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Old 18th June 2008, 09:04 AM   #1
rinx is offline rinx  Estonia
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Default MDF edges and screws

Hei please tell me some ideas:

-how big should be screws diameter (I am doing box 20cm x 45cm x 150cm box. And how often should I use screws.

Why I am asking, is that my last speakerbox I used screws diameter about 3mm and they arent long. I put MDF pieces together and used autospackel to cover connection places, but even when I spackled twice between one week, after waiting a week I felt under fingers that the line where two pieces of MDF were together has some stage. I was wondering , maybe the MDF is playing on this connection line and pieces are removing eachother?! Any ideas?!
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Old 18th June 2008, 11:03 AM   #2
Shaun is offline Shaun  South Africa
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No need to screw if the joints fit snugly. Just glue with wood glue and clamp while it sets.
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Old 18th June 2008, 11:36 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
end grain of MDF does not glue well.
Use screws.
End grain of MDF does not screw well.
Use a timber batten inside all the corner joints. Screw the batten into the faces of each MDF panel.

You can use long screws into MDF end grain but they do not survive shock loads. The MDF splits and leaves the joint open/weakened/damaged/leaky.
Try 4mm chipboard screw (very deep thread profile) with 50mm of penetration into the end grain pre-drilled to about 3mm diameter and use glue in the hole as well as in the joint. But this does not give a robust fixing.
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Old 18th June 2008, 11:47 AM   #4
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Hi Andrew,
By shock, do you mean dropped from a high place? A properly glued joint, using the right glue, even on MDF end grain will not fail from the loads put on it from the driver.

Your message makes it seem that glue will be useless and can be avoided with the use of corner cleats and screws. Not so.
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Old 18th June 2008, 11:58 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by MJL21193
Hi Andrew,
By shock, do you mean dropped from a high place? A properly glued joint, using the right glue, even on MDF end grain will not fail from the loads put on it from the driver.

Your message makes it seem that glue will be useless and can be avoided with the use of corner cleats and screws. Not so.
shock load could be dropped from a small height or banged into by a trolley etc.

The batten must be glued and screwed, in hindsight that wasn't made clear.
Quote:
and use glue.......... in the joint
applies to both the battened and unbattened joints.
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Old 18th June 2008, 12:44 PM   #6
rinx is offline rinx  Estonia
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I understand to use corner batten, like Andrew is suggesting. I have done it for PA speakers, but to start using it for tower-speakers its not for me best solution:

1) it takes too much time for every speaker (its faster without it)
2) still I believe some other option.

So, what do you think something about that:


I am using router to cut half of the edge and glue it like it shows (bold lines) and use screws 4mm and longer (50 f.e). of course I am gonna drill 3mm hole.
ps! I will upload picture a littlebit later

But still, do you have any idea, why this stage appear after when I have grind the surface and after couple of days?! Mystica!
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Old 18th June 2008, 12:59 PM   #7
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Quote:
Originally posted by rinx


But still, do you have any idea, why this stage appear after when I have grind the surface and after couple of days?! Mystica!

What type of glue did you use? Yellow carpenter's glue is best. It is basically white PVA with Aliphatic resin to increase strength.

For a box that will not be abused this glue will make a strong enough joint on it's own. As Shaun said above, make the joints tight and clamp it properly. Screws just create too many problems with MDF.

To reinforce the joints, if you think they are not strong enough, use cleats (battens) in the corners after the box is assembled (reach in through the driver hole). Just glue these in place without screws.
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Old 18th June 2008, 01:27 PM   #8
rinx is offline rinx  Estonia
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I am using regular PVA without anything. Its not expensive, but my guess is maybe its not the best glue for MDF - why the spackel is playing like that.

Yes I can use maybe those batterns for corners.

I hope i can put a link also for the picture soon
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Old 18th June 2008, 02:15 PM   #9
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Regular PVA (white) is ok, but the one I'm talking about is MUCH better. It is no more expensive either.See here.

Joints need to be tightly fitted. Use plenty of clamps.
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Old 18th June 2008, 02:26 PM   #10
rinx is offline rinx  Estonia
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Thank You,
I try to find something similar from europe also. Well, and its not very excpensive compare with white PVA I am using.
Do you have any idea, how speakermanufactures are building boxes and what glue do they use. Do they use the technic we figured out, or do they use lots of screws (actually I do not like spackle the plases - other hand it will fix excact place)
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