How many clamps do I need for woodworking?

Measurement is important in woodworking. Accurate measures create the sturdiest craft. This includes gluing up panels with clamps.

When it comes to gluing up panels, you can’t just rely on gut feeling and convince yourself you used the right number of clamps. To get the right fit, you have to make some calculations. And yes, it involves math to know how many clamps for a panel glue up.

Roman Rabiej, a professor in Michigan, developed three keys to clamping success. He has done various scientific studies on wood species, clamping pressure, glue joints, glue types, and more.

The three keys to clamping success are:

  1. Match the clamping pressure to the wood
  2. Make sure that you have enough clamps
  3. Distribute the pressure effectively

The above three keys answer the following questions: (1) What is the optimum force when clamping soft and hard woods?; (2) How many clamps for a panel glue up do you need and how do you arrange them?; and (3) How can you test a sample joint to see if you are getting good results?

Let’s focus on key number 2: the number of clamps you need.

The number of clamps needed for gluing up panels can be calculated using this formula:

Glue surface (sq in) x Required clamping pressure (psi)

Force applied by each clamp (lb./in.)

 

  

= Number of Clamps

Step #1

Multiply Glue surface (sq in) and Required clamping pressure (psi). For instance, you are gluing two boards ¾-inch in thickness and 36-inch in length. When you multiply them, and you’ll have 27 sq in.

Then, the required clamping pressure (psi) to apply is 450.

Multiply 27 sq in with 450. You’ll have 12,150 lb.

Step #2

The force applied by each type of clamp is dependent on your strength, the operator. To determine the force applied by each clamp (lb. in.), conduct a test and get the average force you applied.

Here’s an example after testing three types of clamp:

  • I-Beam Bar Clamp – 1,350 lb.
  • Aluminum Bar Clamp – 680 lb.
  • Parallel Jaw Clamp – 370 lb.

Step #3

Divide the result you came up with Steps 1 and 2. From the above example, 12,150 lb / average clamping force, say 370 lb = 32 Parallel Jaw Clamps. You need 32 Parallel Jaw Clamps to glue up a panel.

However, it would be hard to fit 32 Parallel Jaw Clamps in a 36-inch board. You can add more powerful clamps. For instance, repeat Step 3 but calculate using the average force on “I” – Beam Bar Clamp. That is, 12,150 lb / 1,350 lb = 9 I-Beam Bar Clamp.

The technique is to mix and match types of clamps to get the right fit.

With the formula above, you will know exactly how many clamps for a panel glue up should you use and how you should arrange them. This removes the need for guesswork, which will allow you to complete projects efficiently and produce durable pieces.