I made my benchtop out of MDF. 4 layers 18mm (3/4") each. I used PVA. Just
ensure you have a flat surface to work on as MDF will sag under its own
wieght once the glue is applied. Not a lot, maybe 2 - 4mm on your span, but
can easily be avoided by taking that into account. It also helps to have
another willing helper to glue up and help lift the sheets as they're mighty
heavy. I just placed a load of other wood and tools and anything else at
hand on top to hold it tight while the glue set. It is as solid as a rock,
but much heavier.
vacuum bagin works very well in this application. you get very uniform
clamping pressure with a vacuum bag, and do not need extra boards to
distribute the load, like with clamps. Like the other poster said, just make
sure you have it supported flat when you glue it up. Assuming you get 10
inches of mecury vacuum pressure (which is real easy to do near sea level,
even with an old worn out vacuum pump), you will be applying over 12,000 lb
total force to your glueup. It is hard to match that with weight.
I have used Titebond in this application, and it worked well. If possilbe
tack two corners together with a small brad to keep the baords from shifting
while you apply the bag.
Joe in Denver
my woodworking website:
Not sure your math is quite right here... It's a lot of force, I grant you,
but I don't think it's quite that much. Here's how I figure it:
10" mercury = approx 1/3 atmosphere = approx 5 lb / sq inch.
13"x96" = 1248 sq in.
5 psi * 1248 sq in = approx 6250 lb.
Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?
Depends on how one is measuring -- a 10" reduction "below normal pressure",
or a reduction *to* 10" of pressure.
If it's a reduction _to_ 10" (a circa 20" 'differential' to the outside world),
then the 12,000+ pounds is right. 1296*14.7*(29.92-10)/29.92 = 12,683+ <grin>
I've done quite a bit of this using full size 4x8 sheets of 1/2 CDX.
Did it with TiteBond II, worked quite well.
Would suggest the following:
Spread a sheet of plastic on a bench or on a concrete floor (driveway)
that is flat, then lay down first piece of MDF.
Using a Bondo spreader (plastic piece, 3"-4" wide), apply lots of
TiteBond II (Now you know why I buy in in gallon jugs) to the bottom piece.
Lay top piece on glue surface, align pieces, and clamp on each end with
2" spring clamps.
Now weight the two (2) boards down using full size (8x8x16) concrete
blocks as weight, removing spring clamps as you go and allowing excess
TiteBond to ooze out.
Allow to cure at least 24 hours before removing blocks.
There are several better techniques to get and hold registration between
the two (2) pieces, but the above was good enough for my job.
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