Press Tooling Essentials: Design, Operations & Safety
Limitations of Press Tool Usage
High Initial Cost: The design and manufacture of press tools involve significant capital investment.
Limited Flexibility: Press tools are generally designed for specific parts, making them unsuitable for small batch production or frequent design changes.
Tool Wear and Maintenance: Press tools experience wear and tear and require regular maintenance and sharpening.
Material Restrictions: Only materials with suitable ductility and thickness can be processed effectively.
Safety Concerns: High-speed operations can pose safety hazards to operators if proper safety measures are not followed.
Space Requirements: Large presses and associated tools require considerable floor space.
Press Tool Materials & Selection Criteria
Common Materials:
Tool Steels (e.g., D2, O1, A2) – Used for punches and dies due to their hardness and wear resistance.
High-Speed Steel (HSS) – For high-wear parts requiring good heat resistance.
Carbides (Tungsten Carbide) – For high-volume production and abrasive materials.
Spring Steel – For springs and stripper plates.
Mild Steel and Cast Iron – For die sets, holders, and backing plates.
Aluminum and Bronze – Used for guide posts and bushings for lower friction.
Selection Criteria:
Hardness and Wear Resistance: For longevity and maintaining sharp edges.
Toughness: To withstand shock and avoid chipping.
Cost: Balanced against production volume.
Machinability: For ease of tool making.
Heat Resistance: Important for high-speed operations.
Corrosion Resistance: For longer tool life in humid environments.
Press Tonnage: Calculation & Factors
Press Tonnage refers to the maximum force or load that a press machine can exert during an operation, measured in tons.
How it is Decided:
Material Type: Stronger materials require higher tonnage.
Material Thickness: Thicker sheets increase the required tonnage.
Perimeter of Cut: Longer cutting edges mean more force is needed.
Operation Type: Operations like blanking or deep drawing require more force than simple piercing.
Formula (for blanking/piercing):
Tonnage = Perimeter × Material Thickness × Shear Strength
Importance of Developed Length Calculation
Importance of Developed Length Calculation:
Accurate Blank Sizing: Ensures the right amount of material is used for bending/forming operations.
Minimizes Material Waste: Prevents excess or shortage of material.
Tool Design Precision: Critical for die design to ensure proper fit and function.
Predicts Final Dimensions: Accounts for stretch and compression during forming.
Cost Estimation: Helps in estimating raw material costs accurately.
Press Selection Factors for Applications
When selecting a press for a particular application, several factors must be considered to ensure optimal performance and efficiency:
Type of Operation: Blanking, piercing, bending, drawing, etc., each may require a specific type of press (mechanical or hydraulic).
Tonnage Requirement: The press capacity must match or exceed the maximum force needed for the operation.
Stroke Length: The maximum travel of the press ram must accommodate the die and material thickness.
Shut Height: The distance between the ram and bed when fully closed should match the tool design and allow for proper die installation.
Bed Area: The press bed must be large enough to support the die and material adequately.
Speed of Operation: Higher speed is preferred for mass production, while lower speeds are necessary for precision operations.
Type of Drive: Flywheel, servo, or hydraulic drive, depending on the required precision, force control, and energy efficiency.
Operator Safety and Ergonomics: Ensure the press design and features promote safe operation and comfortable working conditions.
Press Overloading: Causes & Prevention
Overloading occurs when the force applied exceeds the rated capacity of the press, leading to:
Damage to press components (like crankshaft or frame).
Inaccurate part formation.
Tool breakage or excessive wear.
Significant safety hazards for operators.
Ways to Avoid Overloading:
Accurate Force Calculation: Use correct tonnage calculations based on material properties and operation type.
Use of Load Monitors: Install sensors to monitor real-time load during operation.
Press Selection: Always choose a press with a rated tonnage slightly higher than the maximum required for the operation.
Tool Maintenance: Keep dies and punches sharp and properly aligned to reduce required force.
Proper Operator Training: Train operators to recognize and avoid potential overload scenarios.
Use of Overload Protection Devices: Implement safety features such as hydraulic overload relief valves or clutch systems that disengage the press when capacity is exceeded.
Key Specifications of Press Tools
Press Capacity: Maximum tonnage (force) the press can exert.
Die Clearance: Gap between punch and die, which affects cut quality and required force.
Stroke Length: Maximum travel of the press ram.
Shut Height: Distance between the ram and bed when fully closed.
Number of Stations: How many operations can be performed in one progressive die.
Stripper Force: Force applied to remove the blank from the punch.
Feed Length: Length of strip fed per stroke.
Material Thickness: Maximum thickness the tool is designed to handle.
Press Machine Safety Devices
Fixed Guards: Rigid covers over dangerous parts to prevent access.
Interlocked Guards: The press stops if the guard is opened, preventing operation when exposed.
Die/Safety Block: A physical block inserted to prevent ram drop during maintenance or setup.
Two-Hand Control: Requires both hands to operate the press, keeping them away from the danger zone.
Light Curtains: Infrared beams that stop the press immediately if interrupted by an operator’s body part.
Pressure-Sensitive Mats: Mats placed around the press that stop the machine if stepped on.
Clutch-Brake Mechanism: Stops the press quickly during overload or emergency situations.
Hydraulic Overload Protection: Releases hydraulic pressure if it exceeds safe limits, preventing damage.
Safety PLCs: Programmable Logic Controllers that monitor all safety systems and stop the press if faults occur.
Hand Protection Sensors for Press Operations
Light Curtains (Photoelectric Sensors):
Uses infrared beams arranged in a curtain. If the operator’s hand interrupts the beam, the press stops immediately.Pressure-Sensitive Mats:
Mats placed around the press. If stepped on, they send a stop signal to the machine.Two-Hand Control Sensors:
Require the operator to press two buttons simultaneously, ensuring both hands are away from the danger zone during operation.Capacitive or Proximity Sensors:
Detect the presence of a hand or object near the dangerous area and stop the press if detected.
CNC Press Controllers: An Introduction
A CNC press controller automates and controls press tool operations using programmed instructions.
It precisely controls ram movement, stroke length, feeding, and timing.
Offers high precision, repeatability, and significant error reduction.
Enables multi-axis control, automated tool changes, and advanced diagnostics.
Commonly found in turret punch presses and modern press brake systems for enhanced automation and flexibility.
Shaving vs. Trimming Die Comparison
| Feature | Shaving Die | Trimming Die |
|---|---|---|
| Purpose | Improves the surface finish and dimensional accuracy after blanking | Removes excess material from the edges of a formed part |
| Precision | High precision and fine finish | Less precise, primarily removes flash or unwanted material |
| Operation Stage | Secondary operation | Final finishing operation after forming |
| Tool Design | More complex and sharp tolerances | Relatively simpler |
| Application | Used where tight tolerances are required | Used in deep drawing and forming operations |
Shaving Die: Components & Operation
Parts:
Punch: Shears the excess material (flash) after forging or blanking.
Die: Supports the workpiece and provides the cutting edge for shaving.
Stripper: Removes the workpiece from the punch after cutting.
Guide Pillars: Ensure precise alignment of the punch and die.
Working:
The punch moves down, shaving off a thin layer of excess material (flash) from the forged or blanked part to achieve a smooth finish and accurate dimensions. The die holds the part firmly, and the stripper pushes the part off the punch after the shaving operation is complete.
