• Sheet Metal Guidelines - Tolerance
• Material Impact on Sheet Metal Design
• Sheet Metal Process Selection & Tolerances
• Sheet Metal Design Features & Considerations
• Considerations for Sheet Metal Post-processing

Sheet metal part design is more complex than it might seem – sheet metal parts are, after all, relatively simple in appearance compared to an electronic component. However, there are a large number of considerations that go into designing and optimizing the design of sheet metal parts, from considering the built-in tolerances of the base material, to selecting the proper fabrication process and compensating for it in the part design, to selection of surface finishing options.

This guide underscores the impact of Design for Manufacturing (DFM) strategies, a blend of theoretical and practical approaches that enhance precision, reduce errors, and optimize production timelines. It offers an in-depth analysis of sheet metal manufacturing tolerances, a critical aspect ensuring the consistency and quality of every piece manufactured.

Each section of this guide is crafted to provide insights into the multifaceted world of sheet metal design and fabrication, from the initial design phase to production.

Tolerance is considered the cornerstone of precision in sheet metal fabrication, serving as a measure of the acceptable variation between the initial design and the final product. It is a delicate balance that ensures the functional and structural integrity of the sheet metal part while allowing for minor deviations that do not compromise its quality or performance.

Precision in tolerancing is not just a technical requirement but a critical factor influencing the alignment and assembly of the finished product. While advancements in fabrication techniques and equipment have enabled the achievement of narrow tolerances (down to ±0.05 mm), it is essential to approach this with a balanced perspective. A tighter tolerance equals higher cost and generally increases manufacturing complexity, so the best approach is to default to looser tolerance requirements wherever possible.

A pragmatic approach involves aligning the tolerance specifications with the actual requirements of the sheet metal part, avoiding unnecessary precision that does not add value to the function or structure of the product. Each fabrication process comes with its inherent tolerance levels, making it imperative for designers to have a nuanced understanding of these variations and their implications on the overall design.

Consulting with your sheet metal manufacturer is advised to align the design specifications with the practicalities of fabrication, ensuring that the established tolerances are both achievable and cost-effective. This synergy between design precision and fabrication reality underscores the essence of optimal tolerance in sheet metal manufacturing, ensuring quality, functionality, and cost efficiency.

It's crucial to emphasize that the numerical values and guidelines outlined in this document are intended for general reference in the context of sheet metal design. They should not substitute for the specific, tailored tolerances provided by your custom fabricator. Variabilities in material type, equipment, tools, and other nuanced factors necessitate a customized approach to defining tolerances and clearances for each unique project.

Engaging in a collaborative dialogue with your fabricator is essential to align design aspirations with practical manufacturing capabilities and constraints. This alignment ensures that the established tolerances are not only theoretically sound but are also practically achievable, balancing precision, quality, and cost-effectiveness.

Every project presents a distinct set of requirements and challenges. Adapting tolerances and clearances to these unique parameters ensures that the final product aligns with the intended functional, structural, and aesthetic objectives while adhering to the highest standards of quality and efficiency.

Specific tolerance guidelines integral to sheet metal fabrication are detailed in the table below. Each entry details the acceptable variance for various methods of fabrication and assembly. With values expressed in both millimeters and inches, the table serves as a reference, covering aspects from forming and bending to the relationships between holes, edges, and hardware.

These defined tolerances bridge the gap between theoretical design and practical manufacturing. They are instrumental in ensuring seamless alignment and assembly, enhancing the overall aesthetic and functional output. By adhering to these guidelines, professionals can navigate the complexities of material and process variances, ensuring each product exemplifies excellence in structural integrity and performance.

Actual tolerances in sheet metal fabrication are influenced by various factors, underscoring the necessity of engaging with your fabricator at the initial stages of design. Ensuring CAD designs are aligned with Design for Manufacturing (DFM) standards is pivotal in establishing realistic and attainable tolerances, taking into account the complexities of material, equipment, and fabrication processes.

