Cutting foam.

[Dave Plowman (News)]

I've been driven crazy - or rather more than usual - by a resonance on a pair of pro speakers I've bought. It's a tweeter fault by the frequency - and I actually bought new voice coil assemblies for them with no improvement. What it appears to be is the damping material behind the dome has failed causing the resonance. It is a yellow foam - a segment of a sphere - simply glued to the central pole piece. Roughly 25mm across the flat and about 10mm high. It acts to damp the resonant frequency of the cavity formed by the diaphragm and pole piece. I'm told. ;-) Which is what I'm hearing.

Any clever ideas about how to make a replacement?

I can buy complete new tweeters, but that isn't DIY. And I'm too tight.

[Martin Brown]

The self adhesive acoustic foam sold for damping studios and PC cases carved to roughly match the shape you need.

Or wetted foam deep frozen and turned on a lathe? (behind a shield in case it comes loose enexpectedly!)

[Baz]

Cut it with an electric carving knife and a sanding disc to shape it. Baz

[Jeremy Nicoll - news posts]

What do the manufacturers say?

Why are you considering repair, rather than returning the goods?

[michael adams]

A hot wire is often the best way to cut foam otherwise you can get everything covered in fine dust and bits, as being light it gets everwhere. From memory I think you can rig up electric hot wires with a battery or maybe you can buy them.

Or alternatively if you've got any fret saws or bow saws knocking around you could maybe stretch suitable wire across the jaws of one of those and possibly heat the wire with a blow torch as you go. Not that I've tried it.

michael adams

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[Steve Firth]

Easy to make. Get a length of bicycle brake cable. Unwind a single strand of wire from the cable. Grip each end of the wire with pliers or mole grips. Pull the wire until it stretches slightly and work hardens. You now have a nice straight cutting wire. Clamp in a frame - I used to make them from ply as a "C" shaped frame like a coping saw. Wire up to a 12v power supply preferably with a rheostat to control temperature.

Won't work worth a damn. I have tried it.

Adams mutter, trolling everywhere.

[michael adams]

It probably just takes a bit practice to get the temperature just right so the foam doesn't catch fire etc . After all, its possible to cut foam with a discarded carving knife blade held in a pair of pliers and heated over a gas flame. The only problem is getting all the black gunge off of the kitchen ceiling afterwards.

michael adams

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[polygonum]

A really dumb cluck stupid suggestion. If you had actually checked what type of foam, and that hot wire cutting does not poison the person doing it, it would simply have been bad.

The other day I needed to cut some foam for a cushion. Clamped the slab in a Workmate-like device. And cut along the clamping line. Not quite perfect but perfectly adequate. Possibly entirely useless for the OP's requirement.

[michael adams]

It probably just takes a bit practice to get the temperature just right so the foam doesn't catch fire etc . After all, its possible to cut foam with a discarded carving knife blade held in a pair of pliers and heated over a gas flame. The only problem is getting all the black gunge off of the kitchen ceiling afterwards.

michael adams

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(reposted with format corrected: the 76 character line length was mentioned last week but this was ignored as the machine the post was read on was set to

1. I have three machines on the go one of which, this one is normally, rarely used for posting and its on this one that the line length was wrong )

[Dave Plowman (News)]

They no longer exist. Or rather the complete speaker ones don't - the tweeter one (audax) has re-emerged from the ashes of the original company.

They're also some 30 years old. ;-)

[The Natural Philosopher]

no one is going to die from a widddly bit of PU foam

irement.

>

[michael adams]

As a matter of interest, do you have access to any independentally sourced statistics as to the number of people who have irreperably damaged their health by cutting up the odd piece of foam ? Of whatever composition ?

michael adams

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[John Williamson]

The problem with using a wire and a flame that wire has a very low thermal capacity and a high surface area in relation to its mass, and will be too cold to cut within a couple of seconds of being moved out of the flame. The carving knife blade has many, many times the thermal capacity of the wire, and so will stay hot enough for a minute or so.

[polygonum]

No. And why should I? It is up to you in making your suggestion to identify any such issues.

There is lots of information about the production of cyanide products from polyurethane. One of the reasons that quite a number of hospitals maintain ultra-high dose vitamin B12 (e.g. 5000 mcg hydroxocobalamin) for treating people exposed to combustion products of polyurethane and other substances.

[The Natural Philosopher]

the way to sculpt styrene foam is a soldering iron with some fat copper wore wound to the tip.

[michael adams]

Only if I thought there was any possibility of there being any danger. Which I don't.

That's for house fires and similar where victims will have breathed in large volumes of noxious combustion products within a confined space quite possibly while they were asleep.

I don't condone missaplications of Darwin, and in any case most people cutting up foam will already have passed on their genes. But IMO anyone working on a piece of foam who chooses to breathe in any of the acrid and noxious combustion by-products being produced maybe shouldn't be allowed out on their own or allowed near sharp tools in the first place.

michael adams

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[The Other Mike]

That is the equivalent of a *very* large jar of Marmite

[polygonum]

If I have my decimal point in the right place - around 33 kilograms. :-)

[John Williamson]

Been there, done that. It dunn'arf reek.

It works because there's a constant heat flow from the soldering iron to the wire. What our self confessed troll seems to be suggesting, though, is the use of a flame to heat the wire, then use the residual heat of the wire to carve PU foam.

[michael adams]

Firth said I was a troll.

Nothing to do with me at all. I've not "confessed" to anything.

While possibly wrong headed, my suggestion was made in all sincerity.

The troll in this thread was the person suggesting using a sander which potentially could fill the room with particles within seconds.

michael adams

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