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Thread: Laminating deck beams: Spring back

  1. #1
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    Default Laminating deck beams: Spring back

    I am planning to laminate deck beams for my 32'Friday Island Ferry. Plans call for a 3" camber over a 11'-3" length.
    I was planning to use 1/2" strips of Douglas fir glued with epoxy clamped over a plywood mold.
    My questions are:
    1-Should I anticipate any spring back from this method and build in compensation with the mold?
    2-Are 1/2" strips more likely to spring back than 1/4" or 3/8" laminations?

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    Default Re: Laminating deck beams: Spring back

    You don't say how thick the beams will be, but note that using five layers or more in the lamination will minimize springback, as will thinner layers.
    My deck beams are similar in size to yours, although they have more camber. I used five layers of SYP. Springback was minimal, maybe 1/4 to 3/8" over the length of the longest beam.
    Little enough that I just ignored it. Worked out fine.

    You may want to consider sawing the mold out of a couple of 2 X 12s rather than using ply. It takes a lot of pressure to make the bend. The mold needs to be rigid and strong.

    Last edited by kc8pql; 12-25-2011 at 09:13 AM.

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    Default Re: Laminating deck beams: Spring back

    Using more thinner laminates will reduce springback. When I was laminating frames I used five 5/16" oak laminated for each frame. I had essentially zero springback. JUst make sure you leave them on the jig until the epoxy is fully cured.

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    Default Re: Laminating deck beams: Spring back

    There is a formula for bent lamination springback calculation:

    The basic formula is y=x/n2

    (y is the amount of springback), or the amount x will be reduced after the lamination comes off the mold.

    (x is the vertical distance between the chord and the top of an arc in inches, aka camber)

    (n is the number of laminations)

    (n2 meaning n squared)


    If you look at it, you'll note that the largest factor is the # of laminations. If you were making a 2" high beam, and used 1/2" lams, there's be 4. If you were springing it to a 3" camber, your springback would be 3/16"

    y = 3/(4*4) = 3/16


    If you doubled the # of laminations -

    y = 3/(8*8) = 3/64

    OR... doubling the # of laminations reduced the springback by a factor of 4


    Now... this is a rough formula. The overall length of the beam can play in. The species used can certainly play in. But this'll get you close and give you a sense of how much to expect. Yes... I'd use it to adjust the layout of your forms.
    Last edited by David G; 07-14-2022 at 08:52 PM.
    David G
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    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

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    Default Re: Laminating deck beams: Spring back

    I have found that 6 joints(7 laminations) or more result almost no springback. I used 9 layers of laminations on my deck beams and 7 laminations on my trunk cabin beams and the springback amount between the two was virtually the same- maybe an 1/8" inch in the camber. In both cases the laminations were between 1/4 and 5/16 inch thick

    http://northseabuilder.blogspot.com/

    Richard

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    Default Re: Laminating deck beams: Spring back

    Answers to 1) and 2) are both "yes". The formula is essentially correct. Double the number of lamination layers, cut springback by a factor of four. Ten or eleven layers will give you less than 1% springback.
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    Default Re: Laminating deck beams: Spring back

    Pardon my ignorance, but why not just cut them out of solid wood?

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    Default Re: Laminating deck beams: Spring back

    Quote Originally Posted by Eddiebou View Post
    Pardon my ignorance, but why not just cut them out of solid wood?
    Length, thickness, strength and material waste.

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    Default Re: Laminating deck beams: Spring back

    Thanks all and particularly David G for the helpful formula.

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    Default Re: Laminating deck beams: Spring back

    BUMP: revisiting this one with a couple of questions for clarification and to see if anyone can offer a bit more on how to calculate compensation for the springbuck - other than a trial and error approach......

    Would the amount of spring back not be somewhat dependant on the type of timber used, as well as the type of glue used (to a smaller extent)?

    Does the width of the laminate (across) have any effect on the amount of springback?

    Does the length of the chord not have an effect and, if so, how do you account for it?

