A sheet metal component can look correct on a CAD screen and still create problems during fabrication. One common cause is poor sheet metal hole placement, especially when holes sit too close to bends.
During forming, the material stretches and compresses. These forces can distort nearby holes, move critical mounting points and create alignment problems during assembly.
Good sheet metal design rules account for material thickness, bend radius, bend clearance, hole function and manufacturing tolerances before production starts. For engineering teams, getting these details right early can reduce rework and improve repeatability across a production run.
Why Sheet Metal Holes Distort During Bending?
Sheet metal bending is a controlled forming process, but the material must move to achieve its final shape. When a press brake forms a component, the sheet experiences different forces across the bend area.
Features positioned within or close to this deformation zone can move with the surrounding material. This makes hole distortion a design issue as much as a fabrication issue.
From our experience working with fabricated components, small drawing changes made before production can prevent much larger problems during inspection and assembly. Moving a critical hole away from a bend may be all that is required.
How Do Tensile And Compressive Forces Affect Hole Shape?
During bending, the outside of the bend stretches under tensile force while the inside compresses. Material between these areas experiences less strain, but the geometry around the bend still changes.
If a hole sits too close to the bend, the material surrounding the opening cannot maintain its original geometry.
A round hole may become oval. A slot may change shape or position. A threaded or locating hole may lose the accuracy required for assembly.
Consider an equipment mounting bracket used on an industrial production line. The flat sheet is laser cut and passes its initial dimensional inspection. After forming, one mounting hole no longer aligns with the machine frame.
The cutting operation may have been accurate. The problem can originate from sheet metal hole placement that failed to provide sufficient clearance from the bend deformation zone.
Common Assembly Problems Caused By Hole Distortion
The consequences depend on the function of the hole.
| Hole Application | Potential Issue After Bending |
| Threaded mounting hole | Fastener engagement or thread alignment problems |
| Shaft or guide hole | Binding or poor movement |
| Component mounting hole | Assembly misalignment |
| Precision locating hole | Position or tolerance failure |
A distorted hole can lead to additional machining, longer assembly times, extra inspection and production delays.
These problems become more significant on repeat orders. Engineering and manufacturing teams need a component to perform consistently across the complete production run, not just on the first part.
How To Determine Suitable Hole-to-Bend Clearance?
Correct bend clearance should be considered during the design stage rather than corrected after forming.
The required distance depends on factors such as:
- Material grade and thickness.
- Inside bend radius.
- Hole diameter and geometry.
- Hole function.
- Forming method and tooling.
- Required positional tolerance.
This means a single clearance value should not be treated as a universal rule for every sheet metal component.
Consider Material Thickness And Bend Radius Together
Thicker material and larger bend radii can increase the area affected by forming. Hole placement should account for both.
A useful design review asks a practical question: will this feature remain outside the area where the material must stretch and compress during forming?
The answer depends on the specific part geometry and production process.
For example, a 1 mm stainless steel bracket and a heavier mild steel mounting plate may require very different design allowances, even when both contain holes of a similar diameter.
Rather than relying on appearance in the flat CAD model, engineers should assess the formed geometry and confirm the required clearance against the fabricator’s tooling, bend radius and process capability.
Give Critical Holes Greater Protection From Bends
Not every hole performs the same job.
A loose-fit clearance hole may tolerate some positional variation. A locating hole, tapped feature or shaft opening may have much less room for movement.
Critical features can include:
- Tapped holes.
- Shaft locations.
- Dowel or locating holes.
- Precision mounting points.
- Features controlling final assembly alignment.
These holes may require greater bend clearance or a different manufacturing sequence.
Consider a sheet metal enclosure that mounts onto a machine frame. Several holes may accept loose-fit fasteners, while two locating holes control the enclosure’s final position. Treating every hole with the same clearance could create unnecessary risk at the locating points.
The function of the feature should influence its placement.
Apply Sheet Metal Design Rules For Hole Spacing And Edge Clearance
Good sheet metal design rules also consider the material remaining around each opening.
