Hi, what do you think about not using screws on small and medium MDF enclosures? This material is so good for acoustics, but I found it not very compatible with screws, even if you use special screws, you really can not trust that that thing won't eventually take apart, for that reason I always thought that MDF enclosures only with glued dowels and box joints.
I have been thinking in new ways to make very solid enclosures without worry about screws or the hard method of making box joints, etc, and my mind suggested that wood glues do miracles when joining wood pieces (specially with joints like box joints).
Do you think that treating the contact area before gluing with a drill metal brush, harsh sandpaper, etc, will result in a lifetime enclosure that will never take apart on regular basis? I think yes, although not sure if wood glue or industrial one, but obviously for price, you are not going to use epoxy. I think that would be the trick to not use nails or screws, simply treat the contacts that are going to be glued, and structure will last permanently, at least talking about enclosures, obviously just 2 wood pieces forming a L letter, would be affected easily if not using nails or screws.
I have been thinking in new ways to make very solid enclosures without worry about screws or the hard method of making box joints, etc, and my mind suggested that wood glues do miracles when joining wood pieces (specially with joints like box joints).
Do you think that treating the contact area before gluing with a drill metal brush, harsh sandpaper, etc, will result in a lifetime enclosure that will never take apart on regular basis? I think yes, although not sure if wood glue or industrial one, but obviously for price, you are not going to use epoxy. I think that would be the trick to not use nails or screws, simply treat the contacts that are going to be glued, and structure will last permanently, at least talking about enclosures, obviously just 2 wood pieces forming a L letter, would be affected easily if not using nails or screws.
Screws don't hold too well in MDF unless you pre-drill and countersink the screw holes. Coarse thread drywall screws work well in that application. If you're also using either Urethane or yellow carpenter's glue to assemble the cabinet, then screws are basically clamps until the glue dries. If you can, use rabbit or rabbit/dado joints when building the box. Makes alignment easier and you'll increase the surface area for the glue by 50% or more, depending on the joint.
To produce stronger joints, use bisquit joints. They consist of grooves routed in the mdf, and bisquits placed in the grooves.
How To Cut Biscuit Grooves With a Router
For that matter, you don't really need to use anything else than glue. My old man and his work buddies tested normal chipboard, joined end to end with normal carpentry glue. They laid one plate on a table, and put weights on it, and the added weights to the free end. Eventually, they got a break, but the break came a few cm besides the joint. You just need the patience to wait until the glue has hardened, as Dick said.
Johan-Kr
How To Cut Biscuit Grooves With a Router
For that matter, you don't really need to use anything else than glue. My old man and his work buddies tested normal chipboard, joined end to end with normal carpentry glue. They laid one plate on a table, and put weights on it, and the added weights to the free end. Eventually, they got a break, but the break came a few cm besides the joint. You just need the patience to wait until the glue has hardened, as Dick said.
Johan-Kr
FWIW, much of the MDF manufactured in NA is now classified as NAUF - no added U-F, which doesn't mean there won't be residual amounts of naturally occurring Formaldehyde.
I'd avoid using drywall screws for cabinet assembly - not all have coarse enough thread to securely bite into sheet good cores, and many are brittle enough to snap off if not properly piloted or if set with too high a torque setting on driver drill.
There are coarse thread screws (Lo-Root, etc) that are specifically designed for use in MDF, PB and plywoods. Some even have nibbed heads to aid in countersinking(not shown below) , but it's always a good idea to use a proper countersinking pilot bit. And avoid screwing any closer than 2" from a vertical edge to avoid splitting.
Of course you could avoid use of screws on the outside of the box altogether by using brad nailer - much smaller holes to fill if you plan on paint finish. But then your cabinets wouldn't pass AWMAC inspection
Not all PVA glues are equal - the yellow cabinetmakers glues are probably a better bet, and it doesn't hurt to lightly sand the flat heat tempered surface of MDF to encourage further core penetration -whatever the adhesive.
I'd avoid using drywall screws for cabinet assembly - not all have coarse enough thread to securely bite into sheet good cores, and many are brittle enough to snap off if not properly piloted or if set with too high a torque setting on driver drill.
