I am considering doing some built-ins with 3/4″ MDF, and if you have worked with this material, I have some questions for you. Everything will be primed and painted upon completion.
What screws work best for cab box assembly, where the screws will be driven through a face into an edge?
What prep method works best for painted edges?
Replies
Watch out for MDF dust...nasty stuff.
Screwing into the edge of man made board materials always present something of a problem, Mr. Micro. Whether it be a plywood, chipboard, or MDF, the materials are laid down in layers bonded with glue. So any tendency of a screw to force the layers apart can result in delamination, i.e., failure. There is a similar problem with screwing into the end grain of solid wood for much the same reason.
A typical standard woodscrew won't generally be effective because it has a fat shank and comparitively shallow threads with a low pitch. What you need are screws with narrow shanks with a thread pattern of a coarse high pitch with a sharp biting profile.
This configuration is the least 'wedge' like for want of a better term, and the threads have the best chance of cutting a clear non-deforming pattern. For the life of me I can't drag up a brand name of the type of screw you're after for the most effective holding power used as you describe, except I that I know Hafele produce such a screw designed specifically for this purpose, as do other hardware suppliers.
Woodworker's Supply, Rockler, et al, I think, sell such screws at a premium. Look in the area of particle board screws in their online catalogues and see if you can find a match. The websites of both companies are easy enough to find using a search engine. Slainte.
PS. Edit. I suddenly remembered just after posting that Confirmat is the brand name of just such a fastener for need you have. It requires a buying a special drill bit as well as the screw, and can be bought at Rockler, her-- http://www.rockler.com/ecom7/product_details.cfm?sku=652 Rockler are expensive, as are all those types of companies for this kind of thing, but if you're not a professional buying the stuff by the bucketful, they are convenient. The screws are supplied by Hafele at about 75% of Rockler's price if you need a lot of this kind of stuff on a regular basis.
Website
Edited 11/8/2003 2:13:32 PM ET by Sgian Dubh
Thanks. I have found a source for the confirmat screw and the special stepped bit required for the holes. The website for the seller, Professional Hardware, is http://www.profhdwr.com I went to WOODWEB, and read through a lot of posts that offered great advice on finishing MDF sawn and machined edges. I am on the right track now.
Confirmats are special purpose screws, mostly for knockdown type furniture. The production screws, as represented by the SPAX and others, or in a pinch, sheet metal screws will work just fine and be considerably less expensive than confirmats.
There are some other secrets to the successful use of screws in MDF. The pilot hole in the lower (or base) piece should be of shank diameter. The threads are thin, strong, and sharp and will not split the material as long as the pilot will accomodate the screw shank.
The hole in the upper (or attached) piece needs to be of thread diameter to avoid "bridging" of the two pieces by the threads. It should also be countersunk at both ends, normally at the top and a shallow countersink at the bottom. This makes a space for the inevitable "digout" of MDF material from the lower piece and allows the two pieces to mate firmly together. It is a good idea also, to put a shallow countersink into the pilot hole in the lower piece for the same reason.
I hope this info helps you with your project. Best of luck and post pictures when it's done.
--
Lee in Cave Junction, Oregon
Gateway to the Oregon Caves
Generically, they're simply called "Production Screws." And, indeed, they are best understood as being quite straight (no taper, save for the very sharp point), a narrow shank, and a straight root with large, sharp threads that stand clear of the ENTIRE shank from top to bottom.
Regular production screws have heads quite similar to traditional screws. Drywall, deck and pocket-hole screws are all deriviatives, but have bugle heads instead of true flatheads (while pocket-hole screws have pan heads). So make sure you get the right kind. Drywall screws, for example, typically have black oxide coating, frequently have a double-thread, and are often threaded all the way to the head. They're also rather brittle, and not up to much driving through wood.
These kinds of screws were initially developed explicitly for mass production in furniture factories and cabinet shops, and work well not only in solid wood but -- specifically -- also work great in composite composite materials (plywood, particle board and later MDF).
McFeely's and a lot of other hardware providers do have them, often with a choice between square drive, phillips and combination.
One other tip: if appearance won't be affected, you can always drill dowel holes across the face of the MDF you're planning to screw into the end of. Glue in a dowel, cut flush to both faces. Now, when you drive your screw, you'll be driving it through the side of the dowel, making it far less likely to pull out. Lot of extra work, though.
-- Roger
In Europe we call them carcase screws (as beloved of kitchen fitters everywhere), and MUCH cheaper than Conformats - but ALWAYS drill a pilot hole before screwing. Another good screw to use on MDF is the Spax screw.
Scrit
Mr. Micro
Agree with everyone on the screws. I use the Spax Scrit mentioned which are available readily through Highland Hardware. As Sgain touched on the fact that MDF is glued layers, Scrit nailed the fact that a pilot hole is very important to keep those layers from separating. I always drill a pilot hole one bit size smaller than the actual screw. Since I use #8's pretty reverently on MDF, the pilot hole is drilled with a 1/8" bit.
http://www.highlandhardware.com
Good Luck...
sarge..jt
Edited 11/9/2003 12:00:06 AM ET by SARGE
There was a great article in FW Just a little while ago. But since everyone has tackled the screw question, I'll touch on the paint / sealing question.
You'll need to seal all the edges before you prime and paint. Otherwise the edge will roughen as it soaks up the paint. same with any routed edge detail, any good wood sealer will do.
I use a lot of MDF for painted case work, but I usually end up with solid wood face frames anyway. (Poplar) Justus Koshiol
Running Pug Construction
Justus
Good point. Most of my MDF projects are for shop tables, jigs, fixtures and bases. The edges will rough out and suck up more than the almost closed pore sides. I usually have to put several coats on the edges to close them. Not critical with what I use MDF for as similar to you face frame it for concealment. But, if it's going to be exposed, sealing is a necessity that should be attended too.
BTW, one more point. Those sides are so closed pore, I usually rough them out slightly before painting or applying what-ever finish for adhesion. That's some "slick" stuff.
Regards...
sarge..jt
There was an article not to long ago in FHB on this very subj. Other than the MDF screws biscuits got a pretty high rating also. I think they came in 2nd behind the screws.
Darkworksite4:
Estamos ganando detrás el estado de Calif. Derrotando a un #### a la vez. DESEA VIVO LA REVOLUCIÓN
Yellow PVA glue thinned 4to1 or 5to1 with water makes a great sealer for cut edges.
Dewaxed shellac is another option.
Both will still require some light sanding to knock off the nub.
Bondo makes a great dent fix for MDF.
Jim Blodgett had a great article in FH #154 (April/May 2003) on working with MDF.
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