Hemming dies
Press-brake hemming stages, flat/open/teardrop profiles, tool variants and product-specific load limits.
Tags: tooling, sheet-metal, press-brakes, hemming
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Mate's guide for 1.0 mm mild steel at 60,000 psi tensile strength lists 15 metric tf/m for a 3.5 mm-high teardrop hem and 40 metric tf/m for a 2.0 mm-high flat hem. A uniformly loaded 0.50 m length gives 7.5 tf (73.5 kN) and 20.0 tf (196.1 kN) of reference closing demand. Compare those demands with tool and press capacity for the actual setup. Mate hemming tonnage guide.
Purpose and forming stages
The minimum hem length depends on the flange that the first acute bend can form. WILA hemming applications
Planishing follows the acute pre-bend and creates a flat, open-parallel or teardrop hem. Mate’s February 22, 2016 article describes knurled grips resisting lateral movement during closure, with possible surface marks. Mate hemming-tool explanation
One assembly can perform both actions. Check pre-bend angle, nose radius and clearance for repositioning.
Hem profiles
The table interprets the three outcomes named by Mate; these are descriptive shapes, not dimensional standards. Mate hem shapes. WILA links open-hem results to pre-bend radius and closing depth. WILA open-hem explanation
| Profile | Cross-sectional interpretation | Consequence |
|---|---|---|
| Closed or flat hem | Returned flange flattened against the sheet | Approximately two layers outside the nose |
| Open-parallel hem | Parallel return with a gap | Gap and outside height matter |
| Teardrop hem | Rounded pocket retained at the nose | Height differs from a flat hem |
Interpretation: a 180° return does not alone specify the gap, nose radius, closure length or finished thickness. These shapes can have different forming loads even in the same material. Hemming also does not establish that an edge is suitable for a particular handling or structural requirement.
Tool arrangements
| Arrangement | Sourced operating principle | Applicability |
|---|---|---|
| Spring-loaded combination die | Mate uses an acute V opening for the first bend, then the die's open gap for planishing | Its illustrated 30° die is rated for 1.5 mm maximum material thickness |
| Hydraulic or spring-loaded table | WILA's Dutch bending tables have front flattening jaws with hydraulic or spring actuation | A table's actuation system is distinct from the press-brake tooling clamp |
| U-shaped combination or hammer tools | WILA's U-shaped tools pre-bend and close; flat-faced hammer tools flatten a previously bent flange | The required tool movement and workpiece clearance differ |
Mate's March 25, 2016 description specifies a punch with the same angle as the V opening and a small nose radius. It discusses variants with different thickness limits; the illustrated 1.5 mm limit is not a family-wide maximum. Mate spring-loaded hemming tools. The WILA arrangements above follow its application descriptions. WILA hemming solutions
As a concrete hydraulic example, WILA's current DD-005 HI page lists maximum thicknesses of 3 mm mild steel, 3 mm aluminum, 2 mm stainless steel and 3 mm galvanized sheet. It gives a maximum tool load of 100 ton/m, a maximum V opening of 40 mm and hydraulic lifting up to 90 bar. Piping and power pack are excluded from delivery. These specifications apply to that product description, not all WILA hemming equipment. WILA DD-005 HI
Interpretation: hydraulic lifting pressure is neither hemming force nor the machine's rated capacity. Product thickness limits, tool load limits, punch strength, holder capacity and press capacity constrain different parts of the assembly.
Forming load versus rated capacity
Mate's American Precision Style catalog, LIT01186 Rev C2 PN 2018, provides a hemming guide for 60,000 psi tensile mild steel. Its metric 1.0 mm row gives 15 metric ton-force/m for a teardrop hem of height 3.5 mm, and 40 metric ton-force/m for a flat hem of height 2.0 mm. Mate hemming tonnage guide
Worked example: for a uniformly loaded 0.50 m hem under those mild-steel guide assumptions, the reference closing loads are:
Teardrop: 15 tf/m × 0.50 m = 7.5 tf ≈ 73.5 kN
Flat: 40 tf/m × 0.50 m = 20.0 tf ≈ 196.1 kN
The conversion uses 1 metric ton-force = 9.80665 kN. The flat-hem reference is 40/15 ≈ 2.67 times the teardrop reference. These estimates describe demand under the guide assumptions; a maximum tool rating describes capacity.
Geometric comparison: two nominal 1.0 mm layers give a 2.0 mm stack where they touch. This does not describe the whole teardrop envelope or predict local thinning, coating compression or springback at the folded nose. Blank development also requires bend geometry rather than merely doubling a flange dimension.
Material, finish and coverage limits
Alloy, condition, thickness and the final closure profile affect hemming feasibility. A pre-bend that forms successfully does not establish that the subsequent tight fold will remain crack-free. Local strain and final closure require application-specific assessment; neither the catalog load example nor a generic die name establishes allowable minimum radius for every material.
WILA warns of marking and protective-foil damage during hemming; this is its stated process guidance, not proof of the finish produced by every tool option. The desired finish and allowable witness marks therefore remain part-specific requirements. WILA hemming FAQ
The two Mate articles carry their February 22 and March 25, 2016 publication dates; its cited catalog explicitly carries revision C2 and 2018. No issue date was established for the cited WILA web pages. URL and crawl dates are not editions. This reference covers press-brake hemming tools, not roller hemming or complete automotive panel-hemming systems, and it supplies no universal production settings or material approval.