# Clamp pressure



## stinger (Jul 6, 2014)

Well I've heard all kinds of stuff about clamp pressure. 
What is enough? 6" spring clamps all around a frame?
C Clamps CRANKED? 
In the vise cranked? 
Thank you in advance


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## quarterinmynose (Jan 21, 2012)

I would avoid "CRANKED".

Moderate, even pressure should be fine for most anything as long as Your surfaces are even/flat to one another.

I use plastic harbor freight spring clamps almost exclusively. You will get some variance in pressure from old or just plain crappy ones, but i keep them in two groups...sissy, and strong. Just give 'em a squeeze before clamping to get a feel.

This stuff really isn't rocket science, just don't clamp too tight or you can squeeze all your glue/adhesive out.

You tube is your friend when getting this stuff down, go watch a bunch of wood working videos that seem to apply to given techniques....then build, and build again. You will learn quite a bit from jumping in a getting to it.

this is just for most common woods mind you,

it gets just slightly more complicated with more difficult/exotic materials where special adhesives and preparations are necessary.


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## Cr2O3 (Aug 30, 2014)

Most manufacturers will have a recommended pressure printed on the label. Some adhesives require a lot of pressure, others moderate. If you prepare your joints properly too much pressure will produce a stronger joint vs too little pressure. The line between excessive pressure and strong clamping pressure depends on the individual.


Clamping pressure depends on the type of adhesive. In my experience thicker adhesives require less pressure than thin adhesives. Often we assume the opposite to be true, but based on the manufacturers recommendations thin or low viscosity adhesives require higher pressure.
Titebond original wood glue (the yellow stuff) can be bonded properly with moderate pressure. Between 100 and 300 PSI per manufacturers recommendations. 
Titebond III which is a much lower viscosity. In my personal experience this glue requires a little more pressure than the Titebond original. I have not seen any documentation backing this up, it is a personal observation.
Epoxies require much less clamping pressure even though they are thinner than some wood glues. Since epoxies are not water based, the fluid does not abscorb quickly into the wood fibers. Because of this I recommend applying epoxy to both surfaces, and wait for a little bit before you clamp them together. If you are using a slow 20 minute curing epoxy, you can afford to allow the surfaces to stand for 5 minutes or so before clamping. For fast cure, I suppose there is no option to wait.
Plastic resin glue (also called formaldehyde glue or urea glues) are thin low viscosity glues which are very sensitive to the surface texture. High pressure in the range of 150 to 300 psi is recommended if the surfaces are rough however if the surfaces mate well and are properly prepped the clamping pressure required can be as low as 50 psi.

Clamping pressure depends on the type of material. Hard woods require higher pressure. Soft woods require less than half the clamping pressure by comparison.
One reason for the difference in pressure is the wide range of porosity encountered between different species of wood. Softer woods like pine, poplar, and even some species of cherry have a spongy composition which allows the glue to easily penetrate. Hard woods like oak, ash, hard maple, and exotic woods like ebony require higher clamping pressures to generate enough fluid pressure to penetrate the fibers. 
The second reason I'm aware of is deformation. Most gluing is done between two boards which have been planed. A planer will take very small circular cuts which to the naked eye look to be flat but if you were to run your fingers across the surface it would feel like a miniature washboard. Softwoods will easily deform under pressure. Some hardwoods will not deform without damaging the wood with the clamps. So with hardwoods, it is necessary to apply 2 or 3 times the force you would apply to a softwood even with the same adhesive.

