For CNC-cut cabs - Can countersunk Bolt Circle Diameter be a perfect fit?

oidua5

Member
2016-05-18 2:45 am
Hello! Just getting started on my first DIY speaker project, based on the Auratone-style cab in this schematic...

https://imgur.com/HlUBacg

The material will be Baltic Birch plywood. I'm planning to cut the baffle hole using a CNC machine and then make a second cut around that in order to countersink the speaker (or the "Mounting Hole BCD") into the panel. The possibility is there to make BCD fit perfectly, even though it is not perfectly round...

Canada's largest online supplier of speakers by Celestion, Eminence, B&C, Quam and McBride.

Would it be safe/advisable to make the BCD a perfect/tight fit, or would it be better to leave a 1-2mm gap to allow for part size variation and/or to allow the cab to expand/contract from humidity?

Any suggestions greatly appreciated. Thanks!
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
put the driver face down on a scanner with rulers (or a square) on x and y axes. Insert the image into a cad program and trace the outline after making sure it is square and to scale.

For one or two cabs, probably easier to make a template with registration marks and do it with a freehand router..

You could also ask the mfg for a cad file
 
The material will be Baltic Birch plywood. I'm planning to cut the baffle hole using a CNC machine and then make a second cut around that in order to countersink the speaker (or the "Mounting Hole BCD") into the panel. The possibility is there to make BCD fit perfectly, even though it is not perfectly round...
Would it be safe/advisable to make the BCD a perfect/tight fit, or would it be better to leave a 1-2mm gap to allow for part size variation and/or to allow the cab to expand/contract from humidity?

As long as you are going to all the trouble to set up a CNC cutting file, you might think about cutting the entire project out with the CNC router. Plywood does shrink and expand a little and the plywood edges are smooth, but not all that smooth. Also, router bits may not be exactly 0.25 inches in diameter. Leave a couple of mm slop and you will be much happier because cutting the hole a second time with CNC, especially after removing it from the machine is a bugger.

And while you are at it, drill the 4 holes for mounting the speaker, and cut out the panels and use a 45 degree bevel bit to cut the bevels. The front and rear edges of the sidewalls can also be recessed to hold those panels and drill holes in the front and back panel for their mounting screws. You can even bevel the front panel edges to hide the raw edges.

This would be a great project to learn to use a CNC machine and the software that controls it.

However, if you just want to make a round hole int he front panel, it is a lot faster to use a hand router and trammel.
 

Gnobuddy

Member
2016-03-01 4:10 pm
It should be an engineering truism that perfect fits aren't ever perfect. :D

I wouldn't trust the cheap stamped-steel baskets in those speakers to be all the exact same size, never mind variations in the mounting hole locations, the thickness of the paint coating on the basket, and the fact that wood (even plywood) moves a little with age and humidity.

Add in the fact that the material will relax and change shape a bit after the cutting forces from the CNC machining are gone, and the internal strains and stresses find a new equilibrium. I recently sawed some solid pine planks to length to build a shelf, and was shocked when the formerly flat planks both developed a substantial amount of twist, just from my having cut them to a shorter length.

Definitely leave some tolerance - there is nothing to be lost, and it will avoid a lot of unnecessary headaches!

-Gnobuddy
 

Gnobuddy

Member
2016-03-01 4:10 pm
I think that's called an interference fit.
My machine-shop skills are fairly rudimentary, but years ago I took a short machine-shop course in college so I could make bits and pieces for my actual college projects. The machine-shop instructor showed us how to make interference fits: machine a precise hole one-thousandths of an inch (this was in the USA, hence the Imperial units) smaller in diameter than the object to be pressed into it.

In other words, interference fits are not perfect, but rather, one thousandths of an inch off! :D

-Gnobuddy
 
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Unless there's something seriously wrong with the machine itself or the CNC program, the patterns should be as close to "perfect" as you can measure.

BUT -you don't want to know how many times I've relied on manufacturer's engineering drawings for rebating of drivers / hardware terminal plates etc, and had them either not fit at all, or be far too tight to install and remove without damage. I always allow at least .5mm diameter tolerance over drawing or even hand measured dimensions, and test fit in a piece of scrap.

You'd think it should go without saying -but that kind of assumption can be very costly - that it's also critical to confirm actual tool bit diameter. Our own industrial machine has automated tool changer for about a dozen bits, some of which are resharpened several times during their life spans, and corrections for variations for individual bit size or wear on tool bit holder can be made at run time.

As I happen to build a lot of speakers & kits for Fostex drivers with similar pin-cushion shaped frames as on the McBride, I'd long ago taken the time to carefully draft CAD files for the 3 most popular sizes. They've been saved in the CNC's console PC, and can be dropped into virtually any size / shaped baffle with the push of a few buttons. However, there's a good argument that while it can look more professional, rebating such a thin frame as the Fostex and many others may be an unnecessary elaboration.
 
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