Two presses might be identical in size on a factory floor but have vastly different pressing capabilities since a 20-tonne press has the capacity to handle a much lighter load than a 100-tonne press. A decision on the choice between them is not just an issue of purchasing the stronger machine.
The force required varies with the sheet metal, the operation being performed and the tooling. Before comparing prices, production speeds or machine features, manufacturers should be aware of tonnage when they are looking at a power press machine for sale.
Choosing the incorrect size can impact part quality, break tooling and pose significant safety hazards. By knowing the basics, manufacturers are better equipped to make purchasing decisions.
What Does Power Press Tonnage Actually Mean?
Tonnage is the highest pressing force that a machine can deliver under a set of operating conditions.
The capacity of mechanical presses in many cases is given in metric tonne-force. A metric tonne-force is roughly equal to 9.81 kilonewtons.
Rated capacity is not necessarily equal to the maximum force that the press can exert at each point of the stroke.
The rated capacity point for mechanical presses will typically be just before the bottom of the stroke. The maximum force allowed will vary according to design and manufacturer specifications of the machine.
This is important because force is necessary during various stages of the press movement for different metalworking operations.
Why Do Different Sheet Metal Jobs Need Different Pressing Forces?
Suppose a thin metal sheet is cut out to form a small circular piece. Now compare this with thicker material to make a deeper component.
The two operations are both performed with a press, but they require very different forces.
Blanking and Punching
Blanking is used to form a complete shape from a sheet of metal and punching is used to form a hole or opening.
The required cutting force mainly depends on:
- Total length of the cutting edge.
- Thickness of the sheet.
- Shear strength of the material.
- Tooling design and cutting conditions.
A longer cutting perimeter or thicker material generally increases the required force.
Bending Operations
Bending changes the angle or shape of sheet metal without separating the material.
Force requirements depend on material strength, thickness, bend length, tooling geometry and the bending method.
Two sheets of identical thickness may require different forces if their material properties differ.
Forming and Drawing
Forming operations reshape metal into the required component profile.
Deep drawing, for example, involves pulling sheet metal into a die cavity.
The required press capacity depends on several interacting factors, including material properties, drawing depth, tooling design and friction.
These operations require careful engineering assessment rather than relying on a simple tonnage estimate.
How Is Pressing Force Calculated for Sheet Metal Cutting?
For basic blanking and punching operations, manufacturers can estimate cutting force using:
Cutting Force = Cutting Perimeter × Sheet Thickness × Material Shear Strength
Consider a simplified example:
- Cutting perimeter: 200 mm
- Sheet thickness: 1 mm
- Assumed shear strength: 300 N/mm²
The estimated cutting force becomes:
200 × 1 × 300 = 60,000 N
That equals approximately 60 kN, or 6.1 metric tonnes-force.
This calculation provides an initial estimate, not a final press recommendation.
Additional requirements may include stripping force, tooling loads, safety allowances and the press’s rated force position.
Before selecting a power press machine for sale, manufacturers should have the complete operation reviewed by a qualified tooling or press engineer.
Why Is Choosing a Higher-Tonnage Press Not Always Better?
It may seem safer to purchase a press with significantly more capacity than required.
However, unnecessary capacity can increase equipment investment, space requirements and operating costs.
A larger press may also be unsuitable for certain tooling arrangements or production requirements.
More importantly, tonnage is not necessarily the determinant to whether a machine can carry out a specific operation.
Also key features are:
- Bed size: The bed should be large enough to allow the die and its support equipment.
- The stroke length: The length of the press movement should be adequate for the operation.
- Shut Height: The available tooling space should be the same as the die requirements.
- The operating speed should be appropriate for the material, tooling and handling arrangement (stroke rate).
- Rigid frame: The press should have a rigid structure to resist the expected load otherwise there is deflection which is unacceptable.
These specifications should be assessed in conjunction.
What Happens When a Press Operates Beyond Its Rated Capacity?
It’s not something to be taken lightly when overloading a power press.
Excessive force can cause damage to dies, cause machines to become misaligned, or cause undue stress on mechanical parts.
Overloading can also diminish the reliability of equipment and pose risks to safety if the equipment is overloaded repeatedly.
Every effort should be made by an operator to never try to “work around” the machine’s limitations by making unauthorized adjustments.
If it’s an operation that’s over the maximum allowable load, the answer is to go back and look at the process, tooling or machine selection.
How Should Manufacturers Select the Right Press Capacity?
The selection process should start with component selection, and not with the machine catalogue.
The first step in the process for manufacturers is to record the material’s grade, sheet thickness, component dimensions and intended use.
They should then calculate or derive appropriate levels of force, using appropriate methods of calculation or engineering analysis.
Additionally, when looking at a power press machine for sale, buyers should also look at:
- The amount of force it can produce and where the force is located.
- Allow for off centre loading of parts and tooling.
- Size of the machine and the space at the factory.
- Operating requirements and safety features.
- Maintenance solutions and technical data.
Technically suitable press is one which can do the job at hand in the operating range for which it is designed.
Choosing Press Capacity With Confidence
Power press tonnage is one of the key specifications, but it’s not the only factor when selecting press equipment. Completely consider the forming or cutting process before investing by the manufacturers.
When you are considering buying a power press machine, don’t just assume capacity, start with the right material specifications, tooling requirements and engineering calculations.
Tinmaco (Unity Metals) is a manufacturer of power press machines for industrial use. View our range of machinery and ask about the equipment available to you and talk about your sheet metal manufacturing needs.
Frequently Asked Questions
1. What does tonnage mean in a power press?
Tonnage is the pressing force of a machine when tested at certain conditions. It assists in deciding if the press is able to exert enough force to accomplish a specific metal working process.
2. How do I calculate the tonnage required for sheet metal punching?
A simple calculation is made with the cutting perimeter and the thickness of the sheet material and the shear strength of the material. Further equipment selection considerations include additional loads, tooling and machine specifications.
3. Can a 50-tonne power press handle every job requiring less than 50 tonnes?
In addition, the suitability is determined by the location of the force during the stroke, tooling dimensions, off-centre loading and operating limits.
4. Does thicker sheet metal always require higher pressing force?
Generally, the thicker the cutting geometry, and the properties of the material being cut, the higher the cutting force. But the actual requirements also will depend on the operation and the material properties.
5. What should manufacturers check besides tonnage when buying a power press?
Such parameters are stroke length, bed size, shut height, operating speed, frame rigidity, tooling adaptability, safety devices, maintenance.
