# How much pressure does glue need?



## SJB (Oct 16, 2020)

Hi, I want to glue two boards together and they are rather long and I don't have enough clamps for the length. If instead of clamping I put some heavy weight on top of them, how much weight do I need to make a good glue?

The boards are about 6' long and about 24" wide. I have about 200 pounds of water bottles in cases, if I put those on top would that be enough?


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## difalkner (Nov 27, 2011)

Are the solid boards or plywood? How thick are they? Are they flat/straight? In other words, do they have any warp, twist, cupping, etc. that will keep them from making a good glue joint? If they're really flat and lay against each other 'perfectly' then your water bottles will probably suffice, especially if you strategically place the clamps you have.

Another thing you can do is use some 2x4s or other boards as cauls to span the 24" cross section and that will work well. I've done that and used some cardstock in a section in the center to make sure it has force there with the ends maybe raised 3/16" off the boards to be glued. That makes it sort of a spring board ensuring fairly even pressure across the width.

David


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## woodnthings (Jan 24, 2009)

*How many ways can you glue these .... 2*



SJB said:


> Hi, I want to glue two boards together and they are rather long and I don't have enough clamps for the length. If instead of clamping I put some heavy weight on top of them, how much weight do I need to make a good glue?
> 
> The boards are about 6' long and about 24" wide. I have about 200 pounds of water bottles in cases, if I put those on top would that be enough?



You did not say which way you want to glue them :surprise2:


You can glue the faces together or the edges. :|


if you are gluing the faces, the water bottle will work, but so will anything heavy..... cement blocks, car tires/wheels, javck up you car and put them under .....


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## m.n.j.chell (May 12, 2016)

To iterate what's stated above, if the boards fit together with no gaps, then very little pressure will be required. Just enough to get wood to wood contact through the glue, and you should be fine.
If, on the other hand, the boards are warped or bowed, etc., then you'll need enough force to take all that out, again getting wood to wood contact.
Without the wood to wood contact, the glue joint won't be very strong at all.


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## woodnthings (Jan 24, 2009)

*How many ways can you glue these .... 2*



SJB said:


> Hi, I want to glue two boards together and they are rather long and I don't have enough clamps for the length. If instead of clamping I put some heavy weight on top of them, how much weight do I need to make a good glue?
> 
> The boards are about 6' long and about 24" wide. I have about 200 pounds of water bottles in cases, if I put those on top would that be enough?



You did not say which way you want to glue them :surprise2:


You can glue the faces together or the edges. :|


if you are gluing the faces, the water bottles will work, but so will anything heavy..... cement blocks, car tires/wheels, jack up your car and put them under the tires .....


If you are gluing them edge to edge, that's way more complex. :sad2:
You must have almost perfectly straight and square mating edges.
You must keep them flat with braces or cauls until the glue sets.
You should have as much pressure forcing them together as possible. If you arrange them in a vertical orientation you can drape heavy bags across them for added pressure. Ratchet straps will also add pressure BUT they will have a tendency to curve them under tension and they won't stay flat across without braces/cauls clamped to them. There is really no substitute for clamps, BUT you can make clamps using a long, wide board, with screwed on strips and two wedges inserted to force them together ....cheap!:smile2:


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## redeared (Feb 7, 2019)

With edge gluing once I see uniform squeeze out I stop but that takes practice in the amount of glue you apply. If you glob the glue on you will get a lot squeeze out with very little pressure and not getting a good to wood to wood contact. I seen many people over clamping even to the point of bowing the clamps, you want good even pressure but not over powering.


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## Rick Christopherson (Jun 27, 2016)

SJB said:


> The boards are about 6' long and about 24" wide. I have about 200 pounds of water bottles in cases, if I put those on top would that be enough?


Because you are talking about water bottles, I assume this must be a face-to-face glue-up that is 72 inches long by 24 inches wide. If that assumption is correct, you could park 4 full-size pickup trucks on your boards and just barely have enough weight to equal atmospheric (i.e. vacuum) pressure. So absolutely no; 200 pounds would not do a thing.

Even if this was an edge-joint, it would still take 800 pounds just to equal atmospheric pressure. 

None of this even begins to address the fact that atmospheric (vacuum) pressure is not sufficient for standard wood glues. You would need 5-time the weight to bring you up to the minimum clamping pressures recommended by the manufacturers.


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## SJB (Oct 16, 2020)

Rewriting my answer below. I had replied to Rick's comment before I realized there were other answers here. Thanks to all for your replies - it seems my question wasn't well written in that I didn't provide enough detail. Rewriting below.


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## GeorgeC (Jul 30, 2008)

SBJ, as has been stated, all is dependent upon the quality of the boards. If they are nice and flat/straight it will not take a lot of pressure. Your 200# would be fine is spread out well.


