During the actual use of a bending machine, there may be some errors in bending accuracy to a certain extent. Sometimes it may be due to mechanical factors of the machine that result in insufficient bending accuracy, or it could be due to external factors such as the bending mold, material thickness of the bending sheet, and even some human-operational factors. This article will analyze the factors that affect bending accuracy from various aspects and propose solutions for some common situations.
Common Factors Introduction
Mechanical Factors
1. Straightness of the clamping opening of the bending machine slide (in the Y and X directions).
2. Relocation accuracy and arbitrary positioning accuracy of the left and right slides.
3. Whether the clearance between the sliding block and the frame guide rail of the bending machine is reasonable.
4. Verticality and inward inclination of the frame.
5. Connection between the hydraulic cylinder and the sliding block.
6. Strength and precision of the frame and sliding block.
7. Relocation accuracy and arbitrary positioning accuracy of the backgauge system (in the X and R directions).
8. Whether the computer system is adjusted properly.
9. Whether the hydraulic system is adjusted properly.
10. Compatibility between the hydraulic system and the computer adjustment.
Mold Factors
1. Whether the upper and lower molds have accurate precision.
2. Deformation, damage, and wear of the molds will affect all aspects of the bending forming. Once discovered, they should be reported and corrected in a timely manner.
3. Different knife positions between the upper and lower molds can cause dimensional deviations in bending. When positioning the knives, ensure they are properly aligned.
4. After the lateral movement of the backgauge, the relative distance to the lower mold changes. Use a vernier caliper to measure and make minor adjustments with the backgauge screws.
5. Whether the compensation device of the lower mold is accurate and matches the design of the frame.
6. The precision of the upper mold fixture should be high.
7. The size of the V-groove in the lower mold is inversely proportional to the bending pressure. With a fixed sheet length and thickness, a larger V-groove requires less pressure. Therefore, when processing different thickness workpieces, the correct size of the V-groove in the lower mold should be used according to the specifications.
8. Bending with a single-side load on one end of the bending machine not only affects the bending pressure but also causes damage to the machine tool. It is strictly prohibited. When configuring the mold, the force should always be distributed in the middle part of the machine tool.
Sheet Factors
1. Straightness of the reference surface of the sheet.
2. Uniformity of stress in the metal sheet.
3. Consistency of sheet thickness.
4. Insufficient parallelism between the workpiece and the lower mold during bending, causing the workpiece to rebound after the upper mold applies pressure, affecting the bending dimensions.
5. Material properties and thickness can affect the bending angle, so each workpiece must undergo initial inspection and strengthen random sampling during bending.
Human Factors
1. Inconsistent pushing force from left to right when bending the sheet metal.
2. Failure to adjust the data errors within the system when using it.
3. Insufficient angle in the first bending affects the dimensions in the second bending.
4. Cumulative bending errors can lead to increased dimensional errors in the workpiece shape. Therefore, ensuring the accuracy of single-side bending is particularly important.
5. Different lengths and thicknesses of workpieces require different pressures. The length and thickness of the sheet are directly proportional to the required pressure. Therefore, when there are changes in the length and thickness of the workpiece, the bending pressure must be readjusted.
Maintenance Guidelines for Molds
Based on the above issues, it is evident that the precision and usage of the molds have a crucial impact on bending accuracy. To address potential problems with the upper and lower molds on the bending machine, we have summarized the following maintenance guidelines to share with everyone.
Basic Maintenance of Molds
Maintenance of continuous molds should be carried out carefully, patiently, and step by step, avoiding blind actions. When troubleshooting and repairing molds, it is necessary to attach material strips for problem identification. Open the mold and inspect its condition by comparing it with the material strips, identify the cause of the malfunction, and locate the problem before proceeding with mold cleaning. Only then can the mold be disassembled.
During mold disassembly, the force should be evenly distributed. For molds with unloading springs positioned between the fixed plate and the unloading plate, and with the unloading springs directly pressing on the inner guide pillars, the disassembly of the unloading plate should ensure that it pops out in a balanced manner. The tilting of the unloading plate may cause the upper mold to fracture within the mold.
