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Are Most Benchtops Woefully Underclamped during Glueup? (probably not)

by Nicholas Hall
posted 12-06-2012 05:46 PM


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88 replies so far

View Monte Pittman's profile

Monte Pittman

15501 posts in 1090 days


#1 posted 12-06-2012 06:09 PM

Part of my concern would be the pressure needed to crush the wood. I find it easy to comprehend that different woods require different pressures or that grain orientation makes a difference. But I don’t want to ruin the wood trying to create the perfect glue joint. Just a thought.

-- Mother Nature created it, I just assemble it.

View crank49's profile

crank49

3524 posts in 1723 days


#2 posted 12-06-2012 06:16 PM

Air bags.
A 12” diameter air bag exerts 12,400 lb of force at 110 psi.
A row of 8 bags could be arranged to push a big caul along one side of a bench/table top glue-up.
That would be 99,525 lbs of force applied over an 8 ft length.
And the units could all be piped together so you would only need one valve to control it.

May be a little spendy; the bags are between $200 and $800 each.
The structural frame will be a couple grand at least.

But hey, quality costs.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View David Craig's profile

David Craig

2135 posts in 1860 days


#3 posted 12-06-2012 06:30 PM

I think there is a little confusion here on how this article translates. From the math, I get the feeling that there is an assumption that the clamping pressure of one clamp is only providing force for one square inch. The clamp is providing pressure across the whole board but the concentration of pressure is within a smaller radius. Keep in mind that sometimes too much pressure causes failed glue-ups. Some people exert so much force that the joint is starved for glue.

-- There is little that is simple when it comes to making a simple box.

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#4 posted 12-06-2012 06:42 PM

@ Monte: It’s actually not that much more force than I typically use on a kitchen table-top. For a kitchen tabletop with a finished thickness of 1” x 6 ft long, made out of cherry, with the quartersawn grain in the glue joint, the table above would give us 250psi*72 square inches or 18000 total pounds of force exerted. That means I need 17 clamps to do a 6 ft long tabletop. I currently use 10, just because thats how many I have.

The trouble comes when you get to cherry and oak, because these have more demanding force requirements for an optimal glue joint. But by the same token, they are much stronger woods that are more resistant to the force so no wood is going to be damaged, particularly with when using cauls.

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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crank49

3524 posts in 1723 days


#5 posted 12-06-2012 06:43 PM

Well, the chart in the article says pressure required per Sq.In. for oak is 900 Lbs for flat sawn face glued.

A bench top glue joint is 4” x 96” = 384 sq. in.

384 sq. in. x 900 lb required per sq.in. is 345,600 total lbs of force.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#6 posted 12-06-2012 06:52 PM

@crank: In my family business PRC Industrial Supply in Porland ME, we used airbags to generate pressure for the vulcanization of conveyor belts. The system used four 10”x6” H beams and an airbag. We would charge the bag to 100psi and it would exert 151,200 pounds of force on the splicing area heated to 300 degrees. That’s how you splice 6 foot wide industrial conveyor belts in paper mills. It works like a champ, but the splicing press is wee bit costly as you suggested!

@David Craig: It’s possible that I read the article wrong but I don’t think so. What he does say specifically in the article is that it is very nearly impossible to “starve a glue joint” with too much pressure, because most people have never seen enough clamps to accomplish this feat, let alone own enough clamps to do it themselves.

Pounds per square inch seems fairly unambiguous. If you have 384 square inches, and you need to generate 900 psi for a glue joint, you need to generate 345,000 pounds of force to accomplish this. At least that’s how I interpret it.

Are there any engineers out there with access to Fine Woodworking.com who can take a look? I’m really curious at this point.

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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David Craig

2135 posts in 1860 days


#7 posted 12-06-2012 06:58 PM

crank, I am not disputing your math in regards to total force, but the assumption that the clamp is only providing 1050 pounds of force. The clamp is providing 1050 pounds of force per square inch. Lets say you have 3 12 inch wide boards pressed together. The clamp optimum is in the center which is providing 1050 pounds per square inch of force. Wouldn’t you now have 37,800 (edited due to poor math) pounds of force applied?

-- There is little that is simple when it comes to making a simple box.