    I have 8 deck frames to laminate and each of them has a slightly different chord length due to the curve of the cabin sides as well as a slightly different arc height (x) due to the sheer of the deck and cabin sides.

    I have been aiming for my deck frames to be @36mm deep x 40mm wide.

    I am using Australian Scented Rosewood and I had pre-cut the pieces to allow for two 18mm layers of laminate, before realising that the springbuck would be just too significant.

    I am gluing them up with epoxy.

    I still have enough rosewood that I can get away with taking them down to 12mm layers for three laminates (I hate wasting that 6mm) but I don’t think that I have enough stock available to take them down to anything suitable for any more layers to get the same deck beam strength.

    I’d loose around 4mm on the table saw and thicknesser trying to split the 18mm lengths in half, leaving me with 7mm widths, so 4 laminates would only give me 28mm deep frames (possibly just OK) and I wouldn’t have enough stock to go to 5 layers.


    So....if I stick with three x 12mm laminates can I calculate, rather than guess, how much to compensate across the arc of the frame when I set up the moulds? If so how do I do that before I waste any more timber and epoxy?
    Larks

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  11. #11
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    Default Re: Laminating deck beams: Spring back

    Quote Originally Posted by Larks View Post
    BUMP: revisiting this one with a couple of questions for clarification and to see if anyone can offer a bit more on how to calculate compensation for the springbuck - other than a trial and error approach......

    Would the amount of spring back not be somewhat dependant on the type of timber used, as well as the type of glue used (to a smaller extent)?

    Does the width of the laminate (across) have any effect on the amount of springback?

    Does the length of the chord not have an effect and, if so, how do you account for it?

    I have 8 deck frames to laminate and each of them has a slightly different chord length due to the curve of the cabin sides as well as a slightly different arc height (x) due to the sheer of the deck and cabin sides.

    I have been aiming for my deck frames to be @36mm deep x 40mm wide.

    I am using Australian Scented Rosewood and I had pre-cut the pieces to allow for two 18mm layers of laminate, before realising that the springbuck would be just too significant.

    I am gluing them up with epoxy.

    I still have enough rosewood that I can get away with taking them down to 12mm layers for three laminates (I hate wasting that 6mm) but I don’t think that I have enough stock available to take them down to anything suitable for any more layers to get the same deck beam strength.

    I’d loose around 4mm on the table saw and thicknesser trying to split the 18mm lengths in half, leaving me with 7mm widths, so 4 laminates would only give me 28mm deep frames (possibly just OK) and I wouldn’t have enough stock to go to 5 layers.


    So....if I stick with three x 12mm laminates can I calculate, rather than guess, how much to compensate across the arc of the frame when I set up the moulds? If so how do I do that before I waste any more timber and epoxy?
    Re-read #4. I believe most or all of your questions are answered there...
    David G
    Harbor Woodworks
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    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  12. #12
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    Default Re: Laminating deck beams: Spring back

    Quote Originally Posted by David G View Post
    Re-read #4. I believe most or all of your questions are answered there...
    #4 is what prompted me to ask these questions David, it’s very helpful but it raised more questions than answers for me because of the variations that I mentioned (which you did refer to). I’m also finding the formula/calculation a bit confusing - not being particularly mathematical minded. Using that calculation, does it flow as:

    120mm = 4.72”
    Y = 4.72 /(3x3) or 120 /(3x3)
    Y = 4.72/9 or 120/9
    Y = @1/2” or 13.3 mm
    Larks

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  13. #13
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    Default Re: Laminating deck beams: Spring back

    Quote Originally Posted by Larks View Post
    BUMP: revisiting this one with a couple of questions for clarification and to see if anyone can offer a bit more on how to calculate compensation for the springbuck - other than a trial and error approach......

    Would the amount of spring back not be somewhat dependant on the type of timber used, as well as the type of glue used (to a smaller extent)?

    Does the width of the laminate (across) have any effect on the amount of springback?