Maintenance of Upper and Lower Molds
When disassembling the upper and lower molds, attention should be paid to the original condition of the molds for easy restoration during subsequent mold assembly. If any shimming or repositioning is done, the thickness of the shims should be marked on the parts and recorded. When replacing the upper mold, test-insert the unloading block to check if it moves smoothly, and test-insert it into the lower mold to ensure uniform clearance. When replacing the lower mold, test-insert it into the punch to ensure uniform clearance as well. After grinding the upper mold and shortening its length, shims should be added to achieve the required length. It is necessary to check if the effective length of the upper mold is sufficient. When assembling the upper mold, check if there is sufficient clearance between the upper mold and the fixed block or fixed plate. If there are pressure blocks, check if there is enough clearance for movement. When assembling the lower mold, it should be placed horizontally and lightly tapped into position using a copper rod on a flat iron block placed on the lower mold surface. It must not be forcefully hammered in at an angle. The bottom of the lower mold should be chamfered. After assembly, check if the lower mold surface is parallel to the mold surface. After the assembly of the upper mold, lower mold, and core, necessary checks should be made against the material strips. Check if any parts are assembled incorrectly or in reverse. Verify if the lower mold and lower mold shims are installed correctly, if the material discharge holes are blocked, if new parts need to be trimmed, and if the necessary trimming is sufficient. Ensure that the locking positions of the mold are properly secured. Pay attention to the confirmation of the locking of the discharge plate screws. When locking, apply force from the inside to the outside and cross-tighten evenly. Do not lock one screw first and then the other, as it may cause the discharge plate to tilt, leading to upper mold fracture or reduced mold accuracy.
Adjustment of Mold Clearances
The positioning holes for the core may wear due to frequent assembly, resulting in increased clearance (looseness) or uneven clearance (positioning deviation) after assembly. This can lead to poor section shape after punching, easy breakage of the upper mold, burrs, etc. Proper clearance adjustment can be done by checking the section shape after punching. When the clearance is small, there are fewer sections, and when the clearance is large, there are more sections and larger burrs. Shifting can be used to achieve a reasonable clearance. After adjustment, appropriate records should be made, such as marking on the edge of the lower mold, to facilitate subsequent maintenance work. In daily production, it is important to collect and preserve material strips from the molds in their optimal conditions. These can serve as references for mold maintenance when production becomes problematic or when the molds show variations. Additionally, auxiliary systems such as the material stopper pin and its wear, the ability to stop the material, and the wear of guide pins and bushings should be checked and maintained.
Causes and Countermeasures for Common Mold Failures
In actual production on bending machines, it is necessary to analyze the specific causes of low bending accuracy and implement effective countermeasures to fundamentally solve the issues. This is essential for reducing production costs and achieving smooth production. The following analysis provides causes and countermeasures for common occurrences of low bending accuracy in production for reference and use.
Burr Formation
Causes: a. Blade wear; b. Ineffective results after grinding due to excessive clearance; c. Blade chipping; d. Unreasonable vertical or horizontal displacement or looseness of the clearance; e. Misalignment of the upper and lower molds.
Countermeasures: a. Grind the blades; b. Control the machining accuracy of the upper and lower molds or modify the design clearance; c. Repair the blades; d. Adjust the punching and shearing clearance, check for mold cavity wear or dimensional accuracy issues; e. Replace guiding components or reassemble the mold.
Chip Jump and Compression
Causes: a. Excessive clearance; b. Improper feeding; c. Excessive oil droplets during stamping, causing oil adhesion; d. Failure to demagnetize the mold; e. Upper mold wear, with chips adhering to the mold; f. Short upper mold, insufficient insertion length into the lower mold; g. Harder material with simple cutting shape; h. Emergency measures.
Countermeasures: a. Control the machining accuracy of the upper and lower molds or modify the design clearance; b. Trim the material strip when it reaches the appropriate position and clean the mold in a timely manner; c. Control the amount of stamping oil and consider changing to a less viscous oil; d. Demagnetize the mold after grinding (especially for iron materials); e. Repair the upper mold blades; f. Adjust the length of the upper mold insertion into the lower mold; g. Change the material or modify the design. Modify the top end face of the upper mold with a tapered surface or arc (pay attention to the direction). Reduce the contact area between the upper mold blade end face and the chips; h. Reduce the sharpness of the lower mold blade, reduce the amount of grinding on the lower mold blade, increase the roughness of the straight surface of the lower mold (coating), use a vacuum cleaner to remove waste material. Decrease the punching speed to reduce chip jumping.