If boards are not of good quality then the amount of pressure required would much more. How much more depends upon how much gap has to be closed.


I would just guess that your boards are "pretty good" and again guess that somewhat more than 200 # is needed.


george


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## Rick Christopherson (Jun 27, 2016)

SJB said:


> How far apart would I need to have my clamps at a minimum to get a good bond over the whole surface?
> .


You're not going to like my answer. Here's a glue-up that was only 10 inches wide by 16 inches long, and I still didn't have enough force/pressure to do it properly. As I turned this on the lathe, I found I still had a visible glue line closer to the center in one area.

When you do the math, you see how difficult the problem can be with a face-to-face glue-up. The amount of force that you need is equal to the desired pressure times the surface area of the joint. So in your case, it would be 72 inches times 24 inches (1728 square inches) times 14 psi = 24,192 pounds of force. And that's only for atmospheric pressure. Normally, I recommend 100psi clamping pressure, which would result in 172,800 pounds of force.


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## Rick Christopherson (Jun 27, 2016)

GeorgeC said:


> SBJ, as has been stated, all is dependent upon the quality of the boards. If they are nice and flat/straight it will not take a lot of pressure. Your 200# would be fine is spread out well.


To address the several people saying 200 pounds would be sufficient, let's do the math. 200 pounds of force spread out over 1728 square inches results in a clamping pressure of 0.11 psi. 

That's equivalent to 1 ounce of weight sitting on top of a 1 inch square. Which is also the equivalent of how much pressure your cell phone applies to the top of the desk when you set it down. (My cell phone weighs 5 ounces)


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## SJB (Oct 16, 2020)

Hi all, let me rewrite this as it wasn't at all clear in my first go-around.

As you can see in these two photos, the cost of lumber is quite steep here; a (fir) 7.75' long 2x4 is US $16.69:



















What I'd like to do is sandwich a few 200 x 60 x 1.8 cm boards (.75") to make a board that is 1.5" that I can then cut lengthwise into laminate 2x4's and 2x6's.










As you can see from the stack, they are more or less flat. Some bowing occurs when you remove them from the stack but I'll cherry pick the best ones I can. I want to more or less glue them as they lay like this in the pile:










Stacked like that with 200# of water, good enough?


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## SJB (Oct 16, 2020)

Rick Christopherson said:


> You're not going to like my answer. Here's a glue-up that was only 10 inches wide by 16 inches long, and I still didn't have enough force/pressure to do it properly. As I turned this on the lathe, I found I still had a visible glue line closer to the center in one area.
> 
> When you do the math, you see how difficult the problem can be with a face-to-face glue-up. The amount of force that you need is equal to the desired pressure times the surface area of the joint. So in your case, it would be 72 inches times 24 inches (1728 square inches) times 14 psi = 24,192 pounds of force. And that's only for atmospheric pressure. Normally, I recommend 100psi clamping pressure, which would result in 172,800 pounds of force.


To the contrary Rick, I _do_ like your answer. You make a good point and I am not sure that water bottles is going to get a consistent glue squeeze out across the length of every board in the stack. If I am to cut these into 2x4's and 2x6's, then I need 100% (not even 99%) glue coverage and bonding. I don't want one of my laminant 2x4's to separate later. 

If this run is successful, I plan to repeat this going forward because it is the only way I can afford 2x4's and 2x6's with the lumber prices here. Two of the 2m x 60cm x 1.8cm sheets is ~ 13.50 Euros, so 26 Euros for the pair. I can cut these into 6 boards that are close in dimensions to a 2x4. The price for me would drop from Euro 14.25 to 4.33 (26 Euros / 6 boards).

Looking at your photo and the price of pipe clamps here, your rig would cost me (not including pipes) about 600 Euros (USD $702). While your rig sure looks stout, it isn't quite in my budget this month :wink:

I could run (underneath) a row of perpendicular 4x4's with another row on top, the two tied together a 10cm (.40") bolt (threaded rod), some washers, and wing-nuts. 

I made a rough sketch of my clamp idea:










I can buy a garden 4x4" (9x9cm, 2.4 meters long for EUR 13.50) and get 3 per board. I would need 2 pieces for each "clamp" for a wood cost of 9 Euros and hardware of about 2 Euros (1 meter long 10mm threaded rod is .95). So, about 11 Euros per clamp. If spaced 12" apart, I would need about 6 of these (66 Euros), if spaced every 8" then 10 of them (110 Euros). Certainly beats 600+ Euros and I think would get the job done.

I appreciate everyone's input - thanks to all.


