Ask the Stamping Expert: How can we maximize our tooling’s hits per service?
Q: We are running high-strength, low-alloy (HSLA) brackets for a large aerospace firm. We cannot get the hits per service we need from our tooling and are struggling to meet production requirements. Any advice?
A: Regardless of how many hits per service you get off a tool, you always want to maximize results through continuous improvement. A number of factors contribute to premature or inconsistent tool wear:
- Stamping press integrity. In a sloppy press, the tool guidance system has to do more work to keep the punches and cutting dies aligned.
- Lubrication. Limitless mixes of lubricant can be matched to an application.
- Tooling construction. Tool life depends on design robustness (thickness of the die plates and the size and quality of the guidance pins and ball cages), quality and accuracy of design component details, and the component manufacturing process (wire electrical discharge machining [WEDM] or surface ground) and resulting surface finish.
- Tooling raw material. While the mechanical properties of the material you’re running might limit your options, in general, powdered steel is a better choice for tooling than mill-rolled steels, and carbide is preferred over steel if the application warrants it.
These factors are the ones most commonly considered when tool wear is a concern. Without question, depending on the application, tooling material, press accuracy, and proper lubrication can have a significant effect on hits per service. But there are other, more elusive factors to consider: friction and vibration.
Friction. Amontons’ first and second laws state that the force of friction is directly proportional to the applied load and is independent of the apparent area of contact. Friction resists relative lateral motion of two solid surfaces in contact. The force of friction is exerted by each surface on the other. It is parallel to the surface in a direction that is opposite to the net applied force.
The concern in stamping is the friction on the sides of the punches as they enter and retract through the material. One way to minimize the load is to maximize the punch-to-die clearance; during piercing, this causes the material to stretch during cutting and then relax at snap-through, resulting in a through-hole that is larger than the punch. This minimizes friction on the side of the punch and, thus, tool wear.
Vibration. On the cutting edges of steel punches, the molecules are pounded with every tool stroke and worn off by the material being cut. A combination of rigid tool guidance, proper cutting clearance, excellent surface finish, and dimensional accuracy yield incremental improvements in hits per service.
However, carbide does not wear like steel. For carbide, vibration is the silent killer. Carbide punches that have rounded cutting edges look worn, but in fact they have disintegrated from impact and shock.
A good surface finish, achieved through wire EDM or grinding in the direction of the piercing or forming operation, can minimize galling and wear and, thus, vibration. For good results, try to achieve a surface finish of roughness measurement system (RMS) of 8 or better when grinding a metal cutting surface. In addition, the tool guide pins and bushings must be robust. This is one of the most commonly overlooked areas when addressing hits per service.
Case Study. Consider a company that was cutting 0.005-in., half-hard 301 stainless steel sheet with wire-EDM-cut CD 650 carbide tooling. The tooling was yielding about 500,000 hits per service. The company changed the die guide pins from 1 in. OD using standard ball guide bushings to 2 in. OD using roller guide bushings. It also maximized punch-to-die clearance.
Changing the wire EDM punches and dies to split ground dies and sweep ground punches improved the surface finish from 16 to 4 RMS. Tolerances for the components and stripper guidance were +/-0.0002 and changed to +/- 0.00005 in. The company shortened the press stroke as much as possible, staggered the punches to minimize the snap-through during piercing, and ensured the press ram was running square and parallel with minimum backlash. These efforts resulted in an improvement of 4 times the hits per service.
STAMPING Journal is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping.