Last week we spoke about construction of the pine boxed frame. This post continues on that, exploring the next steps which are:
rounding over edges and sanding down frame
preparing the baffle and the rear panel
preparing the front and rear panel mounts
Rounding over the edges and finalising the boxed frame This doesn't have to be done at this stage but it is quite fun and good to get out of the way. I used a 1/2" radius round-over bit to round over all the outside edges. There's no special consideration needed here, it is pretty self explanatory if you've ever used a router before. I guess the only advice I can give is to make sure that the frame is square to begin with and all the surfaces are relatively smooth so that the guide of the round-over bit can ride on it smoothly.
Sometimes when the dovetailed edges are rounded over, you may find some gaps in the dovetailed joints (depending on the quality of your dovetailing). In this case I would just fill it with something like wood putty, or glue and sawdust if it's a tiny gap. This will then have to be sanded down smooth.
For the inside edges of the box you can use a round-over bit as well, maybe a 1/4" radius. I just used sand paper to break the edges.
After that, I would just smooth out all surfaces of the boxed frame with some sandpaper. There's no need to go down too fine a grit because the whole thing will be tolexed anyway. I would stop at 120grit.
Preparing the baffle board and rear panel The baffle board is just a board that your speaker attaches to, which is then attached to the cabinet. One would assume that its method of attachment would be integral to the quality of cabinet, and its material would be integral to the sound of it.
This being the first speaker cabinet I've built from scratch, I did not have opinions or preferences on mounting type or material. So I did some research online and decided to use 1/2" void-less birch ply. I also decided that I would mount it using bolts attached to a mount that is glued/screwed on two sides of the cabinet. I believe this is referred to as a 'floating mount', however don't quote me on that. It was the simplest design and that was the main reason why I chose it. We will get more into the mounting of the board in the next section.
The 1/2" plywood sheet will need to be cut down to size so that it fits inside the cabinet. When sizing the board it is important to leave enough of a gap between the edges of the baffle board and the inside of the cabinet for the speaker grille cloth to fit. There will also be tolex on the inside of the cabinet so allowance has to be made for that too. I can't tell you what gap I ended up with, but I think it was something like 4mm all around. So, measure the inside of your cabinet, minus 4mm all around the edges, and cut your baffle board to that dimension. If you find this to be too big later, you can always shave it down with a hand plane to fit perfectly. It doesn't have to fit too snugly either, there can be some room.
The next thing that is needed is to cut a hole for the speaker. I wanted the speaker to be placed in the centre, so I drew a line from one edge to its adjacent edge, and repeated for the other set of edges. This gave me an 'X' which marks the centre. Once this is done you can use an awl to mark it with a dimple.
Next, use a compass to draw a circle representing what waste needs to be cut out. Be careful here, and measure with your speaker. Make sure the hole is not too large such that the speaker cannot be mounted. Once you have drawn the circle, place the speaker on top of it and double check that you do have enough space for mounting.
I know some people will be critical about the sizing of the hole. I wasn't. All I can tell you is that I'm sure it matters but I'm also accepting that there is quite a lot of room for error in this. I have seen 12" speakers mounted in baffles meant for 10" speakers and they don't sound terrible.
Once it is marked out, and you have triple checked your measurements, it's time to cut out the waste. I used a router and a gerry rigged jig (pictured), but you can use a jigsaw or any other method you can think of. Just be careful.
For the rear panel, I used the same plywood that I had left over from the baffle board. Similar to the baffle board, measure the inside of the cabinet, then minus the space needed to fit in tolex (remembering that there will be tolex on the panel as well as tolex on the inside of the cabinet, so two layers of tolex). If you have some tolex you can do a dry fit and adjust the size as needed with a hand plane.
Preparing the mounts The mounts are just bits of wood added to the inside of your cabinet such that your baffle board and your rear panels can be attached to it. They are simple to install however there are a few important things to work out before gluing anything down.
The first thing you will need to work out is where the mounts will be generally located. As usual, I took the simplest approach. For the rear panel I decided on installing mounts all the way around. This will give me some versatility - if I ever want to convert it to a closed back cabinet, I can easily mount a second rear panel to close the gap.
For the front baffle I chose to install the mounts on the 2 sides of the cabinet. There are no mounts on the inside top and bottom of the cabinet. As alluded to above, I believe this technique is called a floating baffle, but I'm not too sure.
Second thing - work out how you will attach the baffle and the rear panels to the mounts. I chose to use T-nuts and bolts to attach the baffle; and stainless steel screws with cup washers to attach the rear panel(s). Work out the length of the screws and bolts you will need, and this will tell you how wide your mounts will have to be.
To install the T-nuts to the baffle (pictured below), I used a spade bit of the same diameter as the T-nut and created a depression for the T-nut to lie flush. This will be important as we will be installing spacers on top of it later. The spacer is a thin strip of wood that is installed on the front of the baffle (where the speaker points out) so that the grille cloth will sit away from the baffle when pulled taut. It only needs to be maybe 1/4" or 1/2" and can be glued directly on the baffle or nailed in, or both. This spacer will obviously add to the overall thickness of the baffle and it is important to consider this and take this into account when figuring out the mount location.
I don't have any pics of the spacer installation, but I will add a pic below that shows what it looks like after it's installed. Pardon the jump ahead in the build process, but this is just for display purposes:
Unfortunately I didn't record the dimensions of the mounts but they should not be too hard to work out. For the baffle mount I used thicker stock than the rear panel mounts. If you own any combo amps or speaker cabinets in a similar style you should be able to take them apart and work out the rough measurements.
Next, work out how deep the mounts need to be. To do this for the front baffle, consider that the baffle needs to have a spacer added to it, and covered in grille cloth. The the whole baffle is set back a small distance from the front of the cabinet. For the rear panel, consider that it has to be wrapped in tolex and is almost flush with the rear of the cabinet (doesn't have to be flush, but shouldn't be proud of it anyway). Once you have calculated how deep the mounts need to go, mark it out with a pencil so that the mounts can be positioned there.
To attach the mounts to the inside of the cabinet, I used wood screws (glue added at a later stage). Again, be careful with the placement of the screws and make sure they do not intersect where you want the mounting screws for the baffle/rear panel to go. I find with these types of stages in a project build, it's really easy to lose focus and go on autopilot because these mounts will not be visible on the outside. However it is critical to pay attention to what you're doing and consider that what you are doing now may affect later stages of the build. This is why I advocate only using wood screws at this stage until the dry fit of all components can be done - if there is a mistake you can always unscrew and adjust the position. Only glue it down once you are sure.
Photos below show the gluing/clamping of the mounts, as well as how the baffle and the rear panel are attached to them. Hopefully this clears things up. Ignore the fact that I'm gluing up, these photos are not in order and my recommendation is to only glue up when you have all the components fitted. The photos also show where I've positioned my screw holes for the rear panel. These should be self explanatory.
To be continued....