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Double Sides, the facts revealed
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Author:  Tim McKnight [ Sat Jan 19, 2008 1:47 pm ]
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There was a recent thread (on another guitar forum) discussing Double Sides. I made a statement that I have been using double sides on all of my guitars and that [my] Double Sides were much stiffer than [my] single sides of the same thickness.

Someone said "Tim, how can doubled sides of the same thickness as a single, be any stiffer? Lamination doesn't make wood stiffer for the same thickness.

This piqued my interest since the only evidence I had was how the completed Double Side set "felt" in my hands. So I set out to gather some data to either prove or disprove my previous statement.

I had recently re-sawn some Black Walnut sets so I took the cutoff end of one plank and re-sawed 3 consecutive boards from it. The 3 boards were all the exact same width and length with the only variable being the final thickness.



Board #1 was sanded to .090" thickness which is the average thickness of a single solid side.



Boards #2 & #3 were sanded to .045" final thickness.





Then boards #2 & #3 were joined together with structural adhesive and allowed to cure for 24 hours.




The next step was to perform Deflection Testing" of the #1 solid board and of board #2/3 which were now permanently bonded together. As you can see a 5 pound weigh was used to measure the deflection and board #1 measured .470" bend or deflection.



Board #2/3 was placed in the exact same location and the same 5 pound weight was used to record it's deflection of .360" which proves the laminated board is significantly stiffer than a one piece solid board.



The next step was to weigh the two boards. Board #1 weighed in at 117.5 grams while board #2/3 weighed in at 234.5 grams. The mass changed because 17 grams of adhesive was added during the bonding process.





Fellow engineer and guitar builder Brad Way, stopped by just as I was preparing to perform these tests.

Brad was impressed by the test data and the results. Although as Mario pointed out it really doesn't make sense why two thin boards can be stiffer than a single solid board. Brad and I discussed this and the only explanation that we could come up with is that as the adhesive penetrates the wood pores, during the clamping process, the adhesive actually forms tiny I-beams in the wood pores. These micro I-beams are what is adding to the structural strength of the laminated Double Sides.

It was a fun experiment and I thank the person [who asked the question] for challenging me to gather data to support my statement. Even though intuition is a pretty good indicator it is also reassuring to see that the data supports my instincts.
Author:  KenH [ Sat Jan 19, 2008 1:59 pm ]
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Interesting data Tim! Thanks for sharing it. I may have to rethink my stance on laminated sides as a result of this.


One thing that I am curious about is if laminating the sides produce bends that have no ripples or cupping?

Author:  crich [ Sat Jan 19, 2008 2:14 pm ]
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Interesting. Do you bend after or before laminating? Clinton
Author:  Shane Neifer [ Sat Jan 19, 2008 2:20 pm ]
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I could be wrong but I think the lamination thing is that it changes the compression/tension point from the centre of one board to making that point firm and now having that 'slippage' point now on two boards  The same technology exists in laminated beams which are stronger than single member beams.  I am no engineer, just a simple carpenter so I am sure the structural engineer guys can explain this in more technical terms.  It works the same for using two 2x12's nailed together versus a 4x12.  Just the process of nailing them together, if I recall correctly, makes them 'stronger'.

Shane
Author:  Bob Garrish [ Sat Jan 19, 2008 2:23 pm ]
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Did you sand them back to the same thickness. Two 0.045 pieces put together with any glue won't make one 0.090 piece.

To get the 'why', you'd need a third and fourth piece. One slicked with the adhesive on one side then sanded back to 0.090 (taking off only wood), and one 'soaked' in adhesive and then the excess adhesive sanded off both sides (leaving only the 0.090 of wood with whatever soaked into it). That'd tell you if the adhesive line has structural properties, if it's the process of impregnation that makes the wood stronger, or if it's actually just the lamination process itself.

Good on you for doing some real research on the topic! If you ever get the time, I'd LOVE to hear the results of the above (because the 'why' is even more useful than the 'what')
Author:  Tim McKnight [ Sat Jan 19, 2008 2:26 pm ]
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Before
Author:  Tim McKnight [ Sat Jan 19, 2008 2:34 pm ]
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[QUOTE=Bob Garrish] Did you sand them back to the same thickness. Two 0.045 pieces put together with any glue won't make one 0.090 piece.

