Die Acceleration Methods

If you have a roll former and it is coil-fed, a cutoff device must be added to the exit end of the machine. Some type of measuring system is added to size the parts and activate the cutoff press. It is possible to stop the metal so that a stationary cutoff die can be used but this is inefficient for most applications. The vast majority of roll forming machines use a flying die so that the material can be fed at a continuous speed and productivity is maximized.

In order for a flying cut to be made, the die must be traveling at the same speed as the material. Thus every machine needs some means of accelerating the die from a speed of zero up to the speed for each cut.

Die Acceleration by Gauge Bar

The simplest form of die acceleration occurs by allowing the material itself to push the die forward. This is done by abruptly engaging the die to the material. One way this is done is with a gauge bar. The gauge bar has a target located exactly one part-length away from the die. The leading edge of the roll formed material hits the target, pushing the gauge bar forward and the die forward along with it. The die then trips a limit switch, which activates the cutoff press. The cut occurs while the material is pulling the die forward until the cut part falls away. With the part now gone, the die is pulled back to its home position by either springs or an air cylinder.

  Pros:
  • Accurate

Cons:

  • Line speed is limited
  • Much Downtime for part length changes
 

Die Acceleration by Material

A second method of material acceleration is used on machines with electrical measuring systems. These can be machines with simple flag switches or those with electronic measuring wheels. On these systems, when the desired length of material is past the die, the press is activated. When the press cycles, the die begins to engage the material. The material pushes the die forward as the remainder of the cycle is completed. Once the cut is complete, the die retracts away from the material and returns to its home position.

  Pros:
  • Simple Mechanical Design

Cons:

  • Part may become damaged at higher line speeds
  • Repeated impact may damage die
 

Die Acceleration by Mechanical Kickers

It is possible to accelerate a die with mechanical “kickers”. A ramp is located on a fixed position of the press. As the press closes, a roller on the die contacts the ramp and the die is pushed forward. A spring or air cylinder returns the die to the home position. This system is simple and inexpensive to install. However, the angle of the ramp must be carefully adjusted to match the material speed. Any mismatch of speed will cause damage to the part or the die.

  Pros:
  • Simple Design
  • Inexpensive to install

Cons:

  • Difficult to adjust angle of ramp
  • Must be adjusted for different line speeds
  • Material Speed MUST remain Constant
 

Die Acceleration by Die Boost Cylinders

To alleviate the impact of the die on the material, boost cylinders can be employed. These are either pneumatic or hydraulic cylinders that push the die forward with each cut. The cylinders are activated just prior to or at the same time as the cutoff. The same cylinder is used to return the die to its home position at the end of the cut. The advantage of such a system is that it is inexpensive, easy to implement, and effective as a die accelerator. The disadvantages of such a system are that it is difficult to control the boost rate and there can be a substantial amount of inaccuracy introduced by this device.

  Pros:
  • Inexpensive
  • Easy to implement

Cons:

  • Lack of control over boost rate
  • Low accuracy
 

Die Acceleration by Closed Loop Die Accelerator

A closed loop die accelerator system uses a positioning device that can accurately control the position of the die across the entire stroke length of the press. When a cut is made, the positioning device moves the die so that it is directly over the desired cut point and traveling at the same speed as the material. Tracking continues throughout the entire press cycle. Once the material is cut, the positioning system returns the die to its home position and awaits the next target. Since the die and the material are traveling at the same velocity, each cut is made at virtually zero speed, just as if it was a standing cut. This results is a clean cut with no strain on the material or on the die.

  Pros:
  • Higher line speeds
  • Part and Die move at virtually zero relative speed at cutoff point
  • Highly accurate
  • No stop required to change part length or line speed

Cons:

  • High initial cost
 

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