What’s the best technique for doing this?

[Louis Brandt]

This one will probably sound silly and elementary, but here goes.

I just finished “laminating” two pieces of 1/2 inch thick MDF, to give me a 1 inch thick plank. The pieces were 48 x 15. I “laminated” them, but without glue. I used 28 (equally spaced across the width and length) #6 brass 3/4 inch Phillips flat head wood screws, and I (deeply) countersunk all of them. I got the job done ok, but in order to keep the screws from forcing the MDF apart as they went in, I put the two pieces of MDF on top of another piece of 3/4 MDF to provide resistance. Then, to provide more weight, I sat down on the whole thing as I drove the screws down. I suppose that I could have clamped the two pieces together, but I didn’t, since I couldn’t have clamped any further in towards the middle than two or three inches.

My question is, is there a foolproof way to do what I was doing that will insure that the screws don’t force (even slightly) the two pieces apart as they go in? Is there a different type of screw that I should have used or a better technique?

Louis

[James K Peterson]

I've done similar things, but I normally drill a pilot hole and countersink before screwing the pieces together. I think what you did will work fine. Was there a reason you didn't just glue the pieces together?

Thanks
James

[Matthew Hills]

I thought the typical approach for a tight screw joint was to have a larger diameter hole in the topmost piece. The threads pull the bottom piece up, and the top piece is pressed down by the screw head.

The other common issue is if the hole is not predrilled, the screw displaces wood as it goes in, causing the top of the wood to pucker up.

Matt

[Jason Whelehon]

I've found that predrilling and/or using a self tapping screw works...

[Louis Brandt]

I guess that I didn't explain the situation well enough. I did predrill and I did drill deep countersinks before I drove the screws down. The whole thing came out ok, but there were still a couple of places along the edge where I can see a tiny gap, where the pieces didn't come together tightly enough. My question really is, was my technique bad, or was I using the wrong type of screw?
Louis

[Dick Bringhurst]

I've heard of people predrilling the top piece then turning it over and counter sinking the bottom of the top piece so there is room for the MDF to expand into when the screw enters the bottom piece. Dick B.

[Joe Scharle]

I use a hand countersink and relieve all the holes in MDF to prevent jacking.

[Tony Bilello]

Was there a reason you didn't just glue the pieces together?

Tony B

[Faust M. Ruggiero]

Lewis,

A couple things can cause the pieces to separate when screwed together. First, if the hole, though pre-drilled, in the top piece was not drilled to a large enough diameter to allow the screw to spin without the threads biting , the two pieces can separate. Most countersink drill bits, especially the tapered bits do not make the hole large enough in the top ply. The only way to do it most of the time is to pre-drill both plies then enlarge the holes in the top ply.
The second thing that might cause the problem even with a large enough hole in the top ply, is if the drill bit pushed a piece of drill waste between the two plies. It doesn't take much to let that happen, especially when laminating sheets when you can't really apply enough pressure to keep the plies together when drilling.
Laminating sheets without the help of a vacuum system or veneer press is not as simple as it appears.
By the way, screws don't hold very well in MDF.
Faust Ruggiero

[Louis Brandt]

Thanks for the good replies. From what I think that some of you are saying, it sounds as if my countersink bit didn't do a good enough job. Maybe what you're saying is that, instead of just relying on a simple countersink bit, I could have just used a larger diameter bit and "opened" up the top portion of the hole more.

And Faust, are you saying that some countersink bits aren't tapered?
Louis

[mreza Salav]

And Faust, are you saying that some countersink bits aren't tapered?
Louis

I think "most" of them are NOT tapered, only higher-end (and more expensive) ones are tapered. http://www.leevalley.com/wood/page.aspx?c=1&p=32309&cat=1,180,42240

[Faust M. Ruggiero]

Louis,
Tapered bits have a tendency to burn since the tapered point has narrow grooves and can't clear the chips well. though they do make the pilot hole narrow where it needs to be narrow. Still, they can leave the hole in the top piece narrow enough that the screw threads still bite. Straight bits can't allow for clearance in the top hole at all or they would make the pilot hole to large in the bottom piece.
In a perfect world someone would create a step drill like the type used for pocket holes but would be sized to solve our problem. Unfortunately, such a very wide variety of sizes would be required that doing so might be impractical. Google General Tools and check out their Pilot Bit set. It comes the closest to making two different size holes plus a countersink. Unfortunately, the step drill portion breaks off quite easily in hard woods. It will, however, show you the "correct" way to pre-drill when you can't avoid the problem you had any other way.
By the way, you can buy screws with no thread for a portion of the screw just under the head. There is a popular screw catalog who calls them his "optimized thread length" design. PM me if you don't know the retailer I am referring to.
Faust

[Dino Makropoulos]

I thought the typical approach for a tight screw joint was to have a larger diameter hole in the topmost piece. The threads pull the bottom piece up, and the top piece is pressed down by the screw head.

The other common issue is if the hole is not predrilled, the screw displaces wood as it goes in, causing the top of the wood to pucker up.

