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Sensing Strategies for Part Ejection

By: Drew Stevens

Thursday, April 01, 2010
 
Arguably one of the most important events in the stamping process is the stamping exiting the die. Damage caused by hitting double material thickness can range from simply shearing the die to destroying cutoff sections and breaking or bending strippers.

Third-party sorting, express shipping of replacement parts and repairing die damage can get very expensive very quickly, so preventing double hits is a necessity. This is another application dictated by the material, part geometry and how it is ejected from the die, so let’s first examine some of the problems caused by improper part ejection. Then we’ll look at solutions for error-proofing the exit side of the die.

Stampings made from thin material often have high

Part-out sensors
Part-out sensors such as this model from Banner Engineering Corp., Minneapolis, MN, features response speeds as low as 0.8 msec., making them ideal for applications where high-speed miniature object detection is required.
aesthetic quality requirements. Examples include electronic connectors or parts used for reflection or refraction, such as automotive headlight components. Slight scratches or shallow slug marks can create a quality-assurance issue that many times won’t be detected by the operator, due to rushed periodic checks in a pressroom with less-than-adequate lighting, so preventing scratches and slug marks in the first place is critical.

Thicker material—in excess of 0.125 in.—poses a threat if the part stays in the die after cutoff. Although cutting clearance becomes greater as material thickness increases, hitting one part on top of another in the cutoff section not only can destroy the punch and die section but also can damage the bearing cages or the press crank, in a worst-case scenario. Obviously, these are not situations conducive to a lean environment.

Granted, some of these issues can be caused by foreign objects inside the die. Part counters are often based on the number of strokes made by a press, as opposed to the number of parts actually exiting the die. Therefore, monitoring the press for parts that fail to escape the die also can aid the operator in keeping accurate part counts.

Part Ejection

Many elements contribute to parts staying inside the die, whether they remain in the working area of the tool or pile up on the die shoe. Blanked parts often behave as do pierce slugs when exiting the die. Because of the land in the lower blanking section, a blanked part can remain in the cavity until enough parts have pushed it past the land and into the drafted area of the die section, where it can fall freely. If one part hangs in the cavity or becomes cocked, the rest of the parts behind it can stack up, eventually resulting in several bent parts blasting out of the bottom cavity or cracking the die section to relieve the built-up pressure. This can create unnecessary downtime and, in the case of an inherited tool, manufacturing a new die section without drawings can pose a time-consuming challenge.

One of the most obvious solutions in this case would be to ensure that blanked parts never hang anywhere in the die section. Here are some steps to take to prevent this.

• Add ejectors to the blank punch. This will prevent the blank from sticking to the punch in dies where the punch is magnetized or coolant creates a hydro-lock. Pulling the blank back out of the cutting section can damage the cutting edges as well as create the potential for hitting double material on the next hit when the blank stays in the die.

• Polish the draft on the die section. Polishing the wire-cut surface on the die section will prevent parts from hanging up just below the land. Be careful not to hit the cutting edge.

• Take the sharp edge off of the cutting section. Adding a small parabolic radius to the cutting edge of the cutoff punch and the blank section will remove unnecessary stress to either detail during the plastic-deformation stage of blanking. Dead-sharp corners fracture easily, creating the need to sharpen the sections early in the production run.

• Ensure sufficient punch entry. Shallow punch entry will cause blanks to pull out of the die section instead of falling free of the die. Entry into the cavity should be at least double the material thickness.

Unfortunately, even seasoned die makers eschew these simple provisions, so part ejection remains a common problem. Monitoring the blank section for part ejection will require ingenuity, mainly because blanks don’t fall out of the die on every hit. We’ll discuss solutions for detecting blank ejection toward the end of this article.

Parts May Need Help Leaving

Cutting off the part at the end of the die or cutting a part free from a carrier or onion cut is another common method for separating a part from the strip. When the part falls free off the backside or the end of the die, it can still hang up somewhere, especially when part features such as hook-shaped forms or large notched-out areas grab onto the side of a chute or ramp. These features also can cause the part to catch on conveyors, but our main concern is making sure the part exits the die before the next hit.

Depending on where the cutoff occurs, the part may need some help exiting the die, which is why many operations use air blowoffs to eject the part before the strip feeds into location for the next hit. Using compressed air is expensive, so timing the blowoff during the stroke is a necessity. To prevent the use of excessive air, stampers can use a programmable cam output from the press controller or a rotary cam switch to activate the blowoff. These devices can be programmed to close a solenoid connected to the airline and allow air to blow the part off during a certain number of press degrees or for a length of time in milliseconds.

For example, consider a part cut from a strip at 180 deg. or a few degrees before. If the part has cleanly exited the die by 220 deg., there is no need to continue blowing air at the cutoff. Setting the blowoff cam from 170 to 230 deg. should be sufficient. Optimally, aim the air so that the part blows straight off; some operators place one line on either side of the part. While this can work, if one air line has more pressure than the other or if only one side has air, the part will likely try to exit the die a from the center. The air must be controlled evenly so that the part leaves the die straight.

Small spring-loaded lifters or plungers also can be used to help eject

Ring sensors
Ring sensors find use in monitoring part ejection from the press. Here, an inductive ring sensor tracks the successful ejection of drill bits as they pass through a hose.
the part after cutoff, especially if the lifters or plungers are located under the back of the part so that gravity will help the part fall out of the die. Also, some dies are designed to allow the feeding of the strip to help eject the part. While this also can work, it invites a host of problems, such as lifting or cambering the strip if the part stays in the die.

Dies that produce more than one part at a time or right- and left-hand parts require some attention on the exit side as well. In the case of right- and left-hand parts, each part must be contained apart from the other, and each side can create its own ejection problems such as those mentioned above. There are several methods to detect parts exiting the die, whether single or multiple and blanked or ejected.

Finding a Sensor Solution

Photoelectric sensors work well for part-out applications. If the part passes through the beam or, in the case of a diffuse-reflective sensor, reflects the beam back at the sensor, the sensor changes state. For blanked parts, using a through-beam sensor is very effective, especially if the press is programmed to look for a change in sensor state only during certain angles. If a part were to hang up inside the cavity, the sensor would detect it by staying on longer than expected. The press controller then would send a stop signal to the press.

Retro-reflective sensors obviously will not work very well mounted under the die shoe, because finding a to protect the polarized reflector from the rigors of die handling would be difficult. Also, fitting a part conveyor or chute under a reflector would be difficult. One method to detect parts is to embed a diffuse-reflective switch in the shoe underneath the cutoff-die section. As long as the opposite side of the cavity does not cause the sensor to switch and there are no dead spots where the part can miss the beam, this can be one of the best s to detect parts after cutoff. Mounting the sensor between the shoe and the die section protects it from damage, and the section can still be maintained without any danger of cutting a wire. A bit of milling on the shoe is necessary to provide room for the sensor and wires.

Inductive sensors also can effectively detect parts, as long as the parts exit the die within range of the sensor. Parts blanked off the end can be monitored by using a block-style proximity sensor underneath a nonmetallic ramp. Depending on the size of the part, an inductive ring can be used effectively if the part can be directed through it by the use of controlled air or a transparent tube.

Regardless of the method used to detect parts, the method should als be thoroughly bench-tested. If a die-protection application fails just one time at the press, then it can’t be trusted to be effective at all. MF

 


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