How to Improve Press Brake Accuracy: Basic Guide

Errors that result in the failure of press brake accuracy can arise from various factors, including mechanical issues with the sheet metal bending machine, as well as external factors like bending tools, material thickness, and human operational errors. In this article, we will examine the various factors that impact accurate bend and provide solutions for some of the common scenarios encountered.

Introduction to Press Brake Basics

Press brakes are indispensable in the metal fabrication industry, serving as the cornerstone for transforming flat sheets into parts with complex geometries. The mastery of press brake operations is a blend of art and science, requiring a thorough understanding of the machine’s mechanics, the material’s behavior, and the intricacies of the design to be executed.

The Role of Press Brakes in Metal Bending

• Functionality: The essence of a press brake lies in its ability to transform metal sheets into desired shapes with meticulous control over the bending angle and radius. The force exerted by the press brake is both powerful and precise, allowing for a wide range of bending operations from acute angles to complex, multi-angle folds.
• Types: The diversity of press brakes, from traditional mechanical models to advanced hydraulic and electric versions, offers manufacturers the flexibility to choose a machine that aligns with their specific production needs. Each type brings distinct advantages, such as the speed of mechanical presses, the power of hydraulic systems, and the energy efficiency of electric models.
• Tooling: Selecting the right dies and punches is critical for not only achieving the desired bend but also for minimizing wear and tear on the machine. Tooling must be chosen based on the material thickness, bend angle, and required bend radius to ensure optimal results.

Controlled Processes for Precision

• Programming: The integration of CNC controls has revolutionized press brake operations, enabling operators to program complex bend sequences and store them for future use. This digital precision ensures that each bend is executed with consistent force and angle, contributing to the overall accuracy of the finished product.
• Accuracy: Achieving precision in bending is a symphony of controlled speed, pressure, and back gauge positioning. The ability to fine-tune these parameters allows for minute adjustments that can significantly impact the quality of the bend.
• Repeatability: The true measure of a press brake’s capability is its consistency. The ability to replicate bends with unwavering accuracy is what sets apart high-quality fabrication from the rest. This repeatability is crucial for maintaining uniformity across production runs, especially for large-scale manufacturing.

Achieving Precision in Bending

Precision bending is a holistic process that extends beyond the press brake itself. It encompasses the operator’s expertise, the material selection, and the machine maintenance. Each element plays a pivotal role in the final outcome, and a lapse in any one area can compromise the integrity of the bend.

Material Selection and Preparation

• Material Properties: The selection of material is not a decision to be taken lightly. Factors such as tensile strength, ductility, and thickness dictate the bending process and the force required. Materials with higher tensile strength require more force to bend, while those with greater ductility allow for tighter bend radii without cracking.
• Preparation: The condition of the metal surface can greatly influence the bending process. Surface treatment such as priming or coating, along with thorough cleaning, ensures that impurities do not affect the metal’s behavior during bending. This preparation is essential for achieving a clean, sharp bend.

Advanced Techniques and Considerations

• Air Bending: The air bending technique stands out for its minimal contact between the tooling and the workpiece, which translates to less wear on the tools and greater flexibility in handling different material thicknesses.
• Angle Measurement: The use of laser angle measurement systems provides real-time feedback, allowing for on-the-fly adjustments to ensure the bend angle is precise. This technology is invaluable for maintaining high standards of accuracy.
• Tool Maintenance: Regular inspection and maintenance of the tooling are non-negotiable for preserving the quality of bends. Dull or damaged tools can lead to defects in the workpiece and reduce the lifespan of the press brake.

Understanding Press Brake Bending Accuracy

Defining Bending Accuracy

Press brake bending accuracy is the cornerstone of precision metalworking. It is the degree to which the machine can match the intended bend specifications, which includes the angle, radius, and position of the bend on the sheet metal. This accuracy is paramount as it determines the fit and function of the final assembled product. Even a minor deviation can lead to assembly issues, functional defects, or aesthetic discrepancies.

Key Factors Influencing Accuracy

• Material Thickness: The thickness of the material being bent plays a pivotal role in the bending process. Thicker materials require more force and may spring back more than thinner ones, necessitating careful calibration of the press brake to compensate for these factors.
• Machine Setup: A well-calibrated machine setup is essential for achieving precise bends. This involves selecting the correct tooling, such as the punch and die, and ensuring that the press brake is programmed accurately for the job at hand. The setup must account for the material type, thickness, and desired bend angle.

