August 8, 2007
High-strength, low-alloy (HSLA) steels require new ways of thinking about extending punch life. At the same time, there are many tried- and-true techniques that still work.
High-strength, low-alloy (HSLA) steels are revolutionizing the automotive and construction industries, and stainless steel is tremendously popular in appliances. These steels are stronger, tougher, and often more advanced than traditional materials. To be successful, manufacturers must be equally advanced in their stamping techniques.
New materials require new ways of thinking about extending punch life. At the same time, many tried-and-true techniques still work. Below are six of the best proven methods, followed by six progressive approaches, to get the most out of every punch.
Sharpening punches usually becomes urgent when burrs increase or hole quality is affected. But regular maintenance also is an important factor in extending punch life and reliability. Taking the extra time for punch regrinding is well worth the effort. Make it part of your stamping routine.
Punch head breakage is one of the most common problems stampers face. That's why it's critical to know the ins and outs of alterations that make heads less vulnerable. Drawing back the punch head to a consistent 40 to 45 Rockwell hardness C (HRC) makes it less brittle and prone to breakage. Consider making punch heads thicker and adding a larger-than-normal radius under the head. A chamfer on the back of the punch head will minimize side loads caused by tool misalignment.
Stamping tools succumb to tough materials when they lack rigidity. Starting with die set design, engineer it with larger plates, pins, and bushings. Using the largest, shortest (stubbiest) punch design adds stability to the point diameter that pierces the hole, resulting in longer tool life. Together, these steps toward "bigness" will create a more rigid, durable stamping environment.
The stress of high-speed stamping can cause punches to work into the retainer, damaging both the punches and retainer. To prevent this, try using backing plates behind the punch pads or retainers that match punch head hardness (same HRC). This helps to prevent the punch from working into the die and minimizes vibrations.
Stampers often avoid using punches with shear angles (seeFigure 1) because die maintenance is difficult. Sharpening a punch with shear angles can be tedious. Multiple surfaces need to be maintained, and convex or concave surfaces require extra time. Toolmakers have to work with angle plates, sine chucks, or dress a wheel to maintain the shear desired. In comparison, a regular flat punch is easy to set up and sharpen on any surface grinder.
However, any headaches associated with die maintenance of punches with shear angles are well worth the results. Shear angles significantly reduce the shock (tonnage) and recoil within the punch. Reducing tonnage not only increases tool life, but also reduces press wear and tear. Reducing recoil minimizes punch head breakage and protects the die from slug pulling.
Once offered at a premium, CNC machining has considerably reduced the cost of back tapers. Back tapers improve stripping, reduce galling, and increase punch wear resistance. When a slug is removed from the sheet or coil, the material wants to spring back or grip the punch, so the material needs to be stripped away using either a spring, urethane, or solid stripper. When the punch retracts through the material, the friction between the punch and the shear portion of the hole creates heat and punch wear, so if a punch is straight, the material will drag along its entire length.
A punch with a back taper reduces this heat and friction, resulting in longer punch life. Before the emergence of CNC punch manufacturing, punch tips were ground on manual machines. Adding a back taper or an extra back taper required a setup change, and the additional cost prohibited many toolmakers from taking advantage of this feature.
In some instances, an extra back taper may be warranted. Galling is one such instance. Stampers of aluminum, galvanized steel, or galvanneal steel have seen significant gains in punch life simply by adding extra back taper, again reducing friction and heat.
Prolonging punch life must begin in the engineering and purchasing phases. Getting personnel in these departments to think differently about punches may be the most important factor in extending punch life. They need to understand that upfront cost should not always be the deciding factor. Yes, alterations to punches, such as coatings, back tapers, shears, or alternate tool steels, will definitely add cost to the initial tool build and to tool maintenance. However, most toolmakers would agree that adding a shear or another alteration upfront is worth the headache of removing a $10 punch from the press, tearing it down, and replacing it. The cost-savings opportunities lie in the toolmaker's time and press downtime, not the few dollars spent upfront.
Although punches and dies are consumables, they certainly are not a commodity, and everyone needs to understand the value of improving tool life from the beginning. When stampers understand the value of specifying shears, head alterations, tool steels, coatings, and other features, their stamping becomes more productive, long-term costs go down, and everyone looks smart.
A2 tool steel represents the "old school" of thought about punches, in which cheaper is better. But A2 is overmatched when stamping HSLA and other advanced materials. Some punch suppliers now provide M2 steel as standard on all punches, as well as offer powdered steels such as M4, 3V, 10V, and 15V to excel in specific applications. Stainless steel is difficult to work with because of punch breakage, head breakage, or chipping, yet it's a popular metal for many applications.
The standard steel most punch suppliers offer has been A2, with D2 available as an upgrade. Powdered steels have been available, but until recently no one really understood that there are differences among powdered steels: Some provide increased toughness (i.e., ability to absorb energy without breakage), while others increase wear resistance (i.e., ability to resist punch deterioration caused by abrasion).
Stampers have experienced significant success using 3V steel on stainless, which provides high toughness with wear resistance similar to M2. Some tool steel manufacturers offer several different powdered steels for specific applications. By investigating various tool steels, stampers can realize longer runs between sharpenings, or in the case of stainless steel, between punch replacement.
To maximize punch life, a general rule is to choose a tool steel that matches the material being punched. Tough material requires tough tool steel, while more abrasive material (i.e., prone to causing excessive punch wear) calls for greater wear resistance (see Figure 2).
Inexpensive, general-purpose coatings are prone to flaking and tend to wear out fast. For better results, look into coatings such as TiCN that are specifically engineered to maximize the life of punches (see Figure 3). Like tool steels, a superior coating can be well worth the investment in the long run.
Ejector punches are great for minimizing slug pulling, but their hollow interior takes integrity away from the punches, making them more prone to damage. If ejector punches become problematic when stamping HSLA materials, switch to a solid punch. Rely on slug-retaining dies to prevent slug pulling instead.
Straight-line grinding is machining a punch parallel to its Y axis for a smoother finish. This reduces friction and pressure when stripping and produces a cutting edge that distributes force more evenly. The result is reduced galling and tool wear, as well as a cleaner punch with a more accurate hole diameter.
Traditionally, punches are guided through a stripper system only in very tight-tolerance or clearance applications. With HSLA and other new materials, the reduced punch floating and added stability at the point of contact that a stripper system provides can be beneficial. By guiding the punch at the point of contact, the stripper system creates a very rigid and precise environment that limits punch side loading, thus maintaining proper die clearance. This reduces friction as well as punch breakage and can result in a better finished product.
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