Making 12 same holes on a line...

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I would use a hole saw, drill press, and a straight edge. And of course a good quality ruler (or yard stick), some paper, a pencil, and maybe a calculator or some algebra.

When drilling, make sure that the work piece is clamped firmly to the drill press, and drill small pilot holes first for the hole saw bit to grab (it will be hard to see with the hole saw attached, better to feel it). Pilot holes should be ~2/3 the size of the holesaw holes. You don't need to push hard on the drill, that will just encourage the last layer of plywood to splinter.

The hardest part about drilling nice round holes is finding a hole saw in exactly the right diameter.

For the 95mm holes, you may wish to us a 3 3/4" inch hole saw, that will be 95.25mm. The 71mm holes could be drilled by a 2 7/8" hole saw (73.025mm); that will leave an extra mm all the away around, though (but you may not be able to find anything closer).

Bosch part numbers PC334 and PC278. I think Home Depot carries Bosch.

Make a lot of little squares on the baffle - 95mm./71mm.
This will give you an idea what it looks like. If it's a go then draw X's in the boxes this will be your center. Get a protractor and draw the circles.
If you like what you see then cut the holes I also draw the outer circle along with the speaker cutout circle. After the 'cut' the outer circle is my guide for centering the driver and drilling the mounting screw pilot holes.

That's the business end of a router:
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You can do almost anything with a router (not just round holes), they are very easy to use with template jigs. But a good router and bits will probably set you back more than $200... and the holes won't be any rounder than the ones you'll get with the hole saws ($15 or so). Not to mention that it is harder to learn to use a router than a drill...


I just realized, are you trying to counter-sink the drivers?

If so, the job will be much more difficult. You have really only two options in that case: get a router and get a skilled craftsman to teach you how to use it, OR drill the outer hole to a specified depth using your drill press, and remove the in-between material with hand tools (chisel, brass hammer, files, sandpaper...)

The second way above is much more labour intensive and will not yield as good a result as doing the counter-sinking with a router, but it will be cheaper in terms of tools, and is really the only recommended way (by me) to go if you don't have someone to teach you how to use the router properly. Designing a jig, doing the cuts, blah blah, is NOT something you'll get right (well, before the 10th try) by yourself.

I suppose a third option exists for counter-sinking the drivers, see if you have a friend with a CNC (computerized) milling machine. Then it's all software. :D

Wow, those are pretty precise requirements.

For what it's worth -- not even a professional carpenter will be able to cut holes exactly 71.4mm in diameter. Heck, even clearances inside automotive engines are listed with plus/minus tolerance values. And those parts are built by computers.

You need to decide what reasonable tolerances are for your assembly, or you will never complete it.

For what it's worth, I don't believe for one minute that the human eye can discern down to less than ~0.75mm without another point of reference. What kind of sonic difference could 1mm make?

Hmm. f = Lv, f = 0.001m * 1000000 m / 3600s = 0.27Hz phasing effect? The thickness of your head introduces a far larger error.

OBTW, don't *ever* trust my math skills. For anything. Really.

You mention 6mm which happens to be the wavelength at 20kHz. Is this spacing you specify to prevent comb filtering? Because it is center to center spacing that relates to that.

A 94,4mm housing + 6mm spacing corresponds to a wavelength of about 3.3kHz above which comb filtering may be a problem.

Shouldn't you be placing those tweeters as close together as possible?

Have you read this?

Design issues aside, the Jasper circle jig, a router, and a 1/4" cutting bit is the best way to cut your holes aside from having access to a CNC mill.

If being able to cut holes in 1/16" increments (you could cut a 93.66mm hole or a 95.25mm hole) is not close enough, you could make your own circle cutting jig out of a piece of acrylic or polycarbonate although it would take some trial and error to get the pivot points correct.

Can you provide a picture or link for this driver?
I use one of these to cut holes - it's fully adjustable and cuts to 30mm deep - because it drills a pilot hole, you can just flip the wood over and drill from the other side using the pilot hole as a guide if you need to cut a depth more than 30mm. It'll cut perfect holes up to 220mm through 25mm MDF in less than 1 minute.

It works with a std drill, it's very quick and easy. :)

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