To get the 'why', you'd need a third and fourth piece. One slicked with the adhesive on one side then sanded back to 0.090 (taking off only wood), and one 'soaked' in adhesive and then the excess adhesive sanded off both sides (leaving only the 0.090 of wood with whatever soaked into it). That'd tell you if the adhesive line has structural properties, if it's the process of impregnation that makes the wood stronger, or if it's actually just the lamination process itself.

Good on you for doing some real research on the topic! If you ever get the time, I'd LOVE to hear the results of the above (because the 'why' is even more useful than the 'what')[/QUOTE]

Good points Bob. No, I did not sand the lamination back. I just measured it (BRRRRRRRRR) and the thickness varies between .0905" - .091".
Author:  Brad Way [ Sat Jan 19, 2008 2:38 pm ]
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Great test Tim. Sounds like Bob has a good idea for round two of testing.
Author:  Bruce Dickey [ Sat Jan 19, 2008 2:44 pm ]
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Author:  Dean [ Sat Jan 19, 2008 2:52 pm ]
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Tim,

Do you the same epoxy to lam your sides?

Dean
Author:  Dean [ Sat Jan 19, 2008 2:54 pm ]
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Where's the edit?
 Too much vino!

Dean
Author:  Rod True [ Sat Jan 19, 2008 3:02 pm ]
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23% stiffer (based on deflection differences)

12% heavier

Not to bad at all I'd say.

Now what is the reason for making double sides Tim? To allow the top and back plates to do all the vibrating (or more of the vibrating)?
Author:  Tim McKnight [ Sat Jan 19, 2008 3:15 pm ]
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There are many benefits:
- Crack resistance is greatly reduced.

- Stiffer sides will support the neck system better.

- I can't put my finger on the tonal advantage but I believe there [may] be one. I don't think the sides play a large role in the sound production of an instrument but if you would compare the tap tone of a double side rimset verses a single ply rimset the DS will ring like a gong.

- I did notice a difference in my DS guitars vs the single sides BUT I did not start using DS until I came back from Ervin's class so there is a LOT of other things that were changed at the same time too ...

Dean, Yes, I used structural epoxy for the sides.
Author:  Bob Garrish [ Sat Jan 19, 2008 3:19 pm ]
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[QUOTE=Tim McKnight] There are many benefits:
- Crack resistance is greatly reduced.
[/QUOTE]

Not a benefit

I'm looking forward to trying these on my next acoustic, IIRC the reason I was first interested is that they hold shape more or less perfectly while not having any distinct disadvantage. That alone helps a lot with repeatability and allows for neater jigs.
Author:  WaddyThomson [ Sat Jan 19, 2008 3:25 pm ]
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I'm getting into this kind of late, but I think, and I'm no expert, that the laminating process generates stiffness based on two pieces of material thet cannot slip against each other.  For instance.  If you took those two boards prior to lamination, and did the test on them together without gluing, then they would be weaker, because they would slide against each other.  When you glue them, you create a non-slip laminate.  That, in itself, makes them stiffer.  I think that's what Shane was saying.  2 2x12's nailed together can't slip against each other.  Also, I think they would be even stiffer had they been bent prior to lamination.  I would bet that you would get something in the 35 to 40 percent stiffer range with the curved surfaces glued.  Mostly because a bent board is stiffer than a straight one.  Does that make sense?
Author:  grumpy [ Sat Jan 19, 2008 4:22 pm ]
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But the solid piece also has no "slip". It is, after all, glued together naturally, as one solid piece... Oh, and yes, I was the one who said it shouldn't be so <g>. But for the epoxy.

I'd point to the adhesive as being the X factor here. Bob's tests would be very nice to see, and answer this final one.

Nicely done test!
Author:  Rick Turner [ Sat Jan 19, 2008 4:28 pm ]
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With laminating like that there's less distortion of wood fibers...less compression on the inside of curves and less stretching on the outside, so the wood is theoretically less compromised.   

This is essentially the WEST system...Wood Epoxy Saturation Technique...that has been so successful in high performance boat building.   WEST-built boats are winners, and the stiffness to weight ratio is much more favorable than old standard fiberglass construction.   I'm telling you, these boat builder guys have a lot to teach us.
Author:  Steve Saville [ Sat Jan 19, 2008 4:44 pm ]
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Tim - Great work that is sparking some thought.
You got the engineer part of me is doing some thinking. I often get in trouble for that.

Here is what I'm thinking.