• Material Selection: The type and thickness of the material chosen have inherent tolerances. These natural variances play a crucial role in determining the overall tolerance of the finished part.
• Process Complexity: The number of processes a part undergoes directly impacts the ability to maintain tight tolerances. Tolerance stackup occurs as variances accumulate with each additional process, complicating precision.
• Equipment Precision: Different tools and machines used in fabrication have varied accuracy levels. The equipment’s precision capability is a determining factor in achieving specific tolerance levels.
• Fabrication Companies’ Capabilities: Each custom fabrication and assembly company has unique equipment and skills. The diversity in capabilities influences the company’s ability to meet particular tolerance requirements, making pre-engagement essential for aligning expectations and capabilities.

Material tolerance is an intrinsic aspect of sheet metal parts, characterized by the built-in variances that occur due to factors like material properties and fabrication processes. These tolerances are particularly influenced by the thickness of the material, with each thickness category exhibiting distinct tolerance levels.

Material Tolerance - Thickness

Selecting the appropriate sheet thickness is a pivotal step in sheet metal fabrication. This choice influences a variety of factors including the part's overall strength, weight, and integral design features such as minimum bend radii, hole and slot sizes, and flange length. Ensuring uniform thickness throughout the sheet metal parts is essential to guarantee consistent quality and performance.

Guidelines for Material Thickness

A common trend in material tolerance is a lean towards negative tolerance. This indicates that the actual dimensions of the finished part may slightly undershoot the specified measurements in the design. It’s a critical aspect to consider in the design and fabrication processes to ensure the functional and structural integrity of the final product.

For a detailed and specific understanding, referring to a thickness tolerance chart is advisable. Such a chart provides comprehensive insights into the expected variances associated with different materials and thicknesses, serving as a valuable resource for designers and fabricators aiming for precision and quality in the finished products.

Material Tolerance - Properties

Depending on the material and the process used for sheet metal production, sheet metal material can also exhibit differences in physical properties such as surface tension, spring back, tensile strength, etc.

One of the aspects when selecting a material is to consider the amount of variability and safety factor the design allows. As an example, hot rolled carbon steels generally show more material variability than cold rolled varieties (though they are also more costly due to the secondary processing). For more precision bending, cold rolled steel would be the better selection to limit difference between material batches and improve bending consistency.

Precision and quality in sheet metal fabrication are heavily reliant on detailed and well-considered design, particularly when involving laser cutting and CNC bending technologies. The responsibility of the designer is to ensure that each element of the design is not only innovative but also compatible with the practical constraints of fabrication technologies. Every aspect, from the dimensions to the intricacies of curves and bends, must be tailored to align with the specific capabilities and limitations of the employed fabrication methods.

They are instrumental in ensuring that designs are tailored to the specific capabilities and constraints of the fabrication technologies, aligning innovative design aspirations with the practical aspects of material behavior, tool precision, and process efficiencies.

Sheet Metal Process - Laser Cutting

Laser cutting is a favored technique in precision sheet metal fabrication, known for its accuracy and efficiency. It’s especially suitable for projects demanding high precision, and is capable of achieving tolerances as tight as +/- 0.127mm. This method is versatile, handling materials up to 20 mm (0.78″) thick, accommodating a wide range of applications.

However, it's essential to consider the inherent limitations of laser cutting:

• Hole Size Limitation: The minimum hole sizes achievable are constrained by the size of the laser beam and the thickness of the material. In general, when the hole diameter is less than the material thickness the pierce point will often cause the hole to be out of tolerance.
• 3D Feature Constraints: Laser cutting is not suitable for creating tapered cuts or intricate 3D features due to the linear nature of the laser beam.

These constraints necessitate careful planning during the design phase to align project requirements with the capabilities of the process. It underscores the importance of a balanced approach, where design innovation is harmonized with practical execution to ensure the final components are both high-quality and fit for purpose.

