The forces of folding

or: “Why folding 3mm Aluminium is out-of-reach of most home builders

There are two main tools for bending sheet metal, a bending brake or a press brake. In an industrial manufacturing environment, the press brake is most commonly used, as it is the more adaptable and, with the computer control, can produce a set of sequential bends to swiftly produce multi-bend parts.

Bending Brake

Bending Brake

  • Press brake: The two dies close to put the sheet in three point bending along the bend line. In air-bending, where the sheet doesn’t touch the sides of the die, the top jaw doesn’t have a radius; the bend radius is determined by the jaw width and sheet thickness. The stroke depth determines the bend angle.

An Industrial Press Brake

An Industrial Press Brake

Press Brake Air-Bending

Press Brake Air-Bending

Usage:

For the bent parts in Atomic Duck, a bending brake is not suitable for two reasons. One, the metal thickness is too high; heavy duty, manual bending brakes might bend 14SWG/2.03mm steel (22SWG/0.711mm is typical), but the Atomic Duck parts are 3mm thick, tempered, aircraft grade aluminium. And two, the bend profiles for many of the parts are too close together, and will foul in a bending brake for the secondary bends.

Using a brake press is possible, and the parts are designed to be formed this way. The clearance around the upper die is enough to allow parts to be formed with close, perpendicular bends; as shown in this video:

Press Brake Air Bending Video.

(Source.)

Is possible to use press brake in the home workshop? Yes. Press break dies can be fitted to a vise or bearing press, but the limiting factors are bend width and jaw closing force. The closing distance must also be carefully controlled to bend to the correct angle.

For example: the longest bend length for any part in Atomic Duck M0.1 is around 1m, the bending force for 3mm thick aluminium at 7mm radius is approximately 10 Tonnes/metre (98.07 kN/m). So the total die closing force required then is 98078N (10 tonnes).

While this load and bending length is just not possible in a vice, it may be possible to use a 20 tonne workshop press (similar to this one from Harbour Freight) with a press brake kit (like this one from Swag Offroad).


Incidentally, with its 2” wide bottom die, the Swag Offroad kit would produce approximately an 8mm radius for 3mm thick Aluminium.

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2 Comments

  1. alphydan
    30th September 2011 at 11:42 am

    Given the price of the harbour fright press plus the press brake kit (about £200 GBP) … one could be purchased per country and then shipped around to each builder …

    (totalling a less than 2% of the total cost as soon as there are a few orders). One could buy the materials + plans + rental of press for 1 week from the last person who used it. Does that business model make sense? It seems it would really bring the cost down. And I would be willing do to something like that (including renting the van ~50GBP) … to get a < £1000 velomobile.

    Can the design be adapted to the 8mm radius for 3mm?

    • Patrick Fenner
      5th October 2011 at 12:31 pm

      Actually, the lower jaw width (2″ or 50.8 mm) of that press brake kit is almost the correct size. A lower jaw width of 48mm gives an 8mm radius.

      If the price of the press is £350–460, and the Press Brake kit would be around £200–300, (delivered to the UK, with VAT and the cost of having it welded). Obviously, this is an additional £550–760 above the material and component cost, but at the end, you would have a workshop press that could be sold to another builder, or to someone else to recoup a large proportion of that cost.

      As it is, I don’t have the capabilities to set up a hire service, but I think the actual build cost will still be just above £1000 (especially with the bent tube front axle — without cutting — being £200). However, I can get a price for a laser cut set of templates if you’d like to build this way?

      One of the things I’m doing with the current development is to eliminate all the industrial bending requirements though. That should make a sub-£1000 build a realistic possibility.

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