Speakerbuilder's Rabbeting Bit

- Allows for precise flush mounting of tweeters and woofers in a broad range of incremental steps -

One day I was thinking about replacing the SS 9300 tweeters in my Rhyos with Vifa XT25 tweeters.  It would have been a direct replacement if it weren't for the fact the the flange on the XT25 is much thicker than the SS 9300.  I had no simple way of deepening the flange recess with any of my circle cutting jigs since there was no longer a pivot hole.  I had considered using a rabbeting bit but almost all commercial rabbeting bits come with only 7 bushings.  And none of the bushings that came with my bit could cut the proper sized flange recess that I needed.  This got me thinking about a different way to recess drivers.  The Speakerbuilder's Rabbeting Bit (SRB) does not replace a good circle cutting jig like the Jasper Jig.  In fact it requires a circle cutting jig to first cut the through hole.  However, it does allow you do to some things that no circle cutting jig or standard rabbeting bit can do.

The flange width on most loudspeaker drivers averages about .5" (some more, some less).  Many  tweeters and 8"  woofers can be as much as .6".  Many 10 to 12 inch woofers have a flange width of .75" or greater.  Almost all commercial rabbeting bits will only cut a rabbet as wide as .5".  The Speakerbuilder's Rabbeting Bit will cut rabbets from approximately .25" to .73" in 1mm increments on the diameter (.5mm, or .0197", or 1/51" on the radius).

In the past, whenever I've needed to cut a driver recess I've had to make some test cuts on scrap wood just to be sure that I've got everything set up right.  This is most true whenever using a circle cutting jig with a sliding trammel point.  I've spent many frustrating afternoons in the garage loosening the set screw on the trammel point, sliding it in or out by a hair, making another test cut only to discover that my flange recess hole is either too big or too small.  It's often difficult to test for a good fit without being able to drop the woofer or tweeter into the recess in it's proper orientation.  This can't be done until the through hole is cut and once the through hole is cut you can't make any more flange cuts because you've now lost your center point.   The Jasper Jig, though a great tool, does not help much with this situation.  My Jasper Jig is always off by a bit.  I've learned to never trust it completely.  Besides the Jasper Jig is only accurate to a 1/16 of an inch on the diameter.

With the SRB I can cut a through hole big enough to allow the driver to surface mount.  Then I can choose a bushing that I'm sure will be a little too small for a proper flange recess.  Then I can make incrementally larger flange recess cuts until the driver drops into the hole.  Then I can adjust the depth of cut to allow the driver to lay flush with the baffle.  Later on, if I should choose to add in any sort of gasket material I can deepen the recess.  This technique also allows me to veneer an enclosure after I'm sure it's going to fit the design goal.  Trimming the veneer only requires one to use the same bushing used to cut the flange recess.

The Speakerbuilder's Rabbeting bit and 25 bushings on work bench display stand.

Nylon is not the best material for machining but I got it cheap at the local scrap yard.  Delrin would have been a better choice but the Nylon worked out fine.  PS: I make no claims of being a master machinist.

First I cut a bunch of disks of the same diameter.  Then I drilled them out to .5005" so they would have a nice fit on the flanged bearings. 

Once I had a collection of Nylon "doughnuts" of the same diameter, I then made a mandrel out of aluminum so I could start sizing the rings.  Each bushing was mounted in the mandrel and cut down in 1mm diameter increments.

For the smallest bushings, part of the aluminum mandrel had to be cut away.  The smallest bushing is shown above and has a wall thickness of about .5mm.  You can't really tell from the picture, but the Nylon is fairly translucent.

Each bushing is held in place by the same two sealed flanged bearings.  This method allows the me to use just two bearings for all the bushings instead of permanently inserting a bearing into each bushing.  The cutter is a Freud 56-112 slot cutter measuring 2" in diameter and 1/4" thick.  The cutter is held on a Freud 60-102 slot cutter arbor.

Keith Kidder's Audio Site - Home Page

"The way your Dad looked at it, that speaker-builder's rabbeting bit was
your birthright. He'd be damned if any slopes were gonna put their
greasy yella hands on his boy's birthright. So he hid it in the one
place he knew he could hide something. His ass. Five long years, he
wore this speaker-builder's rabbeting bit up his ass. Then he died of
dysentery, he gave me the speaker-builder's rabbeting bit. I hid this
uncomfortable hunk of metal up my ass two years. Then, after seven
years, I was sent home to my family. And now, little man, I give this
speaker-builder's rabbeting bit to you."

NOTE:  The above text does not reflect the opinion of Keith Kidder.