In a recent Woodworker's Journal e-zine several readers responded to the question of how to measure the pressure a clamp places on wood it is clamping.  Over the years the question of clamping pressure comes up regularly.  The answers are almost all pretty standard however there seem to be a lack of understanding the concept of clamping in the first place.  Let's start with the basics by simply stating that the purpose of clamping is to hold objects together.  It does not matter if it is wood metal, glass or any other material.

     Now take the concept a step further and the reason we clamp is to hold things together in order to perform some other action such as gluing, welding, or forming something (concrete blocks for example).  For us in woodworking I venture to say that most of the time we use clamp to perform glue-ups. Probably the second most popular task is to clam wood for sanding, planing, drawing parts together, etc.

     Before we discuss the gluing aspect of clamping let's talk about clamping blocks and clamping pressure.  Whenever we place something (clamping block) between the clamp "jaws" and whatever we are clamping, regardless of how much we tighten down on the clamp, the pressure applied will never be the same as if we clamped directly to the object.  Clamping pressure will always vary depending on what material the clamping blocks are made of and whether the blocks are being used as cauls or not.

     With regard to gluing and clamping, we are concerned with three classes of joinery.

     1.  The first and probably most "common" class is the mortise and tenon.  In this class we have the mortise and tenon joint, the dovetail and box joints, tongue and groove joints, and the half lap joint.  While there are other joint types in this class, the key to identifying the class is that one board goes into another and a shoulder on at least one of the boards is usually involved.  Generally speaking the amount of clamping pressure is not that important.  The purpose of clamping mortise and tenon joints is to insure that the tenon remains well seated while the glue dries.  In a properly formed mortise and tenon pressure is applied only to the shoulder of the tenon. Thus there is no need to "clamp down"  with a lot of pressure while the glue dries.  The desire in these types of joints is to keep the wood in place. 

     2.  The second class of joints involves gluing boards together usually for either length or width.  The edges and ends of the boards are usually butted together, usually end-to-end, end-to-edge, or edge-to-edge.  End-to-end and end-to-edge joints are usually referred to as butt joints.  These joints are weak and often require bracing or some other additional support.  Even if two boards are finger jointed, gluing them end-to-end will not provide a strong joint.  Also, If finger jointed, the boards are usually pressed together but not clamped. 

     Joining boards along their edges however is a different story.  This type of joint may or may not involve the use of biscuits or splines.  The boards are often clamped together and pressure is a concern.  Too much clamp pressure will thin out the line of glue between the boards making the joint weak.  The same is true if too little pressure is used.  The type of glue and wood properties also play an important role in how strong the joint will be.  The key in this type of joint is to apply a little pressure with the clamps but not too much.  The rule of thumb I tell my students is that if the boards require the use of cauls across them in order to flatten them, then too much pressure is being used. In many third-world countries, where clamps may be hard to obtain, woodworkers will often joint boards on their edges but stack the boards horizontally on edge.  This practice often provides enough downward pressure on the joints that a good bond between the boards is made.

     3.  The third class are joints are those that lap each other but may or may not have shoulders,  The miter joint is an excellent example for this class.  The open lap joint is also an example of this class as is the rabbet joint.  Joints in this class will require at least two (2) clamps.  The process of gluing and clamping these joints is essentially the same as for edge-to-edge gluing and clamping.  The same rules apply and the major difference is in the number of clamps that are used.

     Hopefully viewing the gluing and clamping of joints from this viewpoint will help alleviate some concerns for how much pressure to use when clamping.

Phil Rasmussen,US Army-retired
The Mountain Woodworker
Hendersonville, NC 28791
www.mountainwoodworker.com
828-890-8058

Great explanation Phil.

I've seen guys that have a project they're trying to glue that don't have tight fitting joint and they clamp the fire out of it only to have the joint fail and I've heard them say "I guess I didn't clamp it tight enough".

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www.thepatriotwoodworker.com Proud Supporter of Homes For Our Troops

Good advice Phil!!!

On all my projects I try to use just enough pressure to put and keep the wood in place while the glue sets and have even taken some boards out of the clamps before the glue dried completely on straight grained wood.

 

But there are times when a piece of wood gets stubborn and needs a little extra "persuasion" to stay put.

 

Then feel and past experience goes a long way in those cases.

Great post Phil! Thanks for the time you put into this.

I saw you touch on glue starvation a tad.

There are two schools of thought about that, old school which states that too much clamping pressure will squeeze the glue out and starve the joint causing it to weaken, and in the past decade or so, folks consider it a myth that glue starvation due to high pressure clamping causes weak joints.

Here is an old 1952 article from US Dept of Agriculture addressing the issue. The article is in PDF format below.

And to support the idea that starving a joint is a "Myth" here is a link to a good discussion about that. Fine Woodworking

Personally, I clamp the living bejeezits out of every thing. I do not like glue lines at all. Building my rockers I put as much clamping pressure on everything I do. Fine clear lines are a must with zero glue lines. No issues thus far!

There has to be a zero glue line in these joints, I don't know of any other way to accomplish that other then putting the almighty force into the clamping.

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John Morris
The Patriot Woodworker
Proud Supporter of Wounded Warrior Project and Homes For Our Troops

Like you I am not fond of glue lined but you don't have to clamp the "bejeezits' out of the wood. glue lines result because for one of two reasons. The primary reason is that the joint was not made correctly and the second is that usually the joint was not clean when the glue was applied. A third reason is that there was not enough pressure on the clamp to begin with. Assuming that none of these reasons apply to your glue lines, then there is a little "trick" that can be used. When making the joint, you slightly cut/scrape,sand the shoulder so that there is a "burr" on the edge - similar to what you put on a scraper.

Regarding glue starvation, this is definitely a problem. I do not advocate the practice but I also do not advocate using so much glue that you have squeeze out. To understand this better you need to know how glue works. The way I explain this to my students is that glue is not like a sheet of paper that is but between two boards and it "magically" holds them together much like a magnet between two pieces of steel. Instead it is more like sewing cloth together. The thread (glue) intertwines between the wood fibers and forms a bond with them. Alone the fibers are not strong enough to hold the boards together, neither is the glue. But together they work synergisticaly to form a stronger bond. If you look at joints that fail you will find either that the wood is still in tact and there is glue around it (too much glue), or that there is wood with glue on it but where the break is there is no glue, or that if the wood is intact there is no glue on it (too little glue or air space). For the glue to hold the joint, it must "soak" into both pieces of wood. If it does not then there is a problem with the joint itself or perhaps the wrong glue is being used. When using white/yellow/carpenter's glue here is a trick that will help with the gluing. Before applying glue to the boards slightly dampen the edges that will be glued together. This will help the glue soak into the boards. With regard to polyurethane and PVA glues such as Gorilla glue this is a requirement. Water is also a cheap activator for CA glues.