jsreric roll forming machine

Roll formed components often require hole punching -- after all, that’s probably one reason why you chose roll forming to begin with, right?

This slot, hole, notch, or custom pattern is typically cut out along the length of the part using an in-line process. These features can be set at regular or irregular intervals for both custom and standard roll formed shapes.

But what is in-line prepunching, and how does it differ from other metal punching? Keep reading to learn more about those differences and how different punching techniques are used in the roll forming process.

IN-LINE PREPUNCHING VS. PUNCHING IN ROLL FORMING
The entire purpose of the punching process is to create holes in sheet metal materials. Using hardened tools with sharp edges, force is applied to shear out the desired area. The force can be applied manually, but is typically performed through mechanical, pneumatic, or hydraulic means.

In-line prepunching occurs before the metal is bent into its final shape (as it does in most sheet metal fabrication). The reason? It’s simply easier to stamp a flat, 2D piece of metal rather than a finished profile with a hollow design. It also saves you from having to spend more money and time on secondary punching or machining operations.

The engineering behind finding the exact location to punch holes in a flat piece of metal so they line up correctly after being formed is VERY specific -- especially in roll forming. Doing this takes a combination of engineering skill and understanding how local bending forces may want to deform the original punched shape. 

When it comes to roll formed in-line punching, we must design the punch placements outside of the range affected by forming strain. This practice exists in all types of metal forming, but when roll forming, both linear (in-line with the process) and transverse strain occur simultaneously. 

As a result, we must be concerned not only with the placement of the punch on the strip, but the shape of the punch anticipating any “stretch” deformation that may occur. In-line roll form prepunching can be compared to prepunching for hydroforming and progressive stamping - especially along spherical regions of a final shape.  
 

IN-LINE PREPUNCHING FOR A CLEAN CUTOFF
A common reason for prepunching in roll forming is to remove material that is difficult to access with a cutoff die. Imagine a simple shearing operation being imposed on a flat strip compared to a hollow tube. No worries on the flat strip; however, a totally flat, smashed end of a tube is different. 

In-line prepunching can be used to remove the upper half of the tube while still in a flat strip, allowing a gap for a shear blade to cut the rest of the profile. Ideally, this prepunch would remove the least amount of material possible allowing for maximum forming integrity of the continuous forming process - and consistent tolerance of any profile’s leading and trailing ends.

By prenotching complex profiles in the correct locations you can achieve a clean cutoff with no deformation at the cut edges. Not only does this make your final product better, but for certain complex hollow profiles this method is the only way to avoid sawing pieces to length (very common in the commodity tubing industry).

Another use of prepunching for the benefit of cutoff is considering the tonnage of the roll forming line cutoff press. If the press capacity is exceeded by the total cross sectional area of a profile, some material can be removed from the flat strip precisely at the cutoff points to allow for a lower force, post-formed cut.  

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