July 9, 2009
Usually the first place stampers look to reduce cost is the labor burden per part, which leads to the inevitable pursuit of producing parts faster. The speed capability of a stamping die is determined by how fast the part physically can be produced and at what speed the tooling fails. The strength of the carrier, how high the part must be lifted, and the mechanical limits of the springs and side action cams (mandrels) limit how fast the tool can run.
In today's competitive environment, stamping companies are looking at every imaginable way to reduce cost. Usually the first place stampers look to reduce costs is the labor burden per part, which leads to the inevitable pursuit of producing parts faster.
High-speed stamping is a relative term. What is considered high speed varies depending on the size of the part. For an automotive fender, 100 parts per minute (PPM) would be very fast. In comparison, stamping a small, simple washer at 100 strokes per minute (spm) would be very slow. High-speed progressive stamping dies for forming very small, complex parts, such as electrical connectors, can be designed so the parts can run at 1,200 to 1,500 SPM.
In most cases, when higher speeds are desired, more working stations are used, resulting in higher die cost. Based on this premise, one of the first steps in designing a progressive die—and, ideally, the quote process—should be to establish how fast the tool is going to run.
Often, somewhere in the process, someone will state how fast the parts need to be run, without basing that rate on manufacturing capabilities. Usually this target is based on what speed a part must run to produce a profit. Unless used cautiously, these estimates can cause many problems.
Before looking at the tool design for speed capability, you should first understand the speed capabilities of the press and supporting equipment that will run the die. The tool's speed capability is irrelevant if the press feeder, raw material payout, vision inspection, sensors, take-up reel, and conveyors cannot keep up.
When determining the capability of some of this equipment, you need to convert a linear speed such as feet per minute (FPM) or inches per second (IPS) into strokes per minute. To calculate the maximum speed of the equipment in SPM, you must divide the linear speed by the progression.
Two factors establish a stamping die's speed capability. The first is simply how fast the part can physically be produced. The second is at what speed the tooling fails (breakage).
The strength of carrying features (carrier), how high the part must be lifted, and the mechanical limitations of the springs and side action cams (mandrels) physically limit how fast the part can be made. Although problems associated with these items can cause tool breakage, they do not originate from tool breakage.
Design to Counter Physical Limits. In one way or another, each of the physical speed-limiting factors is dictated by the configuration, or design, of the part. When possible, it can be very helpful for you to work closely with the product designer to identify product compromises that make the part more "speed-friendly."
1. For example, a part may be currently designed with very few areas where it can be attached between progressions, making the carrier too weak to feed the parts at high speeds without collapsing (see Figure 1). A minor design change that makes the carrier more robust then can increase the speed capability.
2. Other design compromises can be made without compromising part quality. For example, if the product design can be altered to eliminate the need for or reduce the travel of side action cams, or reduce the distance that springs need to travel, valuable increases in speed can be achieved.
3. Reducing how high a part must be lifted within the progressive die can help attain higher speeds. The most common way of limiting lift is by using more forming stations. A high form can be redesigned with multiple stations using lower forms that require less lift (Figure 2).
Tool failure is also very challenging. The cost associated with tool breakage makes it a hot topic. Although applying commonsense practices of robust tooling design certainly is helpful, there is no substitute for experience when you are designing tooling to prevent breakage. Often, when you are stamping smaller, more intricate parts, addressing breakage is easier said than done.
Not Just Thicker Parts. Designing dies to run at high speeds is not as simple as making a die component thicker in a specific area. In fact, sometimes that approach backfires, because it results in more reciprocating mass, therefore actually increasing the potential for breakage.
Careful analysis of the forming and blanking pressures and their effect on the tooling is critical. Using strengthening techniques, such as the strategic placement of radii and chamfers on the tooling, is more effective.
Know the Breaking Point. To effectively stamp at high speeds, you must understand tool breakage and the point of diminishing returns. This is very tricky to balance. Because experience is crucial to establishing design practices that prevent breakage, you must first understand and balance between the long-term benefits of running faster and the short-term benefits of running slower. Because tool breakage is costly—downtime, design hours, and detail manufacturing—it is natural to feel compelled to reduce the speed so tooling breakage won't occur.
Conversely, if breakage is carefully analyzed and redesigned, the knowledge attained can be used to improve the speed capability of future tools.
It's common to hear frustrated toolmakers say, "Do you want to run the tool at 1,000 SPM all day with no problems, or do you want to run it at 1,200 SPM and have it break down every two hours?" The short-term benefit of running parts at slower, proven speeds is that you will not incur unplanned downtime and tooling costs caused by breakage.
Of course, the disadvantage of running at slower speeds is that you will never learn the weak points in a tool and therefore never attain the knowledge needed to maximize the speed it can be run. If you never push the speed limit, how will you ever know what the highest attainable speed is?
Die designers' thirst for knowledge can drive the push for higher speed limits. That having been said, stampers are in business to make a profit, and pursuing high stamping speeds at any cost is irresponsible. Working closely with the pressroom personnel to analyze breakages and improve the tool designs is likely to net the best results.
Whether you are stamping a large fender or a tiny electrical pin, effectively increasing the speed of your stamping tool will result in cost savings. Once you know the capability of the supporting equipment, you can begin focusing on what can be done within the tool design to increase stamping speeds.
The combination of refining the product design so it is more speed-friendly and eliminating speed-related breakages are major steps in the process. It is also of utmost importance that everyone involved in the designing, building, and running of the stamping dies is on the same page.
Each individual must understand where the company stands on the balance between short-term breakage costs and the long-term benefits of pursuing higher speeds. If anyone within the chain is pressured by others in the group to either run the tool too fast or slow, he or she ultimately will likely take the path of least resistance, whether or not it is in the company's best interests.
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