Ensuring Consistent Accuracy

Importance of Consistency

• Uniformity: Consistency in bending ensures that every piece produced by the press brake adheres to the design specifications. This uniformity is crucial in industries where metal components must fit together seamlessly, such as in aerospace or automotive manufacturing.
• Quality Control: A consistent bending process is a key aspect of quality control. It minimizes the risk of producing non-conforming parts, which can lead to waste, increased production costs, and potential safety issues.

Maintaining High Precision

Machine Maintenance

• Regular Inspections: Routine inspections are necessary to identify any signs of wear or potential issues that could affect the press brake’s performance. Components such as hydraulic systems, electrical parts, and tooling should be checked regularly to ensure they are in good working condition.
• Adjustments: Over time, press brake components may require adjustments to maintain optimal performance. This could include realigning the back gauge, calibrating the ram, or adjusting the pressure settings to match the material’s properties.

Operator Expertise

• Training: Operators must receive comprehensive training on the press brake’s setup, operation, and maintenance. This training should cover the nuances of working with different materials and the importance of precision in the bending process.
• Measurement and Inspection: Operators should use precision measurement tools, such as protractors, angle finders, or laser-based systems, to verify the accuracy of each bend. Regular inspection of the parts during production helps catch any deviations early and allows for immediate correction.

By focusing on these detailed aspects of press brake bending accuracy, manufacturers can ensure that their products meet the highest standards of precision and quality. The combination of a well-maintained machine, skilled operation, and rigorous quality control processes is the key to achieving and maintaining bending accuracy in metal fabrication.

Understanding Press Brake Bending Accuracy

Defining Bending Accuracy

Press brake bending accuracy is the cornerstone of precision metalworking. It is the degree to which the machine can match the intended bend specifications, which includes the angle, radius, and position of the bend on the sheet metal. This accuracy is paramount as it determines the fit and function of the final assembled product. Even a minor deviation can lead to assembly issues, functional defects, or aesthetic discrepancies.

Key Factors Influencing Accuracy

• Material Thickness: The thickness of the material being bent plays a pivotal role in the bending process. Thicker materials require more force and may spring back more than thinner ones, necessitating careful calibration of the press brake to compensate for these factors.
• Machine Setup: A well-calibrated machine setup is essential for achieving precise bends. This involves selecting the correct tooling, such as the punch and die, and ensuring that the press brake is programmed accurately for the job at hand. The setup must account for the material type, thickness, and desired bend angle.

Ensuring Consistent Accuracy

Importance of Consistency

• Uniformity: Consistency in bending ensures that every piece produced by the press brake adheres to the design specifications. This uniformity is crucial in industries where metal components must fit together seamlessly, such as in aerospace or automotive manufacturing.
• Quality Control: A consistent bending process is a key aspect of quality control. It minimizes the risk of producing non-conforming parts, which can lead to waste, increased production costs, and potential safety issues.

Maintaining High Precision

Machine Maintenance

• Regular Inspections: Routine inspections are necessary to identify any signs of wear or potential issues that could affect the press brake’s performance. Components such as hydraulic systems, electrical parts, and tooling should be checked regularly to ensure they are in good working condition.
• Adjustments: Over time, press brake components may require adjustments to maintain optimal performance. This could include realigning the back gauge, calibrating the ram, or adjusting the pressure settings to match the material’s properties.

Operator Expertise

• Training: Operators must receive comprehensive training on the press brake’s setup, operation, and maintenance. This training should cover the nuances of working with different materials and the importance of precision in the bending process.
• Measurement and Inspection: Operators should use precision measurement tools, such as protractors, angle finders, or laser-based systems, to verify the accuracy of each bend. Regular inspection of the parts during production helps catch any deviations early and allows for immediate correction.

By focusing on these detailed aspects of press brake bending accuracy, manufacturers can ensure that their products meet the highest standards of precision and quality. The combination of a well-maintained machine, skilled operation, and rigorous quality control processes is the key to achieving and maintaining bending accuracy in metal fabrication.

