Experiments with Glue and Wood

I wanted to glue up a workbench top 90” long, 30” wide, 4” thick using laminations of 1 1/4" southern yellow pine (SYP). To do it with clamps every six inches would require 14 clamps. I only had four clamps over 30" long, so I wanted to try another way. I got advice on the SawMill Creek forum and decided to try applying glue, screwing each board to the next one, then after the glue dried, removing the screws and gluing on another board.

I wasn’t sure if the screws would apply enough pressure, so I experimented.

Experiment 1:

Two pieces of SYP each 9" x 7”.
Screws centered on long axis 1.5” and 4.5” from one end.
#10 x 2-3/8" ProMax Round Washer Head Dry Lube Screws - from McFeely’s
Titebond glue spread with the objective of just getting a bit of squeeze out after clamping

As you can just see in the lower left corner, the screw force caused the two pieces to bend away from each other on one side. In the right picture, the 9 x 7 grid has been cut into 63 pieces for testing.

I laboriously started measuring the breaking force for each joint.

It's not a sophisticated system, but I was able to fill the bucket hanging at the end of that cord with junk until the joint broke, then weigh the bucket. I soon noticed that there was a lot of variation in measurements but there were just three kinds of breaks: 1) at the glue line, 2) partly at the glue line, and 3) all in the wood. I stopped weighing the bucket and started breaking the blocks and recording the kind of break.

Here are the results of experiment 1:

Clearly, the bent boards I mentioned above caused the upper board to bend away from the lower along the left edge and there was little or no clamping force there.

Conclusions from Experiment 1:

The screw force was sufficient to make a good glue joint.
The clamping force appeared to extend along the grain at least 4 1/2”
The clamping force appeared to extend across the grain 1 1/2".
Wood bends when you squeeze it.






Experiment 2:

Two pieces of SYP each 13" x 4”.
Screws centered on long axis 2.5” and 4.5” from one end.
#10 x 2-3/8" ProMax Round Washer Head Dry Lube Screws
Titebond glue spread with the objective of just getting a bit of squeeze out after clamping

Here’s a SketchUp version of the setup.














I cut one inch squares the long way and short way. Because my circular saw doesn’t cut deep enough, I finished the remaining short cuts with a handsaw. It wasn’t necessary to finish the long cuts.

You can see in the right side picture that the square on the right edge of the glue-up, furthest from the screws broke off cleanly with no adhesion.




Here are the results:

There is a definite anomaly here. It was caused by an oily knot which was on the bottom of the top board. I had hand-planed both boards, but I had not wiped them with naptha as is suggested for SYP.

Conclusions from Experiment 2:

The clamping force appeared to extend along the grain at least 8 1/2”
The clamping force appeared to extend across the grain 2 1/2".
Oily spots, usually related to knots, do not glue well.















Experiment 3:

Based on the data above, I had a theory. A screw puts a point load on the top surface of the wood where the head rests. Force spreads from that point in an oval which is substantially longer in the direction of the grain. For a 1 1/2" board, it covers about 4” side to side and about 12” along the grain. Because it’s an oval, it doesn’t cover the corners of that rectangle.

Based on that theory, I thought I could glue up my 4” wide board with screws every 5” staggered from side to side and an extra at each end. It would look like this, but it would be 90" long.



Two pieces of SYP each 26x4.”
Screws centered on long axis 2.5” and 4.5” from one end.
#10 x 2-3/8" ProMax Round Washer Head Dry Lube Screws
Titebond glue spread with the objective of just getting a bit of squeeze out after clamping

This is what it looked like after I had broken each 1" square and put all the samples back in place. All had broken in the wood, meaning that the glued joint was stronger than the wood itself.


Because the screws were removed after the glue dried and before the next board was glued on, there is no metal in the top. And, it worked. It's been about six months and my top has not shown any cracks nor has it warped at all since the first month.

Conclusion: A $7.50 box of screws can substitute for $500 worth of clamps. But it will take a whole lot more time and effort.