In wood, as most materials, resistance to bending is due to the fibers that are in tension, or the outside fibers of the bend. The inside fibers are in compression, which adds juts a little, but that addition is relatively small. With one piece of wood, you have one group of outside fibers resisting bending in tension, with the double side, you have two sets. That double side does not act as one board just because it is bonded together.

I would love to this repeated with deflection measured for the 2 .045" boards before you bond them together, individually, then stacked up, and finally bonded together. That would show exactly where the strength is coming from.
If the theory above is correct, the glue will not add significantly to the stiffness.
Author:  Zach Ehley [ Sat Jan 19, 2008 4:49 pm ]
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My enginerdtuition tells me that the added strength of a laminated side
has something to do with the fact that you have two seperate grain
patterns that when bound together are better able to resist bending than
a single grain pattern. Because the glue is at the center of the beam, it
has very little affect on the strength. The center sees no stresses, the top
is in compression and the bottom is in tension.

Another possibility is that you have actually made the board thicker and
that in in self makes it stiffer. You could measure th laminated baord and
do the calcs based on that.

You would really have to do this a bunch of times to see what the average
result is. A sample size of one dosnt really tell you much.
Author:  grumpy [ Sat Jan 19, 2008 5:18 pm ]
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Agreed that the glue line, being in the neutral axis, adds nothing, but the glue saturation can have a effect. The only explanation I can come up with it that the epoxy -in- the outer most parts is tying the wood's fibers together.

where's Larry Davis? He does the acrylic impregnating thing, and he may have data on if/how much that process affects stiffness.

In metals, and other predictable mediums, laminations of the same thickness are not stiff than a solid.
Author:  Billy T [ Sat Jan 19, 2008 6:15 pm ]
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Wow!!!! You just discovered.... PLYWOOD!

   The boards have a lot more stiffness because of resistance to compressive and tensile loads. Bend first and it will be a lot stronger! Common understanding in boat building, plywood is a lot stronger than solid!

It's cool to see it done though!!! Are you going to make a guitar out of some double sides??
Author:  jeffhigh [ Sat Jan 19, 2008 6:23 pm ]
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Ok, ex engineer here too.


Firstly, I would suspect that the saturation of the wood by the epoxy would be the sole significant reason for the increase in bending strength. There would be some increase in crack resistance due to this epoxy saturation and also due to discontinuity of the grain.


The analogy to structural timber beams is not relevant, Timber beams are laminated for these reasons.


- to allow use of smaller higher grade pieces of Timber free from cracks and knots


To avoid the problems of cracking and long times, involved in drying large crossection timber.


As far as the method of testing goes, I would recomment the following modifications


-knife edge supports at the ends (perhaps an inverted steel angle) and maybe a bit longer between.


-loading distributed across the full width of the board rather than at the ends of the dumbell only. At the moment the board is being loaded only on the edges and not across the width.


(perhaps two vertical guides each side at the midpoint with steel bars added between as load)


-Preloading to settle the board on the supports before taking the Deflection zero reading and then apply the test load and measure.


At the moment any "wind" in the board will affect the deflection reading significantly


Jeff
Author:  SniderMike [ Sat Jan 19, 2008 6:47 pm ]
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Tim, if your theory about the epoxy making little I-beams is correct, then
simply pore-filling with epoxy should add significant strength as well,
right?
Author:  Bruce Dickey [ Sun Jan 20, 2008 12:37 am ]
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Not boats or guitars, but sheds.

My students built Gambrel buildings as part of their first year carpentry class.

Doors were cheaper if fabricated onsite.

Typical of most high schools, they were bare minimum, a light 2 by 2 frame covered with the plywood siding of the building. Very Weak construction.

As I studied the matter, we took some scrap OSB, oriented strand board, and screwed and glued it to the interior surface of the door. Bingo. The inner skin combined with the 2 by 2 skeletal frame and outer plywood siding, made it very strong. No more warped doors.

So, I agree that two sides are only approaching a beam type construction. It requires one more layer, much like the doors on our sheds. I can't imagine anyone wanting to thin three pieces to .o30 and laminating those together. But I do think that would be stronger still than two at .045.

I like the demo and what it shows us. It's definitely a lot more work.
Author:  Bruce Dickey [ Sun Jan 20, 2008 12:39 am ]
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Tim, Is that the new sound system over Brad's right shoulder?   
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