Factors Influencing Press Brake Accuracy

Machine Factors

There are several factors affecting bending accuracy of a press brake bending machine. These include:

• Straightness of the Ram Opening: The clamping opening of the ram is straight in both Y and X directions. Repositioning accuracy and arbitrary positioning accuracy of the left and right ram. The accuracy of the ram opening in both the Y and X directions is crucial for precision of bending. If the ram opening is not straight, it will lead to deviations in the bending angle and position.
• Clearance Between the Ram and Frame Guide Rails: The clearance between the press brake ram and the frame’s linear guide should be reasonable. Proper clearance between the ram and the frame guide rails ensures the stability of the ram during movement, thereby improving bending accuracy of press brakes.
• Perpendicularity and Inclination of the Frame: Perpendicularity and inclination of the frame. The perpendicularity and inclination of the frame affect the distribution of bending force during the bending process, which in turn ensure bending accuracy.
• Connection Between the Cylinder and Ram: The connection between the cylinder and the ram must be consistent to ensure uniform bending force distribution during bending in press brakes.

There are also other factors affecting the precision press brake bending, like strength and accuracy of the frame and ram, repositioning accuracy of the back gauge system in both X and R directions, proper adjustment of the computer system, adjustment of the hydraulic system, matching between the hydraulic system and computer adjustment.

Tooling Factors

1. Precision of the Upper and Lower Dies:

The accuracy of the upper and lower dies is critical. Deformation, damage, wear, and other issues with the dies can impact all aspects of metal bending. If discovered, it should be promptly reported and corrected. So regular inspection and maintenance are necessary.

2. Die Alignment:

Misalignment between the upper and lower dies can result in deviation in the bending size. Ensure that the toolings are properly aligned during tool setting.

After the left and right positions of the back gauge move, the distance between the lower die and the back gauge changes. This can be measured using a vernier caliper and adjusted using the back gauge screw.

The accuracy and compatibility of the lower die compensation device should be verified, matching the design of the frame. The upper die fixture should be highly precise.

3. V-Die Opening Size:

The opening size of the V-shape die and the bending pressure are inversely proportional. When the plate length and thickness are fixed, the larger the opening, the lower the pressure required. Therefore, when working with different thicknesses, the appropriate V-shape die opening size should be used.

When processing with a unilateral load, such as at one end of the press brake, the bending pressure may be impacted, harming the machine. This is strictly prohibited. The middle part of the machine should always be stressed when assembling the tooling.

Sheet Metal Factors

The straightness of the reference plane of the sheet metal must be checked. Ensure that the stress on the metal plate is uniform. Verify that the sheet metal thickness is consistent.

Inadequate parallelism between the workpiece and the lower die during bending can cause the workpiece to spring back after the upper die is pressed, affecting the bend size.

Springback refers to the phenomenon where the material tries to return to its original shape after bending. The tensile strength, thickness, tooling, and type of press brake all influence springback. Effectively predicting and evaluating springback is crucial for handling tight bends and thick, high-strength materials.

Material properties and thickness can impact the bending angle, so proper inspection and spot checking must be performed on each workpiece before bending.

The following are the material properties in detail:

• Yield Strength: Yield strength is the maximum stress that a material can withstand before it begins to deform permanently. Understanding the yield strength helps determine the maximum force that can be applied during bending to avoid damaging the material.
• Elastic Modulus: The elastic modulus indicates a material’s stiffness and determines the amount of deformation under a given stress. Knowing the elastic modulus helps predict the amount of spring back after bending.
• Material Thickness: The thickness of the material significantly affects the bending process. Thicker materials require more force to bend, and a larger bending radius is needed to avoid cracking or deformation.
• Ductility: Ductility refers to a material’s ability to undergo plastic deformation without breaking. Materials with high ductility are easier to bend and less likely to crack or tear during the process.

Bending Operation Factors

1. Consistency of Operator’s Actions:

Check that the left and right forces are consistent when the press brake operators push the material to bend. Inconsistent bending force can lead to deviations in bending dimensions.

2. Adjustment of System Data:

Ensure that the system data error is properly adjusted when using the system. An insufficient primary bending angle can affect the secondary bend dimension. Accumulated errors in bending will increase the error in the workpiece’s outline dimension.

The pressure required for bending varies with the length and thickness of the workpiece, and the length and thickness of the plate are proportional to the required pressure.

It is important to adjust the bending capacity accordingly when the length and thickness of the workpiece change.

Impact On Work Accuracy

The influence of bending machine worktable flatness (longitudinal) on the straightness of bending parts

1. Effect of workpiece angle accuracy

When there is a longitudinal flatness error on the workbench, it will affect the depth of the upper die into the notch of the lower die.

2. Effect of workpiece straightness accuracy

When there is a flatness error on the work surface, the straightness of the bent part is equal to the flatness error.

