Joining two MDF for the baffles

[qguy]

Do I need to pre drilll the holes for the screw that holds the inner and outer baffles ?.

Wondering if 1.5 inch nails could do the job ?

Yes, there would be glue in between the two layers

[planet10]

glue and clamps is really all that is needed. Brad nails would do the job, fatter might be an issue. Screws need predrillig, include countersink.

dave

[DSP_Geek]

I tried drilling for screws, but the MDF still swelled and forced the layers apart when driving screws into the material.

Here's something which worked beautifully for me. Drill quarter inch holes in the wood and use hex nuts & bolts to hold the layers together. Bolts with a plain area below the head of at least an inch, and better yet an inch and a quarter, are preferred. You want to use washers on both sides, of course. Here's a tip: wrap the washers in duct tape to keep excess glue from sticking them to the MDF. For some reason bolts loosen easily from the holes but steel washers glue surprisingly well.

Once the glue is dry - I waited a week in summer, three weeks in a California "winter" - you can plug the holes with quarter inch dowel pins and cover any remaining gaps with your preferred screw hole filler. The technique is a bit more involved than using screws, and a bit of a pain in the neck when working around long ducts, but the results are worth it.

[qguy]

Thanks for the warning on the swelling part.

[jimbro]

I agree with Dave that glue and clamps are all that's needed but if you wanted to use screws just countersink the front and back of the pre-drilled holes if possible. That way when the mdf swells that back countersink will allow the baffles to clamp together tightly.

[nigelwright7557]

Glue and screw.

[aussiephil]

If you are going to screw it together to hold it then I support jimbro, countersink both sides.

In addition I would suggest removing the slightly shiny finish fron the sheet faces to be joined, a quick sand with 60/80 grit on a ROS will do the trick. Spread PVA glue of you choice (i use exterior grade always) evenly over the surface and then just clamp together, for wide, long sheets throw a couple cement bags in the middle to add pressure/weight. With a supply of clamps screws are just not needed.

The last lot of speaker cabinets i built have no screws at all holding the MDF joints, just biscuits and glue.

Phil

[mill23bay]

I used construction adhesive but put in pilot holes before gluing, to make sure everything is lined up, and were countersunk. Warm up the adhesive before using and use thin lines of the stuff - about 1/8" wide and 1/2" apart as even as you can. I used a 1/2" wide piece of wood to draw lines and that made it easy. Try on a scrape piece of MDF and you will see how strong this stuff is!
Peter C.

[Cal Weldon]

Do not use screws. There is no reason to use them and they can cause problems. Take the previous advice of glue and clamps and a little sanding ahead of time. Consider also full panel coverage with the glue using a putty knife, not just ribbons. You want a nice thin film. Pilot holes drilled for alignment is fine but are not really necessary and you will have to redrill them anyway to clear them of glue.

[chrisb]

Originally Posted by Cal Weldon

Do not use screws. There is no reason to use them and they can cause problems. Take the previous advice of glue and clamps and a little sanding ahead of time. Consider also full panel coverage with the glue using a putty knife, not just ribbons. You want a nice thin film. Pilot holes drilled for alignment is fine but are not really necessary and you will have to redrill them anyway to clear them of glue.

As Cal suggests, for best adhesion and to prevent slipping when clamps are applied, pre-sand the case-hardened surfaces of MDF ( 80-100G would be fine) . Personally I use a lint-free velour paint roller for spreading glue to both surfaces - the result is a thin very lightly stippled film which is far less likely to creep than the ribbons. A couple of brads from an air-gun will help keep the panels in alignment while clamping up - and don't forget to use 2x4 etc cauls to evenly distribute pressure