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PostPosted: May 11th, 2015, 5:47 pm 
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Joined: October 21st, 2013, 6:53 pm
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Hi all,
I want to build a speaker with 2 layers of MDF and I need to laminate 2 panels together to get the thickness I want.

I have done this several times for baffles. What I do is pour some wood glue on a cup, and use a paint brush to brush on a layer of glue. Then I assemble the panels together, line them up, and leave them alone to dry.

It works acceptably, but I have found when I cut my driver holes, sometimes the two pieces of the scrap circular plug I'm cutting out in the center of the hole separate from each other. The baffle itself stays pretty solid, but this suggests that my glue job is not very good.

Is there a better way?

Does anybody recommend I use anything thinner than 2 layers of 3/4"?

I have used 1" before, and that sounds pretty good, but it's a little bit more difficult to get a hold of.

Thanks,

Chris


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PostPosted: May 11th, 2015, 5:52 pm 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
I have a stack of weights I use to apply pressure when gluing panels. Some guys use caulls a cross the surfaces and clamp the ends. It helps to have thicker in the middle to apply more pressure in the middle.


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PostPosted: May 11th, 2015, 7:08 pm 
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I have built a lot of enclosures using 3/4" MDF. In my opinion you are much better off using the 3/4" and then braces. The walls are not only stiffer but you can place the braces at different distances from the edges which will further reduce the tendency to develop resonances in the material. The added mass of laminating multiple sections only changes the resonance points. Breaking the panels up into different sized smaller secrtions minimizes the chance of a given frequency exciting any significant radiating area.

If you also look at laminating two 3/4" sheets together the maximum thickness is then 1 1/2 ". If you cut strips of MDF for braces of 4-5" wide and then glue them end on, you have a structure much more resistant to bending. That is the philosophy behind "I" beams. Also when gluing pieces of MDF together you should use biscuits. MDF is not very structurally strong and the end on grain of the biscuits greatly enhances the ability of the joint to withstand stress. I have a Porter Cable plate joiner to cut the slots for the biscuits if you need to borrow it.

Tom


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PostPosted: May 12th, 2015, 3:50 pm 
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Joined: March 5th, 2013, 9:35 am
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Location: Highland, MD
I agree with Pelliot123 about using whatever is heavy around your house to weigh the panels down. It would be real nice to have vacuum bags - that's what the pros use to apply veneers: then glue panels or cabinets up, slip them into a bag, seal the bag, and suck the air out so the atmosphere applies a constant pressure over the panels.

I also agree with Tom that adding internal bracing and tying opposed panels together will raise the resonances of the panels. Like Tom says, don't be an Engineer and make the braces symmetric - instead place them in somewhat different places on different panels so you don't create new resonances.

Just my $0.02 (no cents sign)

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PostPosted: May 12th, 2015, 8:55 pm 
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Joined: October 21st, 2013, 6:53 pm
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Thanks for the responses.

I have built a subwoofer with a sophisticated skeleton of braces, and I think I kind of understand what they do, but I think I'm going to stick with the design I have because it's relatively proven. The 2 layers of 3/4"MDF is just a minor modification to eliminate the hassle I have to go through to get 1" MDF and build the design like I have previously.

I'm thinking of buying tome ratcheting bands for the gluing, and I can put some thicker pieces of material near the centers of the panels as suggested.


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PostPosted: May 12th, 2015, 9:05 pm 
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MDF is not very structurally strong and the end on grain of the biscuits greatly enhances the ability of the joint to withstand stress. I have a Porter Cable plate joiner to cut the slots for the biscuits if you need to borrow it.

Tom[/quote]
Thanks Tom.
My joints are still a little bit crooked to use a precise tool like that. I'm not sure if the panels are crooked when I buy them, or my saw cuts crooked, but my joints are always a little bit off. I'm not sure if I want to try to fix the problem because I have developed a way of coping that isn't really noticeable when the speaker is finished.
Just to clarify though, in a right angle butt joint, the biscuit goes in the edgrain of one panel, and into the sidegrain of another. This makes the joint stronger than gluing alone?

Chris


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PostPosted: May 12th, 2015, 9:42 pm 
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That is not quite correct as the MDF has no grain. That is why it is weak structurally. The glue is very strong and when the joint fails it is generally because the glue has pulled the surface off the MDF sheet. The biscuits go deep into the sheets by means of the slots and the biscuits are so constructed that the grain runs in the direction into the slots. Therefore they are only subjected to tension forces where they are the strongest. It is really quite a good way to join MDF pieces together.

Tom


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PostPosted: May 13th, 2015, 7:59 am 
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Location: Highland, MD
I think it's always good to experiment. When I assembled my speaker cabinets, I used two-part epoxy because I was afraid the glue wouldn't soak into the MDF like it does with natural grained woods. The cabinets are certainly strong and airtight, so it was successful in that way. I also used biscuits for added strength, and they let me dry-assemble the cabinets before applying adhesive to check all the fits.

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PostPosted: May 13th, 2015, 8:05 am 
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Those crooked cuts drive me nuts! :wtf: Like many endeavors, preparation takes the most time.
Sometimes we have to use a dial gauge to compare the guide slots in the saw table to the blade to see it we have to shim the blade mounts to get the blade parallel to the guide. To date I have not been able to get my saw really true. There are articles and videos online discussing how to setup and tune table saws.

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PostPosted: May 13th, 2015, 8:53 am 
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Generally the biggest problems with saws is a poor rip fence. If the blade is not parallel to the fence you will cut on the front side and rasp the wood with the backward motion of the teeth on the back side. However, even though the cut will be rough and wider than the blade it should not wander. That is usually the sign of a rip fence that is not rigid enough. If the fence cannot keep the work tightly aligned to itself and in register the cuts will not be straight. There is a difference in pressure on the fence as you move the work through especially when handling large sheets. If you were perfect in the way you handle the wood it would not be as big an issue but I know that when I handle large sheets I am not consistent in the sideways pressure I exert on the fence when moving the work through. If you are not careful even with the best fence you can still have problems.

Tom


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