3.The influence of press brake table flatness (lateral) on the angle and straightness of bent parts

The influence mechanism of the flatness (transverse direction) of the press brake table on the angle and straightness accuracy of the bent parts is the same as the “longitudinal influence” mechanism.

Work Accuracy Inspection

Inspection conditions

Test piece requirements

Length requirements (length deviation is ±3mm)

Width requirement: The width of the test piece shall not be less than 100mm. (*Width deviation is ±0.2mm.)

Thickness requirements

Material requirements: The test piece material is Q235A steel plate, and the tensile strength of the material is σb≤450MPa.

Number of pieces: The number of test pieces should not be less than 3.

Mold requirements: The opening size of the lower mold for the test piece is 8 to 10 times the material thickness.

Position requirements: The test piece should be placed in the middle of the workbench.

Bending angle requirements: The bending angle of the specimen is 90°.

Measurement position: Start measuring 100mm from the end of the test piece.

Inspection tool requirements

Qualified flat ruler, universal angle ruler, and feeler ruler.

General requirements

Before working accuracy testing, the equipment needs to be leveled, and the horizontal and vertical directions should not exceed 0.2/1000.

The press brake is one of the key equipment for forming sheet metal products and ensuring the processing accuracy of sheet metal products. The accuracy of the bending equipment will directly affect the processing accuracy of sheet metal products. The impact of insufficiently accurate bending equipment on product deviations is long-term and objective. If the accuracy status of a bending equipment can be detected, it is very necessary to provide equipment accuracy data for the company to accept and repair the bending equipment.

The national standard GB/T14349-2011 “Precision of press brakes” stipulates accuracy inspection, inspection allowance and detection methods for the accuracy of press brakes. The inspection items of bending equipment include the inspection of geometric accuracy and working speed of the press brake, which are explained as follows.

Test Piece Discount Rate Test

Charge or difference

0.05mm over 1000mm length.

Detection method

Place the 1000mm flat ruler detection surface against the edge of the bent specimen (see the picture below), and use a feeler gauge to measure the gap δ between the two. The error value is calculated based on the maximum gap value within 1000mm.

Secondary bending test piece

If the material stress of the test piece is large, the test piece can be bent twice (see the figure below). Test the working accuracy of the bending machine.

Press Brake Calibration: Enhance Accuracy and Productivity

Press Brake Calibration Steps

How well the press brake is calibrated can make or break your whole bending process. A miscalibrated machine can produce uneven products that can kill valuable time through rework.

Don’t get intimidated here. Just follow the following steps, and you will be good to go.

• Collect the Required Tool

First, you should get some tools ready. Tools like press brake tooling, calipers or micrometers, precision angle finders, and materials for testing bending are some examples.

• Check the Specification

Get information about the press brakes’ specs, like tonnage, bending length, and punch speed, from the manufacturing menu or files. Verify that these requirements can meet your needs for certain tasks.

• Clean the Machine

Calibration will not be as accurate if the machine gets dirty. So, keep it clean and guarantee there is no dirt, dust, or debris.

Carefully examine the press brake’s bedding, ram, and back gauge. That’s because these are the parts most likely to touch metal sheets. Besides, make sure that all of the safety and security equipment is where it should be and it works properly.

• Checking

There are three types of checking processes that you will have to conduct at this stage of the calibration.

1. Checking the machine: Check the device for damage, spoilage, or wear because it can affect press brake accuracy. Replace any broken parts before continuous calibration. Additionally, make sure that the hydraulic system, the electrical connection, and the controller all work correctly.
2. Check the punch and die: Look at the punch and die for any strange issues (e.g., noise). If there are any problems with any of the components, please replace them so that the bending is done correctly.
3. Check the oil level: Assess the oil level in the hydraulic system. Low oil levels will make the machine work badly. The bending results will be poor in that case. So, ensure the oil is enough to run the machine before each use.

• Testing

This is the last step before the actual calibration. Use the sample bending on waste material to test the press brake.

• Bending check: To find the final bending angle made by bending a metal sheet, use a reliable protractor or angle gauge. The bending tools should be adjusted to the right angle if the result is wrong.
• Distortion check: After you bend something, check to see if the press brake moves. Abnormal deflection will affect the accuracy of the bending. If necessary, please follow the manufacturer’s instructions to keep deflection to a minimum.
• Calibration

It is time to set up the machine for calibration. This includes setting up the clamps, back gauge, and bending tool.

