Peter Ulintz Peter Ulintz
PMA Technical Consultant

Improving Progressive Die Performance—Part 3

November 29, 2021
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In this final installment of a three-part series, we explore the effects that coil-feed height and pilot-release timing have on process efficiency and accuracy.

Feed Height

Fig-1-Establishing-feedFeed height, also known as pass line, refers to the distance above the bolster plate that the coil material exits the press-feed unit and enters the die. Progressive dies with little or no strip lifting have a feed height equal to the height of the lower die surface. Stampers should adjust feed height at each setup so that the strip material enters the die exactly parallel with the height of the lower die surface—especially important when running thin materials. Slight bending of the material due to differences in die height and feed height reduces the strip stiffness, making it susceptible to buckling.

One method to ensure parallelism: Measure the die surface height with an indicator, then set the feed bridge (material support) and feed unit to the exact same reading (Fig. 1).

Set feed height for dies using stock lifters or lifter rails differently. Here, feed height should measure exactly parallel to the height of the lifter rails to allow smooth feeding of the coil strip. Avoid feeding the material downward or upward into the die. These conditions may cause the material to bind or jam in the lifter rails.

Feed Roll (Gripper) Lift

Fig-2-Feed-roll-liftAfter the coil stock has moved to its predetermined progression (pitch), the feed rolls should open after the tip of the pilot punch travels deep enough into the material to prevent the strip from slipping backward, but before the full diameter of the pilot punch has entered the material. This allows the coil strip to be positioned accurately using the pilot pins in the die (point A in Fig. 2). This commonly is referred to as pilot release open (ON). If the pilot release opens too early on the downstroke, the strip can pull backward. Should the feed rolls open too late, the pilot punch can break off, oblong the hole or wear out quickly.

The feed eventually must close again to grip the material for the next feed cycle. This is referred to as pilot release closed (OFF). The combined feed-roll opening and closing sequence is referred to as pilot-release timing.

Where in the press stroke that the feed closure occurs depends mostly on the die-feed height.

Feed Roll (Gripper) Close

For dies with very little or no strip lift, the feed roll can close after the pilots have become fully engaged, with the stripper holding the strip securely against the die to prevent movement. This usually occurs near bottom dead center (BDC), or at 180 deg. of rotation.

For dies with lifter rails, closure timing occurs later in the press cycle. Fig. 3 illustrates what occurs when the feed rolls close at BDC for a die with 4 in. of lifter travel. As the die closes, it must pull an additional 0.222 in. of material from the feed unit positioned 36 in. away. If the feed roll (or gripper) closes at BDC, this extra material becomes trapped between the die and the feeder. As the die begins to open, the strip material between the die and feed is forced to compress or buckle as it rises to its feed height. When the pilots exit the strip, the trapped material relaxes and pushes the strip 0.222 in. forward in the die. The feed then moves the coil to its next progression, resulting in a strip position that measures 0.222 in. too long. Should this problem be identified incorrectly, the press operator may begin making program adjustments to the feed to compensate for the timing error. The length error worsens should the die be positioned closer to the feed unit (at 24 in.) but is much less significant with a shorter (1-in.) lifting distance.

To avoid trapping material between the die and the feed, roll (gripper) closure must be delayed until the stock lifters return to their full up position. The feed must close completely before the pilots fully extract from the strip (point B in Fig. 2).

Lag Time

Fig-3-Die-to-feedtable 1For feed units that rely on pneumatic cylinders to lift and close the feed rolls or grippers, consider the delay time from when the signal is sent to open the feed and the actual time that the feed takes to open. The time required to charge the solenoid coil, activate the switch, exhaust air from one side of the air cylinder and pressurize the opposite side to lift or close the feed roll may take about 35 to 45 msec.

Assuming that a press running at 60 strokes/min. takes approximately 2.8 msec for the crank to turn 1 deg., we can evaluate the effects of pilot-release timing relative to press speed and lag time.

table 2Referring to Fig. 4, should the feed roll (gripper) be programmed to lift at 125 deg. in a press running at 60 strokes/min., it will not actually lift until 137.6 deg. assuming 35-msec lag time, or 141.2 deg. assuming 45-msec lag time. The same delay will occur when signaling the feed to close. If press speed decreases or increases, the pilot-release timing also changes.
Increasing progressive die efficiency and feed accuracy requires close attention to feed-height settings, die-lifter travel and pilot-release settings. Lag time for feed units relying on pneumatic cylinders to provide roll lift (gripper) and closure also must be considered. MF

Industry-Related Terms: Bending, Center, Compress, Die, Feed Unit, Grippers, Plate, Stripper, Stroke, Surface
View Glossary of Metalforming Terms

Technologies: Tooling

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