Cutting Irregular Recesses for Truncated Frame Drivers

A number of loudspeaker drivers have irregular shapes, making them difficult to flush-mount onto a baffle. Thankfully, the majority of these have a shape that is based upon a circle, with two or four truncated edges. The method presented here can handle truncated circles, but cannot be used with drivers having oval, elliptical, or irregular shapes. For the more involved steps required to do these other shapes, visit http://www.exquisiteaudio.com/baffle.html (NOTE February2018: Link appears to be obsolete.)

The photo below shows a flush mounted Peerless driver in a Linkwitz Pluto loudspeaker. Using a router pattern (or "template" if you prefer) allows the driver recess to be cut into a typical rectangular baffle or into one of almost any outside shape, including this unusual loudspeaker.

Step by step, with pictures...

Measure the round diameter of the driver or use the published specifications. Unlike the Peerless driver shown in the finished example above, this Aurum Cantus driver has four truncated edges. Only the corners retain the original circular shape.

Find some 1/4" thick hardboard. Cut a square piece that is the driver diameter plus 6". This piece, made for a ~6" diameter driver, was 12 inches square. Find the center by drawing diagonal lines from the opposite corners. Drill a hole in the center for a pin that fits your router's circle jig.

Mount the hardboard to a sacrificial backer board for routing. Use counter-sunk screws or double-stick tape. If you don't use a backer board, you'll cut into your table or stand.

The Jasper circle jig uses 1/8" diameter pins as a pivot, and that's shown installed in the picture below. The hole for the pin goes through the hardboard into the backer board to leave ~1/4" proud.

Route a circular opening to hold the driver using a 1/4" spiral upcut bit. I took two shallow passes here in 1/4" hardboard to cut through. This is a very familiar process for speaker builders.

Elevate the driver a bit off the table surface (using cardboard shims, etc) so that it doesn't rest on the surround. Lower the hardboard with the round hole over it. Fit, then glue strips of hardboard so that the strips touch the flats on the driver. Optional thin cardboard or tape shims placed between the driver flats and the hardboard pieces will provide a little extra clearance if you want it. When the glue dries, lift the assembly off the driver. The opening now reproduces its shape. Unfortunately this thin template can't be used directly on the baffle in most cases, however this will become a temporary pattern to make the final pattern.

The temporary pattern is now ready to be used to make the final pattern using a small 1/2" cutting depth pattern bit. This particular bit must have its bearing mounted as shown to work. An end-mounted bearing typical in most flush-trim bits will not work.

The bit pictured here is an MCLS #16509 Katana pattern / flush trim bit.

With the bit mounted in the router, adjust the cutting depth so that the ball bearing straddles both upper and lower pieces of hardboard.

Attach the temporary pattern to the final pattern material smooth side up. I used flush-mounted deck screws to clamp the pieces together, but I suspect double-stick tape would work OK also. I used a scrap piece of oak-veneered MDF in the pictures below for the final template material.

Place this assembly onto a backer board so that you don't cut through into your work surface. Use screws or double-stick tape to make sure that nothing can shift.

Start routing well inside the opening to stabilize the router, and then carefully move the spinning bit to an edge where its bearing will make contact. Use the bearing pressing against the pattern edge to guide the cut around the periphery. The cut will be about 3/8" deep in the work material - deep enough that the temporary pattern isn't needed after the first pass.

TIP: Route away some extra pattern material to enlarge the width of the cutter's path. Do this while the temporary pattern is still in place to reduce chances for an accidental cut outside of the desired shape. The initial pass of the router around the edge leaves a narrow path that may bind on the guide bearing when cutting deeper in the next step. It's not necessary to route away the whole center (although you could), but just enough to widen the path. Pay attention to the direction of router rotation vs. work direction when doing that or the router can grab and yank itself dangerously.

Once the first pass is made, the routed shape should be deep enough (~3/8") to allow deeper cutting without needing the temporary pattern anymore. After removing the temporary template, adjust the router bit to cut deeper for the final cut-through passes. Be sure that the bit's guide bearing rides on the already-cut edges!

Here's the finished 3/4" thick template after detaching it from the sacrificial backer board. Use this template to mill recesses into the actual baffle surfaces. You can use the same router bit.

This final template, or pattern, will be clamped to the actual baffle material, and used to cut the recess for the driver. The same router bit used in the previous steps can be used to do this.

Using the Pattern Template

For my Linkwitz Pluto project, you can see the routed recess made with its final pattern template in the next two photos. It required only two flats to fit the driver, but the temporary pattern (not shown) was made just as described above. Set the depth of cut to allow the driver flange to sit flush with the baffle surface. After cutting the path next to the pattern edge, remove a little more of the center material away. You want enough removed to ensure that the flange area is cleaned up.

The round through-opening in the center will be cut later using the traditional method of cutting driver holes with a circle jig. You can spot the pre-drilled 1/8" hole for the Jasper jig in the photo below.

It's important that the template is firmly attached to the baffle material when routing. Counter-sunk screws or double-stick tape can be used to do that. For the Pluto, I used screws located outside the final baffle area. I routed the recess first on a larger piece of wood, then cut it down to the final desired shape. The screw holes were located in the waste. With double-stick tape, there will be no holes to worry about although I sometimes worry about the strength of the bond.

The photo at the top of the page shows the completed Pluto baffle with the driver installed. It's a nice, accurate fit.

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Created 7/20/2014
© 2014 William Schneider - All Rights Reserved