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Step 14The top, step two - laminating the MDF
The next step is to laminate the two sheets of MDF that will make up the lower layers of the top.
First, trim the MDF to slightly oversize. You'll want room to clean up the edges after the pieces are joined, but you don't need more than a half-an-inch on each side for that, and there's no point in wasting glue.
If you're lucky enough to have a vacuum press, use that. Otherwise drill holes for the screws in the bottom layer at all the points you had indicated in your layout. You'll also want to either drill a row of screws around the outside edge, in the bit you're going to trim off, or you'll need clamps all around the edge. I just added more screws.
The screw holes should have sufficient diameter that the screws pass through freely. You want the screw to dig into the second layer and to pull it tight against the first. If the threads engage both layers, they will tend to keep them at a fixed distance.
If you're using drywall screws, you'll want to countersink the holes. Drywall screws are flat-head, and need a countersink to seat solidly. If you're using Kreg pocket screws, the way I did, you won't want to counter-sink the holes. Kreg screws are pan-head, and seat just fine against a flat surface. Both drywall screws and Kreg pocket screws are self-threading, so you don't need pilot holes in the second sheet of MDF.
Regardless of which type of screw you use, you'll need to flip the panel and use a countersink drill to on all of the exit holes. Drilling MDF leaves bumps, the countersink bit will remove them, and will create a little bit of space for material drawn up by the screw from the second sheet of MDF. You want to remove anything that might keep the two panels from mating up flat. I set a block plane to a very shallow bite and ran it over what was left of the bumps and over the edges. The edges of MDF can be bulged by by sawing or just by handling, and you want to knock that down.
After you have all the holes clean, set things up for your glue-up. You want everything on-hand before you start - drill, driver bit, glue, roller or whatever you're going to spread the glue with, and four clamps for the corners. You'll need a flat surface to do the glue-up on - I used my hollow core door on top my bench base - and another somewhat-flat surface to put the other panel on. My folding table was still holding my oak countertop, which makes a great flat surface, but I want to make sure I didn't drip glue on it so I covered it with some painters plastic that was left over from the last bedroom we painted.
Put the upper panel of MDF on your glue-up surface, bottom side up. Put the bottom panel of MDF on your other surface, bottom side down. (The panel with the holes drilled in it is the bottom panel, and the side that has the your layout diagram on it is the bottom side.) Chuck up in your drill the appropriate driver bit for the screws your using. Make sure you have a freshly-charged battery, and crank the speed down and the torque way down. You don't want to over-tighten the screws, MDF strips easily.
Once you start spreading glue, you have maybe five minutes to get the two panels mated, aligned, and clamped together. So make sure you have everything on-hand, and you're not gong to be interrupted. Start squeezing out the glue on one MDF panel, and spreading it around in a thin, even coating, making sure you leave no bare areas. Then do the same to the other MDF panel. Then pick up the bottom panel and flip it over onto the upper panel. Slide it around some to make sure the glue is spread evenly, then line up one corner and drive in a screw. Line up the opposite corner and drive in a screw there. Clamp all four corners to your flat surface, then start driving the rest of the screws, in a spiral pattern from the center.
When you're done, let it sit for 24 hours.
First, trim the MDF to slightly oversize. You'll want room to clean up the edges after the pieces are joined, but you don't need more than a half-an-inch on each side for that, and there's no point in wasting glue.
If you're lucky enough to have a vacuum press, use that. Otherwise drill holes for the screws in the bottom layer at all the points you had indicated in your layout. You'll also want to either drill a row of screws around the outside edge, in the bit you're going to trim off, or you'll need clamps all around the edge. I just added more screws.
The screw holes should have sufficient diameter that the screws pass through freely. You want the screw to dig into the second layer and to pull it tight against the first. If the threads engage both layers, they will tend to keep them at a fixed distance.
If you're using drywall screws, you'll want to countersink the holes. Drywall screws are flat-head, and need a countersink to seat solidly. If you're using Kreg pocket screws, the way I did, you won't want to counter-sink the holes. Kreg screws are pan-head, and seat just fine against a flat surface. Both drywall screws and Kreg pocket screws are self-threading, so you don't need pilot holes in the second sheet of MDF.
Regardless of which type of screw you use, you'll need to flip the panel and use a countersink drill to on all of the exit holes. Drilling MDF leaves bumps, the countersink bit will remove them, and will create a little bit of space for material drawn up by the screw from the second sheet of MDF. You want to remove anything that might keep the two panels from mating up flat. I set a block plane to a very shallow bite and ran it over what was left of the bumps and over the edges. The edges of MDF can be bulged by by sawing or just by handling, and you want to knock that down.
After you have all the holes clean, set things up for your glue-up. You want everything on-hand before you start - drill, driver bit, glue, roller or whatever you're going to spread the glue with, and four clamps for the corners. You'll need a flat surface to do the glue-up on - I used my hollow core door on top my bench base - and another somewhat-flat surface to put the other panel on. My folding table was still holding my oak countertop, which makes a great flat surface, but I want to make sure I didn't drip glue on it so I covered it with some painters plastic that was left over from the last bedroom we painted.
Put the upper panel of MDF on your glue-up surface, bottom side up. Put the bottom panel of MDF on your other surface, bottom side down. (The panel with the holes drilled in it is the bottom panel, and the side that has the your layout diagram on it is the bottom side.) Chuck up in your drill the appropriate driver bit for the screws your using. Make sure you have a freshly-charged battery, and crank the speed down and the torque way down. You don't want to over-tighten the screws, MDF strips easily.
Once you start spreading glue, you have maybe five minutes to get the two panels mated, aligned, and clamped together. So make sure you have everything on-hand, and you're not gong to be interrupted. Start squeezing out the glue on one MDF panel, and spreading it around in a thin, even coating, making sure you leave no bare areas. Then do the same to the other MDF panel. Then pick up the bottom panel and flip it over onto the upper panel. Slide it around some to make sure the glue is spread evenly, then line up one corner and drive in a screw. Line up the opposite corner and drive in a screw there. Clamp all four corners to your flat surface, then start driving the rest of the screws, in a spiral pattern from the center.
When you're done, let it sit for 24 hours.
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As you say MDF is very responsive to moisture which will contribute to your oak top distortion. Quality plywood is much more stable.
My suggestion would have been (and this is the project I have in mind…) to use marine grade plywood as a base and epoxy the whole panel (as we do in boat building) and glueing the oak top altogether with the same epoxy coat. This should greatly reduce the risk of distortion of the top and will allow not to oil the top. Any work surface that is coated will inevitably at one point or another mark the piece you'll be working on … a good bench top should be bare wood… But I understand that a top with a 5cm thickness made of absolutely dried oak is something impossible to find and too expensive these days (and quite heavy too : but that is also the quality of a good bench : that is why I do not intend to fix levelers at the bottom of its legs)…
This question apart, this is a great job and I can only hope that mine will approach yours ! …
I'd not have used levelers, if I'd not needed them for the floor I was putting it on.
but you get plenty of dust in your lungs from MDF !!!…
Actually I felt I made somewhat a fool of myself when asking this question to someone who made such a great job.
If I had written it in my mother tongue I know I would have found a way to talk that would have been less "up front", but I'm not sure I would have been understood ! …
Have a nice week.
Could you help me out here? Thanks.