When holes sit too close to each other, an edge or a corner, the remaining section of metal becomes narrower. This ligament must withstand manufacturing loads as well as forces applied during the component’s service life.
Poor spacing can contribute to deformation, cracking or reduced strength.
Maintain Sufficient Hole-to-Edge Distance
A hole close to the edge of a sheet leaves a narrow strip of material between the opening and the component boundary.
Whether that distance is acceptable depends on material thickness, hole diameter, loading, manufacturing process and final application.
Corners can require particular attention because several geometric features may concentrate stress in the same area.
Imagine a mounting plate used on industrial equipment in Victoria. The component may experience repeated vibration, load cycles and changing operating temperatures. A marginal edge distance might survive fabrication but still create a weak point during service.
Instead of applying one generic ratio to every design, engineers should confirm that enough material remains to support both fabrication and operating loads.
Maintain Enough Material Between Multiple Holes
Hole patterns are common in:
- Equipment covers.
- Ventilation panels.
- Electrical enclosures.
- Mounting plates.
- Machine guards.
Closely spaced holes remove material from the sheet and can reduce stiffness. This becomes more important when the hole pattern also sits near a bend.
Extruded holes require additional consideration because the extrusion process forms material around the opening. Sufficient surrounding material must remain available for that forming operation.
The correct spacing depends on the material, thickness, feature geometry and production method. Where spacing is tight, the design should be reviewed with the fabricator before drawings are released.
Design Slots Without Creating New Weak Points
Slots provide useful adjustment during assembly. They can compensate for expected installation variation and allow components to move into the correct position before fasteners are tightened.
They can also remove considerably more material than a round hole.
A slot should have a defined purpose. Adding long slots simply to compensate for uncertain dimensions can reduce stiffness without fixing the underlying design problem.
Position Slots According To Their Function And Load
Slot orientation affects how a connection responds to operational loads.
If a slot permits movement in the same direction as a repeated load or vibration, the joint may have greater opportunity to move after installation. Designers should consider the slot orientation alongside the fastener, clamping arrangement and expected loading.
A machine guard bracket, for example, may need adjustment during installation. A short slot can provide that adjustment. Its position and orientation should still account for machine vibration and the strength of the surrounding flange.
Engineers should review:
- Expected load direction.
- Required adjustment.
- Fastener size and clamping arrangement.
- Distance from sheet edges.
- Distance from bends.
- Remaining material around the slot.
A long slot placed close to a bend can combine two potential sources of weakness: material removed by the slot and material stressed during forming.
Do Not Use Slots To Hide Tolerance Problems
Slots work best where adjustment is part of the intended design.
If several mounting points require oversized slots simply to make an assembly fit, the underlying dimensions and tolerances may need another look.
Before increasing slot length, review bend tolerances, hole positions, assembly sequence and final installation conditions.
For repeat manufacturing, fixing the source of the dimensional problem usually provides a more consistent result than building additional movement into every component.
Consider The Manufacturing Process When Positioning Holes
The cutting and forming sequence can influence final hole accuracy.
Laser cutting, CNC punching, forming and secondary machining each have different process characteristics. The relevant question is not simply whether a machine can produce the hole in the flat sheet. Engineers also need to consider what happens to that feature during the remaining production stages.
A hole that requires high positional accuracy after bending may need different treatment from a general clearance hole.
For example, a critical feature may be produced or finished after forming where the final tolerance cannot reliably be maintained through the bend operation.
The best sequence depends on the component, material, tolerance and production volume.
Plan Threaded And Precision Holes Around The Final Requirement
Tapped holes require enough material and positional accuracy to support the intended fastener.
Where a threaded feature sits close to a bend, designers should consider whether forming could affect its geometry or alignment.
The same principle applies to precision holes used for shafts, locating pins or alignment.
A pre-production review should establish:
- The required final hole tolerance.
- Whether the hole should be produced before or after forming.
- Whether secondary machining is required.
- How the bend may affect its final position.
- Whether the drawing gives the fabricator enough information.