Fixed vs. Spring-Loaded Strippers
| Aspect | Fixed Stripper | Spring-Loaded Stripper |
|---|---|---|
| Operation | Rigid, fixed position | Moves with spring action to apply pressure |
| Effectiveness | Suitable for simple stripping | Better for removing material in deep or complex shapes, or when holding the material flat is critical |
| Cost and Maintenance | Simple and low cost | More complex and requires regular maintenance |
Compound vs. Combination Dies
| Aspect | Compound Die | Combination Die |
|---|---|---|
| Operations per Stroke | Two or more operations on the same strip at the same stroke (e.g., piercing and blanking simultaneously) | Two or more operations at the same station and stroke but possibly on different areas of the part (e.g., blanking and bending) |
| Tool Complexity | Simple arrangement, often concentric punches and dies | More complex design, integrating various forming and cutting elements |
| Number of Stations | One | One |
| Examples | Piercing + blanking of a washer | Blanking + bending of a bracket |
Combination Die: Construction & Working
Construction
Punch: The tool that moves downward to cut or form the sheet metal.
Die Block: Fixed part with cutting/forming edges that works in conjunction with the punch.
Stripper Plate: Removes the blank or formed part from the punch after the stroke.
Die Shoe: Base plate supporting the die components.
Guide Pillars and Bushes: Ensure precise alignment between the top and bottom halves of the die during operation.
Punch Holder: Holds and supports the punch securely.
Working Principle
When the press ram descends, the punch moves downwards toward the die.
The cutting edges perform operations like blanking or piercing on the sheet metal.
Simultaneously, other parts of the punch or die perform forming operations such as bending or drawing.
After the press stroke, the stripper plate pushes the material off the punch to avoid sticking.
All operations are completed in one stroke at a single station, significantly improving efficiency and accuracy.
Push-Through vs. Inverted Draw Dies
Push-Through Draw Die:
The punch pushes the sheet metal through the die cavity.
The drawn cup is ejected from the bottom of the die.
Common for deep drawing operations.
Parts: Punch, Die, Blank holder, Die shoe, Stripper
Inverted Draw Die:
The punch remains stationary and the die moves upward over the punch.
The cup is formed in the upward direction.
Often used when ejection from the top is easier or for specific part geometries.
Parts: Punch, Moving die, Blank holder, Die shoe
Progressive Die Fundamentals
A progressive die has multiple stations arranged in sequence.
The sheet or strip material moves through each station with every press stroke.
At each station, a specific operation (e.g., piercing, forming, cutting) is performed on the material.
The final part is completed only after passing through all designated stations.
The strip is precisely indexed (fed) step by step after each stroke to align for the next operation.
Press Tool Design Considerations
Key considerations in press tool design:
Punch and Die Clearance: Critical for accurate cutting/forming, minimizing burrs, and reducing tool wear.
Die Profile Radius: A smooth radius avoids tearing during deep drawing and improves material flow.
Stripper Plate: Ensures the material stays in place during the operation and aids in removing it from the punch after the stroke.
Guide Pillars and Bushes: Maintain precise alignment between the top and bottom halves of the die set, crucial for tool longevity and part accuracy.
Tool Material: Selection of appropriate wear-resistant materials (like D2 steel or carbide) for punches and dies.
Ejection Mechanism: Necessary to reliably remove formed or cut parts from the die cavity.
Fastening and Rigidity: Proper fastening of punches and dies ensures tool safety, stability, and longevity during high-force operations.
Punch Mounting Methods
Threaded Shank: The punch is screwed into the holder, providing a secure and easily replaceable connection.
Clamped Punch: The punch is held using clamps and bolts, offering strong retention.
Shoulder with Set Screws: The punch fits into a recess with a shoulder and is secured by set screws, allowing for precise positioning.
Welded Punch: The punch is welded permanently to the holder (typically used in non-precision or heavy-duty applications where replacement is less frequent).
Interchangeable Punches: Punches are inserted and secured with dowels or locking pins for quick and easy replacement, common in modular die systems.
Common Defects in Bent Parts
Springback: Material tries to return to its original shape after bending due to elasticity, resulting in an angle larger than intended.
Cracking: Usually occurs on the outer radius due to excessive tensile stress, especially with brittle materials or small bend radii.
Wrinkling: Happens on the inner surface due to compression during bending, causing folds or ripples.
Distorted Cross-section: The section may deform from its intended shape, particularly in hollow profiles or complex bends.
Incorrect Bend Angle/Radius: Due to poor tooling, inconsistent material properties, or inadequate process control.
Surface Scratches or Galling: Caused by friction between the die and the workpiece, often due to insufficient lubrication or rough tool surfaces.
Material Thinning: Excessive stretching on the outer radius, leading to a reduction in material thickness.
Bending Defects: Causes & Prevention
Cracking:
Caused by a bending radius smaller than the minimum allowable for the material, or by using brittle materials.
Precaution: Increase the bend radius, preheat the material, or select more ductile materials.
Warping:
Uneven bend forces or residual stresses cause distortion in the part.
Precaution: Use uniform pressure, proper tool design, and controlled bending speed.
Marring:
Surface damage due to improper tooling, rough tool surfaces, or lack of lubrication.
Precaution: Use polished tools, soft tool inserts, and adequate lubrication.
Common Defects in Drawn Parts
Wrinkling: Caused by excessive compressive stresses in the flange; material folds during drawing if not properly constrained.
Tearing: Material rupture due to overstretching beyond its tensile limits, often at the punch radius.
Earing: Uneven flange height around the drawn cup’s rim, caused by anisotropy (differences in material properties based on grain direction).
Thinning: Excessive reduction of material thickness, particularly on the sidewalls or at the punch radius, weakening the part.
Load-Stroke Curve in Sheet Metal Cutting
Definition
A Load-Stroke Curve graphically represents the force applied versus the stroke (punch travel) during a sheet metal cutting operation.
Stages in the Curve:
Approach: Low load as the punch moves toward the material, making initial contact.
Elastic Deformation: Load rises rapidly as the material resists deformation and deforms elastically.
Plastic Deformation: Load continues to rise but at a slower rate as the material yields plastically and the punch penetrates.
Crack Initiation: Load reaches its peak, and then drops slightly as cracks begin to form within the sheet from both the punch and die sides.
Fracture/Separation: Load decreases sharply as the material finally shears apart, and the slug separates from the strip.
Return Stroke: The punch retracts with minimal or no load as it moves back to its starting position.
Spring-Back in Bending Operations
Definition
Spring-back is the elastic recovery of the sheet metal after bending, which causes the bent part to slightly open or return toward its original shape after the forming force is removed.
Causes of Spring-Back in Sheet Metal:
Elastic Recovery:
When bending, both plastic (permanent) and elastic (temporary) deformation occur. Upon removal of the load, the elastic deformation recovers, leading to spring-back.
Material Properties:
Materials with high yield strength and high elastic modulus (e.g., stainless steel, spring steel) show more significant spring-back due to their greater elastic recovery.
Bend Radius:
A larger bend radius leads to more spring-back because a smaller proportion of the material is plastically deformed.
Sheet Thickness:
Thinner sheets are generally more prone to spring-back because they have less plastic deformation relative to elastic deformation for a given bend.
Tooling and Bending Method:
Air bending typically causes more spring-back than bottoming or coining, where the material is fully pressed into the die, maximizing plastic deformation.
Reducing Spring-Back in Bending:
Use a smaller punch radius or sharper dies to induce more plastic deformation.
Apply overbending to compensate for the expected spring-back.
Utilize bottoming or coining methods to set the bend angle more permanently.
Choose materials with lower yield strength for reduced elastic recovery.
Consider techniques like stress relieving or hot forming for certain materials.
Centre of Pressure in Sheet Metal Cutting
Definition:
Centre of pressure is the point where the total resultant force acts during sheet metal cutting. It is the balance point of all individual cutting forces exerted by the punch on the material.
