Custom Parts Made to Your Drawings

  Introduction Sheet metal unfolding size errors often happen during the translation from a 3D folded model to a 2D flat pattern. A CAD model may look correct, but the laser-cut blank can still become too long, too short or misaligned after bending if material stretching, material compression, springback and press brake limitations are not considered. For custom sheet metal parts, the flat pattern is not only a software output. It is the result of material thickness, inside bend radius, K-factor, bend allowance, bend deduction, material grain direction, tooling selection, bend sequence and tolerance strategy. SOLIDWORKS documentation explains that sheet metal bend calculations can use K-factor, bend allowance or bend deduction, and GoEngineer notes that flat patterns may be calculated automatically by these methods, but the selected method can affect whether the final part fits. [1][2] For overseas buyers, unfolding errors may create repeated samples, poor assembly fit, hole deformation, angle deviation and delayed delivery. Before sending files for fabrication, engineers should verify the following points with both the CAD model and supplier-side DFM review. Problem Sheet metal flat-pattern errors usually come from a mismatch between CAD assumptions and the real bending process. The most common causes are: Using default CAD K-factor values without checking actual material and tooling data. Calculating bend allowance or bend deduction with incorrect material thickness, bend angle or inside bend radius. Ignoring material stretching and compression around the neutral axis during bending. Designing holes, slots, hems or cutouts too close to bend lines or sheet edges. Specifying an inside bend radius that does not match available press brake tooling. Ignoring material grain direction, material temper, yield strength and ductility. Failing to consider springback, especially for stainless steel, aluminum alloys, high-strength steel or thicker plates. Applying overly tight tolerances to non-critical dimensions, which increases cost and inspection risk. Using a flat pattern from CAD without first article verification or supplier-side bend deduction data. The result is simple: the flat blank may be cut accurately, but the formed sheet metal part may fail after bending. Why Sheet Metal Unfolding Size Errors Happen 1. K-Factor Is Treated as a Fixed Number K-factor defines the location of the neutral axis through the sheet thickness during bending. It is used to estimate how much material stretches during forming and directly affects flat pattern length. SOLIDWORKS states that bend allowance with K-factor is calculated using bend radius, thickness and K-factor. [3] The risk is that many CAD templates use default K-factor settings. A value such as 0.44 may be convenient for early design, but it should not be treated as universal production data. K-factor changes with material type, material temper, sheet thickness, inside bend radius, bending method and tooling precision. Xometry Pro notes that K-factor varies based on material properties, inside bend radius relative to thickness, bending method and tooling precision. [4] For practical DFM review, the K-factor should be checked against real shop-floor bending results. Autodesk also describes calculating K-factor by measuring sample sheet metal parts after bending, which supports the idea that verified bending data is more reliable than default CAD values. [5]   2. Bend Allowance and Bend Deduction Are Not Matched to Production Bend allowance and bend deduction are both used to calculate the developed flat length, but they represent different calculation approaches. If the selected method or values do not match the real process, the flat pattern can be wrong. For example, if a CAD file calculates a flat pattern using one bend allowance rule but the supplier bends the part with a different die opening or actual inside radius, the final flange length may shift. This is why many sheet metal shops build bend deduction tables based on their own material, tooling and press brake data rather than relying only on theoretical formulas.    3. Inside Bend Radius Does Not Match Available Tooling Inside bend radius is one of the most important inputs for sheet metal unfolding. It affects the bend allowance, bend deduction and final outside size. A zero-radius or completely sharp bend should not be used as a normal design assumption. Sharp bends can increase cracking risk and may not match practical tooling. For many standard sheet metal designs, using an inside bend radius at least equal to material thickness is a safer starting point, although the final value should be confirmed with the supplier based on material and tooling. 