Brines Acoustics

   

3/4"/18mm Plywood

Construction Details

Last Modified 7/6/13

NOTE: This is the second of my three construction methods. This method may be applied to plans designed for the original MDF construction. The only consideration is that the panel stiffeners should be made from quality plywood, not MDF. Plans drawn to this second method my be constructed using my current thin-wall construction method.

Materials:

The preferred material is 18mm Baltic birch plywood. Real 18mm Baltic birch plywood has 13 plies. Almost all Baltic birch that is readily available is shop grade – BB or BC – not the finest finish. It is easily paintable, but I prefer to use veneer. Raw Baltic birch can be stained, but again, it's not the finest of finishes.

18mm Baltic birch is hard to get out here in central Arkansas. I can get all of the 12mm I can stand but not 18mm. A very good substitute that just became available is TigerPly©. TigerPly© is a high quality Chinese hardwood plywood that comes in 18mm 13 plies, although the finish plys are paper thin.  This product is being used by local cabinet makers as a superior substitute for 18mm China birch, which I would not use on a bet. TigerPly© is generally superior to cabinet grade birch plywood and much less expensive. If you are dead set on using “Made in USA” plywood, cabinet grade 3/4” 5(7)ply birch or maple plywood will work just fine.

Then there is MDF. MDF usually comes in 18mm, although occasionally you may come across 3/4”. Measure the thickness carefully. It could be 19mm and then again, you may find 18mm, 19mm and 3/4” mixed in the same stack. MDF has about half the stiffness of hardwood plywood but is half again as heavy. There are those who think that they can hear low level noise coming from MDF cabinets. I can't, but who knows. The real downside to MDF is its weight – you will probably need a helper to manhandle whole sheets of  3/4” MDF – and the dust. The MDF dust WILL get everywhere, and don't even think of  machining MDF without a good quality face mask. MDF is usable for these M-series cabinets.

One consideration if using 18mm material: The plans are drawn assuming 3/4”. That means when glued up, the front to back depth and the total height will be 2mm short. Therefore, you should align the back piece flush with the sides and after glue-up, you can trim the excess side panels at the front. Likewise, make the bottom brace flush with the bottom of the side piece. Measure the position of the INSIDE surface of the top from the bottom. After glue-up, the sides will stand 2mm proud of the top. Trim this off. If you use 3/4” material, everything should glue-up flush, but if you are not careful, the front or back will be proud of the sides and you have a problem. It is best to cut the sides 1/8” wider than drawn and the sides, front and back 1/8” longer than drawn if you are using 3/4” material. The you are assured that there will be some material to trim flush.

Cutting the pieces:

I have supplied cutting diagrams, assuming a 4'x8' panel. The pieces are arranged not so much to minimize material usage as to make it convenient to cut the full sized sheet into two or three smaller pieces with a circular saw that can be one-manned through a table saw. Concentrate on making the first cuts square. A little extra length can be trimmed off later. I use a 40-tooth blade in a circular saw that gives reasonably good cuts. I do not cut the large panels to length on the table saw, and a chop saw will not have the depth to crosscut the sides of the larger M-series cabinets.

The side panels can be cut to final width directly from the rough cut panels. It is best to add 1/8” to the width if using 3/4” material and also to the length of the panels. This will allow a bit of overlap that will be trimmed off after glue-up.Cut the remainder of the pieces slightly oversized. You will want to cut the front, back, braces and top to width with a single table saw setup. This will assure that all of the pieces are exactly the same width. Likewise, you will want to cut the braces and top to depth with a single table saw setup. The tops and plinths are made up of doubled pieces. Cut them a little oversized. Glue them together, then cut everything to width and depth.

Braces:

There are two types of braces in the M-series cabinet. There are three window braces – the ones with the large hole in them, and there are four strip braces.

Window Brace

Window Brace

The window braces go at the top and bottom of the driver cutout and at the bottom of the cabinet. The bottom brace is not solid so that access is available to the interior of the cabinet. Also, the bottom brace only has four holes for screw inserts that will attach the cabinet to the plinth. The bottom brace also serves to make the cabinet square.

The hole in the brace can be cut with a router and a circle jig, or even a saber saw. It is not necessary to round the edges of the hole, but it is simply good practice. If you purchase my kits or finished speakers, all of the braces will have a 3/4" bull-nose applied. Since I make a lot of these braces, I build a jig for each different cabinet:

Brace Jig

Brace Cutting Jig

Cut a piece of 3/4" material 2" larger than the brace. Glue on strips of  1/2"x1" material (3/4" material could be used, but I don's like that much cutter exposed -- 1/4" spiral cutters are VERY brittle) that form a loose fit to the finished brace. Cut a hole the required diameter in the center of the jig.

Spireal Bit and Guide

Spiral Bit and Guide

Set up your router table with a 1/4" spiral bit and a guide bushing. Place a brace blank in the jig. Cut out the hole. Note that the resulting hole will be 1/4" undersized. The next step will correct this.

Cutting the Hole

Cutting the Hole

After cutting all of the holes (6), Flip the jig over and place it on top of the brace blank. This allows the bull-nose cutter to run on the rim of the jig. (The first time you use the jig with the bull-nose cutter, it will remove the undercut portion from the jig. No problem. The rim that is left will work for both the straight bit and the bull-nose.

