Thread: MDF Screws

Yep, those are very classy fasteners, and they work well. They're spendy, though.

Wood screws are not good for MDF; they're tapered, so, ideally, you'd have to drill a tapered hole for them.

MDF -- medium density fiberboard -- may also be called particleboard, or may be called MCP -- melamine component panel -- when it has a thin melamine plastic sheet adhered to one or both sides.

The trick to building with it is to drill the right size hole for the fastener. Drilling too small a hole -- or not drilling at all -- can result in split boards. You want a hole very slightly larger than the shank of the fastener, so the only spreading of the wood is done by the threads. Where there is an unthreaded portion -- such as the part of the fastener under the head in the screws linked above -- that part of the hole has to be very slightly larger than the unthreaded portion of the fastener.

The advantage to using the drill provided with the fasteners in the links is that you always get the proper depth, diameter, and countersink. That means three separate parts of the hole -- threaded area, unthreaded area, and countersink -- are drilled in a single operation. You could do the three parts of the hole separately. But the better bet is to buy similar all-in-one bits in kit form at Home Depot or Lowe's or your hardware store, or online from Harbor Freight and others. I bought several sizes, for a few dollars apiece, in a Ryobi Kit at Home Depot, and have seen a DeWalt kit at Lowe's, and a Black & Decker kit in hardware stores.

Low-Cost Alternative
Coarse drywall screws are a good low-cost alternative to those "fancy fasteners". You still have to drill the right size hole for them, but only one size bit may be necessary. The unthreaded portion of the drywall screws is the same diameter as the shank, so the special bits are unnecessary for that part of the hole. You can sometimes skip the countersinking, because the head tends to pull in nicely (depending on the density of the material). Still, it's best to experiment on scrap before tightening one so far you snap it off in your project. I've done that, trying to get the head flush with the surface. In cases where that's likely, I countersink the hole.

If you countersink in particleboard, you can use a standard drill the same diameter as the head width, but be careful -- if you countersink a hole you've already drilled, the wood tends to pull the bit in unless your drill is going at very slow speeds, and you can wind up with too much countersink depth. Practice on scrap. It's best to drill the countersink first, then the main hole, so the wood won't grab the bit so much.

Other Factors
Add a good glue -- I like urethanes, like those from Gorilla or Borden (Elmer's) -- and you can have joints so strong that the joint will hold when the wood around it breaks.

If you have experience with MDF/particleboard/MCP, you probably know that it can't be used in damp areas without sealing, because it sucks up water -- or just heavy atmospheric moisture -- like a sponge. If you build with it, keep it dry or seal it.

Be cautious of Melamine... Not all are constructed equally. Some sheets are not of medium density, but actually the much lower density particle board. A quick look at any end/edge of the sheet will tell you which it is. Also, note that there are different grades of MDF in itself. I guess medium is simply the range of density (which makes sense). But a sheet of different MDF grades will weigh out differently. With the higher weight being better. (in terms of using for a driver(speaker) enclosure)

Urethane Glues
Titebond is the brand of Urethane we use most often. Seems to set up a bit faster than the Elmers or Gorilla.

A few tips I've found to be quite useful with urethane glues:

-To lower the viscosity a bit, microwave the tube for a few seconds on low. Ensure you have all the metal seals off first, and don't use your kitchen microwave. We have two in the shop... One for glue, and one for food.

-Before applying the glue, wipe down the areas which will be in contact with a damp cloth. Yes, damp. It helps the glues soak into the MDF farther, giving the glue joint more "bite", making it stronger.

-Do NOT wipe off the excess glue while it's wet. Wait till it's dry, and then use a scraper of some sort. Personally, I just use a scrap peice of MDF with a straight edge on it. This gets about 95% of the glue off the face, and then with a light sand, it's gone.

Other Fasteners
If you're using normal drywall screws, go ahead and drop the extra money on a good tapered counter-sink bit. Home Depot/Lowes/etc sell them for something like $10. The bit has some taper at the tip, the counter-sink should be adjustable, it yields perfect counter-sinks every time, and with the slight taper, it creates theoretically stronger joint. (more of the screw it "biting")

Not as expensive as the fasteners I listed in my first post, but slightly more expensive than the generic drywall/wood screws, are the "split" wood screws. They will have a "notch" running straight up the thread. They seem to "bite" a bit better, and have less blow-outs. A GREAT suggestion on these, are the collated subfloor screws you can get at Home Depot/Lowes/etc. They have dual threads, and have the tapping notch on the tip. The part that's typically threaded, is a very course thread, and then there's the shoulder portion, but just before the head, there's about a 1/4" threaded section, which pulls the two peices together.

You know, I've only used biscuits once with MDF, and that was on something totally non-related to anything here. They worked beautifully, and I don't know why I never went back to this method. Thanks for that reminder!

On a similar note, we've also used a simple lap joint on some "boxes". If it's something we're building that's fairly repetitious, this can save a ton of time, simply because of the speed you can put them together with not having to worry about lining edges/faces up. The lap joints are either made on the table saw with a simple dado blade, or on the shaper (or router if you don't have that at your disposal) with a rabbeting bit. An advantage to the lap joint is that, technically, it gives you more glue surface.