4. Material Thickness, Gauge and Stock Availability Are Not Verified Unfolding calculations depend on real material thickness. If the design model uses nominal thickness but the actual stock differs, the bend radius, developed length and final formed size can also change. Engineers should also check whether the selected sheet thickness matches standard commercial stock or gauge options. Non-standard thickness may increase material sourcing time, cost or substitution risk. 5. Grain Direction Is Ignored Sheet metal has a rolling direction or grain direction. Bending parallel to the grain may increase cracking risk, especially for harder materials or smaller bend radii. For critical bends, engineers should discuss grain direction with the supplier and, where possible, orient critical bends perpendicular or approximately 45 degrees to the grain. Grain direction is especially important for aluminum alloys, stainless steel, spring steel and parts where bend cracking or cosmetic surface quality matters. 6. Feature Proximity to Bends Is Too Close Holes, slots, countersinks and insert holes can deform if they are too close to bend lines. During bending, material stretches and compresses near the bend region, which may turn round holes into oval holes or shift slot positions. Protolabs highlights that features can distort when placed too close to bends, and its bend radius guidance warns that holes placed close to bends may become oval due to deformation. [6][7] As a practical starting rule, functional holes and slots should often be kept at least 2x to 3x material thickness away from the bend tangent line, and some supplier guidelines recommend even larger distances depending on bend radius, material and tooling. For high-risk features, supplier DFM review should decide the final safe distance.  7. Hole-to-Edge Spacing Is Too Small Holes and slots placed too close to sheet edges can tear, distort or reduce strength. For general design, keeping holes at least 1.5x to 2x material thickness away from the sheet edge is a useful starting rule, but final spacing should follow the part function, material strength and supplier tooling capability.   8. Flange Length Is Too Short for Press Brake Forming A flange must be long enough for the press brake to hold and form the part cleanly. Protolabs states that minimum flange length on sheet metal parts must be at least 4 times the material thickness for its service guidelines. [6] If the flange is too short, the part may slip, bend inaccurately or become impossible to form with standard tooling. This is a common reason why a 3D model looks possible but the real fabrication process fails.    9. Bend Relief Is Missing When a bend is adjacent to an unbent wall, notch, cutout or edge, bend relief may be needed to prevent tearing or unwanted deformation. Bend relief cuts provide space for material movement during forming. For boxes, channels, enclosures and brackets with intersecting flanges, bend relief should be reviewed before laser cutting. Adding small reliefs can reduce cracking risk and improve edge quality after bending. 10. Hems and Curls Are Not Designed with Enough Return Length Hems and curls require enough return length, internal gap and bend radius to close properly. If the return length is too short or the gap is too small, the material may split, bulge or fail to close consistently. As a practical rule, hem and curl features should be reviewed using supplier-specific forming data, especially when material is thick, hard or cosmetic. 11. Tolerances Are Too Tight for Non-Critical Features Overly tight tolerances increase manufacturing cost and inspection time. They can also make a normally acceptable sheet metal process appear unstable. For non-critical sheet metal dimensions, standard tolerances are usually more cost-effective than precision machining-style tolerances. Xometry lists sheet metal bending general tolerances that include larger tolerances for multi-plane bent parts and +/- 1 degree for bend angles, showing that sheet metal bending tolerance expectations differ from CNC machining tolerance expectations. [8]  12. CAD, Drawing and CAM Units Are Not Consistent Unfolding errors can also come from simple file mistakes: mm vs inch mismatch, missing angle tolerance, unclear material thickness, or inconsistent drawing revisions. Engineers should ensure that 3D CAD, 2D drawing, flat pattern and supplier CAM programming all use the same unit system and revision. Ultimate Engineering Checklist Before Sending Files to Fabrication The following checklist can be used before RFQ, prototype production or repeat sheet metal fabrication. Check Area What Engineers Should Verify Why It Matters K-Factor Confirm CAD K-factor is not only a default value. Calibrate it against material, thickness, temper and supplier tooling. Controls neutral axis position and developed flat length. Bend Allowance / Deduction Check whether the flat pattern is based on K-factor, BA, BD or bend tables. Wrong values make flat blanks too long or too short. Inside Bend Radius Confirm design radius matches available tooling and material capability. Wrong radius changes bend calculation and may cause cracking. Material Thickness / Gauge Verify real stock thickness and commercial availability. Thickness affects bend radius, flange length and final size. Grain Direction Review orientation for critical