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#8 posted 12-06-2012 07:13 PM

Here is a pic of an Almex sectional hot vulcanizing press like I used to use. This is a 6 beam unit, but the principle is the same. Those airbags put out some crazy force. Mind you, I’m not proposing doing this by any stretch of the imagination. These cost $10,000. What I’m wondering is if there is a cost effective way to generate 50,000 of clamping force, in order to maple conference room tables. It’s just a though experiment…

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#9 posted 12-06-2012 07:22 PM

@ crank: I here what your saying; it is possible that you are right. I’m no engineer, and I’ve misunderstood more than a few things in my life:). If you want I can email you the article. PM me if you’re interested.

The method for calculating the total force for the glue-up comes straight from the article though. The article’s author also references “Understanding Wood” by Hoadley, and Hoadley comes to similar conclusions regarding the calculation methods for determining total force required for a glue up.

Is there an engineer out there who might shed some light on this?

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

View PurpLev's profile

PurpLev

8476 posts in 2400 days


#10 posted 12-06-2012 07:36 PM

a few things:

1. while many articles discuss not applying enough pressure, I have never seen any reference (with supporting evidence) that a joint can be starved of glue – or over-clamped.

2. there are Optimal numbers, and then there are acceptable numbers, I think what the article refers to are optimal numbers meaning BEST CASE SCENARIO. does not mean than anything below those numbers will result in joint failure. From the wording of some of those sentences one would think that 98% of all woodworking projects should have failing joints, but in reality this is just not the case.

to sum up the above, this is why you should use whatever clamps you have, and clamp those table tops as much as you can – as much as your clamps will allow you to. it’s “the best you can do”. If you mill your wood properly, the friction between the wood fibers will also reduce the chance of those joints failing, the glue is really only an additive.

don’t over think it. History suggests that it’ll be OK ;)

-- ㊍ When in doubt - There is no doubt - Go the safer route.

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bondogaposis

2755 posts in 1103 days


#11 posted 12-06-2012 08:05 PM

Interesting to be sure, and it supports the old adage that you never can have too many clamps. Even if you do have enough clamps you can’t generate enough clamping pressure even if you space them side by side w/ no space between them! 345,000 lbs of force?, you would have to run into it with a pickup truck to generate that much force. I think there is something lost between the research and practical application of the theory.

-- Bondo Gaposis

View DannyB's profile

DannyB

46 posts in 2174 days


#12 posted 12-06-2012 08:09 PM

Stop trying to think in terms of how many individual clamps you need.

If the goal is to exert 900 PSI evenly across the piece, you aren’t going to need 335 clamps.

You need just a few cauls and clamps to hold them

The cauls will distribute pressure evenly up to some amount
For example: http://www.woodpeck.com/ottclampingcauls.html

These say they will deliver up to 225 lbs evenly.
So you’d need exactly 4 pairs of cauls (8 clamps) to get to 900psi.

Now, that may not be possible to fit, but it’s certainly better than 160 clamps

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#13 posted 12-06-2012 08:34 PM

@ PurpleLev: I wholeheartedly agree that there is an important distinction between optimal numbers and acceptable numbers. I’m not suggesting that any joint below optimal will fail. I do however have a cherry kitchen table built by my father-in-law 15 years ago that has 18” glue-line failure. The table is perfect otherwise, there is just the one glueline failure. My guess is he didn’t use enough pressure. I’ve worked with him in the shop so I know for sure he tons of glue (it gets messy). Every time I look at that table, I stare at that glue-line. It’s awful. Non-woodworking friends comment on it. It’s a $1000 table that I only keep around because he made it.

However, if I’m going to sell someone a 2” thick maple conference table that’s 8 feet long, and I know that the optimal force for the glue joints is 230,000 lbs based on the chart above, I’m not sure I’m comfortable using 10,500 lbs using my 10 little pipe clamps. That only gets me 5% of the actual pressure I need.

A good analogy is a car tire. If I look at the tire and see that the optimal pressure is 35psi, but only have a pump that can produce 2psi, should I call it good (that’s about 5% of 35psi). My guess is that even at 2 psi, the car will probably drive acceptably. Just the same, it seems like I’d be cutting it a bit close to the wire.

With all that said, I think you are right. The bench will be fine. I’m going to buy 10 more Jorgenson pipe clamps at $14.00 apiece and call it good. You can never have too many clamps. My experience using high pressure vulcanizing clamps just got me thinking. It’s unfortunately my nature to tinker and think about wierd stuff like this.

Thanks for the reality check though; I need one every once in awhile :)

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

View Nicholas Hall's profile

Nicholas Hall

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#14 posted 12-06-2012 08:45 PM

@ DannyB: The unit is pounds per square inch. The more square inches you have, the more pounds you need to generate the same PSI.