Sheet Metal Process - Sheet Bending

CNC sheet bending involves the use of press brakes to manipulate sheet metal into desired shapes. While tooling is a fundamental aspect of this process, not all bending requires custom dies, offering a degree of flexibility in the fabrication process. Manual operators play a crucial role, and the type of equipment used can significantly influence the bend radius tolerances achieved. Key considerations in CNC sheet bending include:

• Corner Formation: Achieving a true 90° corner is typically not possible; corners will have a radius.
• Bend Angles: The standard tolerance for bend angles is ± 0.5°, ensuring precision while allowing for minimal variance.
• Bend Length: Tolerances for bend length are generally ±0.20 mm (0.010″) per bend, balancing accuracy and flexibility in the bending process

Sheet Metal Process - Stamping

Stamping tolerance refers to the allowable variance in dimensions and shapes during the stamping process. This process involves the use of dies to cut and form metal sheets into specific shapes and sizes. The precision of stamping is influenced by several factors, including the type of material, its thickness, and the complexity of the dies. Key considerations in stamping tolerance include:

• Material Behavior: Different materials respond variably to stamping, influencing the accuracy of the final product.
• Die Precision: The complexity and precision of the dies used directly impact the tolerance achievable.
• Process Speed: The speed of the stamping process can affect heat generation and material deformation, further impacting tolerance.

Sheet Metal Process - Punching

Punching tolerance is concerned with the allowable deviations during the punching process, where holes or shapes are created in the sheet metal. The precision in punching is paramount to ensure the functional and structural integrity of the final product. Factors influencing punching tolerance:

• Tool Wear: The condition and wear of punching tools can affect the accuracy of the holes or shapes punched.
• Material Thickness: Thicker materials may present challenges in maintaining tight tolerances.
• Machine Calibration: The calibration and maintenance of punching machines play a crucial role in achieving desired tolerances.

Sheet Metal Process - Rolling

Rolling tolerance pertains to the allowable variations during the rolling process, where sheet metal is passed through rollers to achieve specific thicknesses or shapes. The uniformity and consistency of the rolled sheet metal are critical and highly depends on the thickness of the material. Key aspects of rolling tolerance:

• Roller Condition: The state and alignment of rollers influence the uniformity of the rolled sheet metal.
• Material Properties: The inherent properties of the material, including its ductility and strength, impact the rolling tolerance.
• Temperature: The temperature during rolling can affect material behavior and, consequently, the tolerance achieved.

Sheet metal parts and designs generally incorporate a number of standard design features that improve part utility, strength, rigidity or other properties based on the final part application and needs. When incorporating these design features, it’s important to understand their role and what guidelines are needed for their use in a given part. We’ve laid out the basics of the most common features and the basic tolerances and tips for their use.

In sheet metal fabrication, the design of bends and flanges is instrumental in mitigating the spring-back effect and reducing the risk of metal tearing. Utilizing standard die sets, including 0.80 mm (0.030″) tools for internal corners, can be advantageous, or alternatively, consulting with your fabricator about the available tools is essential.

Designing bends on the same plane and direction enhances efficiency by eliminating the need for part reorientation during fabrication. Moreover, consistency in bend radius is a cost-effective strategy, particularly crucial when working with thick materials where making accurate small bends can be challenging.

Offsets, characterized by a double bend forming a Z-shape, are common in creating tiers in sheet metal or fabricating brackets and clamps. Like other bends, offsets should adhere to specific design guidelines to ensure structural integrity and functionality.

Key Design Guidelines for Offset Bends

• Inside Bend Radius: Should be at least equivalent to the material's thickness to prevent fractures or distortion. For low carbon steel, the minimum radius is either ½ the sheet thickness or 0.80 mm (0.03″), whichever is greater.
• Flange Length: Should be at least three times the sheet thickness to ensure stability and strength.
• Offset Bends: Maintain parallel planes at least two times the sheet thickness apart to avoid structural compromise.