Example:
For an irregular piercing operation (e.g., a component with multiple holes of different sizes or positions), each hole generates a cutting force. The Centre of Pressure (CoP) is calculated by taking the moment of all individual cutting forces around a reference axis and dividing by the total force. This calculation is crucial for ensuring the press ram applies force evenly, preventing off-center loading and premature tool wear.
Formula:
Automatic Stock Feeding Mechanisms
Definition
An automatic stock feeding mechanism is used to feed sheet metal strip or coil into the press tool automatically for continuous operation. This automation significantly improves production speed and reduces the need for manual handling.
Types of Feeding Mechanisms:
Manual Feeding: The operator manually places and advances the strip after each press stroke. Used for small batch production or simple operations.
Mechanical Roll Feeders: Uses pairs of rotating rollers, often driven by cams or the press crankshaft, to pull strip material into position. Suitable for continuous, medium-speed production and ensures consistent part spacing.
Gripper Feeders: Pneumatically or mechanically operated fingers grip and pull the strip forward in precise steps. Offers good accuracy.
Pneumatic or Air Feeds: Use compressed air to move the strip in fixed steps. Economical but generally less precise than mechanical or servo feeds, typically used for lightweight materials or short feed lengths.
Servo/NC (Numerical Control) Feeding: Utilizes servo motors and controllers for high-precision feeding. These are programmable for variable step lengths and speeds, making them highly flexible and suitable for CNC press operations and high-speed stamping.
Cam or Link Feeding Mechanism: The feed is driven mechanically from the press using cams or linkages, synchronizing feed motion with the press stroke.
Zig-Zag Feeding: Used to optimize material usage by moving the strip in both longitudinal and lateral directions. Ideal for cutting round parts from coils with minimal scrap.
Advantages:
Increases productivity and output rates.
Ensures consistent part spacing and dimensional accuracy.
Reduces manual handling, improving operator safety.
Integrates well with progressive dies for multi-stage operations.
Fundamental Concepts in Press Tool Design
Key Principles Explained:
Punch and die are always hardened:
Reason: To increase wear resistance and maintain the sharp cutting edges under high pressure and repetitive operations. Hardened tools last longer, maintain dimensional accuracy, and reduce the need for frequent replacement.Grain direction matters in strip layout:
Reason: The material’s grain direction significantly affects its formability and strength. Bending or forming across the grain can cause cracking, while aligning operations with the grain can improve part quality and prevent defects.Punch controls hole size:
Reason: In piercing operations, the punch cuts into the material, and the slug (scrap) is the same size as the punch. Therefore, the hole left behind matches the punch size, while the die opening is slightly larger to provide the necessary clearance.Shear is provided on die or punch:
Reason: Shear angles (a sloped cutting edge) are provided on either the punch or the die to reduce the cutting force required and minimize shock to the press. This creates a progressive cut rather than a sudden, full-length shear, which also reduces tool wear.Only two dowel pins are used in press tool assembly:
Reason: Two dowel pins are sufficient to ensure accurate alignment and precise positioning of punch and die halves. Using more than two can lead to over-constraint, making assembly difficult and potentially causing binding due to thermal expansion or manufacturing tolerances.Clearance is provided on punch:
Reason: In blanking operations, clearance is provided between the punch and die to reduce cutting force, prevent damage to the tools, and ensure clean shearing with minimal burr formation. The blanked part’s dimension matches the die opening, while the punch is slightly smaller for clearance.Coil stock is not used for double pass strip:
Reason: Double pass strip operations require the material to change direction and pass through the die again, which is difficult and inefficient with continuous coil feeding. Coil stock is ideal for unidirectional feeding in progressive dies, but not for setups needing reverse or secondary feed paths.For piercing operation shear is not provided on die:
Reason: In piercing, shear (a sloped surface) is generally applied to the punch to reduce cutting force. Providing shear on the die would result in tapered holes and poor dimensional accuracy of the pierced hole, which is undesirable for most applications.