jig from 12 mm or 18 mm plywood, MDF or stable melamine-coated board. Plywood is generally the better choice for a long-lasting workshop fixture because its edges resist crumbling. You will need a flat base, two parallel fences and a removable stop block.

For a typical 20 mm wide tenon, set the fences so the workpiece is held firmly without being forced. The opening should allow the timber to slide smoothly while preventing sideways movement. Use a square to check that each fence is exactly 90 degrees to the base before tightening the screws.

A useful starting size is a base around 300 mm long and 150 mm wide, although the dimensions should reflect your stock. Label the jig with the intended mortise width or router bit size if it will be dedicated to one project.

Building the cutting guide

Clamp the two fences to the base and drill pilot holes before inserting screws. Add a stop at one end to establish repeatable length. If the jig will guide a router, fit a replaceable centre plate or opening that supports the router base and limits its travel.

For chisel work, the fences can act as a visual and physical boundary. Mark the mortise shoulders directly on the timber, then position the workpiece against the stop. A thin sacrificial strip under the stock prevents tear-out and protects the jig when the chisel exits.

Check the guide with a known straight piece of scrap. Measure from each fence to the centreline and confirm that the cutting path is centred. This small verification is worthwhile before using the jig on jarrah or a carefully selected piece of recycled hardwood.

Cutting accurate mortises

Mark the mortise with a marking knife rather than a soft pencil. Define both ends and keep the layout lines visible during cutting. If using a plunge router, remove material in shallow passes, clearing chips between cuts. Dense Australian species can burn when the bit is blunt or the feed rate is too slow.

A chisel can refine the corners after routing. Keep the bevel facing the waste side when paring towards a marked shoulder, and take light cuts instead of trying to remove the entire wall at once. The jig should guide positioning, but sharp tools and a controlled feed still determine the final surface.

Test the mortise with a short sample tenon made from the same species. A good fit should slide in with firm hand pressure and should not require hammering. Timber movement from seasonal humidity in places such as Brisbane or Sydney makes an excessively tight joint risky.

Forming matching tenons

Use the same jig references when cutting the tenon. A table saw sled, router bit or backsaw can establish the cheeks, while the stop block controls the shoulder position. Cut slightly outside the final line, then pare or sand back until the tenon fits the test mortise.

Keep the shoulders clean and square because they determine whether the frame closes neatly. If a tenon is loose, do not compensate with excessive glue. Re-cutting the component or adding a carefully fitted veneer is usually stronger and cleaner than filling a poor joint.

For small timber projects, repeatable joints can make assembly far easier. The same careful layout applies to wooden toy projects, where rounded edges, safe finishes and robust construction matter as much as accurate joinery.

Adjusting for real workshop conditions

Before glue-up, dry-assemble the entire frame and check for twist, gaps and diagonal differences. Number matching components so they return to their original positions. This is particularly helpful when building in a shared garage workshop or preparing furniture for a weekend project in Melbourne.

Let timber acclimatise in the workshop before machining, especially when moving boards between an air-conditioned room and an open shed. For outdoor furniture, species selection and moisture movement deserve equal attention; this guide to outdoor timber offers useful background before cutting expensive stock.

Apply glue sparingly to the mortise walls and tenon cheeks, then clamp over protective pads. Wipe away squeeze-out before it hardens, but avoid flooding the joint. Once the jig is tuned, record its settings and keep a sample joint nearby so future projects can begin with a reliable reference.