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## woodnthings (Jan 24, 2009)

*One way is .....*

If you reduce the area for the same wight, you increase the pressure. So, saw the boards down the length into approximate widths first to be trimmed or resized after they are glued up ... if needed?
Now, you can either buy smaller clamps and more of them, make some as you show with bolts or threaded rods, or innovate with heavier compressive loads. 



Rick's advice is about achieving perfect, invisible glue lines, at least in previous discussions here. If you settle for a visible glue line for your construction application, it will require less pressure. There are differing opinions on how much pressure is required:
https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure


https://www.finewoodworking.com/201...ferent-woods-need-different-clamping-pressure


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## Tom-G (Nov 11, 2012)

SJB - 

If the final glued up boards will have the back side hidden, after assembling with the wood glue, align the boards, put on a few clamps to hold them in alignment, and drive flat head screws from the back side. Just make sure they are short enough so the don’t pop through the front side when fully seated. You will need to plan the placement of the screws for both even coverage and to keep them clear of the cut lines when you rip the boards to final size. The screws will pull the boards together and basically act as permanent clamps.


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## JohnGi (May 9, 2019)

SJB, I'm not sure what you will be doing with this wood. The lengths seem short for building lumber, and the cross sections seem heavy for furniture or cabinet making.

The 44mm lumber looks like ok--ok but not great--framing fir or whitewood. The edge glued 18mm wood looks like some plantation grown hard pine. There's a lot of pith in the end grain. These were small trees. I would not assume these are equal for strength or stiffness if you have a highly stressed structural application in mind.


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## SJB (Oct 16, 2020)

JohnGi said:


> SJB, I'm not sure what you will be doing with this wood. The lengths seem short for building lumber, and the cross sections seem heavy for furniture or cabinet making.
> 
> The 44mm lumber looks like ok--ok but not great--framing fir or whitewood. The edge glued 18mm wood looks like some plantation grown hard pine. There's a lot of pith in the end grain. These were small trees. I would not assume these are equal for strength or stiffness if you have a highly stressed structural application in mind.


JohnGi, bingo, it is pine (pino). With it I'm just going to build some bunk-beds. Not sure I'll be purchasing a vacuum bag before Christmas :wink: Honestly, that's the first time I've ever seen that. But, as someone's sleep will be relying on the glue joint, I'll take Rick's advice and I suppose I'll be using clamps rather than water bottles.

I'll be sure to post some pics for the clamps and the build.


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## JohnGi (May 9, 2019)

SJB, a bunk bed makes sense. The edge of the pile looks like all full length stock. I don't see any finger joints. With finger jointed stock more of the internal stress in the wood is relieved. These full length strips from rapid growth, small diameter trees still have a lot of internal stress in them which is now being held in check by the edge gluing. They will want to move when they are ripped apart.

I suggest you rip the panels into strips before you face glue the strips together. This allows you to selectively match up strips. You can take 2 bowed strips and glue them to mutually pull each other straight. You can select 2 good strips for more important structural pieces like the long mattress rails. You can stagger any large knots so you don't end up with weak spots. Stock selection plays a bigger role in furniture making than it does in building framing, and it's nice to have these options when you are gluing up each piece. I also think you will end up with better glue lines this way.

In the US a lot of this pine is Monterey pine, native to California but transplanted all over the world. It comes back as lumber with different names. From Chile it's radiata pine. One batch from New Zealand was named illiata pine. The rip and cut line hated it so much they called it idiota pine. It was very unstable and moved anytime it was ripped or even when a rabbet was machined into a small strip. Be cautious and get to know this stuff. People do good things with it, but you have to work with it.


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## FrankC (Aug 24, 2012)

All I can say is, if you are a woodworker and happy with your level of expertise, carry on. On the other hand when someone comes along with credentials and you wish to better your work, follow along with an open mind, we are never too old to learn.


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## kwoodhands (May 1, 2020)

If screw holes on the ends of the boards are not objectional, then this would eliminate four clamps. If the boards have a slight cup from end to end then place the high end down. Clamping or screws on the end will put enough pressure on the boards to flatten them. 
Cauls will work for your glue up. Either find a couple of bent 2x3's or 2x4's that have at least 3/4" bend from end to end. Finding this lumber should be easy, seems like 1/4 of the lumber stock is at least this bent.
I have doe this a lot. I used to make fluted columns that wrapped around structural posts. I glued up three sides in the shop. At the site I first installed the U shaped columns. Then added the 4th board using cauls and clamps. This was done vertically. To keep the cauls from tilting over while I clamped I screwed a 1/4" thick plywood to the cauls the width of the column, then 2 brad nails to keep it in place while I installed 2 clamps at the top, 2 in the middle and 2 at the top. I had two more cauls with a 4" wide piece of 1/4" plywood ready in case I needed to clamp the sides at 90° to the face I just installed.
Not sure about the water bottle method, am sure about using cauls.

mike


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