When we use clamps while woodworking, one of the prime questions that arise is regarding the required tightness of a clamp. The basic answer to that question is that a clamp doesn’t need tremendous pressure. If it is to hold glued joints together, then it needs enough pressure to hold the two surfaces together until the glue dries.
What you need to consider is the fact that most joints are not perfect, and sometimes you need more pressure from a clamp to push the joint into place. One estimate that is commonly accepted today is that you need a clamping pressure of 100 to 150 pounds per square inch (PSI) if you are to clamp softwoods. You would need 175 to 250 PSI for clamping hardwoods.
During practical testing, it was found that one-handed bar clamps provided the pressure mentioned above for softwoods or less. But you would need to squeeze such clamps quite tightly. A clamp may produce more pressure than needed. Hence, it is a good practice to make the clamp snug, and not press too hard.
The learning from what we have discussed so far is that the maximum pressure for clamping most joints is 250 PSI. If you press too hard, you would risk exceeding that pressure and could end up damaging the wood or at best, producing a weak joint.
Before You Clamp
Prior to clamping a glued joint, prepping and gluing is a preliminary and critical step. Here are a few tips on how to prep your joint before clamping it:
- Apply glue to both the surfaces of the joint.
- You can make a good joint with a smoothly-scraped joint, which can be as good if not better than if you sand the surface, but don’t clamp it too hard.
- As long as you fill gaps in the joint with glue adequately, your joint will not be adversely affected.
- Glue is only a substance that gives partial mechanical support. You need to create a strong joint wherein the two components fit snugly into each other.
- When you apply glue, ensure to use adequate fasteners like nails, screws and staples to position the components well and offer mechanical support.
Clamping Techniques that You Need to Know
A major activity in woodworking is clamping. One of the main skills that a woodworker must master is to use clamps by applying just the correct amount of pressure. You can get clamps that have been tested way beyond the maximum expected pressure. But you don’t need to apply that much pressure. For applying the requisite strength to your clamps, here are a few clamping techniques worth mentioning:
Do not Apply Excessive Pressure
If you apply too much clamping force, you will cause “glue starving.” The clamp squeezes out the glue leaving patches where there is no glue.
Avoid Spring Clamps for Overnight Settings
If you plan to leave your joint overnight to set, spring clamps can vary in tension over time and may distort your joint by pulling the components away from each other.
The advantage of using C-clamps is that you can control the pressure that you exert on them. Once you have set them, you can be sure that they won’t move, disturbing the settings of your joint.
Amount of Glue
A common question is how much glue to use. If you want to avoid sliding, you don’t want to apply excessive glue. The amount of glue also depends on the type of wood you use. Softer woods tend to be more porous and then you need to use more glue.
The Effects of Excessive Clamping Pressure
The excessive clamping pressure can result in excess glue oozing out of the joint. The result is that you have less glue between the two pieces. It weakens the joint.
Another thing that can happen is that if you use excessive clamping force, it could distort your joint. There have been cases when after the joint dried, it had to be broken and redone because the clamp distorted the joint.
Finally, if you use too much force on your clamp, the surface of your workpiece could get damaged. Most clamps for woodworking come with pads to protect the wood. In the absence of protective pads, you could fabricate your own out of soft material like cloth, felt or leather.
How to Apply the Right Amount of Pressure
How then do you ensure that you apply sufficient but not excessive pressure while using wood clamps? Here are a few tips that should put you on the right track:
Be Gentle on Softwood
Tighten clamps cautiously if using them on softwood. Ensure that there is some padding between the wood and the clamp. Better still, you can use non-metallic clamping systems that will be softer and easier on the wood.
Tighten in Steps
While tightening a clamp, do so in small increments and stop tightening it when you see the glue oozing out of the joint. You can wipe off the excess once you complete tightening the clamp.
Distribute the Clamping Force Uniformly
It is said about woodworking that you can never have enough clamps. This is true in the sense that the more clamps you use on a joint, the more evenly the force gets distributed. The best way of gauging the force you apply on your clamps is to observe the way the glue oozes out of the joint. If the flow is uniform, your pressure is roughly uniform across the joint.
Hand-Tighten Screw Clamps
The best way of controlling the force of screw clamps, whatever the type, is to hand-tighten them. If you hammer them, you stand to over-tighten them. When you can no longer turn the screw by hand, it is time to stop.
Use Clamps with Larger Jaws
Although you may need to use smaller clamps with small jaws for fine work, in general, keep your clamp jaws as large as possible. Smaller jaws create more pressure concentrated over a small area. This in turn could cause damage to the workpiece and also result in the force not being distributed evenly. Professional grade woodworking clamps with wooden jaws *like these (Amazon) are a better solution than the small, focused point of the C-Clamp pictured above.
If your clamp jaws are small, you could distribute the load better by using padding with blocks of wood or other similarly soft materials.
Wood clamps are fascinating tools in the world of woodworking. They are as useful as they are intriguing. Every woodworker needs to use clamps at some point or the other during their projects. They are an intrinsic part of the woodworking process.
But you need to know about the different types of clamps and how to use them. If you are a bit creative and resourceful, you can get the best use from your wood clamps and use them correctly. One of the important aspects of using wood clamps properly is knowing how much force to apply while using them.
From our discussions in this post, you should now have a better idea of the force to apply for clamps to work the best. You should be better equipped in using various types of clamps in your woodworking projects.
If you are interested in measuring the force exerted by clamps, you could watch this interesting video which sheds some light on measuring clamp force.