There has been a debate raging for years regarding whether it is possible to make a clamp too tight when joining wood with glue. Some think that you can squeeze out too much glue by over-tightening and end up with a weak joint. So, what is the right pressure for clamping?
For softwoods, clamps should exert about 100-150 psi of pressure. For hardwoods, it should be about 175-250 psi. It is impossible to make a joint weaker by over-tightening the clamps. So long as it is not so tight as to damage the wood itself, a tighter clamp will likely result in a better joint.
There is a lot to consider when it comes to clamping wood joints, and many different experts weighing in. In this article, we’ll go in-depth into the different considerations regarding how tight your wood clamps should be. Let’s get started!
The ‘Glue Starvation’ Hypothesis
Before we get into the expert advice on exactly how tight your wood clamps should be, we have to first talk about an idea that has been circulating for a long time: the idea that making a clamp too tight can squeeze out too much glue and result in a weaker joint.
Most professional woodworkers agree that this is next to impossible to do using a hand clamp. You don’t have the strength to apply that kind of pressure, and it is challenging to force out so much glue with your clamp that the joint would become ‘starved’ of glue. You would probably damage the wood before this could take place.
Why Does the Clamping Pressure Matter?
PVA, the most common woodworking glue, is very adhesive but not very cohesive. That means that it sticks very well to the wood but not so well to itself. That is one reason why a tight clamp results in a stronger joint: by spreading the glue thin, you decrease cohesion. In other words, the more the glue is touching wood, and the less the glue is sticking to itself, the better.
Another consideration is that the glue works by being absorbed into the wood itself. By pressing it tightly, then you force more of the glue into the structure of the wood. Some glue will seep out the sides, but that does not mean you are weakening the joint. You should keep tightening at least until you see glue seeping out of the joint. Some people even say you should keep tightening until the seeping has stopped altogether.
A tight clamp is better because as the glue releases moisture, it shrinks and leaves gaps. The extra moisture also causes the wood to swell, which can open the joint up. The extra pressure compensates for this and holds the joint together as the wood tries to swell and the glue shrinks.
It also spreads the glue out and causes it to fill in the gaps caused by the wood being slightly uneven on a small scale.
How Many Clamps Do I Need?
The magazine ‘Fine Woodworking’ released a much-discussed article in 2010, which gave a formula for finding out how many clamps you need.
- First, you find the area which needs gluing in square inches.
- Next, multiply that by the total required clamping pressure. That can be found by reading the instructions on the glue. Again, it is about 150 psi (pounds per square inch) for softwoods and about 250 psi for hardwoods.
- Finally, divide the number you have just calculated by the force applied by each clamp. That is measured in pounds per inch and can be found by looking at the user manual for each clamp. Different clamps have very different strengths, so it is worth checking each type individually.
So, when you multiply the square inches that need gluing by the total clamping pressure, divide that by the force of each clamp, you get how many clamps you will need to use.
Clamping Pressure and Different Types of Wood
Like many things, the answer to the question ‘how tight should wood clamps be?’ depends on many different factors. One of the most important is the species of wood you are gluing. Wood is broadly divided into softwoods (like pine or cedar) and hardwoods (like beech or oak).
The recommended clamping pressure is lower for softwoods than for hardwoods since it is easier to damage softwoods by applying too much pressure.
Hardwoods, which are generally more expensive and last longer, can withstand up to 250 psi of pressure or more. For most softwoods, the max is about 150 psi. To avoid damaging the wood, it is always good to put a bit of scrap wood between the clamp and the wood you are working on. It should never be necessary to clamp so tightly as to damage the wood.
The Effect of the Wood Surface
Another important factor in how well the joint will stick is what the wood’s surface looks like before you start. Most people agree that the smoother, the better. That is because fewer imperfections in the wood mean the joint requires less adhesive, and less adhesive results in fewer bubbles and imperfections in the final product.
Burnishing is the technique of rubbing two pieces of wood together to make the surfaces more smooth. While smoother is better, burnishing will stop the wood glue from properly bonding. This technique should only be used for Hide glue. (check out my article on Hide glue here)
Bob Behnke, a Senior Technical Specialist at Franklin International, recommends putting a drop of water on the wood’s surface. If the water doesn’t soak into the wood, you should sand the surface until it absorbs the water.
How to Clamp Wood Correctly
Now that we have gone through some of the general considerations around clamping wood, we can look at the correct way to clamp wood from start to finish. Here are the steps:
- Before applying the glue, make sure that the surface of the wood is smooth and dry. It is also a good idea to press the joint together without glue to make sure there are no large gaps.
- Apply the glue using a flux brush, like this Shark 14069 Flux Brush Set. (Link to brush on Amazon Here) Too little glue and the joint will be ‘starved’ and weak; too much glue and you will make a big mess when you clamp it. Rub the glued edges of the wood together to get a better spread.
- Using the formula described above, find out how many clamps you need. A good rule of thumb is that you will need about one clamp for every 10in. (25.4cm) of wood.
- Tighten the clamps until glue seeps out. How much glue should seep out depends on how much you put in. If you do not see some seepage, you have likely not used enough glue.
- Wait about 30-60 minutes for the excess glue to turn into a gel, then gently scrape it off with a chisel. If you try to remove it while it is still wet, you may make a mess. Try and remove it when it is too hard, and it will become quite tricky.
It is impossible to over-clamp a joint by hand to the point where it will become glue-starved. In fact, tighter clamping will result in stronger joints. Remember that the pressure you will apply will vary depending on what type of wood you are working on. Make sure to follow the guidelines mentioned in this article and that the wood is smooth and dry before gluing.