Force radiates away from clamp in a 45 degree angle. The point of a caul is to increase the surface area over which a single caul is able to apply force. They don’t multiply the force, they divide it. Take a look at the diagram below to see what I mean.

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

View derosa's profile

derosa

1557 posts in 1587 days


#15 posted 12-06-2012 10:05 PM

Get half the number of clamps but get ones with significantly more strength and use good cauls. I beam clamps can apply 3 times as much pressure as a pipe clamp; by using cauls to change the location of the pressure you can really crank down on the clamps.

-- --Rev. Russ in NY-- A posse ad esse

View Cosmicsniper's profile

Cosmicsniper

2199 posts in 1910 days


#16 posted 12-06-2012 10:32 PM

I believe you can over clamp a joint. So dies, Matthias here…

http://woodgears.ca/joint_strength/glue_methods.html

-- jay, www.allaboutastro.com

View Steve Peterson's profile

Steve Peterson

263 posts in 1834 days


#17 posted 12-06-2012 10:53 PM

Nicholas,

That is very interesting data, Nicholas. I wonder what the chart would look like regarding glue strength vs clamping pressure. You get the maximum glue strength in flatsawn sugar maple at 1200 psi. This does not imply that you only get 10% as much strength at 120 psi and 1% as much strength at 12 psi.

It certainly does not seem possible to starve a joint of glue by applying too much pressure. The only way I can see to starve a glue joint would be to squeeze out most of the glue then reduce the pressure while the glue dries.

I did notice a statement on a bottle of veneer glue recommending something like 750 psi. A vacuum press can only generate a max of around 14 psi and this is the most pressure that an average woodworker can produce in their shop.

-- Steve

View Dan Krager's profile

Dan Krager

1742 posts in 986 days


#18 posted 12-06-2012 11:01 PM

...BTW Nicholas, 345,000/10,500 = 32.8 not 335. You might have said that tongue in cheek, but I’m a math major so… go figure!
There are a LOT of factors besides pressure that contribute to glue line failure. I have used a lot of exotic techniques for edge gluing and have so far not had a glue line failure that I noticed for any of those techniques. The most intriguing one is to plane the mating edges one at a time with a hand plane sharpened with a camber. This puts a tiny (almost microscopic) concave surface on the edge. When mated with another edge prepared the same way, there is a glue pocket formed that does not allow the joint to be starved for adhesive, and at the same time helps with closing the glue line reliably. This is not normally visible in exposed end grain.
Building tension into the glued panel that relaxes as the boards continue to age is done by planing the edges so that the center of the lengthwise joint is open when the boards are simply laid next to each other with both ends touching. We’re talking about a gap of under .005 for a six foot length. Pressure from glue clamping closes the gaps putting tension in the panel deliberately. This tension dissipates over time, but it tends to keep the ends of the panel from separating.
These two techniques have worked reliably for me over the years. I am skeptical of the research you refer to. It is possible to be so heavenly minded that you’re no earthly good. I have repaired more than my share of split tops that were poorly prepared…no glue could have held them together. I have worked with other panels that have not split and ring like a bell when tapped…still retaining some tension.
Don’t overlook moisture content. It is not necessarily uniform as one might assume. This could be a very long list if I tried to spell out all things that contribute to glue line failure. Not going to do it.
DanK

-- Dan Krager, Olney IL http://www.kragerwoodworking.weebly.com

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patcollins

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#19 posted 12-06-2012 11:28 PM

Are workbench glueups underclamped? Do you know any that failed?

I am a mechanical engineer and while I haven’t taken any data myself about glue failure I know one thing, every glueup that I have seen fail it was the wood failing and not the glue. No clamping pressure is going to increase the strength of the wood.

Nicolas your father in laws table most likely had a glue line failure because the wood warped, and unless you left the clamps on forever no amount of clamping pressure would keep the wood from warping because the funny thing about clamps is when you take them off the pressure holding the boards goes away and perfectly flat/straight boards don’t pull apart on their own.

View RussellAP's profile

RussellAP

2966 posts in 1038 days


#20 posted 12-06-2012 11:49 PM

It’s interesting to know that science has found the correct pressure. Beyond that it’s meaningless for woodworkers. Grain direction will skew any pressure pattern you may have. Cups will keep the pressure pattern narrow and crowns will spread them out more.
It’s more important to have straight edges, even glue, good drying habits, and some luck because there is no way you’re going to safely clamp anything at 1200 lb sq in, unless maybe some exploding wood. Maybe some of you pro’s have the time and know how to set something like this up, but for most of us it wouldn’t work.