    Does the length of the chord not have an effect and, if so, how do you account for it?

    I have 8 deck frames to laminate and each of them has a slightly different chord length due to the curve of the cabin sides as well as a slightly different arc height (x) due to the sheer of the deck and cabin sides.

    I have been aiming for my deck frames to be @36mm deep x 40mm wide.

    I am using Australian Scented Rosewood and I had pre-cut the pieces to allow for two 18mm layers of laminate, before realising that the springbuck would be just too significant.

    I am gluing them up with epoxy.

    I still have enough rosewood that I can get away with taking them down to 12mm layers for three laminates (I hate wasting that 6mm) but I don’t think that I have enough stock available to take them down to anything suitable for any more layers to get the same deck beam strength.

    I’d loose around 4mm on the table saw and thicknesser trying to split the 18mm lengths in half, leaving me with 7mm widths, so 4 laminates would only give me 28mm deep frames (possibly just OK) and I wouldn’t have enough stock to go to 5 layers.


    So....if I stick with three x 12mm laminates can I calculate, rather than guess, how much to compensate across the arc of the frame when I set up the moulds? If so how do I do that before I waste any more timber and epoxy?
    Don't know what saw you have but if you could use Freud D0760A Diablo 7-1/4" x 60-Tooth Ultra Fine Finishing Circular Saw Blade, the kerf is less than 2mm. You might get smooth enough cut that only the exposed surface would need to be surfaced.

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    Default Re: Laminating deck beams: Spring back

    Answers ---

    Quote Originally Posted by Larks View Post
    BUMP: revisiting this one with a couple of questions for clarification and to see if anyone can offer a bit more on how to calculate compensation for the springbuck - other than a trial and error approach......

    Would the amount of spring back not be somewhat dependant on the type of timber used, as well as the type of glue used (to a smaller extent)?Yes, but not enough to change the formula for each species. And not enough that you can't tweak the laminate into place.

    Does the width of the laminate (across) have any effect on the amount of springback? No.

    Does the length of the chord not have an effect and, if so, how do you account for it? Yes, but not enough to matter. Longer = less springback.

    I have 8 deck frames to laminate and each of them has a slightly different chord length due to the curve of the cabin sides as well as a slightly different arc height (x) due to the sheer of the deck and cabin sides. That's a lot of form-building. I'd put on my thinking cap to minimize the effort expended.

    I have been aiming for my deck frames to be @36mm deep x 40mm wide.

    I am using Australian Scented Rosewood and I had pre-cut the pieces to allow for two 18mm layers of laminate, before realising that the springbuck would be just too significant. Yes, the more plies the better in general.

    I am gluing them up with epoxy. That'll be fine. If you want to be sure, use G2 - formulated for 'difficult' woods... but more expensive.

    I still have enough rosewood that I can get away with taking them down to 12mm layers for three laminates (I hate wasting that 6mm) but I don’t think that I have enough stock available to take them down to anything suitable for any more layers to get the same deck beam strength.

    I’d loose around 4mm on the table saw and thicknesser trying to split the 18mm lengths in half, leaving me with 7mm widths, so 4 laminates would only give me 28mm deep frames (possibly just OK) and I wouldn’t have enough stock to go to 5 layers.


    So....if I stick with three x 12mm laminates can I calculate, rather than guess, how much to compensate across the arc of the frame when I set up the moulds? If so how do I do that before I waste any more timber and epoxy?
    Solve for 'y'. In #4, I solved that issue for with two different examples. In the first: y=3.16" " to 1/4" tighter radius than the end result desired. Call it 5 mm. For your beams, you can solve it for yourself with your own relevant numbers.


