Roll Forming Line Automation: Flying Dies
Flying Prenotch Dies
Flying dies are dies that are made to move linearly along tracks built into a mechanical, pneumatic or hydraulic press, as opposed to being fixed to the ram and bolster as stationary prenotch dies are. They also can be mounted on tracks mounted on top of bases on which the entire pneumatic or hydraulic press and die assembly moves. One advantage to the flying die or press system is that because the stock is run straight into the roll former from the usually unpowered uncoiler and through the prenotch press (this is called a “tight line”), it takes up less floor space in the direction of travel than a stationary die set up with its’ free loop (sometimes requiring a pit) in and out of the press. Another advantage in the use of the flying die concept is that when running heavier gauges of material we don’t need a potentially more expensive heavy duty servo-feeder with its’ accompanying powered uncoiler or powered straightener (vs. unpowered straightener with a “tight line”).
A bit more care and time must generally be put into the construction of a medium to high production flying prenotch die than is necessary to put into most of the stationary prenotch dies. There are lateral forces exerted on the die components because of the in-line motion as the die is propelled back and forth along the press tracks. The punch holder wants to keep flying when the die shoe hits the backstop position. Leader pins and bushings in die set were originally designed to keep the punches and die openings aligned, but not to take lateral forces in the process. Also, if a mechanical pickup is being used, and if the pickup finger is releasing before the punches are back out of the material being punched, there is pressure on the punches trying to pull them out of position.
All of this means that the guide pins and bushings must be securely fastened with additional support for the guide pins and the additional bearing area in the bushings and that the punches for some applications may need to be guided through a fixed stripper. The additional costs involved in making a properly constructed flying prenotch die are more than offset by the lower downtime and longer die life.
Types of Basic Flying Cut-off Dies
Although there are many variations, there are really only two basic types of cut-off dies used. They are most commonly referred to as (1) the punch cut or blade type die, and (2) the crop-off or slugless type die.
The decision of which type of die to use in each case is made by taking the following into consideration:
- The contour of the shape to be cut
- The gauge of the material to be cut
- Maintenance involved in cutting certain materials
- The line speed to be achieved
- Available press stroke
- Matching prenotched areas
- Lengths of runs planned
Specific Design Principles for Flying Cut-off Dies
It goes almost without saying that die weight is a very important factor in flying dies — in some cases, downright critical. The problem is always hitting a happy medium between die weights and die strength. Only experience, usually a bad experience, will teach a person this in a hurry.
The following can reduce weight:
- Using aluminum die shoes and punch holders.
- Drilling out holes in the die shoe, punch holder, punch pad, die blocks and stripper.
- In the case of high shut heights, put extensions on top of the punch holder to raise the rails.
- Instead of using thick plates for backup of crop-off blades (in slugless dies) use thinner blades with welded gusseting.
The following can increase strength:
- Using gussets whenever possible.
- Running a reinforcement bar across the die shoe.
- Splitting die sections and punch pads so they also act as reinforcing bars.
I have also found that, except for heavy structural-type shapes, a good die fit around the contour of the part is essential for a good cut. The contour should sometimes be ground in according to the part print, and sometimes should be ground to fit an approved sample, depending on the part tolerances. If we grind a radius in the die according to a print, and the finished radius turns out bigger, then we have a gap. When the die cuts off, a burr and distorted radius in that area will result.
Another reason for grinding the shape in after heat treating the steel is to achieve maximum life between sharpening. Hardening steel produces a thin layer of softer oxidized steel at the surface. If this layer is not ground off, the cut edge will become dull much quicker than it would if the surface was ground. This is a more expensive method of making the die, but I have found it to be absolutely essential.
In this new day and age, if the part tolerances and roller die accuracy warrant it, the blade contour can be wire EDM’d to finished sizes (with running clearance and cut timing).