bends; avoid high-risk parallel-to-grain bends when possible. Reduces cracking risk and improves forming stability. Feature Distance from Bend Keep holes/slots away from bend lines; use supplier DFM for critical features. Prevents oval holes, slot shift and insert misalignment. Hole-to-Edge Distance Avoid holes too close to sheet edges. Reduces tearing and local weakness. Minimum Flange Length Check flange length against material thickness and press brake tooling. Ensures the press brake can hold and form the flange. Bend Relief Add relief near corners, notches and adjacent walls where needed. Prevents tearing and unwanted deformation. Hems and Curls Check return length, gap and radius. Prevents splitting and inconsistent closure. Tolerances Use tight tolerances only for functional features. Controls cost and avoids unrealistic inspection requirements. Units and Revisions Match 3D CAD, 2D drawing, flat pattern and CAM units/revisions. Prevents basic but costly production mistakes. DFM Review Points 1. Compare Folded Model, Flat Pattern and 2D Drawing Together Do not review the flat pattern alone. Check whether the folded model, flat pattern and 2D drawing describe the same material thickness, bend radius, bend directions, critical dimensions and revision level. 2. Identify Critical Post-Bending Dimensions For enclosures, brackets and frames, the dimensions that matter are often after bending: mounting hole distance, assembly width, opening size, squareness, flatness and gap control. These should be clearly marked on the 2D drawing. 3. Confirm Supplier-Side Bend Data Ask the supplier whether they will use default CAD output or their own bend deduction table. For tight-tolerance sheet metal parts, supplier-side verified bend data is usually more reliable than default CAD values. 4. Review Feature Deformation Risk Check holes, slots, countersinks, tabs, inserts and cutouts near bend lines. If a feature is functional, it may need relocation, relief, post-machining, special tooling or tolerance adjustment. 5. Check Equipment and Tooling Limitations Review V-die opening, punch radius, press brake capacity, minimum flange length, tool access and potential multi-bend interference. A part may be geometrically correct but not bendable with available tooling. Practical Suggestions 1. Send Both 3D CAD and 2D Drawings The 3D CAD file helps the supplier understand structure and bend direction. The 2D drawing defines material, thickness, tolerances, surface finish, critical dimensions and inspection requirements. For custom sheet metal parts, both files are important. 2. Do Not Treat Default Flat Patterns as Production-Ready Default CAD flat patterns are useful for early design, but they should be reviewed before manufacturing. For critical parts, ask the supplier to confirm K-factor, bend deduction and tooling assumptions. 3. Calibrate K-Factor and Bend Deduction with First Article Data For repeat production, measure the first article sample and compare actual formed dimensions against theoretical values. The supplier can then refine the K-factor, bend allowance or bend deduction table for future batches. 4. Keep Functional Features Away from Bend Lines Move holes, slots and inserts away from bend regions whenever possible. If the feature cannot move, ask for DFM review before cutting material.  5. Use Reasonable Tolerances Apply tight tolerances only where they affect assembly, sealing, locating or appearance. Keep non-critical dimensions under general sheet metal tolerances to reduce cost and lead time. 6. Consider Surface Finish During Design Powder coating, plating and painting can affect slots, holes, grounding areas, threaded inserts and mating surfaces. If the part has assembly-critical areas, consider masking, allowance or post-finishing inspection.   When to Ask Supplier for Review You should ask your sheet metal supplier for DFM review before production if: The part has multiple bends, return flanges, hems, curls or closed structures. The flat pattern was generated automatically by CAD software. The drawing uses a default K-factor or does not define bend radius. The part has tight overall dimensions after bending. Holes, slots, PEM inserts or countersinks are close to bend lines. The material is stainless steel, aluminum alloy, spring steel, high-strength steel or thick plate. The part must fit with CNC machined, injection molded or purchased components. Surface finish thickness may affect assembly, threads or mating surfaces. The part is a prototype that may later move into repeat production. First Article Verification and Parameter Library For critical sheet metal parts, the most reliable way to reduce unfolding errors is to close the loop with first article inspection. After the first sample, measure key dimensions such as overall length, flange height, bend angle, hole position, flatness, squareness and assembly gap. If the result deviates from the drawing, the supplier can adjust K-factor, bend deduction, bend sequence, tooling or springback compensation before larger-volume production. For