    Here's a graphic if that helps ---

    David G
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    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

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    Default Re: Laminating deck beams: Spring back

    Quote Originally Posted by Larks View Post
    #4 is what prompted me to ask these questions David, it’s very helpful but it raised more questions than answers for me because of the variations that I mentioned (which you did refer to). I’m also finding the formula/calculation a bit confusing - not being particularly mathematical minded. Using that calculation, does it flow as:

    120mm = 4.72”
    Y = 4.72 /(3x3) or 120 /(3x3)
    Y = 4.72/9 or 120/9
    Y = @1/2” or 13.3 mm
    hey Greg, I’m not sure how you are fixing them to your cabin side… could you set up the mould in place and fasten them to the cabin sides during the glue up? Essentially eliminating any spring back? It will make finishing them a little harder but doable.

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    Default Re: Laminating deck beams: Spring back

    Quote Originally Posted by Geftb View Post
    hey Greg, I’m not sure how you are fixing them to your cabin side… could you set up the mould in place and fasten them to the cabin sides during the glue up? Essentially eliminating any spring back? It will make finishing them a little harder but doable.
    It’s a reasonable suggestion but the pressure from the springbuck would very slightly splay the cabin sides when the moulds are removed and that would collapse the frames in the middle enough to make a mess of the job. I tried “springing” the test frame in to place and I did manage to get it in but it alone splayed the sides ever so slightly and dropped the centre by about 4-5mm.

    That might not seem much but if I can’t get it spot on for each frame the result will be a wavy deck along the centreline.
    Larks

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  17. #17
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    Default Re: Laminating deck beams: Spring back

    Fair enough. Would keeping them clamped dry for a couple of weeks help? Train the wood. Steam them first?

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    Default Re: Laminating deck beams: Spring back

    Anyway, I think I’ve answered my own question - it looks like increasing the height of the arc by 12.5% “seems" to be the optimal compensation for the spring back when using 3 laminates........... if I’ve worked this out correctly.....???? (It’s a clunky way of doing it and can’t seem to be able to post an Excel file but can someone smarter than me possibly check this?)

    Screen Shot 2022-07-15 at 18.41.38.jpg
    Larks

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  19. #19
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    Default Re: Laminating deck beams: Spring back

    Can’t you just buy more Rosewood and have more lams, don’t skimp now. I used that formula (from Gougeon book) to do my beams that had 5 or 6 lams and about 6 to 10 inches of camber as they went along the roof. I don’t think my roof is as wide as yours. I found the amount of spring back to be reasonably accurate. You may find a little more spring back due to over square dimensions of beam, but by the same token they’ll be marginally stiffer just not as much as if the moulding was increased instead, then that encroaches on headroom. Also (obviously timber varies from one lam to the next, but I spose it’ll average out. Not sure which species they used which I’d imagine can affect things too. Another thing too is that a given roofs scantlings and camber will affect the “give” underfoot.
    But basically I think your only option is to up the number of lams if you want to limit springback. I also wonder about “creep” if the glue up is taken off the jig before glue has fully cured.

    2c
    Last edited by Andrew Donald; 07-15-2022 at 12:09 PM.

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    Default Re: Laminating deck beams: Spring back

    You guys do more calculating for a deck beam than a GD noonsite !

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    Default Re: Laminating deck beams: Spring back

    Since the rosewood is probably some pretty dear stuff, I would suggest that you do a experiment using some less expensive wood. Take your best guess as to how many lams, but the more the merrier. Glue up the experimental beam and measure the amount of springback. This real world experiment will tell you more than crunching a whole notebook of numbers. Afterward, you'll have to adjust for wood species but at least you'll have a firm place to begin.

    Jeff

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    Default Re: Laminating deck beams: Spring back

    One might leave the top lam the half inch,softwood, leaving a bit of meat for tuning it up after instalation.

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    Default Re: Laminating deck beams: Spring back

    Your cabin roof beams molded 36mm, sided 40mm seems contrary to my experience. Generally the sided (width) dimension would be narrower than the molded depth of your beams. The upside to this would be more stiffness for a given volume of wood. On Accolade the relationship is roughly molded 3x vs sided 2x.
    I would think that some temporary support of the roof beams at the centreline and at the cabin sides while fitting would resolve the spring back issues until your ply roof sheathing is installed. imho / Jim

    PS. By the way, while I don’t comment much, I very much admire what you are doing with this rebuild! Good onya Greg.
    Last edited by chas; 07-16-2022 at 05:42 PM.

