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Speaker Foam Cutting

Discussion in 'd.i.y.' started by JemHayward, Mar 25, 2020 at 5:50 PM.

  1. JemHayward

    JemHayward pfm Member

    I've got some 15mm thick speaker grille foam. I now need to cut it to size and the edges will be visible, so it needs to be neat. What is the best method? I'm tempted to measure, flatten with a heavy steel ruler and cur with a stanley knife, but will the flattened edge be straight when it bounces back? I'm worried that if I try to cut it when expanded it might 'drag'...
  2. Vinny

    Vinny pfm Member

    Flattened will produce a rubbish finish.

    If you have plenty to trim off, use what will be the offcut to practise first. If the offcut is going to be thin, tricky to advise, but maybe practise on standard foam????
    Use a steel rule supported on something like wooden blocks a little thinner than the foam, one each end of the foam, the idea being to keep the foam captive but barely compressed. Use a very sharp, long knife, something like a thin carving knife.

    Foam will also cut surprisingly cleanly with a tenon or dovetail saw, if they are sharp, but probably not good enough for your use.
  3. Craig B

    Craig B Re:trophile

    Hot wire cutting of uncompressed foam is the method, hence, very few outfits making these anymore, as the fumes are highly toxic.
  4. juz400

    juz400 pfm Member

    Hmm, this might need a bit of practice first!
    I have only cut foam to go inside my Subwoofers but I found the best result was using a freshly sharpened carving knife, the type you would use on the Sunday Roast
    I didnt squash the stuff, gently seemed to work well as the knife did the cutting without any pressure
    Im sure others here can offer better advice though!
  5. JemHayward

    JemHayward pfm Member

    I did wonder about dipping it in water, freezing it, and then cutting when frozen, but it won't fit in the freezer flat...

    I think maybe I'll make a jig to hold the whole sheet under very sight compression and use double sided tape to stabilise. It's quite long winded, but I do seem to have a lot of time on my hands at the moment...
  6. Craig B

    Craig B Re:trophile

    Cold cut with a bread knife is good enough for speaker stuffing and furniture reupholstery work, but speaker grilles are a fine art.
  7. sq225917

    sq225917 situation engineer

    Hot wire against ceramic straight edge.
  8. Tony L

    Tony L Administrator

    Whilst you’ll never get a truly professional finish I understand an electric carving knife is about the best DIY tool for this job. Don’t compress the foam at all and use an accurate straight-edge and apparently you can get reasonable results.
  9. peterm

    peterm pfm Member

    As Tony says an electric carving knife is the tool for the job.

    The foam will distort if you cut it with a standard knife, even a very sharp one, the blade drags the foam out of shape as you cut it.
    An electric carving knife has 2 blades that oscillate in opposite directions so one blade pushes the foam one way whilst the other one drags it in the opposite direction and cancels out any distortion of the foam.

    I used this method to cut 15 mm foam to size for SBL grilles a few years ago very successfully.

    Oh, and be careful! Electric carving knives can be very dangerous!
    Last edited: Mar 25, 2020 at 8:33 PM
  10. awkwardbydesign

    awkwardbydesign Officially Awesome

    I would suggest having the cut line on the foam on the edge of a straight board, maybe 18mm thick, so the electric carving knife can cut all the way through. Mine doesn't cut cleanly all the way to the tip, so this not only acts as a cutting guide but also avoids using the end of the blade. Also lightly rest a light board or straight edge on top., to stop the foam moving. And do it very gently, whatever you use to cut it. Practice on a spare bit if possible first.
  11. peterm

    peterm pfm Member


    Now you've mentioned it that's how I did it, I seem to remember using a broad steel ruler as my straight edge on top with a board underneath to stop the foam wobbling around. It worked very well!
  12. awkwardbydesign

    awkwardbydesign Officially Awesome

    Smoothly and gently are the watchwords. Like cutting soft bread without squashing it, or wandering offline. Something my wife can never manage. Oh, and sharp!
  13. YNWOAN

    YNWOAN 100% Analogue

    I’ve cut foam neatly by resting a steel ruler on the surface (or other straight edge), don’t compress the foam, just rest it on top. Then use a new craft knife, the type where you can extend the blade and snap of sections to keep it sharp (sold at DIY shops). Extend about an inch of blade and take long gentle cuts letting the knife do the work. Don’t try to cut all the way through with the first cut, cut three or four times and gradually cut through the foam - this results in a very clean cut.
  14. JemHayward

    JemHayward pfm Member

    As I don't have an electric carving knife, this sounds like the best available method... I think I may try stabilising with double sided tape as well.
  15. YNWOAN

    YNWOAN 100% Analogue

    If you use a really sharp knife you don’t need to, but can’t do any harm I guess.
  16. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    I use my band saw and a slow feed rate, if you push to fast it gets sucked into the machine.

  17. a.palfreyman

    a.palfreyman pfm Member

    I now have an image in my head of a hungry bandsaw scoffing foam sheets!
  18. awkwardbydesign

    awkwardbydesign Officially Awesome

    Years I watched some "egg crate" acoustic foam being cut for me. Two big vertical rollers with opposing "hills and dales" which distorted the foam as it moved past the long vertical bandsaw. It all sprang back into shape on the way out, giving two inerlocking sheets with the original flat outer surfaces. Once seen, never forgotten.
  19. retrospective

    retrospective pfm Member

    Just done this a couple of days ago..steel ruler and old skool razor blade
  20. Mr Pig

    Mr Pig ^'- -'^

    Any large print company will have a large guillotine. Not tried it but I suspect one of those would do a good job of cutting foam.

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