Bull-nose Cutter

Bull-nose Cutter

Cutting the Bull-nose

The strip braces are placed in the bottom half of the cabinet. The three long ones go on the sides and back, the short one on the front above the port cutout. These braces are offset from the center line by 3/4”. This breaks up the panels into uneven segments that are less likely to resonate. They are 3/4"x2" material -- use plywood or real wood for these, MDF is not stiff enough. Again, rounding over the edge of these braces is not required but is good practice. Setting up the router fence for a flush cut  and run the braces through.

Cutting the Bull-nose of Long Braces

Cutting Bull-nose on the Long Braces

Finished Long Braces

Finished Long Braces.

Glue-up:

Before you do anything else, dry fit all of the parts to assure everything is the correct size!

Dry Fitting the Parts

What type/brand glue you use is entirely up to you. If you have experience with a favorite product, use it. For gluing the tops and plinths together, I use Titebond I. Any good carpenter's glue will work fine. For gluing the panels together, I use Locktite PL Premium urethane construction adhesive. (I recommend against “Gorilla Glue”. It is a mess to use and has no advantage over the non-foaming urethane glue.) This product has two advantages: It has an open time of about 1/2-hour and will bridge gaps of up to 3/8”. The downside is that once clamped, the structure must remain untouched for at least 12 hours. Yellow glue has an open time of something like ten minutes and can be unclamped in 1/2 hour. Different glues will require different assembly techniques. The 1/2-hour open time urethane glue  allows permits me to glue up three sides, top and braces in one session. The assembly then sits overnight. With yellow glue, one side, the back, the top and the bottom brace are attached at one time. Then, 1/2 hour later, the front and the other two braces are attached. The advantage of the urethane glue is it only takes 1/2-hour to put the three sides together and you are assured that everything is square. The advantage of yellow glue is that you are ready to move on after 1/2 hours. You pick 'em.

Screws, biscuits, clamps, etc: I use clamps only, but it can be done with fair precision and I have the three sided cabinet up in one session. It takes something like twenty clamps. It takes eight clamps to do two sides, top and bottom.

The first order of business is to glue the long braces to the side, back and front panels. The side and back braces establish the positions of the bottom and middle brace. Make absolutely sure that the bottom brace will be flush with the bottom of the panels. The brace on the front panel ends above the port opening. Make absolutely sure that the top end is in the correct position for the middle brace. Do not forget the thickness of the bottom brace.

If you have large handscrew clamps, clamping the long braces is straight forward. If not, then you will have to use cauls to increase the span of of you bar clamps. Like this:

Using a Caul to Clamp the Long Braces

Using a Caul to Clamp the Long Braces

Assembly:

Once the long braces have been attached, the cabinet can be glued together. If you are using PVA glue, I recommend that you glue up one side and the front. As you have as little at 10 minutes open time, I suggest that you proceed as follows:

1. Apply glue to the edge of the front panel. Place the front panel in position on the side. Glue the top to the side and front. Measure the position of the inside edge from the bottom of the side. Make sure the the front and side are flush. Clamp the top down with four clamps. Alternately, if you are using 18mm material,leave the side slightly proud of the front. If you are using 3/4" material, you can cut the sides slightly large to allow for a slight overlap. The overlap will be trimmed off, preferably with a router and trim bit after glue-up.

2. Glue and clamp the bottom brace (the one with the four holes in it) to the side and front. Be sure that the brace is flush with the side and front.

3. If the front panel is slightly bowed (it will be!), bend it into position at the midpoint and clamp down. Apply at least two more clamps spaced along the front.

4. Glue and clamp the other two braces into position. Wait at least 1/2 hour.

5. Unclamp everything. Glue the back to the previous assembly.

6. You did dry fit everything before you started didn't you?

If you are using PL Premium or other slow setting glue, you can glue up three sides in one pass.:

Three Sides Glued-up

Three sides glued-up in one pass

Once the three sides and internals are glued together, apply the lining. The most efficient lining is 1" acoustic fiberglass. This material comes in 2'x4' panels that are semi-rigid. Measure, cut and piece the fiberglass. Wear vinyl gloves, and if you really worry about such things, a dust mask.

Final Assembly:

1. Glue on the final side. Once this panel is set, you can process with the finish of your choice.

2. Set 1/4x20 threaded inserts into the four holes in the base.

3. Mount the terminal posts/cup of your choice. I use individual 5-way posts mounted centered behind the driver. Attach the internal wire pair of your choice leaving at least 6" free outside the driver hole to allow attachment to the driver. If you particular speakers require a filter, mount the filter as close as practical to the terminals.

4. Mount the supra-baffle to the front with strips of double-sided foam tape. Use a much tape as you think you need, but if you ever intend to remove the supra-baffle, don't use any more that necessary. Alternately, carpet tape will work, but does not provide as much isolation.

5. Wire and mount the driver.

6 Set threaded inserts into the plinth that are compatible with the spikes you have chosen. If you do not want to use spikes, you might consider felt feet that have threaded mounts.

6. Attach the plinth to the cabinet with four 2'x1/4x20 machine screws.

Plug them in and enjoy!

(Be sure to observe the manufacture's break-in instructions for the drivers.)

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