• Back Gauge Calibration

To bend metal just right, you must tweak the back gauge settings behind the scenes. Here’s what to do:

• Kick things off by shutting down the machine and getting your safety gear on, like gloves and glasses.
• Pull the gauge’s back endpoint all the way out so it’s easy to get your metal and tools to their location.
• Choose a specific spot on that back gauge to measure. This could be the spot where it pulls back the most or some fixed spot from the bending point.
• Use exact measuring-related tools (think extremely accurate rulers) to find out and note down how far that point is.
• Look at the number the machine throws at you for that spot. If that number is different from what you measured, you go into the computer control bit and tinker with it to match up.
• When you think you’ve got it right, measure at that special spot again, and both numbers—the one you measure and the one the machine tells you—should be twins.
• Wrap it by checking distances over the whole pull and push territory. You need to double-check that the number that appeared is what you’ve got on your measuring tool for each instance.
• Set the Clamp and Bending Tool

The clamp holds the metal sheet in place correctly. Make sure it’s in the correct location. Verify the bending tool is at the right angle to make the bend you need.

• Check Ram Parallelism

Ram parallelism means that the ram is straight across from the bending surface, which could be the press brake’s bed or a workbench. Measure correctly and make changes with the parallelism meter.

• Crowning

Crowning is the curved line on the bed that straightens out the metal sheet pressure that causes it to bend. Simply move the bedding oil cylinder until it forms the right curved line to adjust the crowning.

• Operate the Complete Testing Run

Run the whole testing program on the press brake using different thicknesses of material and angles of bend. Check the accuracy and repeatability by comparing the measured result to what was expected.

• Keep Calibration Records

Keep thorough records of the calibration process, including any changes you make and the final settings you end up with. The file is very useful for troubleshooting and future reference.

• Maintenance Regularly

The press brake will work at its best if it gets regular maintenance. Routinely checking and calibrating the machine is meant to keep it working correctly and safely.

 Importance of Press Brake Calibration

Calibration of the press brake is very important because it has a direct effect on the safety, accuracy, and quality of the metal bending. Here’s how:

• Consistency

Calibration of the press brake can make sure that the machine can bend the metal sheet at a precise and steady angle. Making even parts is possible with accurate bending. As a result, there will be fewer waste parts and fewer changes to the final product.

• Cost Savings

A calibrated press brake can cut down on the waste of material because it only makes a few broken parts. Aside from that, accurate bending can cut down on the need for rework, which saves time and money.

• Safety

A well-calibrated press brake lessens the probability of accidents to a great extent. However, an uncalibrated or miscalibrated press brake can make the tools move unexpectedly during the measurement process. This can turn out to be threatening for the operator.

Conclusion

From the analysis of a set of reference testing data provided by the press brake tool manufacturer, the impact of the longitudinal error of the press brake table on the bending accuracy is about three times that of the transverse error. The impact value of the bending angle deviation is about 3 times the flatness error of the workbench.

The bending angle deviation has little effect on the verticality of the working surface due to the slider stroke and can be ignored. The straightness of the work surface will directly reflect the straightness deviation of the work piece, and the two are almost equal. The bending straightness deviation is less affected by the slider stroke and the verticality error of the working surface. The work table mentioned in this article includes two, one is the machine table, and the other is the geometric accuracy of the horizontal support surface of the upper mold fitting surface. for example:

The length of the working table of a certain press brake is 3200mm. After measuring the working accuracy of this press brake, the following data were obtained: the straightness deviation of the workpiece is 0.06mm, and the bending angle deviation is 0.1720. It can be inferred from this that the flatness of the work surface of this press brake is about 0.06mm.

According to the national standard GB/T14349-2011 “Precision of press brakes”, the flatness margin of a press brake with a table length of 3200mm is 0.08mm. From this, it can be determined that the flatness accuracy of the press brake is qualified. In fact, as a user of a press brake tool, you only need to pay attention to the working accuracy of the machine tool.

Because the press brake accuracy after long-term use is not only affected by the factors mentioned in this article, the transmission clearance and mechanical wear of the machine tool that affect the accuracy of the work piece must also be taken into consideration, and the machine tool should be evaluated and considered comprehensively. That is to say, through the inspection of the working accuracy of the machine tool’s bending parts, combined with the national standards for the accuracy of the press brake, the comprehensive precision processing capabilities of a press brake can be inspected and judged.