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Box stair treads

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Tools for Success by Scott Burton
Scott Burton is the sales and marketing manager for Royce//Ayr Cutting Tools. For more info email: sburton@royceayr.com

There is a growing trend in stairs and railings to manufacture large four to five inch thick wood stair treads, to be mounted on metal stair stringer assemblies.
Image 1 shows an example of a creative stair assembly design incorporating metal, glass, and thick wood, to produce a contemporary style staircase. Thick wooden stair treads can be mounted on metal stair stringers from below or mounted from one side creating a floating stair design. Traditionally, wooden treads are manufactured from laminated wood or solid wood components. However, a new construction method utilizing lock mitre joinery has emerged in stair tread production.
Images 2 & 3 show how parts are assembled to create a hollow box, using ¾ to 1¼ solid wood or wood composite panel products.

Benefits of box treads
Normally, solid wood stair treads have exposed end grain, when mounted on an open stair stringers.  Exposed end-grain is not an issue for rustic style products, but contemporary designers are always looking for ways to refine interior designs with more modern components.  Box tread designs allow for detailed grain matching of various wood species and wood composite products. There are material cost savings realized by producing treads in a box formation, rather than four or five inch thick, solid-wood components. Box treads do require more extensive machining processes and added labour to assemble, so the final cost of the product may not be significantly cheaper. However, most customers demanding these types of products, are generally more interested in functionality and style, rather than cost. Combining box tread designs with metal stair stringers can also optimize the final installation process. The treads can be mounted on the top of the metal stair stringer, or in some cases metal stringers are designed to hold treads from one end to provide a floating stairway appearance. Metal support systems for floating stairs are highly technical to design and produce from a metal working perspective, but box treads designs do provide practical benefits when manufactured to assemble with metal stringers. Box treads have a hollow core, which allows the wood tread to slide over metal structures from one end, to provide the most stable design. Obviously, the tread needs to be manufactured with one end open to allow metal fixture to slide inside, but this can be easily done using lock mitred corners.       

Lock-mitred corners
Image 4 illustrates a lock mitre profile with a 90.5-degree angle and step joinery to locate the angled parts accurately. This system is commonly used in case goods, architectural millwork, and column production, but has recently found a place in stair tread production. Lock mitred corners provide the greatest level of accuracy, joint stability, and assembly convenience. Simple 45-degree components without the lock mitre, are generally tedious to assemble, and do not provide the same level of stability and joint quality. The greatest challenge in lock mitre production, is machining accuracy
to produce high quality joinery.
Even the slightest variation in angle accuracy, will prevent the components from assembling properly. It is critical the outside corners close tightly to avoid any gaps or misalignment, which are difficult to correct once machining is complete.

Tooling required
Image 4 shows a common lock mitre profile machined on precision moulder and end-matcher equipment. It is not recommended to use brazed carbide tooling, as profile distortion will occur during sharpening process. Profile inaccuracy will result in poor quality joinery and large gaps on the outside corner of parts, which is not an option for premium box stair treads. It is highly recommended, to only use precision carbide inserts or polycrystalline diamond cutting tools to ensure optimal cut quality and precise components. 

Machinery required
Although lock mitre profile components are relatively simple in design, they can be very difficult to produce without proper equipment and precision cutting tools. The major production challenge is producing an accurate angle and profile, which will close tightly to ensure a precision joint. It is not recommended to use single spindle shapers or hand router machinery to produce lock mitre components.
The linear parts should be produced on a moulder from the side or top spindle orientation as illustrated in image 4. Production on precision moulder equipment is the only way to ensure accurate joinery and premium cut quality. The ends of parts need to be finished on a multi-rotational end-matching machine. Single spindle shapers and router tables will not provide enough profile accuracy or cut quality on precision lock mitre parts. A multi-spindle end-matcher will ensure profile accuracy, and most importantly, prevent grain tearing on exit cuts.

Critical considerations
Obviously the most important factor in any mitred product, is the visual appearance of the final exterior joint. There are several critical factors which affect final joint quality and long-term stability. The angle of the mitres is very important to ensure the joint closes properly without gaps. All joints require room for glue allowance, so a true 90-degree is not recommended. Many manufacturers use a 90.5-degree or 90.25-degree angle, to allow space for adhesive to bond both parts effectively. If there is not sufficient room for glue, it will either bleed out onto the final product creating finishing issues, or create gaps in the final profile.
Type of adhesive can also play a role in joint quality and function. Lock mitre joints are more stable as a function of their design, which provides additional gluing surface. Therefore, thinner glue products generally provide sufficient bonding without having a negative impact on joint quality. Different wood species and wood composite panel products are also of critical importance. Solid wood species do have different machinability factors, which will have an impact on joinery precision prior to assembly. Varied moisture content and expansion/contraction rates will also be a factor affecting long-term joint stability. Wood composite products are generally more consistent and stable in nature compared to natural wood products. However, wood composite panel products do have machinability features which need to be closely monitored.
For example, plywood core products can be difficult to machine without wood grain tear out. Wood fibres can prevent joinery from closing properly, so profile modifications will need to be made in this scenario. Proper testing with materials, adhesives, and clamping methods, must be conducted priory to production, to ensure optimal joinery.  

Conclusion
Box stair treads provide unique style and functionality to contemporary stair designs in modern home building. There are many beneficial product features that make box treads a viable alternative to solid wood treads. However, the production process and material selection are critical to successfully manufacturing box stair treads. Proper machinery, cutting tool design and adhesives are all critical considerations when manufacturing precision lock-mitred stair tread products.

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