There are coarse thread screws (Lo-Root, etc) that are specifically designed for use in MDF, PB and plywoods. Some even have nibbed heads to aid in countersinking(not shown below) , but it's always a good idea to use a proper countersinking pilot bit. And avoid screwing any closer than 2" from a vertical edge to avoid splitting.
Of course you could avoid use of screws on the outside of the box altogether by using brad nailer - much smaller holes to fill if you plan on paint finish. But then your cabinets wouldn't pass AWMAC inspection
Not all PVA glues are equal - the yellow cabinetmakers glues are probably a better bet, and it doesn't hurt to lightly sand the flat heat tempered surface of MDF to encourage further core penetration -whatever the adhesive.
You will get some disagreement on that. AFAIC only good if it is all you have. SO popular because it is cheap.
dave
Well, many people says that its good for audio because in a primordial view, its fibers pattern make it solid as nothing out there, so this is an advantage on other type of boards. I know it has disadvantages with humidity and screws can split it, but for example, if you are making a cabinet which will be carpeted, you use a water resistant glue, and humidity wont be a problem anymore, the same if you want to finish it with paint, varnish, etc.
I Have tried some type of woods, and saw how mdf is marvelous in all sense, just those disadvantages I mentioned, which can be easily solved, using box joints instead of screws, or dowels. I have not tried fine nails with an air gun, and this is something I actually need to ask, maybe glue and many fine nails from a nail gun its enough to properly join mdf boards, need to try it in the future.
nicasoix2 - I humbly submit that all its meritorious attributes aside, MDF suffers sonically in a comparison to Baltic Birch plywood of identical speaker enclosures and drivers - something I did over a decade ago. I work for a woodworking company from which I could liberate offcuts of MDF from the scrap bin necessary to build all the speakers I had time for.
But for at least 10yrs we've paid for the plywood.
But for at least 10yrs we've paid for the plywood.
MDF is bonded by urea-formaldehyde glue, not PVA. There are EU regulations about the formaldehyde content (still in force in Britain too) - usually less than 1, 000 of the harmful dose (I think it was class E1).
Butt joints with PVA only would provide enough strength to drop the box from the 2nd floor and have it survive. Two 25mm pieces bonded together with Henkel Express (butt joint again) held for 12 blows with a 1lbs hammer and at the end - the MDF itself tore off. I`ve done the same experiment with Soudal glue which is cheaper and it failed consistently, much worse than any other PVA I have used (maybe has experienced frosts, although the selling party claimed it hadn`t).
Screws may or may not hold - depends mostly on the MDF itself. There are porous types made of very fine particles where the outer surface would have high concentration of glue and be very hard, usually the first 0.5mm, in the middle it is porous and weak. These do not hold screws well and if you have to remove the screws, they`d loosen the thread holes and make them flimsy afterwards. Some other MDF is made out of larger fibers (this is clearly visible) and hold screws much better. Best screws I`ve found so far are Spax for MDF and the black ones used by wall fitters. I did a test on the same type of MDF with 25mm thickness with screws and glue and without screws (so glue only) and didn`t find any noticeable difference in joint strength. Mortise and tenon could be better but is usually harder to do. PVA joint strength depends mostly on two factors - quality of glue and pressure applied with the second very important. This type of glue has a disadvantage which makes it very hard to use in some cases (such as veneering) - if metal is introduced, for example by the clamps, it reacts with the tannin in the wood and becomes very dark, almost black. Only oxalic acid may clean the mess a bit but better don`t use it for veneering. It also penetrates veneers.
Besides, why do you need such strength in a loudspeaker?
Not all MDF is bonded with UF glue - the alternatives include Phenol-Formaldehyde, polymeric-MDI and soy resins- but that is only one of the hazards with working with it. Dust and weight are at the top of the list - and the heat tempered surface can hold a machined edge sharp enough to mess you up - not as bad as a splintered piece of GP grade plastic laminate, but certainly enough to "leave a mark".
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