Bending tolerances are a crucial part of your sheet metal fabrication. For parts requiring precision fabrication, be sure you understand what can impact those tolerance.
Today’s bending equipment can be highly precise however the sheet metal itself presents the most significant challenge to achieving tight tolerances.
In particular, air bending has some key considerations that impact bending tolerances:
- Material thickness
- Direction of metal grain
- Material chemical and mechanical properties
Variations in material thickness are one of the most common challenges when achieving bending tolerances.
Sheet metal can have variations in thickness throughout a single sheet or between sheets.
The nature of the rolling process often means sheets are thicker in the middle than the edges.
The gauge of different sheet metals is usually discussed with respect to averages, but in reality, the actual thickness can be anywhere within a specific range for that category.
“A mere plus or minus 0.006 in. variation in material thickness can represent as much as 4 degrees of angular variation.” Source: TheFabricator.com
A very small variation in thickness can impact a bend angle by a few degrees. This has a big impact on results – especially when tight tolerances are required.
Because with air bending, the bend angle is determined by width of the die and the depth of the punch, a variation in material thickness makes precision more challenging.
If a narrow die is used, the impacts of inconsistent thickness is amplified.
Springbackis the “bounce” back of the metal after the press has been applied and removed.
The sheet metal is compressed on the inside, where the press is applied, and stretched on the outside.
Because the material has a higher compression strength than tensile strength, it springs back towards its original shape. It is difficult to calculate springbackaccurately, but it needs to be considered when calculating the bend. Fabricators use the K-factor to calculate the springback factor and better understand how to compensate and achieve tighter tolerances.
The material’s plasticity is also a factor for springback. Plasticity is a measure of the material’s ability to deform without breaking and retain that shape. Higher plasticity metals are often better choices for forming and bending.
Several factors affect springback:
- Materials with higher tensile strength have more springback
- A sharp bend radius usually has less springback
- Wider die openings result in more springback
- The larger the bend radius relative to the material thickness the more the springback