long-term programs, verified material-thickness-tooling combinations should be recorded as an internal parameter library. This helps reduce repeated trial and error and improves consistency between design, quotation and production. Conclusion Sheet metal unfolding size errors are rarely caused by one single factor. They usually come from the mismatch between CAD flat-pattern assumptions and real bending conditions. Engineers should first verify K-factor, bend allowance, bend deduction and inside bend radius. Then they should check material thickness, grain direction, tooling, springback, feature distance, flange length, tolerances and bend sequence. Finally, first article inspection should be used to validate and improve the production parameters. For custom sheet metal fabrication, this DFM review process helps reduce sample revisions, control cost, avoid assembly problems and improve production stability. CTA Need help checking your sheet metal drawing? Upload your 2D drawing, 3D CAD file, material, surface finish and quantity. Our team can review your design and provide free DFM feedback before quotation. Upload Drawing for DFM Analysis 链接到Contact us   Source Notes and References [1] SOLIDWORKS Help – Bend Allowance and Bend Deduction: explains sheet metal bend allowance, bend deduction and K-factor calculation concepts. https://help.solidworks.com/2021/english/SolidWorks/sldworks/c_Bend_Allowance_and_Bend_Deduction.htm [2] GoEngineer – SOLIDWORKS Sheet Metal Bend Calculations Explained: notes that flat patterns can be calculated using K-Factor, Bend Allowance or Bend Deduction. https://www.goengineer.com/blog/solidworks-sheet-metal-bend-calculations-explained [3] SOLIDWORKS Help – K-Factor: provides the bend allowance formula using bend radius, thickness and K-factor. https://help.solidworks.com/2023/english/solidworks/sldworks/c_K_Factor.htm [4] Xometry Pro – Sheet Metal Bending Design Tips: explains that K-factor varies based on material properties, bend radius, bending method and tooling precision. https://xometry.pro/en/articles/sheet-metal-bending-design-tips/ [5] Autodesk Support – How to calculate K-Factor for Inventor Sheet Metal: describes calculating K-factor using measured sample sheet metal parts. https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/How-to-calculate-K-Factor-for-Inventor-Sheet-Metal.html [6] Protolabs – Design Guidelines for Sheet Metal Fabrication: includes minimum flange length guidance and bend design considerations. https://www.protolabs.com/services/sheet-metal-fabrication/design-guidelines/ [7] Protolabs – The Basics of Bend Radii in Sheet Metal: warns that holes placed too close to bends can become distorted due to deformation. https://www.protolabs.com/resources/design-tips/the-basics-of-bend-radii-in-sheet-metal/ [8] Xometry – Metal Bending Service: lists general sheet metal bending tolerances, including angle tolerance guidance. https://www.xometry.com/capabilities/sheet-metal-fabrication/metal-bending-service/

  Introduction Hole distortion is a common issue in sheet metal fabrication when holes, slots or cutouts are placed too close to a bend line. During bending, the metal around the bend area is stretched and compressed, which may cause nearby holes to become oval, elongated, shifted, collapsed or visually deformed. For custom sheet metal parts made to drawings, this problem can affect fastener alignment, assembly fit, grounding reliability, appearance, inspection results and production yield. In many cases, the part may look correct in CAD, but once it is bent, holes near the bend line may no longer match the intended position or shape. At Xu Feng, we treat hole-to-bend distance as an important DFM review point before quotation and production. The goal is to prevent avoidable deformation through design optimization first, process compensation second and post-bending operations only when necessary. Key Point: Hole distortion is mainly caused by insufficient hole edge distance. If the hole is inside the bending deformation zone, tensile and compressive stress may damage the hole shape.   Problem: Holes Too Close to the Bend Line Many sheet metal drawings place holes close to bend lines because the product structure is compact, installation space is limited or the hole is required for mounting, grounding or alignment. The flat pattern may appear simple, but the forming process changes the material condition around the bend. If the hole edge is too close to the inside bend line, the hole may fall into the material deformation zone. During forming, the outer surface of the sheet is stretched while the inner surface is compressed. The hole wall cannot fully resist this local shear, tensile and compressive stress, so the hole may be pulled out of shape. This issue is more serious when the hole is used as a functional feature, such as a mounting hole, locating hole, countersunk hole, threaded hole, PEM insert hole or assembly datum. Common Symptom Manufacturing / Assembly Impact Round hole becomes oval or elongated Fastener fit becomes unstable or visually