Replies

1. 
   DennisS | Sep 12, 2002 12:41am | #1

   Hi Todd -

   There are more experienced voices here to offer advice but I'll relate my experience doing much the same as you describe.

   The project involved a new counter top in the kitchen which spans between two base cabinets with the kitchen table scooted under between them. I sandwiched two 3/4" pieces of MDF together just as you describe. At the time I didn't have enough suitable clamping devices to clamp while glue set so screwing was the obvious alternative.

   The problem I encountered was stripping out screws in the MDF while trying to get the two pieces in intimate contact. I countersunk and piloted all holes but there was just too little "bite" in the screw threads in the bottom layer of MDF to provide a really good grasp for drawing the pieces together.

   Basically, I don't think it would be necessary to screw the pieces together if you have the right kind of clamps and enough of them. You might want to consider the type of glue you use,though, since as I understand it, yellow and white glues will tend to creep over time.

   Dennis in Bellevue WA

   [email protected]

2. 
   billballeza | Sep 12, 2002 12:59am | #2

   I've had no luck getting screws to "bite" in MDF. They spin every single time. What I would do is brush your glue onto both sides.. then use cauls to clamp the pieces together six to eight inches apart.

   If you don't have enough clamps and cauls.. run some plywood or scrap strips underneath the MDF sheets and screw straight through to the wood. When the glue dries.. back the screws out and you should have a tight bond.

   bill

1. 
   LeeEschen | Sep 13, 2002 09:34am | #3

   You gotta use the right kind of screws in MDF. Wood screws and drywall screws simply will not work.

   According to the Shop Talk column in ShopNotes Magazine #64 (07/02), the best screws to use for joining MDF are sheet metal screws. The technique is to drill a shank hole the diameter of the threads, slightly countersunk on the bottom side in addition to the normal topside countersink. This provides clearance for the fibers in the bottom workpiece, which usually lift as the screw goes in, thus allowing the workpieces to mate fully.

   The pilot hole needs to be the same diameter, or slightly smaller, as the screw root (between the threads). The pilot hole length should be slightly longer than the screw, and the screw needs to be about 1/2" - 3/4" longer than you would normally use in wood.

   It is also suggested that the screw be tightened by hand, as a power driver can easily strip threads in MDF.

   Confirmat screws will also work well in MDF.

1. 
   DennisS | Sep 13, 2002 11:08am | #4

   > ... You gotta use the right kind of screws in MDF.

   There's a typr of screw I've seen used in laminating drywall - called a "gyp-to-gyp" screw. Very aggressive threads and a deep root for grabbing the core of drywall. Given propery hole preparation as you describe, I wonder if these wouldn't be a good choice?

   Dennis in Bellevue WA

   [email protected]

1. 
   LeeEschen | Sep 14, 2002 10:23am | #6

   Its the screw's taper that causes problems. MDF has no tolerance for wedge shapes. It splits the material apart. If your gyp-to-gyp screw has no taper, and you prepare as I described, you'd be OK.

   I have yet see one of those screws but a good friend is a drywaller, so I'll check with him for a sample and take a look.

2. 
   billballeza | Sep 17, 2002 12:04am | #7

   Thanks for the tip, Lee. It seems like alot of trouble to go through, especially the tightening by hand, but you do what you have to do.

   bill

1. 
   sorethum1 | Sep 17, 2002 08:55pm | #8

   I experimented some with machine screws using the techniques in a recent Woodsmith magazine (I think), and it doesn't seem worth it.  If the screw goes in too far, it pushes out the back.  If you don't counter both sides of the first board, it doesn't allow a tight fit.  I was more concerned with ending up with bumps from the screws after this experiment, so I'm going to go with just glue.  I'm also wondering if putting a 3/4" board on top of a 1/2" will result in the weight alone of the 3/4 being about enough to get good adhesion.  I will clamp, but getting it even over 30+ inches is a challenge.  Todd

3. 
   HowardAcheson | Sep 14, 2002 12:03am | #5

   Many kitchen counter builders use contact cement to laminate multiple thicknesses of particleboard and MDF. Contact cement can also be used to glue the 1/4" plywood to the substrate.

   I think you will not have to worry about sagging of the MDF. We spanned 24-30" with formica laminated to 3/4 MDF without sagging.