Why use flying shear vs. rolling knife to cut rollformed steel?

any moving material is cut with a rolling or swing knife which starts a cut on one edge of the material and the rotation of the knife speed and angle of the cut is set so the moving material is cut straight and to proper length as it is moving. A shear is when the material is cut only when the material is not moving. example just try to cut a sheet of paper when pushing the paper into the 2 cutting edges from the side. A rolling slitter blade is use in some application to cut widths of cut able moving material not length.

How is the steel being decoiled if it has already been rollformed?

I'm guessing this all part of one process. Material is decoiled and fed straight into roll former and cut to length when it comes out the other end

Working in the steel tube and pipe industry, I am familiar with various types of flying cutoffs used to cut the continuously formed tube into specific lengths. Whether your mill is producing roll formed shapes (non-welded) or tubular products (welded) the type of cutoff you select will be dependant upon four main items: The material being used, your desired cut quality, your production speed, and how much you want to spend. The main type of flying cutoffs are shear (either double or single cut), rotary disc, cold saw or hot saw.

One of our tube mills produces a diameter range of 1" to 3.5" OD and a wall from 0.050" to 0.250" in a wide variety of ferrous materials ranging from 40 ksi yield 1010/1020 through 110 ksi Full-Hard Cold Rolled, to include HSLA and 15B21 grades. Our mill runs up to 500 FPM and can cut lengths on the fly from 10 feet to 30 feet in length.

The type of cutoff we selected for this mill was based on the fact that most of the tubes get recut in off-line cutting cells or for the ones that get sold right off the mill, need to have a machined face, ID and OD chamfer. Since all ends see a secondary operation, the actual cut quality at the main cutoff is less critical so we went with a hot saw... more bang or the buck.

However, due to our production rates and product range, if we could have gotten away with not having to machine the ends of each tube we would have went with a double cut shear (aka nick and shear.) But, since we have to machine the ends anyway and considering a double-cut die set costs almost as much as our entire friction saw, the choice was obvious.

In summary, the choice of a cutoff is dependant upon many factors and as these factors are evaluated, an optimal cutting method for your specific application will rise to the surface.

A "flying shear" or flying cutoff die is commonly used to give a clean cut on rollformed material. As it locks to the material and travels with it,it is "stationary" with respect to the material... releasing and returning to rest at the end of the cut stroke. The beauty of the system is the fact that it is not speed dependant (within reason.) It gives the same cut at 20 fpm or 120 fpm.