Curls in sheet metal fabrication are employed to impart strength to the edge of the sheet and mitigate the exposure of sharp edges, enhancing both safety and durability. Distinct from a hem, curls are characterized by the edge turning inwards, creating a circular roll. They can be designed to be off-center or on-center, depending on the specific application and design requirements.

Key Design Guidelines for Curls

• Outside Radius: It should be at least double the thickness of the material to ensure structural integrity.
• Curl Opening Radius: Utilizing a radius that is twice the material thickness results in a curl opening radius that is equal to the material thickness.
• Opening Size: The opening of the curl should be at least equivalent to the thickness of the material to maintain consistency and strength.

The following table delineates the minimum distance guidelines specific to the relationship between a curl and other features such as holes and bends in sheet metal fabrication.

Each entry in the table provides a precise distance metric, calculated based on the curl's radius and the material thickness, to guide designers and fabricators in achieving optimal spacing between these features, ensuring correct integration and enhanced structural performance.

Hems are an essential element in sheet metal design, enhancing the strength of edges and eliminating sharpness, though they contribute additional weight to the part. Hemming is the process of folding a piece of sheet metal over itself; this can improve aesthetics by hiding edges or burrs, and provide added strength and reinforcement.

The pursuit of flat hems can lead to material fractures; hence, the design should focus on open or teardrop hems to mitigate this risk.

Key Design Guidelines for Hems

Open Hems:

• The inside diameter should be at least equal to the material thickness to ensure structural integrity.
• A cautionary approach is necessary for larger diameters as they may lose shape.
• The return flange should be at least four times the material thickness to maintain stability.

Teardrop Hems:

• These should also have an inside diameter at least equal to the material thickness.
• Openings should be at least ¼ of the material thickness to uphold structural integrity.
• The return flange should be at least four times the material thickness to ensure robustness.

Incorporating these specific design parameters ensures that hems contribute effectively to the structural strength and safety of the sheet metal parts, balancing the enhancement of functional attributes while adhering to safety standards.

The following table outlines the minimum distance guidelines between a hem and other features such as holes and bends in sheet metal parts. These distances, calculated based on material thickness and the hem's radius, are essential for ensuring structural integrity and optimal functionality during fabrication.

In sheet metal manufacturing, the design of holes is a critical aspect that requires precision and adherence to specific guidelines. The diameter of holes should be at least equivalent to the material's thickness to prevent excessive burring and ensure the longevity of the part. Proper spacing between holes is also essential to maintain their shape during subsequent processing steps.

Key Design Guidelines for Hole Design

• Diameter Requirements: Holes and slots should have a diameter at least equal to the material thickness or 1.00 mm (0.04″), whichever is greater. In the case of alloy or stainless steel, the diameter should be at least twice the material thickness.
• Consideration for Material Strength: For materials with higher strengths, the requirement for the diameter of holes and slots increases to ensure structural integrity and functionality.

These guidelines are integral to ensuring that the designed holes contribute to the overall quality, durability, and performance of the sheet metal components.

This table provides specific guidelines on the minimum distances required between holes, slots, and other features in sheet metal fabrication. Each parameter is carefully calculated, considering factors like material thickness and bend radius, to facilitate optimal design and fabrication outcomes.

Notches and tabs are two approaches to get additional space in a sheet metal design to avoid interference between features and to allow for tool access during fabrication. Notches involve the removal of an unwanted section of sheet metal from the outer edge of the part, while tabs are protrusions extending from the edge, often used for bending or joining purposes.

Key Design Guidelines for Notches and Tabs

Notches:

• Must have a thickness of at least 1mm (0.04″) or equivalent to the material thickness, whichever is greater.
• The length of a straight or radius end notch should not exceed 5 times the material thickness.
• For V notches, the length should be restricted to no more than 2 times its width.
• The corner radius of a notch should be 0.5 times the material thickness.

Tabs:

• Minimum width should be 3.2mm (0.126″) or two times the material thickness, whichever is greater.
• The depth of a tab should not exceed 5 times its width to ensure structural integrity.

