How to Calculate Clamping Force

Clamping force plays a crucial role in ensuring the stability and accuracy of machining operations. By understanding how to calculate clamping force, you can ensure that your workpiece remains securely in place during the machining processes.

This article will guide you through determining the appropriate clamping force for your application and provide insights into measuring and calculating clamping forces. Additionally, it offers valuable information on the importance of selecting the proper clamps. It provides recommendations for finding the ideal clamp for your specific needs.

Whether you are a seasoned professional or a novice in the field, mastering clamping force calculation will contribute to improved machining outcomes and overall efficiency.

 

What is Clamping Force?

The amount of force applied to a workpiece by closing and locking a clamp is called clamping force. It is a necessary calculation because it allows a clamp to have adequate capacity to resist external machining forces. To determine the clamping force for your application, consider several factors, including cutting force and workpiece material.

 

Importance of Clamping Force Calculation

Calculating clamping force is vital because if it is insufficient, it can result in the workpiece shifting or moving during machining, leading to inaccuracies, poor surface finish, and even damage to the workpiece or the machine itself. On the other hand, excessive clamping force can also be problematic as it may deform or distort the workpiece, affect its dimensional accuracy, or even cause damage to the clamps or fixtures.

 

How to Determine the Appropriate Clamping Force

You must consider several factors when determining the appropriate clamping force for a specific application.

One of the primary considerations is the cutting force exerted during machining. Cutting forces are generated due to the interaction between the cutting tool and the workpiece material. They vary depending on factors such as the machined material, the tool geometry, cutting parameters, and the depth of cut.

It is also essential to consider the clamp or clamping mechanism used. Different clamps have varying capabilities and limitations when providing clamping force. Factors such as the design, size, and condition of the clamp and the contact area between the clamp and the workpiece can affect the achievable force.

 

How to Measure Clamping Force

When it comes to measuring clamping force, it can be a complex task that requires careful calculations. However, in some cases, an approximate method can provide sufficient information. One way to estimate clamping force is by considering the clamping force ratio and comparing it to the available force from manual clamp straps. These measurements can be instrumental when comparing them with power-clamp forces.

The clamping force ratio refers to the ratio between the force applied to the clamping mechanism and the resulting force exerted on the clamped object. For example, a 2-to-1 clamping-force ratio means that for every unit of force applied to the clamping mechanism, two units are generated to hold the object in place.

See the table below for how much clamping force is available from manual clamp straps of various sizes (with a 2-to-1 clamping-force ratio) to compare with power-clamp forces.

Stud Size Recommended Torque* (ft.-lbs.) Clamping Force (lbs.) Tensile Force In Stud (lbs.)
#10-32 2 300 600
1/4-20 4 500 1000
5/16-18 9 900 1800
3/8-16 16 1300 2600
1/2-13 38 2300 4600
5/8-11 77 3700 7400
3/4-10 138 5500 11000
7/8-9 222 7600 15200
1-8 333 10000 20000

 

*Clean, dry clamping stud torqued to approximately 33% of its 100,000 psi yield strength (2:1 lever ratio).

You can also calculate the required clamping forces based on the calculated cutting force. A simplified example appears below, with the cutting force entirely horizontal and no workpiece stops (frictional force resists the entire cutting force).

Contact surfaces Friction coefficient (Dry) Friction coefficient (Lubricated)
Steel on steel 15 12
Steel on cast iron 19 10
Cast iron on cast iron 30 19

 

Calculating Clamping Force

Use the Carr Lane Mfg. clamping force calculator below to determine the required clamping force for your application.

Adjust the fields to match the numbers associated with your machinery and materials. You can adjust the calculator by typing or clicking.

 

 

 

Complex Clamping Force Calculations

With workpiece stops and multi-direction forces, calculations become much more complicated. To simplify somewhat, intuitively determine the worst-case force situation, then treat the calculation as a two-dimensional static mechanics problem (using a free-body diagram).

In the example below, the cutting force is already known to be 1,800 lbs from previous calculations. The workpiece weighs 1,500 lbs. Unknown forces are:

FR = Total force from all clamps on the right side

FL = Total force from all clamps on the left side

R1 = Horizontal reaction force from fixed stop

R2 = Vertical reaction force from fixed stop

R3 = Vertical reaction force on the right side

N = Normal force = FL + FR + 1500

µ = = Coefficient of friction = .19

The equations below solve for unknown forces assuming that for a static condition:

  1. The sum of forces in the x direction must equal zero
  2. The sum of forces in the y direction must equal zero
  3. The sum of moments about any point must equal zero

At first glance, the example above looks “statically indeterminate,” i.e., there are five variables and only three equations. But for the minimum required clamping force, R3 would be zero (workpiece barely touching), and FL would be zero (there is no tendency to lift on the left side). Now with only three variables, we can solve the following:

Solving for the variables,

FR = 1290 lbs
R1 = 1270 lbs
R2 = 2790 lbs

In other words, the combined force from all clamps on the right side must be greater than 1290 lbs. We recommend a 2-to-1 factor of safety (2580 lbs). Even though FL (the combined force from all clamps on the left side) equals zero, a slight clamping force may be desirable to prevent vibration.

Too much force can be as detrimental as too little. Excess force can cause fixture and machine-table distortion or even damage. Even a small hydraulic clamp can generate tremendous stresses (S).

In the example above, three 4,560-lb Edge Clamps cause some machine-table bending. Using static beam-binding calculations, the maximum distortion at point D is about .0006 inches (probably acceptable).

However, the distortion would be much more significant if the clamping point were higher off the machine table (P dimension). Higher clamps would require adding an intermediate fixture plate to increase table rigidity.

 

Find The Right Clamp For Your Application

Carr Lane Mfg. offers a complete line of industrial clamp straps with clamping force capacities ranging from 300 lbs to 10,000 lbs.

Once you have determined the necessary clamping force for your application, browse our versatile, heavy-duty clamp straps in the Carr Lane Mfg. catalog.