Pocket Screws and MDF

[havighurst]

Has anyone used pocket screws with MDF? I know that I need to use the course thread screws, but I am wondering if the joint is as strong as in plywood. I want to make a few rolling tool cabinets, and I am having difficulty finding decent (and reasonably priced) plywood. MDF seems like a cost effective alternative.

Any assistance would be appreciated.

[cwsmith]

About the only way I'd use MDF would be if to make a frame out of solid wood. MDF is good for use as a solid flat surface, but it has absolutely no integrity using screws that have to bite into the material. If you screw into and edge, it will split. If you screw into the face, it will either pull-out, or, if stress is on the screw at an angle, it will split the MDF.

MDF may be strong if you bolt "through" it, and use a washer or larger head fastener. Basically MDF is nothing more than dust that is mixed with a bonding agent and then compacted under extreme pressure. It has great compression strength, but very little tensile strength because of its lack of grain or fiber. Ply and flake board are substantially stronger with regard to the use of both fasteners and adhesives.

I noticed how much they use MDF on shows like "Home Improvement" and last year decided to give it a try on a couple "cabinet-type" book cases. The stuff was certainly heavy and seemed solid enough. But right off I noticed how dusty it was when cutting and drilling. It certainly gave me none of the sensory pleasures of working with wood. It was pretty easy to paint though, and the surface was smooth enough that you didn't need to do any sanding. However, I immediately ran into problems when I attempt to screw into and edge, as it split with even relatively small screws. Even with very short shelves, it sags and when used for vertical (side of book case) it bows unless cross braced every couple of feet.

CWS

CWS

[hitekrednek]

Pocket hole in MDF

If you look at a number of the MDF mobile bases out there, many of them use pocket hole joinery. I have had no problems with any of the MDF that I have joined with pocket hole screws.

[RayintheUK]

Pocket hole screws are a swift and convenient way of holding together a butt joint while the glue cures. Because of the comparatively short length of screw that bites into the stock, the strength of the MDF joint is almost totally reliant on the glue used. Modern glues are perfectly capable of holding a butt joint securely and I have never had a problem with joining MDF in this way. The screws will have more of an effect on the strength of the joint when using pocket hole screws in wood, although once again, it's the glue that does the real work.

Ray.

[jAngiel]

I built a few cases out of mdf and used pocket hole screws. Worked fine for me. I did see it split some when when I tried to screw into the edge but after that I started drilling pilot holes and it has worked fine since then.

Using pocket holes with self tapping screws (i.e. kreg screws) worked fine without predrilling but when I was using pocket holes with dry wall screws, I found that I needed to predrill. Because if this I typically use kreg screws now, cost more, but more convenient to use.

[sacherjj]

The only other thing I do is double glue. By that I mean I wipe glue on all the MDF edges, then the other surface, then on the edges again. Otherwise, the edges seem to suck up too much of the glue and I don't get a real good bond. I guess you could just put on a bunch, but it seems cleaner to add a thin layer to "wet" the MDF, then another thin layer to work as the glue.

[Scottydont]

I have used MDF for cabinets with great success. There are two different types of pocket hole screws, one of which is suited to MDF. Pocket hole screws work differently that wood screws or drywall screws. Pocket hole screws are designed to bore into the hole where woodscrews will split MDF. The only time I split the MDF was when I overtightended the screws.

As to the strength of the joints MDF will drink up glue on the edges and the flat face does not absorb glue very well. I use rabbets and dados and that remedies that problem. I do like to wet the joints lightly with water before applying the glue nad that seems to help.

I made a workbench and BT mobile cabinet that way and they are built like a tank. I made a router table that I tried to disassemble with a blow hammer and the MDF broke before the joints did.

Be preppared for the dust boy! It gets everywhere and burns the pores of your skin. A dust mask would most definately be recommended.

[crokett]

I have a few shop cabinets built of MDF. One is a drill press cabinet with drawers that I built 4 years ago and roll around once or twice a week. It has held up fine. I've never had screws split it or pull out screwing into the face or the edge. As has been said It is very dusty to work with - work outside and wear a mask if you can.

[Jeffrey Schronce]

Washer Head 1 1/4" Coarse Kreg Screws work fine. cwsmith notes in his post the problems he had. I had the same problems at first. Take a couple of MDF scraps and give it a try. You have to get the right "feel" for driving speed, torq, etc to avoid stripping the wood or crushing the pocket. I find that it forms a very strong joint. I really like Tightbond II or III for MDF joinery. I have not tried the Gorilla type glues with the MDF. That may work well, especially with a water thirsty edge.

[havighurst]

Thanks for all the great assistance!