February 19, 2001
Demands for cheaper, better, and faster tooling for stampings and the pace of business continue to escalate. This trend is not going to reverse itself.
I want it cheaper. I want it better. I want it faster, preferably yesterday. The pace of business in the metal stamping industry continues to escalate. And the trend is not going to reverse. Balancing cost against quality and time can be a real challenge. Add to that the pressures of global competition, and the task becomes even more difficult.
The tooling needed to produce a stamping is a big part of the equation. Annual production requirements should dictate the type and quality of tooling to be used to produce a particular stamping. Decisions on when to compromise between cost and quality greatly impact whether a stamper receives the work and, more important, whether the job is profitable.
Volume requirements vary from industry to industry. Each, however, has what are categorized as low-, medium-, and high-volume quantities. Tooling should be produced differently for each of these categories if a stamper is to be competitive in today's global economy. In the simplest terms, the higher the volume, the higher the quality of the tooling should be.
For purposes of this article, tool quality refers to how a die is designed and constructed. What types of features will the die have that make adjusting and servicing the tooling easier and add to die life? The combined costs of the tooling initially and the sometimes-hidden costs involved in maintaining the die after production begins determine whether a job is profitable.
When production volume is high, decisions on die features are relatively simple. Because the die will spend a lot of time in the press, expensive downtime may result if the tooling is not readily maintainable. A die should be designed so that it can be serviced as easily as possible in such applications. The payback far outweighs the higher initial tooling cost.
For medium or medium-low production volumes, however, all decisions require that compromises be made. All design and construction features in a die have costs associated with them when they are built initially. However, a die that lacks such features may cause additional costs to be incurred during production.
When looking for ways to save money on tooling used for lower-volume stamping, it is best to start with the simplest option — look at the type of steel and the size of the die block. Less-expensive ways to construct a tool should be considered, but shortcuts that will cost more money in the long run should be avoided.
When trying to reach a compromise among competing goals for a stamping tool, an honest assessment of the stamping operation is necessary. What is the quality of the press and feeders? What are the skill levels of the setup people, press operators, and in-house toolmakers? If confidence in these key people is high, it is likely that fewer mistakes will occur during production. As a result, the risks associated with potential maintenance problems will be fewer, even if compromises are made during construction of the tooling.
The materials used in die construction provide one area in which cost savings may be realized. However, trying to save money by using a low-priced tool steel that lacks traceability is unwise. Contaminants or voids may be present that can cause serious problems during the heat-treating process. These stresses could cause a block to crack prematurely at any time, even during final grinding or while the block is undergoing wire electrical discharge machining (EDM).
It is better to use less-expensive grades of quality tool steels. D-2 could be used in place of powdered metal steels, for example, or A-2 could be used in place of D-2. Reviewing material choices for all die components can generate savings.
The thickness of the materials being used in die construction is the next logical area to investigate for cost savings. A rule of thumb always has been to use 1.5-inch-thick die blocks when stamping material up to 0.125 inch thick. In many applications, however, 1.25-inch or even 1-inch die blocks will work just as well, and cost savings will be realized in expenditure for the tool steel, machining, and wire EDM. Reducing the stripper and punch holder thicknesses can create additional savings.
To use a 1-inch-thick die block for blanking thicker material, choices must be made. If half-inch bolts are needed for strength, very little room is left for die life or block strength. One option is to use smaller bolts but to add more of them to the design. Other alternatives include decreasing the Rockwell hardness of the tool steel by a point or two to give it more strength or switching to a more shock-resistant grade of tool steel.
However, thinner die steels can create weaker sections in trimming and forming areas of the tool, which may make it too risky to reduce the thickness at all.
The same problems are encountered in the top half of the die. An additional concern is the amount of room needed for die springs. If these springs are compressed beyond the level specified by the manufacturer for a particular average rated life expectancy, their failure rate increases rapidly. If production volumes are low enough, this may not be a problem. For higher production volumes, the choice is between a lower initial tooling cost and the expense associated with frequent spring replacement and related downtime.
Another area in which to look for cost savings is in the size of components used in die construction. Tooling can be built using very large die sections, which reduces the number of blocks to machine, wire, and mount, reducing the initial cost for the tooling.
Differences in material thickness cause uneven cooling when dies are heat-treated. Thinner sections cool first, but then draw heat from thicker sections.
Of course, a stamper who builds tooling in-house is limited to using die sections only as large as his equipment can handle. Although it sometimes is overlooked by a stamper who plans to purchase tooling, the same consideration applies — die sections of outsourced tooling must not be larger than can be accommodated by the equipment used to maintain it.
Other factors must be considered as well when deciding on the sizes of die components. When larger die blocks break, for example, repairs are almost always more expensive than those of smaller die sections. Using large die blocks also can increase the risk of problems developing during the heat-treating process. Depending on the shape and the size of an opening in the block, stresses can develop that may cause cracking.
Heat treating of tool steels is one area in which shortcuts should never be taken. It is important to double draw, at a minimum. A 25-degree temperature increase is needed with the second draw to ensure proper tempering. Proper heat treatment is especially important when tool steel sections are thin or weak, as is often the case in today's cost-conscious stamping environment.
Figure 1shows a cross section of a pedestal punch. Arrows indicate the direction of the heat flow during the cooling process. The thinnest section cools first; the heat then is drawn from the thicker sections into the thinner sections.The uneven cooling process can cause a block to warp and also creates stress in the block. To slow the heat transfer process, the block can be wrapped in a stainless steel tool wrap or placed on a hot plate during the cooling process. It is important that the block with the thinnest section be placed against the hot plate.
The sharper the corners that the heat is trying to flow around, the greater is the chance of the block cracking. The heat transfer must be slowed to create a block that is stable.
One final issue regarding heat treating and tempering involves the molecular structure of tool steel and how it is affected by the forces generated during the blanking process. As a result of the continual pounding, molecules in the tool steel become compacted and therefore more dense (see Figure 2). Rockwell hardness may even increase by as much as a point.
The continued pounding to which tooling is subjected can compact molecules in the steel, increasing Rockwell hardness by as much as a point.
These dense molecular structures can cause die blocks to chip. Returning die blocks to their original postheat-treated state can help extend die life, which can be important for low-volume tooling that has little die life built into it.
After die blocks are sharpened, they should be retempered. They should be drawn back to a temperature 25 degrees less than the original draw temperature. Exceeding the original draw temperature can cause dimensional changes to occur in the blocks.
When looking for ways to save money on tooling for lower-volume applications, it is best to start with the simplest options. Less-expensive ways to construct a tool may be available, but shortcuts that will cost more later should be avoided.
The potential for cost savings for each tool varies. The largest savings are likely to result from the creativity with which a die is designed and built. Can operations be combined? Can parts be combined? Can processes be eliminated?
Relying on the talent of in-house tooling engineers or developing a relationship with an experienced tool house will give a stamper the best opportunity to cut costs, win bids, and make money on the jobs.