7. The outer edges of the...
7. The outer edges of the MDF fence and mounting ring are notched with a rabbeting router bit for maximum contact area and strength for the top of our enclosure. The ring is securely held in place with four stilts and superglue.
RING BEARER
Now that the sides of our enclosure are in place, the speaker/sub-mounting ring needs to be made. Although many installers still cut their rings using a jigsaw, I made the 1/2"-thick MDF rings using the "Perfect Circle" router jig. This circle-cutting jig is one of my favorite tools, mated to my Porter Cable 690 plunge router with a 1/4" solid carbide spiral bit, to cut perfectly round discs and holes. I highly recommend it to anyone serious about fabrication. Using 1/2" MDF is fine for speakers up to 8", but 3/4" MDF should be used for subwoofer rings. If the speaker/subwoofer is going to be recessed, two rings will be cut: a lower ring for the woofer to mount to and an upper ring to make the woofer sit flush with the surface of the enclosure. The inner edge of the top ring can be shaped with a "round-over" or 45-degree "chamfer" router bit. Both rings should be cut with the same outer diameter so that they can be easily aligned and bonded with wood glue and brad nails.
8. Templates for additional...
8. Templates for additional ribs are made with cardboard. These ribs allow the top of the enclosure to have a unique shape instead of the usual "ring-stretched-in-fleece" look you would have without them.
The outer edge of the speaker/sub-mounting ring is notched using a rabbeting bit. This notch allows for easy application of our fiberglass-shaping material because it serves as a guide for the stapler to follow. It also allows us to build up our top flush without grinding away the strength where the MDF and fiberglass join. This is especially important for an enclosure like this one in which reinforcing the ring from behind with additional fiberglass or reinforced fillers is difficult. The width and depth of this notch is determined by how thick you plan on making the fiberglass top. For a simple single subwoofer enclosure, a 1/4" wide x 1/4" deep rabbet should be sufficient, but beefier enclosures will require a larger notch. Rabbeting bits are dangerous, and the rabbet should be cut in 1/8" increments for safety. For those of you without access to a router table, a rough notch can be cut into the edge with the die grinder/24-grit Roloc disc, Dremel tool with sanding drum, or the hard way with a chisel/file/rasp. The sharp edges of the MDF are knocked down with the Roloc grinding disc to create a smooth transition where the fiberglass and MDF meet.
Once we have our mounting ring fabricated, it needs to be put in place. Before deciding on a final mounting position, make sure to take into account factors such as speaker magnet clearance and the material used to cover the top. Some vinyl and suede-like materials can only stretch a certain amount, making curvy shapes difficult to cover without the help of a professional upholsterer. If you plan to cover the enclosure with a stretchable carpet or paint, you can make the front of the enclosure almost any shape you want. Make at least four blocks or "stilts" with 1/2" or 3/4" MDF to sturdily hold the ring in position. Grinding or sanding the ends of each stilt to sit flat with the joining surface and having at least one of the stilts braced against the enclosure wall will make everything more solid. I highly recommend using superglue to hold the stilts to the shell as well as to the MDF ring. Thick, commercial or hobby grade superglue (not the stuff at Home Depot) with an aerosol accelerator creates a quick and amazingly strong bond, especially when compared to hot glue and other adhesives. The last thing you want is the ring moving or collapsing when you apply fiberglass to the top. Make sure that the ring is mounted exactly where you want it before moving to the next step. Mounting a single ring is relatively simple, but maintaining symmetry in an enclosure with multiple rings requires time, patience and a lot of measuring.
RIBBING
For those of you who want extra-credit points by making an enclosure that has a nice shape to it (rather than the typical "ring-stretched-in-fleece" look), ribs can help contour the top. This is where style and creativity come into play. The 1/2" MDF ribs connect to the ring and side walls and are cut with a jigsaw/bandsaw. Using cardboard patterns make this process a lot easier. So they can be superglued securely, the ends of each rib should be cut and sanded to meet nice and flat with the joining surface. They should be placed so that the fiberglass-shaping material can be stretched smoothly over the top. Deciding where to place the ribs and how many to make comes with experience. Although the layout of the ribs doesn't need to be perfect, the closer your framework is to the intended shape, the less shaping and sanding you will have to do later. Once all the ribs are in place, the framing is finished and our multi-composite enclosure is ready for the next stage.Next time, we'll fiberglass and shape the top of our composite enclosure
TOOLS AND SUPPLIES
If you don't have these tools, you can get them at a home supply store (or find someone nice enough to let you use theirs):
·Jigsaw/Bandsaw with metal-cutting blade
·Die grinder with cut-off wheel
·Angle die grinder
·Brad nailer
·Porter Cable 690 series plunge router
·Wood glue
·Permanent markers
Automotive paint supply store:·3M 2" 24-grit Roloc grinding disc and holder
·Fiberglass-reinforced body fillers (Duraglas [USC] or Everglass [Evercoat])
·Body filler spreaders
·Latex gloves
The "Perfect Circle" router jig, high-quality router bits and commercial grade superglue/accelerator are available at www.mobilesolutions-usa.com, 206.853.0768.
As the application R&D supervisor at Alpine Electronics from 1995 to 2003, Chris Yato was in charge of creating and managing numerous demo vehicles that served as centerpieces at CES. All of them have been featured in this magazine. He is currently the special projects manager/fabrication trainer for Competition Soundworks in California. E-mail him at chris@teamcsw.com.