Sunday, 2 June 2013

A Tale of Two Tables: 4. Starting at the Top

Constructing the table top is a simple theory. Stick several wide planks together. Our table top is based on seven 6" planks, being cut to a square once joined. Rather than just butt the planks up against each other we wanted to join them with a stronger joint. To get a stronger joint you need a larger surface area of the planks in contact with each other. Traditionally that would mean tongue and grove, dovetail or something similar but these methods all require careful joinery with expensive router bits. With 24' of join to make I didn't fancy attempting dovetails! Conveniently there is a modern alternative that provides a reliable joint almost as strong as a well made traditional joint - a biscuit joint.
A biscuit joint is made with a pointed oval biscuit of compressed beech that fits into a cut slot on either side. This reinforces the joint be adding glued surfaces deeper into the join. A biscuit jointer cuts a 4mm high curved slot and is precise, repeatable and fast. A specialist bit of kit but very handy for all manner of things (as you will discover as the project progresses).
Before we even started cutting we laid out all our planks and clamped them to allow us to mark up the square, align the planks and position the biscuits. The outside two planks had alternate double biscuits to add extra strength where it overhangs the frame. all the others had single biscuits.
Having cut all the joints we did a dry run, slotting it all together with biscuits. Fortunately the biscuits resolved the slight bowing that a few of the planks were showing. When clamped it all fitted together well (clamps on alternate sides to prevent curving the top). From here it was time to glue. We took care to glue all the surfaces that would be in contact, including the biscuits. The glueing was also done in stages, few planks at a time and then letting it dry for a while. This was because even with three of us gluing we wouldn't manage to get everything glued and together before it had started to set - the weather was against us!
 To try and ensure a good pressure across the length of the table we used a ratchet strap to hold the centre of the table once it had been clamped, allowing us to move the clamps nearer the end. Note all the blocks of scrap wood to prevent the clamps damaging the surface.

Once dried we then clamped up the ends for cutting. A stress relief cut was made with the router to a depth of 8mm on the reverse, 1cm behind the final edge. This was to ensure the waste wood didn't splinter off with the table top. When cut from the top to a depth of 18-20mm from the top surface the ends gently cracked off, allowing easy trimming of the remaining waste wood. A block of scrap wood was clamped to the outside to try and prevent the edge being splintered by the router. Once cut it was ready for finishing but that is for another post!


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