poor Slot becomes twisted or uneven Assembly adjustment or rework may be required Hole center shifts after forming Mating parts may not align correctly Countersink geometry deforms Screw head may not seat properly Hole edge collapses or tears Part may fail inspection or require redesign Cosmetic deformation around visible holes Appearance quality may be unacceptable after finishing   Why It Happens During bending, sheet metal does not simply rotate around a clean geometric line. The bend area is a deformation zone where the material yields and flows. The outer surface of the sheet is stretched under tension, while the inner surface is compressed. Between these two areas, there is a neutral layer where the material length changes less than the outer and inner surfaces. Any hole located inside this deformation zone will be affected by the stress redistribution during forming. [1][2] When the hole edge is too close to the bend line, the bending force pulls and compresses the hole wall together with the surrounding sheet. As a result, a round hole may become oval, a slot may twist, or the final hole position may shift from the drawing requirement. Main Factors That Increase Distortion Risk 1. Small hole-to-bend distance The closer the hole edge is to the bend line, the more likely it is to be inside the deformation zone and affected by tensile or compressive stress. 2. Larger bend radius or thicker material A larger inside bend radius or thicker sheet may require more clearance because the deformation zone becomes wider and forming force increases. 3. Functional hole requirements Positioning holes, threaded holes, countersunk holes and PEM insert holes are more sensitive because even small distortion can affect assembly. 4. Long slots or large cutouts Long slots and large cutouts weaken the local sheet structure and may deform more easily than small non-critical holes. 5. Material behavior Material ductility, hardness and springback all affect bending behavior. Aluminum, stainless steel, carbon steel and spring steel may require different safety margins. Core Design Rule: Hole-to-Bend Distance The first DFM check is whether the hole edge has enough distance from the inside bend line. In practical sheet metal design, several reference rules may be used depending on accuracy requirement, material, bend radius and hole function.   Design Reference Suggested Use L >= t + R Minimum reference for general holes, where L is the distance from hole edge to inside bend line, t is material thickness and R is inside bend radius. L >= 1.5t + R + 1.0 mm More conservative reference for high-accuracy or critical holes that affect assembly, locating or fastening. Distance >= 2.5 x material thickness + inside bend radius Common sheet metal DFM reference used to keep holes outside the bend deformation zone. [1][3] Slot distance >= 4 x material thickness + inside bend radius Useful for slots or elongated features, which are usually more sensitive to deformation than round holes. [1] 3 to 4 x material thickness away from bend line Practical rule of thumb when there is enough design space and the hole function is important. These values should be treated as design references, not fixed rules for every part. The final decision should consider material, thickness, bend radius, bend angle, hole size, hole function, tolerance and production quantity. DFM Principle: Design avoidance first, process compensation second, post-bending hole operation last.     Manufacturing Risk 1. Assembly misalignment If the hole is used for screws, pins, grounding points or mating components, distortion or position shift may cause assembly difficulty. The fastener may not pass smoothly, or the part may need manual adjustment. 2. Poor appearance For visible panels, covers and enclosures, distorted holes near bends can create a poor cosmetic impression, especially after powder coating, painting or plating. 3. Inspection failure A hole may pass size inspection in the flat pattern but fail after bending. If the drawing controls the final formed part, the finished part must meet the final dimensional requirement. 4. Higher production cost If hole distortion is not found during DFM review, the manufacturer may need extra rework, manual correction, secondary drilling, special fixtures or sorting inspection. 