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Old 24th November 2011, 06:40 PM   #1
diyAudio Member
Join Date: Nov 2011
Default Making holes...speaker jigs

I've been trying to figure out how exactly to do the countersinks.

I have this jig

I'm looking at some diagrams for the box I need to make.

First off, how much easier life would be with the Metric system

I need to make 3.93" bore with a 3.125" through hole and a .18" countersink.

So when I look at fractional conversion charts my only option is 3 15/16's which is 3.94 when rounded up.

I assume I do the 3 15/16's hole with the depth set at 3/16's or 11/64's?

Then do a through hole with it set to 3 1/8's inches.


The directions are in decimal inches but can I just make the assumption that I pick the closest fractional and have it work?
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Old 24th November 2011, 07:21 PM   #2
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Join Date: Oct 2006
Location: Charlotte, NC
Essentially, yes, you are correct. What I do is always start with a test piece and verify the hole I need. Also, you have two choices when using the circle jig (I use this exact model).

One; when cutting the hole pull the router to the outside of the radius, getting just a smidge more wider of a hole.

Two; push the router into the center of the radius, getting a smidge less wider of a hole.

Either of these options will usually give you a hole to match your metric needs to reasonable accuracy, such that visually the end product looks great.
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Old 29th November 2011, 11:36 PM   #3
pski is offline pski  United States
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Join Date: May 2011
Location: Flatrock Community, GA, USA
Here's what I do:

1. Drill the center hole
2. Set the counter sink depth and do a test on a scrap large enough to hold the whole router plate. Dig a few from the edge so you can put the driver edge into the test cuts.
3. Set the nearest larger 1/16" diameter with the center pivot and go round.
4. Subtract 1/4" by carefully moving the center pivot.
5. Repeat until you've cut the counter-sink which is the smallest 1/16" indicated.

If you have a doubt, cut a paper template by using a good (screw) compass and an accurate scale. This will assure you that the outside and inside of the driver (countersink) will fit the hole you are going to cut.

6. Drill through the baffle AND the base of the bench (I use a scrap of MDF between two sawhorses.)
7. Set the router to -1/4" from the last turn and set the plunge to about 1/2 the baffle thickness (or full deep if you have a really thick baffle) and cut a circle to begin removing the center of the hole.
8. Turn the baffle over and put the drill bit through the baffle and the bench. Clamp the baffle to the bench.
9. Cut the circle from the "back" of the baffle.

I do this because the drill bit is not as large in diameter as the peg included with the jig. Using the drill bit to center the baffle and bench (to clamp) and then using the peg to cut the hole makes the router cut from the back of the baffle line up better with the cut we started from the front.

Originally, I tried using the drill bit as the peg in the jig but that allowed the router to drift enough that the cuts from the front and back did not line up well.


Of course, you would want to do both baffles at the same time, cut for cut.

There's also no substitute for a cheap digital caliper. Even higher priced drivers may not be the dimensions on their spec pages.

Last edited by pski; 29th November 2011 at 11:44 PM.
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Old 2nd December 2011, 04:35 AM   #4
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Join Date: Nov 2011
Thanks for the tips, going to work on it this weekend since the kit showed up.
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Old 3rd December 2011, 11:39 AM   #5
dangus is offline dangus  Canada
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Join Date: May 2004
Location: Vancouver Island
I screw a piece of scrap MDF or plywood to the back of the baffle, making sure that the screws are safely away from the cutting area. That will keep the centre cutout in place and prevent something unpleasant from happening on the final cut. And if the hole turns out to be a hair too small when you trial fit the driver, you can screw the cutout back in place and take another cut with the router.

It's best to measure the driver to get the cutout diameter rather than trust the spec sheet. Those are often too generous, which doesn't leave much surface to seal or anchor T-nuts.
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Old 3rd December 2011, 12:43 PM   #6
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Join Date: Apr 2009
Location: Farmington Hills, MI USA
In one case I had a hole to cut for a metric tweeter that was ammost exactly halfway between the available sizes on my Jasper jig. I wanted a close fit since these are exposed tweeters so I drilled an additional 1/8" hole in the jig on a drill press in the middle of the two straddling holes and got a great outcome.
Kevin(ahcc20)...I'd rather have a bottle in front of me than a frontal lobotomy!
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