There is a close, direct relationship of the engineering on a roll forming line to the economic payback of that line. Picking wrong methods or systems can make the line less profitable by operating too slowly, requiring too much maintenance, or requiring expensive secondary operations. To ensure that the line is working as productively and profitably as possible from a die standpoint, it's critical to consider the punch press. In this post, we'll discuss how to choose and set up the best punch press for the most accurate cutoff length.
Punch Press Requirement for Accurate Length
On average, a 6 millisecond variation in the time cycle of the cutoff can cause + 1/16-inch accuracy of a roll forming line. The punch press plays the most important role in that time cycle.
The two most important things to look for in a press to be used for flying cutoff dies are:
- The rotations per minute (RPM) of the press — the faster the better.
- The consistency of the top stopping point (or rest position) of the ram.
The RPM value is important when considering the inherent fluctuations in line speed. If a 100 RPM press is compared with a 300 RPM press, the 300 RPM press will show only 1/3 of the error that a 100 RPM press will show when we get a line speed fluctuation.
The consistency of the top stopping point of the ram is very important and affects the length accuracy whether the line speed fluctuates or not. In a crank-type press, no matter how fast the brake is, there will always be a time error in the brake actuation and some brake slippage. If the ram is set at top dead center, it will sometimes stop before or after that position. This variation in the stopping point means a variation in the time it takes for the blade to come down and engage the stock.
It is better to set the ram brake so that the ram stops beyond top dead center. Thus, it is already on the downstroke in its stop position. If there is a variation in the stopping point of the rain, the error in time would not be as badly affected. The speed of the ram is quicker in the vertical portions of the stroke than at the top or bottom of the stroke, leading to decreased time variation.
The major advantage of an air press, or high-speed hydraulic press, over a crank-type press is the fact that its ram returns to the same starting point after each stroke. Therefore, it is at an advantage when running a flag trip or linear measuring system when the repeatability of the press has a direct effect on the length tolerance. There are some disadvantages to air presses and high-speed hydraulics too, so it's crucial to weigh all requirements when choosing a press.
The faster the press is, the less linear die travel is seen. Therefore, for an additional investment in a faster press, it's possible to run a roll forming line faster with less problems and better length tolerances for the life of the line.
Importance of the Press Stroke in the Cutoff
The obvious reason to use a short stroke press in pre-punch or post-punch operations is when the cycle time must be short because of continuous punching or very short distances between punching.
Another not-so-obvious reason to keep the stroke as short as possible is in the use of blade-type cutoff dies. Blade maintenance is always a problem, and using a longer stroke than necessary only compounds the problem. At any given RPM, the longer the press stroke, the faster the blade travels up and down. It is important to remember that the speed at which a cutoff blade travels through the roll formed shape is a big factor in the gaulling of the blade.
Choosing the Right Cutoff Press for Your Operation
As more competitors turn to roll forming, staying ahead of the pack relies on better engineering of auxiliary roll forming tooling and improved total system engineering. If manufacturers aren't carefully choosing the right methods or systems, including the proper cutoff, their products become less profitable due to slow operating times, increased maintenance, and secondary operation requirements.
As every roll forming line will require a cutoff, it's always critical to ensure it's the right one. As manufacturers continue to make immense investments in roll forming, each business should look critically at its roll forming production methods and consider how refining them could save time, labor, and money.