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Hems, returns, locks and seams · Lesson 3 of 4

Hems, returns, locks and seams: methods and worked application

The general two-step hem, the path from FlashDraft to a finished hem, and worked examples: hem allowances, limit checks by material, a short return and seam movement.

You will be able to: Describe how a hem is formed, then check a hemmed profile against AFS's limits, work out its hem allowance and choose between a rigid and a loose seam.

9 min · 12 slides · 6 questions

All lessons in this module
Slide 1 of 12

Forming a hem: the general method

WILA makes the first step of a hem a 30 deg pre-bend, made as sharp as possible.

The pre-bent flange is then flattened between flat-faced tools.

For an open hem, WILA says the result depends on the radius of the pre-bend and the depth of the hemming stroke.

This is the press brake method from a tooling maker.

The general two-step hemThe edge is pre-bent to a sharp acute angle, then closed flat for a smashed hem or partway for an open hem.sharp acutepre-bendclosed flatsmashed hemclosed partwayopen hem123General method, not to scale

Three sections. 1: the edge of a red sheet is pre-bent to a sharp acute angle. 2: the pre-bent flange is closed flat against the sheet, a smashed hem. 3: the flange is closed only partway, leaving an open hem with a gap. Blue arrows show the closing motion.

1
Pre-bend: the edge bent to a sharp acute angle (WILA: 30 deg)
2
Closed flat: a smashed hem
3
Closed partway: an open hem; the gap depends on the pre-bend radius and the stroke
The press brake method from a tooling maker (WILA), drawn not to scale. It is the general method, not how the AFS folder sequences a hem.

Sources

Videos for this lesson

What this video shows: A Wilson Tool press brake tool forming a teardrop hem in two steps. This is press brake tooling, not how the AFS folder forms hems.

Video: Hemming Tear Drop in the Press Brake by Wilson Tool International, on YouTube. Chosen from the video's published description.

What this video shows: A demonstration at the 2012 SRCA conference of a curved copper double-lock standing seam roof formed with hand tools, a specialty case.

Video: Making a Curved Double Lock Standing Seam Copper Roof Using Hand Tools by Slate Roofing Contractors Association of North America, Inc., on YouTube. Chosen from the video's published description.

Glossary

Pre-bend
The first hemming step: the edge is bent to a sharp acute angle before it is closed.
Hem allowance
The flat material AFS adds to the blank for a hem: twice the return, plus the gap for an open hem or pi x t / 2 for a teardrop.
Complex profile
In AFS's limits, a profile with more than one bend or any hem.
Thermal movement
The change in length of a run as its temperature changes: dL = L x E x dT.