12″ Ported Bass Cabinet. Cutting and preparing the front baffle.

The speaker, and most of the other bits I need to complete the build are now sitting in the workshop, taking up space. I say “most of the other bits” – since I’m still trying to find some suitable speaker mounting bolts to fit the Eminence Basslite frame. The standard M6, hex-drive  type bolts, (in my motorcycle spares kit), seem to fit nicely – except the heads are slightly too wide to sit within the channel of the frame. I really need some T-Bolts – but the local hardware shop is now a big, out-of-town warehouse type concern, and the staff there just look at me blankly. Whatever happened to the good, old fashioned hardware store?

20190906_122933

So – I’ve changed the design a little, from my earlier plans. Looks-wise, I’d originally planned to go down a kind of Fender hybrid route – and to use an oxblood, Fender grill cloth. However – it makes much more sense to follow the Ashdown styling of the head I’ll eventually be using with this cabinet – and since that has a kind of black tolex / white vinyl piping thing going on – I might as well try and follow that lead. Rather than a plain black grille cloth, however – I’m going to use a Marshall style, “Salt and Pepper” type fabric, which should work nicely with the tolex and piping, and give an authentic, vintage-y vibe.

But before I can do any of the styling and dressing up – I need to get the front baffle board cut to receive the speaker and port tubes. The board – originally cut to size by the woodyard with the rest of the components,  is currently a good, snug fit within the cabinet box – but I need to allow a little extra clearance so I can wrap the grille cloth around the baffle edges, and apply the vinyl piping. I take a short length of each – double it up, and tack it to the side of the board. I can then offer it up to the cabinet, and gauge just how much I need to remove from the edges of the board. It looks like about an additional 3mm all around, should just about do it.

20190904_105756

Since the baffle is already a good fit, and is already perfectly squared – the easiest way to trim off the excess is to set up a straight edge, so that a straight cutting bit on the router follows the existing edge perfectly. Moving the straight edge inboard by 6mm, and then clamping it firmly in place – gives me a perfectly straight and square following edge. Two cuts – one each, along the short and long edges of the baffle, and my board is trimmed to size.

20190904_115525

I’ve already worked out the dimensioning and placement of the baffle openings, using a computer illlustration package, (see previous post). But bits have been trimmed off here and there, along the way – so it makes sense to draw out the rough layout on the baffle board – just to identify any possible clashes – perhaps with the softwood battens within the cabinet, or even between the speaker and port tubes. I know the top edge of the speaker frame will just begin to inpinge on the area where the top batten will be screwed to the front baffle. I don’t want any securing screws going in the wrong place. I also don’t want to position any of the speaker mounting bolts here, if I can help it. As I lay the design out on the baffle, I decide to move the speaker down a little bit more – just to be on the safe side. The distance between the edge of the speaker opening and the inner port opening also looks a bit tight, but there’s room enough to slide the ports over another 10mm to the right. Despite changing a few of the dimensions – I’m able to keep the centres of each opening so that they lie slightly off centre, and I’m able to keep the rough proportions of the original design.

20190904_131653

I’m cutting the larger, speaker opening with the router – using a circular guide. I want to mount the speaker and port tubes within recesses on the baffle board, so I can apply the grille cloth over the face of the grille – without seeing anything behind, and without the need for any additional, supporting, frame. The Eminence specifications lay out the exact dimensions of the speaker cutout and fixings required, and I can use the circular router attachment to cut my recess, so that it has a step – into which the speaker frame will sit.

I start by dimensioning the inner extent of the opening – where the speaker actually passes through the baffle board. I first mark the centre of the opening and drill a small hole through the board, so that the centre guide of the circular router jig can sit snugly in place. Since I want to keep this centre mark in place until the end – I plunge the router through the whole 18mm thickness of board in one spot, and then back it out again. This physically marks the position of the inner edge of the opening, and I can later use this to reset my router and then cut the whole opening from the other side of the board – once I’ve cut the, slightly larger, recess shelf for the speaker frame.

20190904_132908

I set the router to a plunge depth of 13mm. That will give me a 5mm shelf to attach the speaker, and allow the speaker frame and gasket to sit just below flush. I cut the shelf using a 16mm diameter router cutter. Obviously – that takes the inner edge further in, than is actually required. The cutting action removes my reference marks – but the through hole I previously cut using the plunge, allows me to flip the board over, re-set the router bit in the correct place, and to then cut the remainder of the opening. All this – flat on the bench. It helps greatly, to be able to clamp the work down firml. Being able to complete a through cut without having to extend the bit all the way down, means that the cut can be made, and that the worktop remains unmarked and uncut.

