Glue Experiments for Laminating a Bench Top

[John Schreiber]

I wanted to glue up a bench 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 long enough, so I wanted to try another way. I got advice here on the Creek (http://www.sawmillcreek.org/showthread.php?t=83320&highlight=glue) and decided to try applying glue, screwing each board to the next one, then removing the screws after the glue had dried.

I wasn’t sure if it would work though. 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

Picture below.
121676
As you can just barely see in the lower left corner, the screw force caused the two pieces to bend away from each other on one side. You can also see how I marked out a 9 x 7 grid to test each of the 63 square inches.
121677
I cut the grid apart and laboriously started measuring the breaking force for each joint.

121678
Not sophisticated, but I was able to fill the bucket hanging at the end of that cord with junk until the joint broke, then measure the weight of 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’s what happened in experiment 1.
121679
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 13x4.”
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 from McFeely’s

Here’s a SketchUp version of the setup.
121681

continued in next post . . .

[John Schreiber]

I cut one inch squares the long way and short way. Because my circular saw won’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 this picture that the square on the right, furthest from the screws broke off cleanly with no adhesion.
121687

Here are the results:
121688

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.

121689
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.
121690

This is what it looked like after I had broken each 1" square and put all the samples back in place.
121691

For the actual benchtop, the screws were removed after the glue dried and the next board was glued on with screws as clamps again. That way, 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: If you are really cheap, a $7.50 box of screws can substitute for $500.00 worth of clamps. But it will take a whole lot more time and effort.

[David Keller NC]

Wow - That's an incredible amount of work to avoid buying a few extra clamps! But well done and well thought out...

[Walt Caza]

Another interesting post from Mr. Schreider.
Good work John, seems you are blazing new trails again.

With Creekers like you making colourful postings all the time...
I am never bored. I enjoyed reading this and the linked original thread.

For those who do not much follow the Creek's off-topic forum,
John regularly contributes well thought out and researched ideas.
His body of work is informative and entertaining.

Very cool work. Please give us a peek if you go ahead with this strategy.
I'm sure many would enjoy pics, if you build a benchtop this way.

We could all benefit from sharp minds like yours...
if you would only put your energy into helping Mother Earth!
be well,
Walt

ps I enjoyed your 'wrenches' pic alot. I'm still smiling...

[Frank Drew]

If it was already January I'd make some comment about the long winters you must have there... :D

[Ken Garlock]

would go though an exercise like that.:D

I would have made some maple cauls and broke the bank for a couple more clamps. But wait, then we have a bridge force distribution problem. Will it never end.:eek:
OH,,, just go buy the top. ;);):)

Anyway, somewhere I missed the pictures of the new bench...

[John Schreiber]

What can I say, I wanted to be a scientist when I grew up but I couldn't even get into a program. Then I wanted to be an engineer, but I couldn't pass calculus (three semesters of Introduction to Calculus - followed by a recomendation from my advisor).

There was an extensive debate here on the Creek about whether this approach would work, so I wanted to find out for sure before I started the work.

Cauls distribute pressure, but they don't create pressure. You need more or bigger clamps. Budget is tight, so if I was going to buy more clamps, I'd have had to postpone the project or cut corners on the bench itself.

The bench is now done (except for drawers). It's a hybrid of Ruobo and Shaker and I've added some special features. It works great. When I get a chance I'll post some pictures.

[David Keller NC]

"What can I say, I wanted to be a scientist when I grew up but I couldn't even get into a program. Then I wanted to be an engineer, but I couldn't pass calculus (three semesters of Introduction to Calculus - followed by a recomendation from my advisor)."

Hmm - Not sure you want to know this, but it's a very rare engineer that actually uses calculus after college. Even with a PhD in that field (engineering), I didn't use it past grad school. I think they put it in there to torture undergrads (sort of like thermodynamics).

[Cliff Rohrabacher]

Indeed Few people use calc' formally. ICBM engineers have to.

Archers and shooters do too but they do it on the fly leaving all those messy numbers and calculations out of the picture.
Lets see I lead the duck by oh this much and then oops the shot is steel now so that changes the ballistic time to the duck. Lead a bit more and squeeze . DINNER~!!

[Logan William]

Agreed that most engineers don't use calc after they get out of school. As a recent(less than 2 years ago) grad I think I've used the calc once, and what I was doing was covered in Gen Calc, primarily anymore its a class to "thin the ranks" so to speak, calc, engineering physics I&II, thermo, fluids, for my major operations research are basically just to build a common foundation of knowledge and lower the class size.....for better or for worse. But sorry for continuing the thread jack, excellent work on the write up and its giving me some ideas for when I build my work bench this fall(hopefully).