Preparation is key. When gluing a joint which that will support a critical load, it is important to properly prepare the surfaces to accept adhesive.
For edge gluing (gluing up wide boards on edge, example: tabletop) It is important to have the glue surfaces dead flat and parallel so they properly mate. Running the boards through your jointer is the first step to get them straight but now it is important to remove the bumps on the surface, this can be done with a hand plane or a sanding block with a guide that keeps it perpendicular to the face of the board. Removing the ridges from the jointer pass is much more critical with hardwoods than it is with softwoods. In the case of pine, it is perfectly fine to glue up your project without flattening the jointer cut.
For face gluing (wider surfaces glued together) it is good to texture the surface so the glue has a chance to squeeze out a little. I use a scraper which is made from a piece of hacksaw blade about 4 inches long attached to a homemade handle. Looks like a paintbrush but instead of bristles, you have the teeth of a hacksaw. Using this tool to create a cross hatched pattern on large surfaces accomplishes two goals. 1) It gives the air a path to escape between the ridges. 2) The cross-hatch pattern increases the penetration depth of the adhesive by exposing more pores in the wood thereby increasing the overall surface area.

Apply a liberal amount of glue to both surfaces.
It is better to have excess glue squeezing out of a joint. True there is more cleanup but it is far more time consuming to fix a glue starved joint than to clean up the extra that squeezes out.
If both surfaces have had glue applied to them it is very difficult to clamp them together with too much pressure. Also if both surfaces are wet with glue, it will give you a few more minutes of assembly time. 

Contact cements, veneer adhesives, and spray-on adhesives do not follow the same rules.
If you are using a contact cement type adhesive, it is easiest to follow the instructions to the letter. All of them will have different amounts of time where the glue must dry on both surfaces before bonding them together. 
With contact cements, there is no such thing as too much pressure. Extremely high clamping pressure produces extremely strong bonds.
Clamping overnight is a plus. Most of the time the instructions tell you to push the two surfaces together and move on. Trust me on this one small thing. If you clamp overnight (especially flexible materials like leather, cloth, rubber) you will have a permanent bond that will not de-laminate.

Determining clamp pressure
Really tough to determine without calculating the surface area using force gauge but heck these glues really do not require exacting pressures. The range in which they will properly bond is huge, anywhere between 50 and 300 psi will give a reasonably strong bond which is often stronger than the material itself.
1 good rule of thumb is to snug up all the clamps, then turn them each 1/2 turn. If the clamps still do not feel tight enough, tighten each one another 1/2 turn until satisfied. Tightening 1 clamp to full pressure then adding subsequent clamps at full pressure is a bad technique. Uneven clamping pressure can generate internal forces which will fight with the glue line and possibly cause a failure down the road.
Another (less scientific method) method to apply the right amount of clamping force is to imagine you are 9 years old when you are clamping. Tighten your clamps with 90% of your strength when you were 9. This should give about the right amount of torque without over-tightening any of the clamps. At the age of 9 I suspect the average strength is about the same between boys and girls so this should work for everyone.


I know this is a long post. I have more information and experience with adhesives, brazing, etc... Just don't always know when to stop..


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## stinger (Jul 6, 2014)

Holy cow Cr203! That's an answer! Thank you for taking the time! As a hunt and peck typist (two hot girls in typing class, I had no chance) you response would take me an hour+ to type! Thank you


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## JonM (Aug 17, 2013)

In other words, depending on the bonding agent you use, too little or too much squeeze out can adversely affect the project you're working on. I experiment on scrap prior to final glue up.


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## shane Wink (Aug 30, 2014)

Dont forget that depending on what your gluing, material wise, the materials may also soak up a great deal of the glue. Slow set epoxies are stronger than fast sets and give you plenty of time for wood or micarta's to absorb some of the glue. More an issue with open pore woods like oak, wenge, black plam, etc


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## Cr2O3 (Aug 30, 2014)

stinger said:


> Holy cow Cr203! That's an answer! Thank you for taking the time! As a hunt and peck typist (two hot girls in typing class, I had no chance) you response would take me an hour+ to type! Thank you


Thanks for not going into a boredom coma while reading it. Took more than an hour to post it.


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## stinger (Jul 6, 2014)

Cr203 I really appreciate it. I've read it three times now. I may print it and put it in the shop!


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