Sheet metal can be surprisingly unpredictable. Bend it in a press brake and it may distort in unexpected directions. To minimize this deformation the sheet metal fabricator needs to know and work with his material. It’s like how a carpenter works with wood. By studying grain and texture he determines the best way to cut out each piece to achieve his end result.
Like lumber, metal has a grain that affects how it behaves. It also varies from sheet to sheet and batch to batch. The variation isn’t what the carpenter might see with oak from different trees, but it still creates challenges.
The trouble with differences
The fabricators life gets a lot harder when a sheet metal piece doesn’t come out exactly as per the print. Assembly and welding longer because the piece needs bending and pulling into shape. Or alternatively, other pieces need altering.
If we’re fabricating multiple supposedly identical pieces – trailer thresholds or fire truck bumpers for example – piece-to-piece variation creates a host of problems. Assembly time increases, and at some point in the future replacement will be a major headache. So we strive to minimize differences between pieces, and that means developing the craftsman’s feel for sheet metal.
Avoiding sheet metal deformation
Creating a bend means stretching the metal over the outer radius and compressing it on the inner. How the metal handles this stretching and compression determines the extent of deformation. Deformation can be reduced, although not prevented entirely, by paying attention to:
- Grain direction
- Work-hardening tendencies
- Hardness and thickness variation
- Hole positions
- Sheet size
1. Grain direction
Rolling metal into sheet form at the mill elongates the metal crystals and gives it a grain. When bending metal along this grain there’s an increased risk of cracking, especially when putting-in a tight radius. It’s always best to bend across the grain.
Bending metal makes the crystals side over one another. In some grades of some materials they move easily but in others they quickly lock together. This has the effect of increasing the metal’s hardness. If you look at aluminum sheet as an example, the 3000 series grades work-harden very little while the 7000’s demonstrate significant hardening.
Work-hardening increases the bending force needed, and also the chance of cracking. One way of reducing the effect of work-hardening is to bend faster.
3. Hardness and thickness variation
Specifications for sheet metal allow for a range of hardness and thickness. As a result, two supposedly identical sheets may show differing degrees of springback. If the press brake operator isn’t alert to this he could end up with some parts that don’t assemble as easily as they should.
4. Hole positions
A hole near a bend changes how the metal stretches and compresses, and that gives rise to deformation. A good rule of thumb is that any holes should be at least three times the material thickness away from the start of the bend.
5. Sheet size
The force needed to put a bend into a sheet metal piece is proportional to the length of the bend. When putting a long bend into a large sheet the forces are enough to distort – slightly – the frame of the press brake. This can result in a bend being not perfectly straight.
Artisanal sheet metal?
When sheet metal fabrications don’t turn out exactly to print, assembly and later replacement become much more complicated. This variation stems from how the metal deforms, which in turn depends on a number of metal characteristics. Our job as providers of metal fabrication services in Indiana is to understand and control the sources of variation so that you get complete consistency. You could say our people are sheet metal artisans.