Matt

The typical and only way for a tight screw joint.
#1. countersink.
#2. predrill both pieces with a (smaller than the screw) drill drill.
#3. predrill the top piece only with a ( larger than the screw) bit.

[Dino Makropoulos]

I thought the typical approach for a tight screw joint was to have a larger diameter hole in the topmost piece. The threads pull the bottom piece up, and the top piece is pressed down by the screw head.

The other common issue is if the hole is not predrilled, the screw displaces wood as it goes in, causing the top of the wood to pucker up.

Matt

The typical and only way for a tight screw joint.
#1. countersink.
#2. predrill both pieces with a (smaller than the screw) drill drill.
#3. predrill the top piece only with a ( larger than the screw) bit.

Before you screw the pieces together, remove the top piece and clean
the bottom from chips.

[Jeff Bratt]

The explanation didn't get across. Especially in MDF, the holes need to slightly countersunk on the inside surface of the upper piece - the one with the clearance hole. Material is "pulled out" when the screw is driven into the other piece - some clearance gives this material a place to go. Otherwise the two pieces will separate no matter how much weight or pressure you apply to during assembly.

You should have purchased 1" MDF or glued the pieces together. Why not?

[Brian Peters]

Permanent: little glue here and there, draw a smiley face if you're feeling good, and use a narrow crown stapler, tack it together. Should hold well.

Removable: countersink/drill combo and two clamps on each side. If you use the right screws there shouldn't be any press back or resistance.

[James Hart]

This one will probably sound silly... to provide more weight, I sat down on the whole thing as I drove the screws down.
Louis

Louis,

A picture would be.......helpful. ;)

Jim

[Louis Brandt]

Thanks again for all of the replies. I now have a much better idea about how this should have been done. I think that to sum up most of what I'm hearing, I should have taken more precautions to make sure that the debris was cleaned out of my predrilled and countersunk holes before I started driving the screws.

As to why I didn't use 1 inch MDF, my local BORGS don't carry MDF thicker than 3/4, and there really aren't any sources of good quality wood in my general area.

And as to why I didn't use glue, I didn't think that I could get enough weight on such a large and wide surface to provide enough pressure for a good glue job.

Louis

[Rich Engelhardt]

Hello,

I didn't think that I could get enough weight on such a large and wide surface to provide enough pressure for a good glue job.

The last thing I glued that was too lage to clamp, I spread the glue - slapped the pieces together an pulled the van on top of them.
Tacked it together with 18 ga staples.
IMHO - it will take an Act of God to seperate the pieces.

[Larry Fox]

If the pieces will fit into the bag I use glue and a vac-press. If they will not I use the countersink both sides of one piece (very slight on bottom) and drill trick.

[Howard Acheson]

>> I didn't think that I could get enough weight on such a large and wide surface to provide enough pressure for a good glue job.

You didn't say what you were using this panel for.

In the shop I was involved with, we used solvent based contact cement to glue particleboard or MDF panels together. We had a contract with a bank chain and made all their teller station countertops and floor customer tables using double thick particleboard or MDF which was then laminated with plastic laminate. Probably did 300-500 over a 15 year period.

I have build a number of router table tops and bench tops using exactly the same procedure.

In the shop, we sprayed the contact cement but in my own shop I either use a roller or a notched applicator.

I don't think I would ever consider trying to screw two composition material panels together.

[Rob Russell]

And as to why I didn't use glue, I didn't think that I could get enough weight on such a large and wide surface to provide enough pressure for a good glue job.


A bunch of cinderblocks would provide more than enough weight for clamping pressure. Just make sure you have a flat surface to layout the material on or you'll be gluing it into a non-flat shape.

[Louis Brandt]

Well, now that I know that contact cement works, I may try that next time. But if I recall correctly, when I posted a similar question about laminating MDF some time ago, I received some replies that suggested that I needed a lot of weight on them to make it work.

In any case, the final result came out ok, but with all of the things that I've learned from the responses, I'll do it different next time. As for what I built the apparatus for, it's just for use as an infeed/outfeed table surface for my RAS.

[Jason Whelehon]

I saw a great idea, empty milk jugs + sand/water

[Tom Esh]

Well, now that I know that contact cement works, I may try that next time. But if I recall correctly, when I posted a similar question about laminating MDF some time ago, I received some replies that suggested that I needed a lot of weight on them to make it work.
I think a lot of us tend to assume (understandably) that it needs the same sort of clamping pressure as other types of joints. Fact is almost all the stress on a laminate joint is shearing load, however if it's sealed to prevent warping and supported adequately to prevent sag you eliminate the shearing stress. I've done quite a few shop fixtures using nothing more than the weight of the mdf for clamping. None of them shows any signs of comming apart and it's been years for some. OTOH, the first shop fixture I did 25 yrs ago I did not support adequately and although still in use it has noticable sag. Not being much of a WW'er back then I through-bolted it with countersunk machine screws, which I'm sure is the only reason it's still in one piece.