Driving screws into MDF

[Grant Edwards]

I'm about to make a sliding fence for my router table (something to attach a box joint spacer fence to). The base is going to be a chunk of malamine MDF with a Kreg miter bar screwed to the bottom. [HD had a pallet of 3/4" thick 11" x 2' melamine MDF shelf boards on special for a $3 each, so I picked up a few.] On top of that goes one of these:

In addition to the aluminum miter rail (which runs along the front side of the table), there will be a hardwood "outrigger" rail that rides along the back edge of the router table top -- similar to the design in Bill Hylton's Ultimate Guide to the Router Table. If you've never seen one, it's much like a table saw cross-cut sled. The side of the miter slot closest to the bit is going to be the reference edge, so I'd sort of like to try making the outrigger sort of spring-loaded, but I haven't quite figured out how I'm going to do that yet.

Anyhow, I've never worked much with MDF and was wondering what tricks there are for screwing stuff to MDF shelf boards.

For example, when screwing things to balsa, it's common practice to put a drop of cyanoacrylate into the pilot hole so that it wicks into and hardens the walls of the hole. You let the CA cure completely before driving the screw -- so you're not gluing the screw in, you're just using the CA to reinforce the walls of the pilot hole.

Any recommended tricks like that for MDF?

[BenignBodger]

I've always found that Confirmat screws are, hands down, the best screws for MDF or particle board. Unfortunetely, they don't come in very many sizes and their shafts are quite thick so they might not work for attaching standard hardware. Spax makes screws for MDF but I've never tried them; their regular screws seem to do the job in other materials so their special ones might be worth a try if you can obtain a few samples. If neither of these work for you then some experimentation with regular screws and various sizes of pilot holes might let you find a good compromise between grip and distortion. Squirting some sort of thin non-water-based glue (to avoid swelling of the fibers) into the pilot holes certainly shouldn't hurt anything even if it doesn't actually help.

[Grant Edwards]

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Based on experience assembling flat-pack stuff that comes with that type of fastener, I agree those do work well in MDF (as well as anything can in MDF).

But, as you say, it doesn't look like those are going to work for things like a miter bar that comes with pre-dtrilled counter-sunk holes intended for standard flat head screws.

It looks like they're only available in 1-1/2" and 1-3/4" lengths, so they won't work for things like the miter rail either.

Yup. It looks like there isn't much option other than regular screws. Since MDF is denser and less compressable than wood, I'll probably try slightly larger than normal pilot holes and some CA or something. I assume you want a hole in the Melamine surface that's large enough for the screw without requiring distortion of the melamine layer.

[Swingman]

All MDF is not created equal. That said, as long as you are screwing into the face, and not an edge, a quality wood screw that is not over tightened has always worked for me for most applications.

Do some tests on scrap and see if that works for your application.

[SonomaProducts.com]

Well after years of loving MDF because it is so flat and easy to work with, I rarely now use it for any long-term fixture, at least sparingly so. It j ust doesn't hold up well.

I use 1/2 mdf for the base of small cross cut sleds but I consider those ki nd of disposable and rebuild new square ones quite often. MDF just doesn't hold up to wear and tear. Screws will always pucker the hole. It glues real ly well so maybe shallow dado's and pre drilled screws to help hold it whil e it dries. Maybe dado and glue in some wood strips where you needs screws to hold well, or even maybe screw in inserts with heavy threads into larger holes to accept machine screws.

[-MIKE-]

Spax and a couple other companies make specialty screws for MDF and they work great.

[Morgans]

"-MIKE-" wrote

Agree on the specialty MDF screws. They are better.

That said, the best screw for MDF is no screw, so the less screw the better. I use glue and pin nails for MDF usually, and sometimes use trim head screws, where I need to have the fastener pull the jig together. Good ole titebond II. If I take the time to build a jig, chances are I don't want it to move about, and I will keep it and not tear it apart, go glue is good, and a fastener holding it together while the glue is setting and setting up things is what the doctor ordered.

If not that, a combination pilot countersink to drill for multipurpose screws. The more coarse threads the better. Where a head sticking up does not matter, Kreig screws work fantastic and hold well. You can usually get them in course and fine thread versions, but not so often choose between multipurpose threads.

[dadiOH]

Use bolts (national coarse thread):

1. drill proper sized hole 2. tap the hole 3. add CA to tapped hole and let dry (until NO odor) 4. tap again

[Grant Edwards]

Thanks for the suggestion. It occurred to me that I ought to be treating MDF more like plastic than like wood. I was thinking of doing something similar: drill and tap a hole for a machine screw -- not so much expecting the threads to hold, but more to get a hole with no slop. Holding strength would be provided by putting either a countersunk T-nut on the opposite side (or just a washer and lock-nut if it doesn't interfear with anything).