Subwoofer box and lots of 5/16 hardwood dowels. - Page 2 - Woodworking Talk - Woodworkers Forum
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post #21 of 29 Old 04-30-2016, 08:28 PM
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Originally Posted by Johanx3 View Post
I countersink it, and anyway happened, as I mentioned it, what I did not do, was to make the countersink in the board which fibers are not running parallel (I mean, in the corner of a box attacking 2 pieces), but I don't know, I made a test with a machine screw and a big washer, thinking that if all head pressure falls on the washer and therefore cover more mdf area, it won't split, but it did it. Is it ok using #10 screw on 3/4 MDF? or should I use #8? I think for this time only going to dowel the screws with glue and drilling a same diameter hole. Next time will think in using plywood or even particulate board, which dont have this problem. I decided to use MDF, because had planned using some 2x2 timbers at every corner, that way I was supposed not to screw on any MDF edge, but plans changed and needed to get rid of the 2x2 timbers and now need to do what I said.
I'm confused now. I thought your project entailed putting a screw into the edge of MDF. What you have illustrated I would use #10 screws and drill a 3/16" shank hole with a countersink through the MDF. The screws would pull down into the 2x2 timber just fine.
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post #22 of 29 Old 04-30-2016, 08:39 PM
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Originally Posted by Steve Neul View Post
I'm confused now. I thought your project entailed putting a screw into the edge of MDF. What you have illustrated I would use #10 screws and drill a 3/16" shank hole with a countersink through the MDF. The screws would pull down into the 2x2 timber just fine.
He's not going to use the timber...
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post #23 of 29 Old 04-30-2016, 09:05 PM Thread Starter
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He's not going to use the timber...
I think you both are right men, and you shoot, I understand your point, I have seen how complex are those screws designed for MDF, I have never seen something like that before, but my point is still there, that although that screw go in correctly without splitting anything, I can't trust that in the future it won't rip... It was not a waste of time for anybody here, I was planning using the 5/16 hardwood dowels, but then I got what you said, that they will weak the board because they mass portion they are going to take, so no doubt that I am not going to use the 5/16 dowels, what I am going to do is pre drill with the same screw thread diameter, and put in some glue in both places, hole and screw, that way the MDf hole will be reinforce with the glue, which is stronger than MDF, guarantying that screw won't cause a future split. It's a little annoying doing this, but will do it just in this project. These woodglues, tends to expand, so there is where I will get the strength, have you seen how hard wood glue is? it dries like rock!. I am going to put one every 3 inches.

Last edited by Johanx3; 04-30-2016 at 09:09 PM.
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post #24 of 29 Old 04-30-2016, 11:09 PM
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Drywall screws will work also, but experiment with them on some scrap to get the correct pilot hole ( they can snap under to much torque.


In MDF, screws are used as clamps while the glue dries..they serve no other purpose.


You could remove the screws afterwards and lose no strength , the strength is in the glue joint.


You stated that you have already glued it ...if its holding ok adding screws or dowels would be pointless.
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post #25 of 29 Old 04-30-2016, 11:47 PM Thread Starter
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Drywall screws will work also, but experiment with them on some scrap to get the correct pilot hole ( they can snap under to much torque.


In MDF, screws are used as clamps while the glue dries..they serve no other purpose.


You could remove the screws afterwards and lose no strength , the strength is in the glue joint.


You stated that you have already glued it ...if its holding ok adding screws or dowels would be pointless.
Of course you lose strength if only glued, because with screws, the masses are connected internally, so the surfaces wont peal off at a heavy use, because this is a powerful subwoofer, not for 8" driver but for 15". Its like gluing a washer in a surface, if you add a screw, then it's harder to remove it. Next time I am going to do 1 inch box joints, that's the strongest way to join mdf and theres no need for screws.

I thought a new method to join mdf, it uses lots of glue, what if you fill the pilot holes with glue instead of screwing them? once glue dries, it turns into rock, you may fill pilot hole several times until glue stabilizes and cover the area, however, I am still with the idea of using #10 screws as dowels.

Will try to make the test of the 6mm pilot holes filled with glue. Titebond dries as rock, I dont know if any other glues are the same hard. Have tried elmers as well, incredible woodglue as well. I may say that this is a better task for epoxy, but this one is expensive in big quantity, and I observed how woodglue has nothing to envy to epoxy, at least in wood usage.

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post #26 of 29 Old 05-01-2016, 03:42 AM Thread Starter
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So what do you think about my method? Here I am going to explain why it has huge potential. In the picture below, theres a MDf board edge, with a hole. Green dot is the screw, red section is the glue. Once the glue is dried, we will find that all the fibers around the screw, have been merged and reinforced, so it won't split even with hard movement of the screw. Few hours ago I read a comment of a person, who usually uses crazy glue in the pivot hole, he got the same idea and saw how convenient is gluing those fibers, I would prefer wood glue instead.
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post #27 of 29 Old 05-01-2016, 08:22 AM
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3/4" is thinner than I'd like, and the overall volume seems very small for that much woofer. I'd be inclined to use at least 1" material with screws and glue, but would put a wood brace in each corner for the screws to grab into.
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post #28 of 29 Old 05-01-2016, 08:46 AM
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This is a bit too technical ...

Is a glue joint strong? Yes, if properly made and clamped, it's stronger than the surrounding wood. Is MDF wood? ...kinda, sorta. If you edge glue MDF to itself will it break at the glue joint IF Twisted ... if pressed down in the joint? If you joint MDF at right angles, how strong is it? If you use screws, is it any stronger? How strong is a glued MDF joint.

Then the quest5ion becomes "Do you need screws in a right angle glued MDF joint?"

Then another question is "In a dynamic loading, like sound waves, what is the effect on glued joints?" AND should they be "flexible" rather than rock hard solid. Maybe PVC glue, having some softness/flexibilty is best? I also wonder about Liquid Nail Construction adhesives...?


Sorry for all the questions, but that's what I'd like to know. Some testing would be required.

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)

Last edited by woodnthings; 05-01-2016 at 10:26 AM.
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post #29 of 29 Old 05-01-2016, 05:01 PM Thread Starter
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3/4" is thinner than I'd like, and the overall volume seems very small for that much woofer. I'd be inclined to use at least 1" material with screws and glue, but would put a wood brace in each corner for the screws to grab into.
Actually yesterday decided to use the 2x2 timbers as corners I discarded before, they represent about 0.2 3F, but I see its a bad necessary, just for the weight and mass they take, what I don't know if 2.1 CF is still good for 15" driver, most manufactures ask for 2 CF for their 15" drivers, but have seen larger 15" boxes.

Total space was 2.3 3F, but I also have a 2x4 cross that links the 5 walls, this cross also connects with some external panels I have in the center of 4 main walls, I thought in this concept some days ago, since the center of the walls is the most flexible part of a speaker box. It will be a heavy box, but with premium sound though.

concept of bracing is something like this picture I made, I also made some external work as a superior belt that reinforces the upper side, just around the driver, double baffle and a cover, so this thing will be a war tank once done. So now with the 2x2 battens there, can link every mdf face without screwing on their edge, but just in their faces.
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