5. Lower batch repeatability Even if one prototype can be adjusted manually, batch production may not remain stable if the hole is located inside the deformation zone. DFM Review Points: What We Check Before Quotation Before production, Xu Feng reviews sheet metal drawings to identify hole distortion risks and suggest practical improvements. The review is focused on final part function, not only flat pattern geometry. Hole edge distance from inside bend line We check whether the hole edge is outside the bend deformation zone and whether the distance rule is suitable for the material and bend radius. Hole function and tolerance We confirm whether the hole is a clearance hole, locating hole, threaded hole, countersunk hole, grounding hole, PEM insert hole or other critical feature. Hole size and shape Small holes, long slots, large cutouts and holes with sharp corners are reviewed according to bend direction and local stress flow. Material thickness and bend radius Thicker material and larger bend radius may require more clearance between the hole and the bend line. Bend sequence and tooling access For multi-bend parts, the bend sequence and tooling access may affect stress distribution, hole position and final part accuracy. Surface finish and hardware installation Powder coating, plating, painting, PEM hardware, tapping or countersinking may affect final hole size and assembly fit. Standard Solutions When a hole is too close to a bend, the best solution depends on whether the drawing can be changed, whether the hole is critical and whether the part is for prototype, low-volume or batch production. Design-Side Optimization: Preferred Solution 1. Move the hole away from the bend line The most reliable and cost-effective solution is to relocate the hole so that the hole edge is outside the bending deformation zone. This is preferred for new designs or early-stage prototypes. 2. Add relief slots or cutouts If the hole must remain close to the bend, a U-shaped, corner-shaped or small relief slot can be added between the hole and the bend line. The relief feature helps cut off the stress transfer path and release local deformation. [2] 3. Modify the hole shape If the hole is not a precision locating feature, a round hole can sometimes be changed to an obround hole, enlarged clearance hole or compensated shape to allow assembly after bending. [2] Process Compensation: When Drawing Changes Are Limited 4. Coining line or pre-bending control A shallow pressure line or pre-forming operation near the bend can help control material flow and reduce sliding that may pull the hole during bending. This method depends on material and cosmetic requirements. 5. Laser stress-relief slit A fine laser-cut slit between the hole and the bend area can help isolate bending stress. This must be reviewed carefully because it may affect strength, appearance or sealing. 6. Pilot hole before bending and final enlargement after bending For some designs, only a small pilot hole is made before bending, and the final hole is drilled, reamed or enlarged after forming. This reduces deformation risk while keeping final accuracy. Post-Bending Operation: Accuracy Backup 7. Drill, punch, laser-cut or machine after bending For high-accuracy holes, the most reliable method is to bend the part first and then create the final hole. This gives the best final roundness and position accuracy, but it adds cost and may require fixtures or secondary setup. [4] 8. Improve clamping and anti-slip control Increasing holding force, improving tool contact or using anti-slip pressure surfaces can reduce relative movement between the sheet and tool during bending. 9. Control burr direction When possible, burrs from cutting or punching should be considered during bending. A burr on the outside tensile side may increase tearing risk, especially near holes and narrow bridges.   Solution Comparison Solution Best For Advantage Limitation Move hole away from bend New designs and early prototypes Lowest cost, stable and simple Requires design space Add U-shaped or corner relief Hole must stay near bend Releases local stress May affect strength or appearance Use obround or oversized hole Non-critical clearance holes Improves assembly tolerance Not suitable for precision locating holes Coining / pre-bend control Limited drawing change Reduces material sliding Depends on tooling and appearance requirement Pilot hole then final enlargement Important holes near bends Balances cost and accuracy Requires secondary operation Drill / punch / laser after bending High-precision critical holes Best final accuracy Higher cost and setup time Burr direction and anti-slip control Production stability improvement Reduces tearing and positioning risk Cannot replace poor hole placement Practical Design Suggestions 1. Keep holes away from bend lines whenever possible If there is enough design space, move holes farther from bend lines during the design stage. This is usually the simplest and most cost-effective solution. 2. Use conservative distance for critical holes For holes used for positioning, fastening, grounding, sealing or hardware installation, use a larger safety margin than for non-critical clearance holes. 3. Avoid countersunk holes too close to bends Countersunk holes need stable geometry for screw seating. If placed near a bend, the countersink may deform and cause poor screw fit or uneven appearance. 4. Review slots separately from round holes Slots are more sensitive to deformation because of their length and direction. Slot direction should be checked relative to the bending direction. 5. Use post-bending drilling only for critical holes Secondary drilling or machining improves accuracy but increases cost. It should be used for positioning holes, tapped holes or high-accuracy assembly holes, not every clearance hole. 6. Consider coating thickness in final hole size If the part will be powder coated or plated, the coating may reduce hole size. This should be checked together with bending distortion. 