12″ Ported Bass Cabinet. Installing the reinforcing battens.

I’ve got hold of a few lengths of 1″ square profile, pine timber – to make up my reinforcement battens. Cutting the lengths down to size, and cutting accurate mitre corners at the same time, is easy with a Morso guillotine. The lengths are fed in one side, and the blades cut both corners of a mitre simultaneously. I can also use the length stops to ensure all the lengths are cut precisely. Not only does this make the job much simpler – it also helps get the most out of a length of wood, and cuts down on wastage.

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The mitres are all spot-on too. I cut all the lengths of batten required to fit around each panel opening – eight lengths in all. I cut the initial lengths a couple of millimetres longer than required – so that I can eventually use the micro adjustment on the Morso to shave the lengths down to the exact length required, when fitting. Before I fit the battens, I pre-drill and countersink all the screw holes that will be required – using a drill press. I need to be able to fit the battens without the risk of the wood splitting.

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For the battens at the “rear” of the cabinet – the only screwholes required are those needed to screw the battens to the carcass. For those around the front face – identically spaced holes are required for attachment, but additional screwholes will also be required so that the baffle can, eventually, be screwed into place from the inside of the cabinet. The speaker baffle will be covered, at the front, in a grille cloth – so all of the the fixings will be hidden inside the cabinet. This will also allow a gasket to be fitted around the inside face of the baffle board – against the insides of the battens. This should help provide an air-tight construction – helping the speaker work at its’ most efficient. Pre-drilling these additional holes on the front battens at this stage, helps ensure the fixing screws are placed at the best locations – in the “meat” between the securing screws.

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Each batten is trimmed to the exact required length, using the Morso. It is then glued and clamped into place – using protective pieces of thin MDF to prevent the clamps from digging into the soft pine. The first battens fixed, are those for the rear of the cabinet. I check the position of these to ensure there is an equal, 20mm inset dimension, all around. This should allow the 18mm rear panel to be fitted flush, within the carcass of the cabinet. The extra couple of millimetres allowing for a couple of thicknesses of tolex covering, and a compressed foam gasket. (Note from the future – leave a few extra mm spare next time!)

Once each batten is clamped into place, and secured – I sink wood screws tight into their countersunk holes. I use wood screws with a cutting tip, and a light-ish slip torque setting, on the driver. The screws bite into the wood of the carcass, and pull the battens tight against the construction. Once the glue is dry, I’ll make sure the inside of the seal created by the battens is caulked and fully sealed.

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In the corners – the mitres are also glued as the battens are fixed into place. Once the back panel battens are all in place, and secure – I turn the cabinet around and install the front baffle battens. The process is identical – except in the case of the baffle board, I ensure there’s a 30mm clearance all around. This should allow the baffle board to be inserted within the carcass – leaving the face approximately 10mm behind the edges of the cabinet. For this design, I prefer the idea of having the baffle grille slightly recessed – rather than sitting flush with the front face.

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With the battens now fitted and fixed within the cabinet, I lay the cabinet on the workshop floor, and fit a 9mm roundover bit to my router. I’ve got a few power points located in the rafters of the workshop – and this allows me to work over the cabinet, whilst keeping the power cord well out of the way. I adjust the depth of the router bit so that the roundover falls flush with the top edge, and then slowly work my way over each outside edge of the cabinet with the roundover bit – allowing the following bead on the bit to keep the cutting edge, consistant along each edge. It always pays to go slowly when routing – especially doing roundovers. (It’s all-too easy to forget one edge). Going slowly also helps cut down on any splintering at the corners. Using plywood – a certain amount of splintering is inevitable, because of delamination, and weaknesses in some of the layers of ply and bonding material. Once all of the outside edges of the cabinet have been rounded over – I always go over the work again, to check. In this case – there are a couple of places where the reinforcing dowels have been cut into, and also a few where small lengths of plywood have been pulled out, along the edges. Once the routed edges of the cabinet have been given a rough going over with coarse grit paper – these are places where I’ll have to do a bit of filling – just to make sure there’s nothing which will telegraph through the tolex covering. I’ll use a two-part epoxy filler where necessary – and then sand everything level before covering. It doesn’t have to be super accurate and smooth – just enough to give a solid, consistent look under the tolex.

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And that just about completes the basic construction of the bass cabinet. I test fit the front grille baffle board, and note a good, snug fit. In fact the fit is so tight, I can’t risk fitting the rear panel at the same time. (I need to be able to push the panels out from the inside). I’ll have to trim off a tiny bit of each panel to allow for a proper fit, once they’re eventually covered with Tolex. Meanwhile, I’ll let the glue dry properly, and then caulk all of the internal joints. The next step, after that, will be to cut the side handles and jack plug sockets. I’d better get an order into the suppliers, and start thinking about getting the bass driver on order too.

Whilst I’ve got the router set up – I roundover the cabinet edges for the FBJ extension cabinet as well. The design of the tweed, open-backed speaker cabinet is still under review, and is still evolving – but it makes sense to get the roundovers done at this stage. I’m not in a rush to complete the FBJ cab – it’s entirely a bonus project built around the unused wood from the bass cabinet project. I’ve got an idea to do a amplifier build sometime – and I’m thinking about doing a simple tube head. I’ve got a few off-cuts which would probably make a head cabinet. Perhaps I’ll use the FBJ cabinet as a matching speaker cabinet for the amp build? Maybe. I’ll just park it at the back of the workshop for now, until it’s needed.

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