These specifications are crucial to ensure that notches and tabs are designed and fabricated with optimal strength, precision, and functionality, contributing positively to the overall quality of the sheet metal components.

The table below outlines the minimum distance guidelines for positioning notches and tabs relative to bends, holes, and each other in sheet metal designs. These guidelines are essential to ensure the structural integrity and functionality of the fabricated parts, taking into account the material thickness and specific feature placements.

Corner fillets are employed in sheet metal design to transform sharp edges into rounded ones, mitigating safety hazards and ensuring that the part is easily removed from the tool. These rounded edges not only enhance safety but also facilitate ease during assembly, surface treatment, and packaging processes.

Key Design Guidelines for Corner Fillets

• Size: The size of corner fillets should be equivalent to half the material thickness. This ensures that edges are sufficiently rounded to eliminate safety concerns while maintaining the structural integrity of the design.

Implementing corner fillets with precision contributes to the overall safety, functionality, and efficiency of the sheet metal components during both the fabrication and utilization phases.

Relief cuts are instrumental in managing the deformation of sheet metal, especially in thicker parts with small bend radii. They mitigate issues such as overhangs, tearing near edges, and undesired deformations during bending by channeling the material effectively.

Key Design Guidelines for Relief Cuts

• Width: The width of a relief cut should be at least equal to the material thickness to ensure effective management of material deformation.
• Length: The length should exceed the bend radius to accommodate the material displacement during bending.

Dimples are another feature of sheet metal design, and are small holes through the sheet metal created by custom metal dies. These dimples improve strength and rigidity, reduce weight by removing material, and can improve aesthetic appeal with a regular hole pattern.

Key Design Guidelines for Dimples

• Diameter: The maximum diameter should be restricted to 6 times the material thickness to maintain structural integrity.
• Depth: The depth of a dimple should not exceed half of its inside diameter to avoid compromising the material's strength and durability.

These guidelines ensure that relief cuts and dimples are designed and implemented with precision, contributing to the overall quality, functionality, and aesthetic appeal of sheet metal components.

In the following table specific guidelines for the minimum distances required between dimples and other features like edges, bends, and holes in sheet metal fabrication are shown.

Gussets offer a practical solution to enhance the strength of flanges without the need for welding. These specialized components are integrated into sheet metal designs to reinforce structural integrity, ensuring that the final products can withstand various stresses and strains during their operational life.

The production of gussets typically involves custom tooling, a process that allows for precision and customization to meet specific design and performance criteria. This approach ensures that gussets are tailored to fit and function optimally within the broader design, contributing to the durability and reliability of sheet metal components while also streamlining the fabrication process by eliminating the need for welding.

Outlined below are the minimum distances to be maintained between a gusset and other features like edges and holes in sheet metal components. These guidelines are calculated considering the material thickness and the specific features involved, aiming for a balanced and efficient design.

Lances are a specific feature in sheet metal fabrication characterized by the technique of cutting and bending a piece of material without removing any part of it. This process effectively alters the shape of the material and is commonly employed to create vents and louvers, facilitating airflow through the part.

• Open Lances: These should have a minimum width of 3.00mm (0.125″) or two times the material width, whichever is greater. The maximum allowable width is capped at five times the initial width to maintain structural integrity.
• Closed Lances: The minimum width for closed lances is set at 1.60mm (0.06″) or twice the material width, whichever is greater. In terms of height, it should not exceed five times the material thickness at a 45° angle to ensure stability and functionality.

The creation of lances often necessitates specialized tooling to achieve precise cuts and bends, ensuring that the modified shape meets the design specifications and functional requirements. Below is detailed the rewired distances between a lance and a through hole.

Ribs and embossments are integral features that enhance the structural integrity and aesthetic appeal of the final products. Ribs add strength and rigidity to sheet metal components, reducing the need for thicker, heavier materials.

Embossments, on the other hand, are raised or recessed patterns on the surface that can serve both functional and decorative purposes. These features are produced to meet specific design and performance criteria, ensuring that each sheet metal component is optimized for its intended application.

