Pre-cut vs. post-cut: Choosing roll formers
Any investment can cause a business owner to lose some sleep. When it comes to investing in roll-forming equipment and accessories, let the designer do the worrying.
The pre-cut vs. post-cut dilemma keeps roll-tooling designers up at night and it’s not always a technical issue. Decisions get more complicated when the equipment dictates the cut-off method and financial considerations surely enter into the equation.
Pre-cut pluses
- Eliminates expensive cut-off dies and their maintenance.
- Buff-free ends.
- Simple, low-maintenance pre-cut shear.
- Ability to hand-feed strips or sheets for low-volume production.
Pre-cut minuses
- Possibility of increased end flare, especially in deeper parts.
- Higher strength steels present a larger challenge.
- Requires more forming stations, making the roll former and tooling more expensive.
- Generally requires more floor space.
- Shorter parts are more of a challenge and can introduce part quality issues.
- Difficult multibend parts can be a problem in terms of quality.
Some shapes, such as seam-welded closed parts or lock-seam tubes, perform a lot better when post-cut to suit the accompanying technology required to achieve those results. Deep roof deck profiles should be post-cut to avoid “spring back” or flare inherent in deep profiles. Automotive parts, due to their tight tolerance requirements formed with high-strength steels, are almost exclusively run in a post-cut manner. This last sentence may say everything about the dilemma.
Interestingly, metal wall panels and roofing can be produced with either method. This allows for personal customer preference when designing the roll former. Many other products can be produced successfully using either technology, so personal preference, floor space and/or employee skill level helps determine the design.
Joe Repovs is the founder and chairman of Samco Machinery Ltd., a Toronto-based business that has been producing roll-forming equipment since 1972. Email him at jrepovs@samco-machinery.com.
Post-cut pros
- Production rate is higher with post-cut.
- Better part control in terms of quality.
- Reduces end flare considerably. End flare can be controlled.
- Better results for high-strength steels, such as dual-phase and martensitic steels.
- Lengths as short as 2-3 inches can be produced.
- End straighteners are most effective in post-cut, controlling bow, camber and twist.
- No leading-edge deformation of the part in post-cut.
- Better roll life in post-cut when running heavy gauge and high-strength material because the leading edge in pre-cut hits the rolls with every part.
- Friendlier to material deviation in terms of gauge and hardness.
- Ability to provide punching/notching during the cut-off operation, possibly eliminating one or more secondary operations.
Post-cut cons
- Expensive cut-off press and die.
- Maintenance costs, especially for the cut-off die.
- End burr, resulting in rough end.
- Distortion on product end.
- May require manual feeding of leading end of first part upon introduction of a new coil.
- In many cases, all surfaces must be supported in order to avoid end distortion. This is not always possible due to inability to build robust die sections into the die or the need to leave an opening in a practically closed space.
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