Material Blockage
Causes: a. Insufficient size of discharge holes; b. Large discharge holes causing material rolling; c. Blade wear, resulting in larger burrs; d. Excessive oil droplets during stamping, causing oil adhesion; e. Rough surface of the lower mold straight edge, causing powder and chips to adhere to the edge; f. Softer material; g. Emergency measures.
Countermeasures: a. Modify the discharge holes; b. Modify the discharge holes; c. Repair the blades; d. Control the amount of stamping oil and consider changing to a different type of oil; e. Surface treatment, polishing, and attention to reducing surface roughness during machining; change the material; f. Adjust the punching and shearing clearance; g. Modify the mold to ensure proper discharge; h. Modify the guide plate to prevent material sticking; i. Thinner material may cause warping during feeding; j. Improper mold installation, significant deviation from the vertical alignment with the feeding mechanism.
Dimensional Variation in Material Positioning
Causes: a. Blade wear on the upper and lower molds, resulting in burrs (larger outer dimensions, smaller inner dimensions); b. Improper design dimensions and clearance, poor machining accuracy; c. Misalignment of the upper and lower molds or inserts, uneven clearance; d. Worn guide pins, insufficient pin diameter; e. Wear of guiding components; f. Improper feeding distance, pressure, and relaxation adjustments; g. Improper mold closing height adjustment; h. Wear of the material stopper block, loss of pressure (excessive pressure) function (leading to small punching); i. Excessive pressure from the material stopper block, resulting in larger punching; j. Variation in mechanical properties of the stamped material (unstable strength and elongation); k. During cutting, cutting force causes material traction, resulting in dimensional variation.
Countermeasures: a. Repair the blades; b. Modify the design, control the machining accuracy; c. Adjust the positioning accuracy and clearance during assembly; d. Replace the guide pins; e. Replace the guide pillars and bushings, and pay attention to regular maintenance; f. Readjust the feeding mechanism; g. Readjust the mold closing height; h. Grind or replace the material stopper block to enhance pressure function and adjust the material stopper; i. Reduce excessive pressure depth; j. Change the material and control the quality of the feed; k. Modify the top end face of the upper mold with a tapered surface or arc (pay attention to the direction) to improve the force distribution during cutting. Optionally, add a guiding function to the material positioning on the material stopper block.
Material Jamming
Causes: a. Improper feeding distance, pressure, and relaxation adjustments; b. Variation in feeding distance during production; c. Malfunction of the feeding mechanism; d. Material curvature, excessive width, larger burrs; e. Abnormal stamping on the mold, causing material bending; f. Insufficient diameter of the material guide hole, resulting in material pull by the upper mold; g. Inefficient material removal during bending or tearing; h. Improper setting of the material removal function on the guide plate, material strip adhering; i. Thin material causing warping during feeding; j. Improper mold installation, significant deviation from the vertical alignment with the feeding mechanism.
Countermeasures: a. Readjust the adjustments; b. Readjust the adjustments; c. Adjust and repair the feeding mechanism; d. Change the material, control the quality of the feed; e. Eliminate the material strip bending; f. Repair the upper and lower mold guide holes; g. Adjust the strength of the material removal springs, etc.; h. Modify the guide plate to prevent material sticking; i. Install upper and lower material pressure devices on the feeding mechanism, install upper and lower material extrusion safety switches; j. Reinstall the mold.
Upper Mold Fracture and Chipping
Causes: a. Chip jumping, material blockage, mold jamming, etc.; b. Improper feeding, cutting half of the material; c. Insufficient strength of the upper mold; d. Close proximity of the upper and lower molds, material traction during cutting causing fracture of the small upper mold; e. Excessive sharp angles on the upper and lower molds; f. Small punching and shearing clearance; g. Insufficient stamping oil or using highly volatile stamping oil; h. Uneven punching and shearing clearance, offset, interference between the upper and lower molds; i. Poor accuracy or wear of the material stopper block, loss of precise guiding function; j. Inaccurate mold guiding or wear; k. Improper selection of upper and lower mold materials, improper hardness; l. Wear of guiding components (pins); m. Improper use of shims.