20190904_134813

The guide hole on the flip side of the board allows me to quickly set the correct dimension and plunge depth to finish the cut from the other side. The router does the rest.

20190904_135053

The result gives a perfectly concentric shelf to mount the speaker onto. Looking at it however – it’s going to be a mighty job to ensure each of the bolts sits in the best place, without splitting the wood. I’m beginning to see why some people advise making the baffle board double thick – and use extra long bolts.

To be honest – I’m in two minds as to whether this is a good way to go. I want a plain grille cloth to the front – so a front mounted speaker is out of the question. The gasket on the speaker seems better on the underside of the frame – more rubbery. That leads me to suspect that the speaker might actually prefer to be recess-mounted. The harder plastic of the front gasket leads me to believe that a rear mount might be possible – but perhaps not ideal. Recessing it seems the best solution all-round. I can use the soft rubber gasket, as installed, and drop the face of the speaker down behind the level of my grill cloth.

That said – what I’m actually left with seems an awfully meagre bit of wood remaining, to mount the speaker. When I think about it – I’m not sure if it’ll offer the best sound transference – although the speaker is still securely mounted to the baffle board. My logic tells me it should still be able to transfer soundwaves efficiently. Perhaps that’s the point? Maybe the speaker needs to be held “in isolation”? Perhaps I don’t want the baffle to vibrate in sympathy with the speaker at all? I don’t know. I need to go back to cabinet school and read up a bit more. For now, at least – I’ll proceed with the plan, as I envisaged it. Hopefully the compression effect of the retaining bolts will hold the speaker sufficiently – even if the odd bolt hole does split. If I do need to try another solution – I can always cut another baffle board from my spare plywood pieces, and use a following bead to easily, and accurately, copy the shape of the existing one.

20190904_140936

I can now test the speaker recess and opening, by supporting the baffle at both ends, and by dropping the speaker into place. Looks fine. The only nuts and bolts I can easily source, and which may work, are some standard M6, Hex-head types. Paired with washers and Ny-loc nuts – these 30mm long bolts should be secure enough. They’re on order, and should be with me soon. If the heads won’t fit in the frame channel – I may be able to grind a little bit off – to fashion my own kind of T-Bolt. While the speaker is in place – I can clearly see that the port tubes could do with being moved over a little way. I move the markings for both tubes over a couple of centimetres to the right – then, after putting the speaker safely away again in its’ box – I’m ready to cut the port openings.

The idea is to repeat the same, basic, technique as before – but this time with a much shallower recess. However – there’s an additional problem in that I’m not able to use the circle guide supplied with the router this time. These holes are just too small. I know there’s a way of building a jig – but since this is likely to be a one off, and since no-one will really see the end result once it’s behind the grille cloth anyway – I’m going to try and cut these openings by hand.

I use the router again – but it’s all done by eye. My technique has got better with practice – but every now and again, the router tends to catch the grain of the ply, and it’ll run off line. It’s hard not to look at the finished result and wince a bit, but it’s surprising how accurately you can actually guide the router by hand. It’s just a case of going as slowly and surely as possible. First – the outer recess is cut to a depth of 3mm. I use the 16mm wide cutter again, which is more than enough to create the required shelf for the outside of the port tubes. Once the outer extents of both recesses have been cut, I can then mark out the line of the through openings, using a compass. It’s then just a csae of cutting through the board using a cutting bit – trying as hard as possible to keep the bit online and true.

I won’t win any awards for the finished cuts – but they’re functional, and the openings are accurate enough to hold each tube securely in place. I’ll eventually fix them in place permanently, using a few blobs of epoxy adhesive.

20190906_122728

And with that – the baffle board is cut. I rub off any loose edges and ragged cuts with a rough grade, Scothbrite pad – and then transfer the piece to the spray booth. A couple of coats of grey primer are applied to the front face. I want to paint the board black – so that it doesn’t visually transmit at all through the grille cloth. Making sure the primer is applied from all angles – so that there aren’t any missed bits of recess – the primer will cure for 24 hours, before a matt black paint coat is applied over the top. I’ve still got a bit of paint left over from the Strummercaster paint job. This will use up the rag ends, and stop things going to waste.

The fixing bolts are, apparently, in the post. Hopefully, I’ll be able to look at the final bits and pieces of this project next week. I’m pleased with the way it’s coming together from a visual point of view. But it’s going to be far more interesting to hear how it performs.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s