Key Design Guidelines for Ribs and Embossments

• Rib Inside Radius: The inside radius of a rib should not exceed three times the material thickness to ensure structural stability.
• Round Embossment or Rib Depth: The maximum depth is equal to its inside radius, balancing structural integrity and design aesthetics.
• Flat Embossment Depth: The depth should be equal to the inside radius plus the outside, ensuring a harmonious integration into the overall design.
• V Embossment Depth: The maximum depth for a V embossment is capped at three times the material thickness, ensuring it is structurally sound and aesthetically pleasing.

The table below outlines the minimum distance necessary for the integration of ribs or embossments in sheet metal components.

*Please use these numbers as sheet metal design guidelines only and always check with your fabricator for their recommendations before completing your design.

Welding

Welding preparation is a critical step, especially for certain materials that require pre-welding processes like grinding. The design phase should anticipate these needs to ensure a seamless and efficient welding process. Precision in establishing tolerances is essential, as tighter tolerances can reduce the need for additional welding materials and techniques.

Key Design Guidelines for Welding

• Hand Welding: This technique is recommended for materials with a thickness greater than 20 gauge, ensuring an optimal balance of safety and effectiveness.
• Welding Head Access: The design must accommodate the physical space requirements for welding heads. In confined spaces or internal corners, a minimum clearance of 40 to 50mm is often necessary to ensure the welding equipment can access and effectively weld the designated areas.

These design considerations are crucial to facilitate a streamlined welding process, enhancing both the quality and efficiency of the final fabricated product. In the table below are additional guidelines for consideration.

Countersinks and counterbores play a critical role in achieving a flush surface for seamless fastening between parts. A countersink is characterized by a conical hole, engineered to house screws, ensuring a smooth surface post-assembly.

In contrast, a counterbore is characterized by a straight, flat-bottomed hole, typically designed to accommodate bolts or nuts. It's important to note that these features are not advisable for use in thin materials due to structural integrity concerns.

Key Design Guidelines for Countersinks and Counterbores

• Countersink Depth: The depth of a countersink should not exceed 0.6 times the thickness of the material to maintain structural integrity.
• Countersink-Fastener Contact: There should be at least 50% contact between a countersink and a fastener to ensure a secure and stable connection.

These guidelines are essential to optimize the functionality and durability of the assembled metal parts, ensuring that the countersinks and counterbores are effectively integrated without compromising the structural integrity of the materials involved.

The following table outlines the minimum distance parameters that should be maintained between these features and other elements like edges and bends, as well as between the features themselves.

One of the more common ways of joining sheet metal parts is to include fasteners or rivets to join sheet metal parts and sub-assemblies. While fastener selection is its own complex topic, there are some design considerations to make when fasteners need to be included into the part design.

Key Design Guidelines for Inclusion of Fasteners

• Threaded Fasteners: the thread width and depth need to be considered and calculated based on the thread and screw type being used (machine screws being most common). The material will also need to be thick enough to accommodate sufficient turns (at least 3) when threaded.
• Rivets: Rivets will have a minimum / maximum hole size to be considered when they are installed. Your rivet supplier will have suggested guidelines on hole size depending on rivet selected.
• Hardware Placement to Edge: there needs to be a minimum distance from a threaded hole to the edge of a part to make sure the hardware is secure, no damage will occur during threading and that the part will be well secured.

You should use these numbers as a guideline, but remember to check with your sheet metal fabricator when you pass over the drawing. The PennEngineering website provides additional details and specifications for common fasteners.

With over 20 years of expertise in sheet metal fabrication in China, our skilled team of engineers and technicians at Komaspec is committed to excellence in every project.

With more than twenty engineers and project managers, we provide in-depth reviews and analysis of sheet metal part design, offering comprehensive Design for Manufacturability (DFM) feedback.

Our collaborative approach aims to enhance product functionality while minimizing manufacturing and tooling expenses, ensuring that your project is both cost-effective and of the highest quality.

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