-- A positive attitude will take you much further than positive thinking ever will.

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crank49

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#21 posted 12-06-2012 11:52 PM

Steve, I’d be curious about that veneer bottle statement of 750 PSI.
Many many folks who do veneer use vacuum bagging systems.
And you are correct, they only develop 14 PSI. Can’t do more with vacuum. It’s impossible.
Now on a one foot square area, that 14 PSI effectively produces 2016 Lbs of force, but its still only 14 PSI.

Comicsniper, I have the highest regard for Matthias. He always looks at things from the practical viewpoint and his logic is down right Vulcan.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#22 posted 12-06-2012 11:56 PM

@ Dan: That’s an interesting approach to glue ups I had never of. You are right of course that what I’m proposing is overthinking things a bit. This is really a thought experiment more than anything else. The interesting question to me is how could one exert even 100,000 pounds of clamping force in the home workshop affordably.

@Patcollins: I don’t think anyone needs to worry about a catastrophic glueline failure on a 4” thick ash benchtop. These are 400 benches made to hold 20lb boards. Incidentally, the optimal pounds per square inch is defined as the point at which the glueline is stronger than the wood. The more interesting thing to me is the problem of the visual glueline on thick maple tops that require 1200psi. You are probably right that clamping pressure is not the biggest risk factor for this though. Nice to have an engineer’s perspective.

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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patcollins

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#23 posted 12-06-2012 11:59 PM

Vaccum bag can get 14.7 psi, the approximate atmospheric pressure. I suppose you could put your veneer glueup under water for more pressure.

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#24 posted 12-07-2012 12:51 AM

@Crank: Mathias 10 times the woodworker that I’ll ever be, and he’s smarter too. That said, you’ll note that that he’s exterting a massive amount of pressure in his “high pressure” joint. It looks like he’s clamping maple. He’s using 4 c-clamps on a 1.575”x1.575” area. Cheap ductile c-clamps generate 2800 pounds of force. So we have 11200 pounds of force over 2.48 square inches. That’s 4500 psi. The chart above says 1200 is optimal for maple. The author of the article does say that it’s possible to starve a glue joint, he says it’s unlikely to happen outside of a labratory.

Here is a link to c-clamp force measurement: http://www.newmetalworker.com/reviews/besycclmpvar.html

Still Mathias’ experiment is very interesting. What it lacks is a control of sorts. He massively overclamps some, massively underclamps others, but none of his test cases evaluate the strength of a normal joint that most woodworkers create. He uses 4800 psi for the overclamping an 1psi for the rest. I’d love to know what the strength would be at the optimal pressure.

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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Cosmicsniper

2199 posts in 1910 days


#25 posted 12-07-2012 12:56 AM

Yeah, Matthias is applying a ton of pressure, but the question above was posed if too much pressure is possible. I was just providing some data. When doing large panel glue ups, sure, it’s hard to over clamp. But a lot of people might overclamp low surface area joints, similar to Matthias. Plus, if the panels are thin, like with a guitar soundboard, then those can be easily overclamped as well, IMO.

-- jay, www.allaboutastro.com

View DannyB's profile

DannyB

46 posts in 2174 days


#26 posted 12-07-2012 01:05 AM

@Nicholas
I’m a CS major/Physics minor, so I do understand they are going to spread load further, however, i will point out
1. The cauls in that picture are not cambered properly, so they are not actually spreading load very evenly, as the picture itself shows.

2. you are assuming you can’t apply more force to the ends of a caul than you can to a small area of wood.
Since it spreads out load better, you can in fact, use a lot more force without damaging anything, precisely because it is spreading it out.

So it’s not a simple game of “i can apply 1050psi with a pipe clamp, and thus, with a caul over 48 inches, i can only get 1050/48”

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#27 posted 12-07-2012 01:20 AM

@Danny B: A pipe clamp exerts 1050 pounds of force. The psi that it exerts is a function of the area over which that force is applied. If it is applied to a nail it will be over 10000 psi. If it is applied to 100sq inch caul it will be 10psi. The chart in the post says maple needs 1200 psi. To acheive this over a 384sq in area will require 345,000 pounds of force. My question is not how to distribute the 345,000 pounds of force. My question is how in the heck could a woodworker ever generate that much force in the first place.

