Learn the latest on lock mitre joinery systems with Scott Burton’s article in Woodworking Canada. Mitre joints have always been an effective way to hide solid wood end grain and produce a more aesthetically pleasing product. The challenge facing manufacturers is always structural integrity of the joint and simplicity to assemble parts. See how machinery and tooling innovations have improved the design of lock mitre joints and optimized the assembly process.

lock mitre

The lock mitre bit is the answer when you need to make a strong corner joint with plenty of glue surface. It’s ideal for assembling frames, boxes, boxed posts and columns. The lock mitre bit creates a symmetrical 45º cut on both pieces with one bit, and you don’t have to change the bit set-up. One side of the joint is cut with the piece laying flat on the router table and the other side is cut with the piece on-end, running along the fence.

“The challenge facing manufacturers is always structural integrity of the joint and simplicity to assemble parts.”

Common products that utilize linear lock mitre joinery:

  • Interior or exterior columns
  • Newel posts
  • Cabinetry
  • Range hoods
  • Fireplace mantels
  • Speaker boxes
  • Contemporary furniture parts
  • Solid surface countertops
  • Various plastic and composite box products

Traditional machining methods generally utilized a saw blade or sharper head in an angled orientation to cut both sides of the mitre. Tooling design changes later made it possible to produce a ‘lock mitre joint’ on a shaper, which is structurally superior, provides greater surface area for adhesive, and gives assembly staff a more accurate and efficient method of final assembly before clamping. Lock mitre joints became favourable for ease of assembly reasons, however traditional machining methods were extremely tedious and required skilled labour to achieve any level of repeatability.

Advancements in CNC router configurations and tooling technology have improved the mitre joint process dramatically. Mitre joints manufactured from a variety of materials can now be done with much greater accuracy and repeatability with the proper machine and tooling set-up.

Using a lock mitre set in router bit format will produce a joint with maximum structural stability and gluing surface. The only disadvantage is the material yield and cycle time reduction which results from using two router bits in two separate operations.

If the volume of production dictates a change to improve cycle time and save material, there are two CNC router upgrade options available to optimize the process.

Aggregate Attachment for a 3-Axis Machine

Aggregates for CNC routers are essentially a gearbox designed to change the orientation of the cutting tool to a more advantageous position. In this scenario, the cutting tool can be run on the CNC router in cutter format from above the material. Complexity of programming the aggregate is generally the only challenge for this solution that should be considered. However, the production efficiencies and accuracy that can be achieved with this system, make it a very attractive solution for many products.

Investment in a 5-Axis Router

This option will essentially provide the same benefits as a 3-axis machine, but without the necessity of an aggregate attachment. The tool can be easily run in the same orientation as above with the added benefit of angle flexibility using the fifth axis on the machine.

Take a look at our heavy-duty, high-performance 5-Axis CNC Router

Important Details

Onsrud 37-87: A 90° V-bit

Onsrud 37-87: A 90° V-bit

The exact angle of the cutting tool is obviously critical to achieving accurate results on the finished part. Lock mitre joints are dimensionally critical and therefore the tooling and machining process must be extremely accurate. However, there are many other factors that contribute to achieving a 90 degree finished product.

Something as simple as viscosity of adhesive can be enough to change the angle slightly upon final assembly. Density of material can also be a factor. For example, particle board with a loose core will often leave material fibres behind in the tool path which prevent the mitre from closing properly.

“Advancements in CNC router configurations and tooling technology have improved the mitre joint process dramatically.”

 To address this issue many tooling companies currently stock half degree variations to compensate for differences in material core and adhesive. Alternatively, custom tooling manufacturers generally have extensive experience and have the capability to make custom modifications to suit the application if all the details are provided.

There are endless ways to improve joint accuracy and function if all the factors or the application are considered. Consult our tooling suppler, LMT Onsrud,  to explore creative tooling solutions that utilize lock mitre joinery systems.


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