This keeps the discussion centred on the finished component rather than the flat sheet alone.
Add Relief Features Where Holes And Bends Compete For Space
Compact components do not always provide enough room to move every feature well away from a bend.
Enclosures, brackets and tightly packaged assemblies may force bends, holes and edges into a small area.
In these cases, relief features can help control material movement.
Use Relief Holes And Notches Near Difficult Bend Areas
A relief feature gives the material space to move during forming.
Depending on the design, relief holes or notches can help:
- Reduce local stress concentration.
- Separate functional features from the bend area.
- Reduce tearing.
- Improve forming consistency.
- Protect nearby holes from unwanted deformation.
Consider a small mounting bracket where packaging restrictions place a functional hole close to a folded flange. A suitable relief may help separate the hole from the area experiencing the greatest forming strain.
The relief still needs to preserve enough material for the component’s structural requirements.
Include Bend Reliefs Where Material Movement Is Restricted
Bend reliefs are commonly used where a bend terminates beside an edge, flange or other restrictive geometry.
They can be particularly useful for:
- Narrow flanges.
- Small folded sections.
- Multiple adjacent bends.
- Features close to bend ends.
Relief geometry should match the material thickness, bend radius and forming method rather than being copied from an unrelated component.
These features may look minor on a drawing, but they can make the difference between a repeatable forming operation and a part that requires regular workshop adjustment.
Review Hole Tolerances Before Releasing Drawings
A fabricated component passes through several production stages. Each stage can influence final dimensions.
For holes near bends, the important measurement is often the final position after forming rather than the location in the flat pattern.
Designers should consider the cumulative effect of cutting, bending and assembly when assigning tolerances.
Match Tolerances To Hole Function
Applying unnecessarily tight tolerances can increase manufacturing cost without improving performance. Applying loose tolerances to critical features can create assembly problems.
A useful approach is to classify holes by function.
| Hole Type | Design Priority |
| General clearance hole | Adequate fit and practical manufacturing tolerance |
| Fastener mounting hole | Assembly access and positional consistency |
| Tapped hole | Thread quality and alignment |
| Locating hole | Tight control of final position |
| Shaft or bearing feature | Final size, alignment and fit |
This helps engineering teams direct tighter controls to the features that genuinely need them.
Use A Hole And Bend Design Checklist Before Production
A short design review can catch problems before drawings reach the workshop.
Material and forming
- Confirm material grade and sheet thickness.
- Specify the required bend radius.
- Review bend locations and forming sequence.
- Confirm the intended forming method where it affects critical features.
Hole placement
- Check sheet metal hole placement against bend deformation zones.
- Confirm suitable bend clearance for critical holes.
- Review hole-to-edge distances.
- Check the material remaining between adjacent holes.
- Identify holes with critical final positional tolerances.
Slots and reliefs
- Confirm every slot has a defined adjustment or assembly purpose.
- Review slot orientation against expected loads.
- Check slots near bends for sufficient surrounding material.
- Add bend reliefs or other relief features where required.
Production and assembly
- Confirm whether critical holes should be completed before or after forming.
- Identify secondary tapping or machining requirements.
- Check fastener and tool access.
- Review final assembly clearances.
- Confirm critical tolerances relate to the finished component.
Prevent Hole Distortion Before Production Starts
Preventing hole distortion starts with understanding how the sheet will behave after cutting.
Material thickness, bend radius, hole function, sheet metal hole placement and forming requirements all influence the final result. Good sheet metal design rules account for those factors before production drawings are released.
For manufacturers and OEM engineering teams, an early fabrication review can identify holes that sit too close to bends, features that need additional clearance and tolerances that may be difficult to maintain after forming.
Australian General Engineering provides CAD/CAM design support alongside sheet metal fabrication, laser cutting, brake pressing, CNC machining, welding and assembly. With more than 35 years of fabrication experience and ISO-certified quality systems, AGE works with Australian manufacturers to move components from design into reliable production.
Solving a clearance problem on the drawing is far easier than correcting a production run after the parts have been formed.