7. Ask for DFM review before prototype production Small design changes before production are usually cheaper than rework after parts are bent. Solution Selection by Project Stage Project Stage Recommended Strategy Reason Single prototype Bend first, then drill or adjust critical holes if needed Good balance between speed and final hole accuracy Design validation Move holes or add relief slots before the next revision Prevents repeated prototype problems Low-volume production Use design optimization plus selective post-bending drilling for key holes Controls both cost and assembly risk Batch production Finalize safe hole distance, relief design and stable tooling method Improves repeatability and reduces scrap Existing tooling fixed Use process compensation such as coining line, pilot hole then final enlargement or anti-slip control Reduces risk without major tooling change When to Ask Your Supplier for Sheet Metal DFM Review You should ask for DFM review when: Holes are close to bend lines. Slots or cutouts are near formed areas. Hole positions are critical for assembly. The part has multiple bends and tight hole tolerances. Countersunk holes, threaded holes or PEM insert holes are near bends. The part requires grounding, sealing or accurate fastener alignment. The part requires powder coating, plating or painting. The part is moving from prototype to batch production. You are not sure whether the hole should be made before or after bending. A short DFM review can help identify whether the hole should be moved, protected with relief, oversized, pre-compensated or processed after bending.   Summary Checklist: Hole Distortion Review Check Item Recommended Action Hole-to-bend distance Check whether the hole edge is outside the bend deformation zone. Critical hole function Mark locating, threaded, countersunk, grounding and PEM-related holes clearly. Material and bend radius Use larger safety margin for thicker material, larger bend radius or difficult materials. Slots and cutouts Review long slots and large cutouts separately from simple round holes. Relief design Add U-shaped, corner or stress-relief slots if hole movement is not possible. Secondary operation Use post-bending drilling, punching or machining for high-precision holes. Surface finish Confirm coating or plating thickness impact on final hole size and fit. Supplier review Request DFM feedback before quotation when holes are close to bends. Conclusion Hole distortion after sheet metal bending usually happens when a hole, slot or cutout is placed too close to the bend line. During forming, the outer surface stretches and the inner surface compresses. If the hole is inside this deformation zone, it may become oval, shift position, collapse or lose its functional accuracy. The best prevention method is to keep holes outside the bending deformation zone. When the design is constrained, relief slots, modified hole shapes, pilot holes, process compensation or post-bending drilling can be used depending on the accuracy requirement and cost target. For made-to-drawing sheet metal parts, the most practical approach is to review hole position, material thickness, bend radius, hole function and finishing requirements before production. This helps avoid assembly problems, reduce rework and improve batch production stability. CTA: Upload Your Drawing for Free DFM Analysis Our team will review your sheet metal drawing, hole-to-bend distance, bending risk and possible cost-saving solutions before quotation.   References [1] Geomiq, Sheet Metal Design Guide, gives sheet metal DFM guidance on hole and slot distance from bend features, including references such as 2.5 x material thickness plus bend radius for holes and larger clearances for slots. https://geomiq.com/sheet-metal-design-guide/ [2] Xometry, Sheet Metal Bending Design Tips, discusses deformation risks for features near bends and suggests design approaches such as obround holes, teardrop holes and bend relief features to reduce stress concentration. https://xometry.pro/en/articles/sheet-metal-bending-design-tips/ [3] FiveFlute, Sheet Metal DFM Design Guidelines for Formed and Punched Parts, includes practical DFM rules for maintaining adequate distance between holes and bends to reduce deformation risk. https://www.fiveflute.com/guide/sheet-metal-dfm-design-guidelines-for-formed-and-punched-parts/ [4] Hubs, Sheet Metal Fabrication Design Guide, explains common sheet metal design considerations, including feature placement near bends and possible post-forming operations when accuracy is required. https://www.hubs.com/guides/sheet-metal-fabrication/ [5] Protolabs, The Basics of Bend Radii in Sheet Metal, explains bend radius, bend allowance, material behavior and how bending affects final sheet metal dimensions. https://www.protolabs.com/resources/design-tips/the-basics-of-bend-radii-in-sheet-metal/