The fine-art of sheet metal bending
This is a great 9 page PDF entitled "BendWorks: the fine art of Sheet Metal Bending". This is a basic guide to the main processes involved in sheet metal bending. The PDF file covers the following topics:
1) Bend allowances
2) Reverse engineering the k-factor
3) Principles of bending: Air bending, Bottoming, Coining
4) Tips and tricks
5) Other common types of bending: V bending, U Die bending, Wiping die bending, Double die bending, Rotary Bending
The original source material can be found here: www.ciri.org.nz/bendworks/
Thanks to the author, Prof. Olaf Diegel
for his kind permission to include this material on the Mechanical Design Forum.
It is also possible to download "BendWorks" from the link above. This is a simple calculator to help you calculate bend deductions, and to help you reverse engineer the best K-factor to use when creating sheet-metal parts in you favorite CAD program. It also includes a set of default "rule of thumb" K-factors to use for various materials, radii, and bending methods.
A screenshot of the BendWorks calculator