Here is the text from the article regarding the calculation of necessary force:

“The first step when gluing boards is calculating the
square inches of glue surface. For example, if you
are gluing two boards 3⁄4 in. thick and 36 in. long, a
single glue surface equals 27 sq. in. Even if you are
edge-gluing several boards, you still need to measure
only one glue surface because the clamping pressure
is transmitted across the width of the boards. If you
are edge-gluing flatsawn red oak boards and wish
to apply about 450 lb. psi, then 27 multiplied by 450
equals a force of 12,150 lb. that must be applied. Using
the average of the editors’ clamping forces, this
could be supplied by around nine heavy-duty bar
clamps, a dozen 3⁄4-in. pipe clamps, or 26 quick-grip
clamps. Obviously it would be hard to fit 26 clamps
along a 36-in. board, so add some more powerful
clamps if you have them. It’s fine to mix and match
types of clamp.

Here is a link to the article:
http://www.finewoodworking.com/how-to/article/get-serious-about-clamping.aspx

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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patcollins

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#28 posted 12-07-2012 01:43 AM

I would have liked to seen the results if Mathias just put glue on the boards and layed one on top of them other using only the weight of the wood itself.

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#29 posted 12-07-2012 02:08 AM

@Pat: Just to be clear, I’m not saying that 1200psi is necessary for a strong joint in maple, or that there is any risk of glue line failure on a 4” thick glue joint. On a surface area that large, I think you could probably get away with a half a dozen harbor freight F clamps. It’s a 400 bench strong enough for a tank to drive over being used to hold a 20# board.

I am just curious how it would be possible to generate 1200 psi over 384 sq inches in the shop of a hobbiest like myself. Like I said in the post, it’s a thought experiment. Half of the posts above are arguing whether or not the calculation of the force required is correct, or whether or not clamping cauls can infinitely multiply that force through magic. The other half concede that the calculation is correct, but that it is irrelevant because you don’t actually need the optimal amount of clamping pressure recommended.

Like I said in the post, this is a thought experiment I had at lunchtime. I’m wondering how it would be possible to create the 300,000 lbs of force in my basement workshop for a few hundred bucks. I’m not arguing it’s necessary or practical. It’s just something I was trying to figure out.

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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patcollins

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#30 posted 12-07-2012 02:31 AM

I’m thinking some sort of clamshell clamp made of wood that has a 1/4” bolt every inch connecting it to the other mating half, that should generate the clamping pressure fairly evenly especially if you torqued the bolts all to the same value.

View Dan Krager's profile

Dan Krager

1742 posts in 986 days


#31 posted 12-07-2012 02:55 AM

One of the tricks to generate enormous pressure in a home shop is to use air bags in a trapped enclosure. If a veneer press type cage (think heavy truck) were built with thick plate cauls (stacked plywood) an air bag (think car tire or bigger) will generate monstrous pressure.
I’ve heard of using a section of discarded fire hose as a linear air bag of sorts. Again a containment vessel is required. Lay the prepared and collapsed fire hose (may be folded once) beside the edge of a glued panel within the containment and apply pressure. Edge clamp pressure can be enormous. The hose is prepared by cutting off whatever length is needed, leaving the fitting exterior to containment. The fire hose fitting is then fitted with an air hose supplying regulated pressure. Careful welding may be required for that adapter.
Now, to the math. Suppose you can get 100 lbs PSI in the fire hose (they can hold several hundred psi safely). You’ve got 384 sq in, so let’s see, um, um, wait… I’ll get it… um 38,400, yes that’s it 38,400 lbs against the edge.

Hydraulic pressure is an underused pressure resource in a wood shop. Bottle jacks tend to be cheap, but I guess it’s the frame that gets expensive. Three 20 ton jacks can generate 120,000 lbs, but what’s going to hold it? I have been known to use accessories from a hydraulic ram kit, structural I-beams…whatever it takes.
DanK

-- Dan Krager, Olney IL http://www.kragerwoodworking.weebly.com

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OggieOglethorpe

907 posts in 862 days


#32 posted 12-07-2012 03:03 AM

I built an 8’ long bench several years ago that has (2) 12 1/2” wide, 3 1/2” thick ash slabs. The slabs are glue-ups of 3-1/2×1-7/8 boards, glued face to face. I glued them with Titebond, I parked a 735,000 pound 747 on them as they dried… OK… I didn’t really do the 747 part…

I clamped them with Jorgensen pipe clamps and Bessey K-Bodies, just like every other glue-up I’ve ever done. The boards were properly jointed, so it didn’t take much force to close the joints. Through the rough handling associated with final milling, and several years of hand tool use, the slabs are great!