Countermeasures: a. Resolve issues such as chip jumping, material blockage, and mold jamming; b. Pay attention to feeding, trim the material strip in a timely manner, and clean the mold promptly; c. Modify the design, increase the overall strength of the upper mold, reduce the dimensions of the lower mold straight edge, pay attention to modifying the top end face of the upper mold with a tapered surface or arc, and cut the small parts last; d. Shorten the length of the small upper mold by at least one material thickness relative to the large upper mold; e. Modify the design; f. Control the machining accuracy of the upper and lower molds or modify the design clearance, appropriately increase the clearance for small parts; g. Adjust the amount of stamping oil or change the type of oil; h. Check the accuracy of various formed parts and make adjustments or replacements to control the machining accuracy; i. Repair or replace; j. Replace the guide pins and bushings, and pay attention to regular maintenance; k. Change the material or select appropriate hardness; l. Replace the guiding components (pins); m. Adjust the shims, use as few as possible, and use steel shims. Place the lower mold shims below the shim block.
Bending Deformation and Dimensional Variation
Causes: a. Worn guide pins with insufficient diameter; b. Poor accuracy and wear of the bending guide components; c. Wear (compression) on the upper and lower bending molds; d. Insufficient mold clearance; e. Material slippage, lack of guiding function on the upper and lower bending molds, and no pre-bending applied; f. Poor mold structure and design dimensions; g. Burr formation on the stamped parts, leading to poor bending results; h. Excessive use of shims on the upper and lower bending molds, causing dimensional instability; i. Variation in material thickness; j. Variation in mechanical properties of the material.
Countermeasures: a. Replace the guide pins; b. Resurface or replace the bending guide components; c. Resurface or replace the upper and lower bending molds; d. Inspect and correct the mold clearance; e. Modify the design, add guiding components and pre-bending function; f. Modify the design dimensions, split the bending process, and add forming steps; g. Repair the cutting edge at the material positioning area; h. Adjust and use solid steel shims; i. Change the material and control the quality of the feed; j. Change the material and control the quality of the feed.
Height Variation of Stamped Parts (Multiple Parts in One Die)
Causes: a. Burr formation on the stamped parts; b. Pressing damage on the stamped parts, presence of debris in the mold; c. Wear or damage to the upper and lower molds (bending position); d. Material flipping during stamping and cutting; e. Wear or damage to the material pressing components; f. Inconsistent tearing dimensions in related cutting positions, blade wear; g. Inconsistent pre-cutting depth in related fragile positions, wear or chipping on the upper and lower molds; h. Severe chipping or wear on the upper and lower molds in related forming positions; i. Design defects in the mold.
Countermeasures: a. Repair the cutting edge at the material positioning area; b. Clean the mold to resolve floating debris issues; c. Resurface or replace the damaged components; d. Repair the cutting edge, adjust or add strong pressing function; e. Inspect, maintain, or replace the related components; f. Repair or replace to ensure consistent tearing conditions; g. Inspect the condition of the upper and lower molds for pre-cutting, and perform maintenance or replacement; h. Inspect the condition of the upper and lower molds in the forming positions, and perform maintenance or replacement; i. Modify the design, add height adjustment or additional forming stations.
Improper Maintenance
Causes: a. Lack of anti-misalignment function in the mold, negligence during mold assembly leading to reverse installation or misalignment (referring to different workstations), etc.; b. Failure to restore the insert components that have been adjusted for clearance.
Countermeasures: a. Modify the mold to add anti-misalignment function; b. Use marking methods on the mold and conduct necessary checks and confirmation against the material strip after mold assembly, and make written records for future reference.
In bending production, insufficient bending accuracy is a common issue. To address such problems, it is important to identify the causes based on the previous discussions and find appropriate solutions based on the specific circumstances. Additionally, regular maintenance of the bending machine is crucial. This includes routine checks to ensure the machine and molds are in proper working condition, such as the functionality of various axes, absence of system deviations, pre-operation mold inspections, and confirmation of tightened components. By following these practices, many unexpected accidents can be prevented. It is essential to think before taking action and keep diligent records to accumulate experience.
