Potting threads in foam

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#1

I’ve run across a bit of a problem with a project I’m trying to finish up.

It’s a speakerbox, and a very complex one. I’m building it to be as light as possible. Foam core (lowes, the blue stuff) epoxy resin, and 7oz (6k series?) glass cloth.

The baffle if already done, and cut to it’s proper shape. Between the speaker and the baffle is a spacing ring, also made of the foam sandwich.

The Speaker weight is minimal, at around 5lbs. It’s a 12 inch lightweight neo driver.

I need to figure a way to mount the speaker to this baffle. I’m thinking t-nuts potted in with epoxy/milled fibre. I’m just concerned about them pulling through under all the weight and the cabinet itself being bounced around in transit.

Any ideas?

#2

How thick is the foam core? You said it was already made right? You can add more layers to that area to make it stronger, maybe use carbon fiber. Bonding in inserts like that can work as well. Got pics?

#3

Crap, first few posts and it wasn’t made in the right thread.

Foam is .600/inch thick. Parts are already made.

I was thinking of laying the inserts in, and then waxing up a few bolts and potting in more epoxy to make the thread a full inch long. I’m just wondering if the foam will hold them, or if it’ll pull right throu.

#4

Well, you can maybe just add a hold-down ring from solid carbon, with or without inserts in. Bolt from the speaker, through the baffle, and into the ring (or through if you use simple nut/bolt)
Hopefully the load will be distributed evenly through the ring/speaker/baffle not to crush anything. In the end, nothing will pull out.

#5

Has to remain flat on the opposing side of the baffle.

Think I’ll make a quick test piece up to see how much torque a potted tnut can handle.

I just have to compress the paper gasket on the speaker enough to make an airtight seal.

This isn’t any normal speaker. It’s a horn. More info on it here: http://www.billfitzmaurice.com/Jack.html

It’s being built to be a very good sounding, lightweight bass guitar cabinet.

#6

Seems the foam gives out before the glass cracks. I think a full potted thread all the way through is the way to go.

A test is curing as I type this.

Would a fine thread (#10-32) be better then say, a rough thread?

#7

Rivet them bad boys in. Or use a floating nut plate.

#8

That it will. Foam is, well…foam. Not much strength.
How thick again? You should have at least 4-5 threads in the insert for a good hold. 3-4 if it doesn’t need to be a “strong” holddown.

#9

a fine threaded bolt is stronger than a course thread. course thread has less meat. fewer/deeper threads. plus fine thread bolt are more vibration tolerant. also use nylock nuts

#10

Inserts are #10-32, about 1/2 of thread. Made to be potted.

I’ll try to get some pics up.