I think you can really over think this…

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Nicholas Hall

348 posts in 858 days


#33 posted 12-07-2012 04:49 AM

I like your style dan :)

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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Viktor

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#34 posted 12-07-2012 05:43 AM

“Clamping force radiates outward at 45 degree” – is that diagram from the aforementioned article? At this point I begin to question author’s (R. Rabiej) credentials. It is inconceivable that professor of engineering would write such a thing.
There must be some mistake, because suggested clamping pressures are similar to compressive strength perpendicular to grain (after which crushing occurs) for the listed species.
Patcollins makes an excellent point here: “I know one thing, every glueup that I have seen fail it was the wood failing and not the glue. No clamping pressure is going to increase the strength of the wood”

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patcollins

1006 posts in 1617 days


#35 posted 12-07-2012 02:23 PM

It is inconceivable that professor of engineering would write such a thing.

I didn’t exactly want to go there, and just assumed it was an over simplified readers digest version.

I did notice that he was a professor of industrial engineering, most engineering students that I knew in mechanical, electrical, computer refered to it as imaginary engineering.

All in all it was an article written for a mass market magazine, I have written for some magazines in the past and some of what I wrote was edited out, but the editors did an excellent job rendering the drawings that I did by hand and sent in for them.

View HorizontalMike's profile

HorizontalMike

6968 posts in 1666 days


#36 posted 12-07-2012 02:42 PM

I just hope that no one gets stupid and actually tries to exert the loads discussed here. They could get hurt or even killed trying.

My 2-cents…

-- HorizontalMike -- "Woodpeckers understand..."

View Nicholas Hall's profile

Nicholas Hall

348 posts in 858 days


#37 posted 12-07-2012 05:25 PM

@Patcollins: It is a very over simplified readers digest version but essentially correct if you are trying to glue together a 1”wide by 3/4” thick by 12” long strip to a 6”wide x 3/4” thick x 12” long strip without using a wide caul, you will wind up with great joints where the clamps were and not great glue joints where they in between (like in the diagram in a prior post).

Here is the non readers digest version by the author of the Fine Woodworking Article:

RJ Rabiej, HD Behm (1992) The effect of clamping pressure and orthotropic wood structure on strength of glued boards, Wood and Fiber Science 24:260-273

Abstract:

Reference values for compression strength perpendicular to the grain were determined for radial and tangential sections of samples of sugar maple and ponderosa pine. Samples to be glued were matched according to specific gravity and orthotropic structure and bonded along the grain in tangential or radial sections. Magnitude of clamp pressure was controlled throughout a range of pressures commonly applied in industry, up to about 80% of the compression strength of the wood sample. Tests were conducted on the bonded samples to determine glueline shear strength and percent of wood failure at the bonded surfaces. Results were subjected to regression analysis to ascertain relationships. It was determined that clamping pressure had a different effect on both shear strength and percent of wood failure depending on species and orthotropic section. It is possible to maximize joint strength by applying proper clamping pressure. Results similar in direction but differing in magnitude were obtained with both PVAc and U-F adhesives. A generalized measure of clamping pressure was defined as the ratio of applied clamping pressure to the compression strength (CP/CS) of the wood section to be glued. Using this concept, the optimum clamping pressure for sugar maple was found to be 0.3 times compression strength using U-F glue and 0.5 times using PVAc glue. This approach to determining reliable clamping pressure data can lead to improved gluing practice and more precise testing procedures.

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lumberjoe

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#38 posted 12-07-2012 05:30 PM

When I build my bench. it’s going to be a split top with each section being 12” wide. That’s about 6 boards per section. Would it be a terrible idea to recess lag bolts every 12” or so on the 4 inner boards before clamping?

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patcollins

1006 posts in 1617 days


#39 posted 12-07-2012 05:52 PM

Lumberjoe if the strength/stability of joints is questionable I would include some sort of mechanical fastening whether it be wood screws internally (lag bolts are way over kill imho), biscuits, dowels, or even a tongue and groove geometry.

I don’t do much in way of hand tool work so I dont need a thick workbench but If I were building one I think that I would like to put a few 1” dowels clear through all of the internal boards, smooth them off and then put undoweled boards on the outside. Would it be necessary, absolutely not, more of a piece of mind thing. Like I said above if the wood isnt flat and straight that is far more likely to make the joint fail than the clamping pressure that you use.

Im sure alot of people substitute clamping for properly selected and flattened boards and then expect glue to keep the word from bending back to its natural state.