  24. #24
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    Default Re: Laminating deck beams: Spring back

    As an apprentice in 1972 at Vic Franck’s on Lake Union, we were building a 65’ Bill Garden Motoryacht. We milled Luan boards to 3/4” x 6” and smeared them with Weldwood Plastic Resin glue and clamped them over a deck camber mold.
    I can’t remember if there were 4 or 5 lams.
    Then we ran these thru the table saw to probably 1-5/8”, and planed them.
    Last, we put our master deck beam pattern on the pieces and ran them through the shaper so all were exactly the same.
    These were used for every deck beam and cabin beam on the boat.
    The point being, someone in the misty past at Vic Franck’s had made the master beam pattern, and we just pulled the glued laminations off the form, milled and shaped them, so springback was negated. There were a lot of these deck beams in a 65’ motoryacht.

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    Default Re: Laminating deck beams: Spring back

    For anyone still interested in actually wondering about compensating reasonably accurately for the spring-back:

    Using the 12.5% figure that I came up with in post #18 I used some blocks at the back of my frame 2 jig to adjust for that compensation:

    I added 12.5% to the X measure of 120mm - taking it out to 135mm - and redrew the arc using the same process that I used for the original arc.

    I laid up two laminates just using some cheap pine, one on the 12.5% compensated jig and the other as a sort of “control" on a jig that I did for frame #6 without any compensation adjustment:

    The “control” (on the frame 6 jig):

    [IMG]IMG_3764 by Greg Larkin, on Flickr[/IMG]

    [IMG]IMG_3768 by Greg Larkin, on Flickr[/IMG]

    [IMG]IMG_3771 by Greg Larkin, on Flickr[/IMG]

    [IMG]IMG_3772 by Greg Larkin, on Flickr[/IMG]


    Noting that the spring back calculation that David provided: Y=X/n2 (squared) for this frame #6 came out at 11.28mm in my table above
    Last edited by Larks; 07-16-2022 at 08:10 PM.
    Larks

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    Default Re: Laminating deck beams: Spring back

    And with 12.5% compensation (on the frame 2 jig):

    [IMG]IMG_3763 by Greg Larkin, on Flickr[/IMG]

    with the clamps off showing spring back:

    [IMG]IMG_3766 by Greg Larkin, on Flickr[/IMG]


    Lined up with the uncompensated frame 2 jig

    [IMG]IMG_3767 by Greg Larkin, on Flickr[/IMG]

    [IMG]IMG_3775 by Greg Larkin, on Flickr[/IMG]

    [IMG]IMG_3773 by Greg Larkin, on Flickr[/IMG]


    So about 3-4mm shy of the mark on both ends - but this is pretty much ideal really. It means that when the beams are in place and the centre of the arc pushed down to force the ends out to the cabin sides they will have an upward tension rather than a downward tension (as they would if I had to spring them in) providing just that little more resistance to any weight on the deck later on.

    And if you are wondering why the centre of the arc would be pushed down to force the ends out to the cabin sides, I will clamp the frames down to the uncompensated jig to mark the accurate beam widths for the cabin side and arc heights to cut them before fitting them to the boat. With the “hope” that this will give me a pretty accurate and flat centre cabin height line for the length that I want it (it curves down very slightly just forward of the mast).

    There are two main reasons that I am interested in establishing a way of compensation for spring back:

    Firstly because this whole process of rebuilding this boat is about learning - and in particular learning how to do it properly - not fudging or guessing.

    And secondly because, as I said above, I don’t want to waste any more good wood. I can’t buy more Australian scented rosewood, no-one actually sells it (as far as I have been able to find anyway) this lot was gifted to me buy one of the guys who used my shed for his own build and came from his Uncles stash on a farm. I am hoping for more but that could be a year away and that’s too long to wait.
    Larks

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