Designing and Building the Carcase and Chassis
With the electronics squared away in Part 1, it was time to address the packaging of the system. The line array speaker layout dictated a vertical orientation. The dimensions of the Minimus 7 speaker cabinet helped to set the dimensions of the final PA enclosure/structure..
Four Minimus 7s stacked on their side are a hair over 18″ high. Front-to-back, a Minimus 7 is about 4-1/4″ deep. For strength, I selected to use 1″ dimensional lumber (which finishes out at 3/4″ thick).
1″x5″ dimensional lumber was perfect being 3/4″x4-1/2″ finished dimension. Just a little wider than the speaker depth, the size left room for wiring and gave some protection to the speakers.
Because I don’t own a table saw, but do own a good power miter saw, I set out to design within the width constraints of the lumber using only cross cuts with the chop saw. By using the same 1×5 lumber, I could cut three identical pieces 7-1/4″ long to use as the top, bottom, and front of the electronics enclosure at the bottom of the stack of speakers it would finish out with a total height of exactly 24 inches. The enclosure is cozy being 4-1/2 inches high and 3-3/4″ deep, but with 7-1/4″ in width, there’s room to move things around to make it all fit.
When you add it up and figure the kerf (kerf: the width of the cut made by the saw blade. iow-waste) widths from the cuts, you end up needing a single piece of lumber six feet long. Just so happen that Home Depot carries some superb, knot-free, straight-grained wood (Southern Star brand), in a 1×5 dimension in six foot lengths that, with sales tax, costs a hair over $10.
Because the three short pieces joint together to make a critical structural element and will dictate the “squareness” of the final assembly, it was critical that these elements be as exactly the same size as is possible. Sitting at the bottom of the total structure, any errors or inconsistencies in the lengths of these three pieces would propagate and expand over the total 24″ height of the structure.
Knowing this, I resorted to the furniture-maker’s technique of laying out the cut lines and used a sharp utility knife instead of a pencil. The unit had to be as close to perfectly square and plumb as I could get it and a scribed line with a sharp utility knife is about 100 times narrower than a blunt pencil point line.
Once all the pieces were cut to correct lengths, there was a single waste piece of the original six foot board. It’s about 1-1/2″ wide.
I’m not a big fan of butt-jointed lumber and screws, so I resorted to my favorite system for butt-jointed construction and that is biscuit, or plate joinery. Using a Skil biscuit joiner, I cut all the biscuit slots and assembled the top, bottom, and front of the enclosure with woodworker’s aliphatic resin glue.
Because of the close quarters in the enclosure for the electronics I didn’t complete the assembly with the side pieces. I needed the access from the sides to visually confirm locations of bits of hardware and to get at fasteners that wouldn’t be conveniently accessible after final glue-up.
As the pictures illustrate, I was able to easily mount the power transformer within the enclosure and cut a hole in one of the side pieces for the 12 volt DC fan that extracts hot air from the enclosure during operation.
The metal chassis was formed from some light-gauge galvanized steel sheet that I picked up in Houston, Texas nearly a dozen years ago when my new rear-projection television was too close for my comfort to a corner fireplace in our small apartment and I concluded I needed to apply some sort of heat shield to the side of the television. Well, Houston is a thousand miles away and that television is gone as well, but I kept that sheet of steel as it was pristine and unmolested. Besides, being so easy to store, there was hardly and penalty for doing so.
The chassis is a simple “L” shape that is high/wide enough to cover the back of the enclosure and provide space for screws to fasten it to the PA carcase, and deep enough to cover the floor of the enclosure.
I created a template out of card stock to finalize the dimensions and work out all of the fastener hole locations and all of the holes needed for jacks, switches, controls, etc. Using that card stock template, I transfered all of the marks onto the steel sheet and cut the basic shape with a pair of aviation shears.
All edges were then dressed with a flat file to remove burrs and soften sharp edges. Outside corners were rounded as well and once the piece was safe to handle, it was bent over a piece of wood into the final “L” shape.
Finally, using a center punch, drills, and a Harbor Freight copy of a Dremel tool I created all of the necessary holes as per the template. For the sake of visibility, not aesthetics, I gave the back face of the chassis a coat of gloss white enamel from a rattle-can.
With all of the basic structural elements created it was time to begin final assembly. This I will cover in the final installment, Part 3. Coming soon.
Brett C. Cammack
Sawgrass Data Systems