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Thread: Constrained expansion: What happens if I put wood in the end of a steel tube?

  1. #1
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    Constrained expansion: What happens if I put wood in the end of a steel tube?

    I'm working on a project which may be exposed to humidity, and I'm wondering if my use of wood with metal will cause problems. An 8" x 12" piece of wood is to be placed inside a steel frame with no give whatsoever. Will the expansion and contraction of the wood from humidity and heat cause deformation? Is the use of a lacquer or polyurethane coating required, or can I just use an oil or stain? And are some woods particularly ill suited to the task? (For reasons of cost, I was planning on using 1/4" bloodwood laminated with 1/2" baltic birch ply or MDF - while not especially desirable materials, their consistency makes them great for building speakers.)

  2. #2
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    If the metal tube is strong enough to hold the wood when it expands from moisture, the wood will be compressed. When the wood dries out, it will no longer fit the metal tube - it will be loose in the tube.

    If your design depends on the wood being tight in the tube, you've got a problem. No finish that I'm aware of will prevent the wood from absorbing moisture when the unit gets wet (when it rains on it, for example).

    Mike
    Ancora imparo
    Go into the world and do well. But more importantly, go into the world and do good.

  3. #3
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    You'll see expansion of the wood until either the steel frame ruptures (may or may not, depending on its structural inegrity & mass) or the wood fibers crush (as in the eye of an axe), and you'll see contraction such that the wood draws away from the metal frame, leaving a gap around the rim (as in the eye of a loose-handled axe). It's possible, if that wood panel is tightly bonded to the steel frame, that it might crack somewhere in the middle, too - I constrained a piece of oak not much different in size, once, and was rewarded by a good 1/8" crack for my efforts.

    For choice of woods... there are large charts all over the Web listing various woods & their expansion/contraction radially and circumferentially (usually called "tangentially"), movement in those two directions being different in nearly all woods.

    Certain mahoganies are highly prized for their extremely low movment - that makes them exceptionally well-suited for such as casting.

    The better you can seal that wood, the less will be (because the slower will be) its humidity-related movement. If the steel-framed wood is exposed to extremes of temperature, there will be some difference in thermal expansion coefficients, too... but unless you're talking about something LARGE it's going to be pretty negligible.

    If ya really gotta' constrain it, if ya can't come up with any way around it, then try & use a low-movement wood and seal the SNOT out of it, every way you can. Hardening oil, followed by shellac after the oil is fully cured, followed by (at least) several coats of lacquer.

    Just how heavy a frame are we talking about? A steel frame can't be built that allows NO "give", but if it's built really heavily it may allow very little.

    Don't underestimate the power of expanding wood - that expansion was used to split large pieces of granite off quarry walls; they'd drill holes, drive dry wooden wedges into the holes, and pour water on the wedges. SHOOM.
    -- Tim --

  4. #4
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    It's just as Mike said. Now could you put some sort of rubber ring on the wood to take up the movement?
    It could be worse You could be on fire.
    Stupid hurts.

  5. #5
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    I may have overstated the amount of possible moisture exposure - I'm going to try building a pair of speakers and headphones this way. The former will spend all of its' time inside; the latter may end up getting a bit wet, but neither will be left outside. That said, better safe than sorry!

    I'd rather not use a rubber ring for cosmetic reasons - I'm spending a stupid amount of money on these speakers, and I'd like to make them look nice.

    Here's a rather poor rendering of the speaker in question -


    As you can see, it's basically just some 8" x 12" x 1/4" wall steel structural tube with a wood plug in both ends. I can assure you it will look a lot better in real life.

    On a related note, I'd appreciate it if someone could help me figure out an efficient way to build a wood laminate. Aside from a few extremely dense (and, as such, incredibly expensive) hardwoods, the only wood-based materials suited for making speakers are baltic birch ply and MDF. (High density fiberboard is even better, but I don't know where to buy it.) While using a veneer is the obvious solution, getting neat results with all the seams and holes is quite difficult, and using a 1/4" layer of hardwood would still be very economical. I personally think the deep red color of bloodwood would contrast well with the steel - any other suggestions?


    Quote Originally Posted by Tim Hofstetter View Post
    If ya really gotta' constrain it, if ya can't come up with any way around it, then try & use a low-movement wood and seal the SNOT out of it, every way you can. Hardening oil, followed by shellac after the oil is fully cured, followed by (at least) several coats of lacquer.
    Last edited by Joseph Shaul; 12-16-2009 at 04:09 AM.

  6. #6
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    For the speakers, why not just use ply with a face of whatever species you want? Or if you are veneering, use MDF which won't move. Then run a bead of caulk around the inside and any slight movement due to temperature changes won't be an issue.

    You are making steel tube headphones? Sounds uncomfortable.

  7. #7
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    I'd laminate a bloodwood veneer onto an MDF substrate, me. That'd do what you want. Be SURE for SURE for SURE to remove any burrs (from sawing) from the inside of that tubing. You may need to do some work to square it up, too, since it'll be bandsawn only to a squareness of about 1/8" in 12".

    You've actually got a supply of that sorta' tubing? Pretty COOL. Care to divulge? I'd like to build a lathe stand outta' the stuff - I could use two 36" lengths, fill 'em with concrete. Or just one for the headstock, and something much lighter for the tailstock.
    -- Tim --

  8. #8
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    Quote Originally Posted by Matt Meiser View Post
    For the speakers, why not just use ply with a face of whatever species you want? Or if you are veneering, use MDF which won't move. Then run a bead of caulk around the inside and any slight movement due to temperature changes won't be an issue.
    That's a pretty good solution, and what I was originally intending. However, there's a small problem: Bloodwood veneer fades really, really fast. Also, as mentioned earlier, it's very difficult to avoid exposing the underlaying MDF: Most speakers have a recessed shelf to put the frame of the woofer flush against the baffle, and getting it to look right difficult. (If you're wondering, the picture I showed has surface-mounted components only because I'm not very good at using SketchUp )


    Quote Originally Posted by Tim Hofstetter View Post
    You've actually got a supply of that sorta' tubing? Pretty COOL. Care to divulge?

    It's actually surprisingly inexpensive. Considering that a premade speaker box is over $100, spending $45 on steel tube is not a bad deal - especially considering I can easily swap out the front panel for modifications during testing.

    http://www.onlinemetalstore.com/item...omments=&qty=1

    Quote Originally Posted by Tim Hofstetter View Post
    You may need to do some work to square it up, too, since it'll be bandsawn only to a squareness of about 1/8" in 12".
    I hadn't considered that - it's definitely a major issue. How would you suggest I go about this?

  9. #9
    I have many garden tool that have wood inserted into a metal tube.

    They seem to work fine without problems. Same idea but different purpose.

    --- --- they work if I provide the muscle.
    Remember the tea kettle - it is always up to its neck in hot water, yet it
    still sings!

  10. #10
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    The garden tools I have that have a wooden handle inserted into a socket all have a pin through the socket and the wood to keep the handle in the socket. When the handle breaks, it seems to always break at that pin.

    Mike
    Ancora imparo
    Go into the world and do well. But more importantly, go into the world and do good.

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