Screw type for MDF?

[RJD2]

I am planning on utilizing MDF for building a mobile base for my BT3100. However, what type of screw works best with MDF? Would coarse thread drywall screws work, or is there a better alternative?

[RayintheUK]

I wouldn't rely on any mechanical fixing alone with MDF - glue is better, glue and rebated joints much better, glue with biscuits (or dowels) even better, especially on something structural, like a mobile base.

Ray.

[Tom Miller]

Ray's advice should not be taken lightly -- he's got a lot of mdf experience! Regarding screws, I hear good things about confirmat screws for mdf -- someone else will probably chime in with some info on those. I use coarse decking-style or construction screws, BTW, which work well for me. I think others use drywall screws, too.

My advice regards screwing into, uhhh, endgrain(?), which can cause the mdf to split. What you should do here, after pre-drilling your hole, is to place the faces of a clamp on either side of your screw hole. If you can't place the clamp during assembly, then at least run the screw into the clamped hole before assembly.


Regards,
Tom

[MilDoc]

What I like about Confirmat screws is their shape and flat head that really helps pull pieces together. But they cost more. If a project doesn't need tooo many, that's always my first choice.

I second what Ray said about not relying on any fastener alone. Glue, rabbet, etc.

And i second what Tom said. Predrill holes (I use a clamp while edge drilling too!) and either predrive the screw with a clamp over the edge or clamp it while assembling. O/W risk the edge splitting.

Hope your dust collection is good! Worst dust ever! Wear a dust mask.

[John Hunter]

I use Kreg course thread pocket hole screws but they are there just to hold things together while the glue dries. I would not use just the screws.

[RodKirby]

Ditto - what they've all said!

[RJD2]

Thank you to everyone that took the time to respond, and my apologies for the vague post. I was planning on utilizing biscuits/glue primarily, but wanted that option of screws for certain areas of the build. My experience with MDF is rather limited, and ultimately wanted opinions on what type of screws to use.

[MilDoc]

I wouldn't rely on bisquits and glue. Cutting the bisquit slot weakens the MDF considerably. Might hold if not subject to a lot of stress, but for a saw cabinet I doubt it would stand up.

[RayintheUK]

I wouldn't rely on bisquits and glue. Cutting the bisquit slot weakens the MDF considerably. Might hold if not subject to a lot of stress, but for a saw cabinet I doubt it would stand up.

Sorry to disagree, Paul, but my mobile base is made entirely that way - the only mechanical fastenings are the rail attachments, the castor bolts and the hinges for the flip-up saw support and doors.

Not only is it heavy in its own right, but it stores around 100lbs. of stuff and is as solid as a rock after years of use. The reason it works is that it has a substantial back, resulting in zero racking, so zero sideways stress, so zero joint movement. Same applies to the dozens of wall cabinets and base units I've made.

I'm not sure why you say that biscuit slots weaken MDF - given that, with modern-day glues, in most joints the stock will break before the glue does - cutting a 4mm oval slot then filling it with glue and compressed beech doesn't seem like weakening to me.

The problem with any type of screw in MDF is - generally speaking - that of overtightening, which is so easy to do with mechanical screwdrivers. Because MDF does not have the long fibers of "normal" wood, if you overtighten a screw, the screw then acts like an auger, drilling out the core instead of gripping it. For this reason, it is best to wind in a screw most of the way with an electric driver, finishing off by hand conventionally. This situation is less prevalent in water-resistant MDF (Medite, or similar - which I bang on about quite regularly!), but it also applies to most man-made sheet stock. MDF core can be particularly weak and "fluffy," so is not a good medium for screw-holding power.

Ray.

[just4funsies]

Gluing goes with screwing, or your work will need re-doing...!!!

[MilDoc]

Ray - I tried bisquits on an MDF project and had it fail miserably. Or maybe I failed miserably since that was the first (but not the last) time I'd used a bisquit joiner!

[jhart]

I have always heard that when pre drilling in MDF that the holes should be drilled slightly deeper that the depth of the screw to be used. By doing that, the screw will not bottom out, the pieces will be drawn tight and stay flat with the top or slightly recessed if you want.

[cabinetman]

Here's my .02 cents. I would like to say that if you want to do the glue and screw thing, I usually drill the hole the size of the shank of the screw, and the depth to be ultimately somewhat short of the overall driven length, so that the end of the screw gets a grab on some meat. I allow the screw length into end drilling to be at least 1 1/4". Being more specific, if I am screwing 3/4" into end grain, I will use a minimum of a 2" screw or longer. I use bits that have a countersink. I also give the hole a little shot of glue, which seems to bind up loose material around the flutes of the screw. I use a cordless drill on the slow setting and when approaching tight, just bump it in, without overtightening. I use coarse drywall, flat head phillips screws, or if I feel galantly rich, Grabbers.



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