# Hems

Folded sheet-metal edges: hem styles, gap and return dimensions, forming constraints and finished-part acceptance.

Reviewed: 2026-11-01

Tags: parts-and-design, hems, sheet-metal, forming

## Purpose and terminology

A hem folds an edge back toward the parent sheet. Specify whether the overlap is closed or retains a gap. [TRUMPF bending processes](https://www.trumpf.com/en_US/solutions/applications/bending/), [Autodesk Fusion Create hem](https://help.autodesk.com/view/fusion360/ENU/?contextId=SM-CREATE-HEM-FLANGE)

For two ideal parallel layers of equal thickness `t` separated by a clear gap `g`, the local outside stack is `2t + g`. This is an original geometric relationship for the straight overlapping region, not the maximum envelope of the rounded bend.

## Choose the cross-section first

Autodesk Fusion distinguishes the following modeled forms. These descriptions identify CAD geometry, not a guarantee that a particular fabricator can make it in the selected material. [Fusion hem types](https://help.autodesk.com/view/fusion360/ENU/?contextId=SM-CREATE-HEM-FLANGE)

| Form | Geometric distinction |
| --- | --- |
| Flat/closed | Return rests closely against the parent flange |
| Open | Gap remains between return and parent flange |
| Rolled | Rounded fold provides an edge for gripping or fastening |
| Teardrop or rope | Rounded fold with an extended return; define the actual opening and radius |
| Double | Edge is folded twice, creating additional layers and internal gaps |

A folded edge alone does not specify a sealed joint or a connection between separate sheets. State any joining, sealing, cosmetic or contact-safety requirement separately. For a hinge or captured mating part, define the usable internal envelope and assembly route rather than assuming that a nominal hem radius establishes fit.

## Dimensions and supplier capability

Record sheet material/thickness, hemmed edge extent, return length, gap, inside radius or diameter, fold direction and the reference used to locate the finished edge. Keep radius and diameter distinct. Locate nearby holes, corners and other bends relative to the finished hem, and agree whether the drawing dimensions apply before or after finishing.

Protolabs' live sheet-metal guidelines recommend a minimum inside diameter equal to material thickness and a return length of six times thickness. The page says hem tolerance depends on radius, thickness and nearby features. These are that service's design recommendations, not universal minimums or a stated numerical hem tolerance. [Protolabs sheet-metal guidelines, Hems](https://www.protolabs.com/services/sheet-metal-fabrication/design-guidelines/)

**Bounded arithmetic:** applying those recommendations to an assumed 1.2 mm sheet gives a 1.2 mm inside diameter, corresponding to a 0.6 mm inside radius, and a 7.2 mm return. This only translates that supplier's guidance; it does not prove that the material, tooling, corner treatment or required gap is acceptable. Confirm the return-length measurement reference with the supplier.

## Forming and model handoff

TRUMPF describes complete hemming as force-dependent and gapped folding as path-dependent. Consequently, successful modeling of a tight fold does not establish the press force, available tooling or achievable gap for the real part. Review the pre-bend and closing operations, part access and sequence with the fabricator, especially where nearby flanges can obstruct the tools. [TRUMPF folding and hemming](https://www.trumpf.com/en_US/solutions/applications/bending/)

Fusion exposes gap and radius parameters for applicable hem types, along with adjacent versus tangent bend placement and a direction flip. A changed placement can change the final envelope even when a displayed size stays the same. Inspect the cross-section and folded/flat representations after changing thickness, hem type or placement; preserve the intended finished edge location in the drawing. [Fusion dimensions and bend position](https://help.autodesk.com/view/fusion360/ENU/?contextId=SM-CREATE-HEM-FLANGE)

Treat blank development as a manufacturing input to agree, rather than simply adding the visible return length to the finished panel width. Use the fabricator's accepted development method or a demonstrated sample for the chosen tooling. A CAD preview and a dimensioned finished section answer different parts of the handoff.

## Gap and envelope: a calculated check

Assume `t = 1.20 ±0.05 mm` and `g = 0.30 ±0.10 mm`, with no coatings or local thinning. Nominal stack is 2.70 mm; independent worst-case limits are `2 × 1.15 + 0.20 = 2.50 mm` and `2 × 1.25 + 0.40 = 2.90 mm`. A slot or clip designed around nominal stack alone could therefore interfere. This calculation does not predict forming springback, contact pressure or edge stiffness; the actual finished profile still requires inspection.

## Finished-part acceptance

Use an agreed section and inspection location to distinguish the return, gap and rounded end. Check the hem's extent and direction, final panel envelope, nearby feature distortion, visible cracking and acceptable burrs or surface marks. Where the hem receives hardware, verify assembly with the specified hardware and dimensional limits.

Do not treat “closed” as a specified clamp force, or edge folding as proof of a particular safety or load rating. Agree the allowable opening, finish condition and functional acceptance with the fabricator before release. First-article forming and fit checks are useful when the selected combination has not been demonstrated.

## Related reference

[Part features](index.md)

## Source scope

The cited live TRUMPF, Protolabs and Autodesk pages were reviewed on the date above; no technical revision dates were verified. Their terminology, service recommendations and CAD settings have the distinct scope stated here. Calculations and proposed handoff/acceptance checks are editorial guidance under explicit assumptions.