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Nicholas Hall

348 posts in 858 days


#40 posted 12-07-2012 05:53 PM

@Horizontal Mike: I agree that it could be dangerous to exert these kinds of pressure using air bags. When a compressed air bag blows at extreme pressures, tremendous amounts of energy are released with explosive force. Anyone who has ever seen a tractor trailer truck tire blow can appreciate this. It’s like a ballon popping, only with enough force to send 1” bolts 6” deep into solid concrete. Nobody shoud try this without an airbag professionally engineered and manufactured for this specific purpose.

Using bottle jacks is quite a bit less dangerous. To generate the forces required will only require movement of the cylinder about 1/8” inch. It’s more likely that an i-beam will bend than that anyone will get hurt. That said, I agree that nobody should try it.

That said, if glue joints do in fact benefit from triple the pressure that most woodworkers currently use with their pipe clamps, I think it’s a valid question to ask whether or not there is a better, more cost effective solution to the problem than pipe clamps. Since a $20 bottlejack can exert 40,000 pounds of force, and a $20 pipe clamp can exert only 1000 lbs of force, I don’t think it’s crazy to entertain the following thought experiment:

Is there a safe, effective way to use bottle jacks for panel clamping in the home woodshop?

Put another way: if two $20, twenty ton bottle jacks and $50 worth of hardwood for a total cost of $90 can clamp more effectively than buying 20 rockler pipe clamps at $20 a apiece for a total of $400, I’d be awful curious to know. It’s a thought experiment. It isn’t necessary of course, any more than a 400 pound, $1,500 workbench is necessary. And to your point, it certainly isn’t worth getting hurt over, or risking the safety of others over.

-Nick

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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patcollins

1006 posts in 1617 days


#41 posted 12-07-2012 06:03 PM

Nicolas, bottle jacks have little control. With the pressure they are able to exert I would worry that I have one exerting much more pressure than the others etc and that only where that particular jack was was properly clamped, much like when you put all the clamps on then go back and find that you have to tighten the first few thta you put on.

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Nicholas Hall

348 posts in 858 days


#42 posted 12-07-2012 06:14 PM

That’s true. Bottle jacks don’t have nearly as much fine control as threaded rod. They also don’t have as much tactile feedback either. With a pipe clamp, you can really tell how much force you are exerting in order to turn the handle.

-- Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -Groucho Marx

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DannyB

46 posts in 2174 days


#43 posted 12-07-2012 06:25 PM

@Nicholas

You keep repeating the same thing while apparently missing my points :)
I completely understand it is forced divided by area.
So let’s start with a simple point

”@Danny B: A pipe clamp exerts 1050 pounds of force. “

No.
Some guy said “the average woodworker” using one of his pipe clamps could exert 1050 lbs of force on something.
IE “Further in the article, the author shows that the average woodworker can extert 1050 lbs of force with a 3/4 inch pipe clamp (which is what I’m using)”

This means, using complete and total bullshit methods, some guy came up with some idea of how much force the average woodworker could exert on a pipe clamp on some piece of wood.
Not “the maximum amount of force you can actually exert with a pipe clamp”, but some mythical average woodworker using against some mythical amount of wood.

So, then, my argument, again, is
A. that you can exert more force than this guy claims
B. Even if you couldn’t, the average woodworker stops exerting force on pipe clamps to avoid damaging wood well before they hit the maximal clamping pressure they can exert (At least, i find this to be the case).
C. The average woodworker can thus exert more of this force on the end of cambered cauls than in the middle of a piece of wood, because they won’t feel like they are about to damage the wood.

Again, I am not claiming that if you exert 14000lbs of pressure on the cauls, this will not be properly spread out and divided, and i am not claiming that you can’t exert more pressure over the smaller area.

I am instead claiming that
A. the test performed tells us nothing about the actual maximal force exerted by a pipe clamp
B. The amount of force the average woodworker exerts on pipe clamps is not the maximal force you can exert with a pipe clamp (because they don’t want to crush the wood)
C. The average woodworker could exert more force (closer to the maximal force) on the end of properly cambered cauls because they will not believe they are going to damage the wood.

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lumberjoe

2847 posts in 1000 days


#44 posted 12-07-2012 06:27 PM

Thanks Pat. My concern was getting a good glue up with proper clamping pressure. Since it is 4” thick and 6 feet long, that is A LOT of area to apply pressure too. My goal with the lag bolts was to provide some extra clamping force.

