Glueing harsh surfaces is better? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 12 Old 08-13-2016, 07:42 PM Thread Starter
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Glueing harsh surfaces is better?

Hi, I am building a MDF enclosure subwoofer using 3/4 MDF. This time I will not pretend either using any joinery or screws (which I think screws are not suitable, even those specials).

Why I am doing is to wire brush with a disc drill all the surface that will be glued, I even leave 5mm for the glue at the corner, which is excellent for sealing a subwoofer.

It takes a minute, but was thinking this is going to be so good for gluing pieces together. Just take a look at how it looks after the treatment, clean and harsh, so the glue will hold in a special strong manner. I have to say I have no tested such thing, had no time, but I think I bet for it. I just put enough glue, let it dry a little so it gets viscosity, and put pieces together.

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post #2 of 12 Old 08-13-2016, 08:11 PM
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In my opinion you are making the joint weaker by wire brushing it. MDF is essentially paper, the same paper they make grocery sacks out of and paper glues well without any help.
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post #3 of 12 Old 08-13-2016, 08:33 PM
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I agree with Steve. Wood glues are not designed to fill in gaps/voids.

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post #4 of 12 Old 08-13-2016, 08:35 PM
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glue joint test

Mattias Wandel did a test of glue joints .... some were tightly clamped, others were left a bit loose. If I recall those with a bit of a gap or more loosely clamped performed better....

Look in at around 5:00

If that is true, then by creating more surface area you may be on to something. It is worthy of a simple test.
Glue up two pieces at a right angle and use a game scale to pull down on the horizontal one. Record the test on a video camera to read the scale at the breaking point.


The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #5 of 12 Old 08-14-2016, 01:55 AM Thread Starter
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Those tests Mathias did, don't represent reality, it would be different for a box and also having proper and special care about the gluing process. The trick is to, once applied, let the glue stand few minutes or join but not clamp until few minutes have passed, why? because we should let the material absorbs and saturate of glue, and when we clamp, less glue drip out. My gaps are not deep, if the would I probably will think about industrial glue, which is more dense, but I think regular titebond its ok this time.
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post #6 of 12 Old 08-14-2016, 07:08 AM
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Quote:
Originally Posted by Johanx3 View Post
Those tests Mathias did, don't represent reality, it would be different for a box and also having proper and special care about the gluing process. The trick is to, once applied, let the glue stand few minutes or join but not clamp until few minutes have passed, why? because we should let the material absorbs and saturate of glue, and when we clamp, less glue drip out. My gaps are not deep, if the would I probably will think about industrial glue, which is more dense, but I think regular titebond its ok this time.
You're just worrying about it too much. Many of us have a background in commercial fixtures which uses a lot of MDF. When you are doing it for a living you don't have time to apply glue and let it set up before assembling. You glue the parts and immediately assemble it shooting a few nails in the parts to hold it together until the glue dries. After the glue dries if you try to take the cabinet apart it tears chunks of the MDF. Personally I don't think that would help to let the glue participial set-up however I've never tried it. It's just not the nature of how wood glues were designed to be used.
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post #7 of 12 Old 08-14-2016, 07:54 AM
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do a test

If these are butt joints or 90 degrees, do a test. First sample would be done as normal, no double application, just glue and clamp or nail. Second sample would be wire brush the surface, glue and clamp or nail. Third sample would be wire brush, apply glue, wait a few minutes, apply more glue then clamp or nail.

There are a few woodworkers who do recommend thinning the glue or double applications for end grain applications.

Then apply a measurable force to the end to see at what point the joint fails. Without a test this is all speculation and theory and can be argued until the cat comes home.

Production processes will be different than a small shop so that's not an issue here. If the OP has time to wait for the double application, that's up to him.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

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post #8 of 12 Old 08-14-2016, 02:19 PM
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Doing a test like woodnthings suggested is probably the best idea.

I will say, that, with MDF, none of this might be important. It is possible, if not likely, that the material itself will break about the same time as the glue joint, since MDF is basically a bunch of stuff glued together anyway.

I'd guess that screws would add a lot of strength, only because it would help transfer the load deeper/further into the material. But, that's just a guess, and could be tested using woodnthings method.
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post #9 of 12 Old 08-14-2016, 03:18 PM
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Quote:
Originally Posted by Johanx3 View Post
Those tests Mathias did, don't represent reality, it would be different for a box and also having proper and special care about the gluing process. The trick is to, once applied, let the glue stand few minutes or join but not clamp until few minutes have passed, why? because we should let the material absorbs and saturate of glue, and when we clamp, less glue drip out. My gaps are not deep, if the would I probably will think about industrial glue, which is more dense, but I think regular titebond its ok this time.
Err, how don't those tests represent reality? We're they done in the matrix? His testing methodology took multiple styles of gluing and compared them directly to each other by breaking force, that's a pretty solid testing method.

I'm with the camp that doesn't see any benefit to the wire brush. Mdf has always glued up just fine for me with no surface prep, and the material has always failed before the glue did. Plus, the wire brush has the potential to introduce contaminates, like oils in the wire from the manufacturing process

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post #10 of 12 Old 08-15-2016, 05:56 AM Thread Starter
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I have been thinking on this, and I think that at least on my application, its not as simple as doing any test mentioned before, because we are talking about a cubic structure, so I believe that glue and surrounded fibers won't be stressed at all once everything is hold and tight, specially if using some bracing here and there, or threaded rod to support structure. So what I can do is to simple do my box and stress it with a high potency woofer, or ask for experiences on audio forums, which is I think I will do as well, its not like something wrong here, its just what the type of application demands. I am going to do several boxes, so if I can avoid making box joints or adding dowels every few inches, it would be nice, and I am saying this because I of course think that making 1" box joints on mdf, would be the maximum guaranty that nothing will take apart, that's why I am planning building the box joint jig once I have my table saw, so I easily will be able to do box joints on too many boards at the time in a second. Will do one box with the treated glued surface, and study it with the proper tool (a powerful subwoofer).
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post #11 of 12 Old 08-15-2016, 06:40 AM
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overkill

Box joints in MDF, not needed or stronger. Making box joints in MDF will kill a router bit and take a toll on a saw blade AND create more dust in the air. It's not needed and you won't find a speaker enclosure built that way... JMO.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #12 of 12 Old 08-15-2016, 11:40 AM
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A bit of innovation never hurts i guess, sometimes we learn something, sometimes we are only reinventing the wheel.

There are countless forums on building speaker enclosures with hundreds if not thousands of hours experience to draw from, many of the contributors are professional installers that build very expensive systems that stand the test of time.

Wise men speak because they have something to say; fools because they have to say something -Plato

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