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crank49

3524 posts in 1723 days


#45 posted 12-07-2012 06:27 PM

Think I’ll design a hydraulic clamp. Something a little larger than a Cabinet clamp, but self contained and kept light by the use of aluminum where possible. Also could put a gauge on it to let you know how much pressure it is making. Would that be a waste of time?

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

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patcollins

1006 posts in 1617 days


#46 posted 12-07-2012 06:57 PM

lumberjoe I would be careful, with the force you can exert with lag bolts it might be posssible that the force is fairly pin pointed around the bolt causing the area between bolts to be bowed out some. This wouldnt be that much a concern as far as the glue line integrity to me as it would causing internal stresses length wise in your glue up from the wood wanting to straighten itself out, however slight it is.

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casual1carpenter

353 posts in 1227 days


#47 posted 12-07-2012 08:19 PM

@Nicholas Hall “In my case I’m doing a 4” thick by 96” long top, so the total square inches of glue face is 384 sq inches. The boards I have are flatsawn, with the flatsawn faces in the glue joint. I’m using ash, which is most similar to red oak in density and grain structure, so we look at the chart and see that I need 900 Pounds Per Square Inch (PSI) in order to get the recommend clamping preassure. That means I need to exert 345,000 pounds of force in order to get the recommended 900psi distributed across the glue joint (900psi * 384 sq inches). Further in the article, the author shows that the average woodworker can extert 1050 lbs of force with a 3/4 inch pipe clamp (which is what I’m using). Luckily I have 10 pipe clamps already, so that gets me 10,500 pounds of force. Now I just need to find 335 more sets of pipe clamps and devise a way to get them mounted on the wood. Are most workbenches woefully under-clamped during glue-up?

Considering an Abrams main battle tank weighs in at about 60.4 long tons, one would have to stack and balance 3 tanks atop a 4” thick by 96” long glue up, perhaps one or two could be without ammo. I’m sorry but something is not making sense here.

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HorizontalMike

6968 posts in 1666 days


#48 posted 12-07-2012 08:28 PM

I am biting my tongue on this one 8-P…!!

IMO, the OP has mixed up ”PSI” with ”Total Pressure” exerted on a pair of boards being glued. OK, a pipe clamp exerts 1050 lbs PSI and let us ASSUME that this pipe clamp has 2 sq.in. of clamping area.

So that equals 2100 lb of pressure PER CLAMP and NOT for the board’s length! And NOT PSI for the board’s area either (unless only one clamp is used).

Using 10 pipe clamps = 21,000 lb of total pressure exerted on/across the ENTIRE length of the board, thus this “21,000 lb of pressure” is averaged out over say 100sq.in. of board clamping surface. This is NOT PSI and needs to be noted. So the average clamping pressure on these 100sq.in. is just 210 lb. PSI

I hope this helps.

-- HorizontalMike -- "Woodpeckers understand..."

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lumberjoe

2847 posts in 1000 days


#49 posted 12-07-2012 08:55 PM

This is why I hated mechanical theory classes and why I liked strength of materials classes. That recommendation is 100% theory and is missing a lot of facts. I’d like to know what that “Ideal strength” represents. We can only assume it represents the required force for 100% of the glue you applied to be absorbed into the wood fibers. Sounds awesome in theory right?

How many times have you got some glue on a block of wood and set it down on another piece of wood (or god forbid an unfinished bench) and discovered it the next day? I have. There was NO clamps at all, but when I whack it with a mallet, the wood splits and I have a nice chip missing from my slapped together bench, or I have a chip of wood still glued on. Why?

Glue is stronger than wood.

I’m not sure how much stronger, but that matters. At some point you enter the land of diminishing returns with clamping pressure. Once your glue joint equals the strength of the surrounding wood, that is the best you can hope for. Anything more than that is overkill. Why strive for overkill?

So again, I would like to see the difference explained from IDEAL clamping pressure (full, deep glue absorption) and adequate clamping pressure (the minimum threshold gate where the wood fails before the glue). Only doctors and scientists are concerned about ideal. They make reports, we make furniture.

Also. make sure you pieces fit together perfectly dry. If I have to close gaps with clamps, I know I have more work to do before I get the glue out.

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lumberjoe

2847 posts in 1000 days


#50 posted 12-07-2012 09:13 PM

And just to play devils advocate, this Veneer/Marquetry press was glued with NO clamping pressure whatsoever. I’m pretty sure these joints see a lot more stress than a workbench will ever see

-- www.etsy.com/shop/KandJWoodCrafts

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