expanding world of metals: Not just steel and cast iron, The

Manufacturing Engineering, Sep 2001 by Aronson, Robert B

One of the big benefits aluminum offers is ease of recycling, both during production [scrap and faulty parts] and end of life. But aluminum one of the problems is warping after machining.

"Although a part may be machined well, it may not hold its shape when released from the part holder," explains Sandvik's Mayer. "Also, the pressures and temperatures generated when cutting introduced stress.

The goal is to eliminate all stress raisers, which requires a good finish. Another problem is that the higher the silicon content of the aluminum the better the fluidity of the casting, so a lot of foundries are using those materials to get more complex shapes. But the higher silicon content means a more abrasive, more difficultto-cut material."

Aerospace engineers want tools that are free cutting. Cubic inches removed is the issue. They have a lot of good tools, the chief concern now is the horsepower of the machine tool. On the other hand, the automotive industry is looking at finishing and semifinishing. This means more cells instead of transfer lines.

Ferrous Metals

Steel makers are working to obtain a 25% weight reduction while achieving a good crash-test rating from both the government and the Insurance Institute for Highway Safety.

Most of the research is related to sheet products, with an emphasis on advanced high-strength steels. The goal is to have a material that is formable, yet gives high-strength crash performance. This means a material that is initialed on the "soft," yet work-hardens to give the final strength properties.

There is a group of steels, chiefly steel alloys used to make dies rated above R^sub c^45, that is growing in popularity because customers want the dies to last longer. Overall, cycle times are shorter because products change so quickly. Customers want the dies to last the life of these shorter runs. They don't want to have to repair a die or change it.

Users also want to do the entire machining job in one setup, save the heat-treat steps by prehardening, reduce grinding, and cut overall cycle time, which the harder materials allow," says Mayer. "The result is a product more dimensionally accurate than one produced with a lot of hand work."

The traditional way was to start with a soft steel, rough machine it, then heat treat followed by finish machining. Now they start with hard material and do all of that at once. But machining in such a process is more difficult, and takes more machine power and greater accuracy. Cutting requires lighter cuts made at higher speeds and feed rates with higher cutting forces. Sloppy spindles and bearings can't be tolerated.

"Cutting philosophy has also changed," says Mayer. "The need is for more heat generated at the contact area, not less. That's one reason why CBN works well with hard materials. It can break strong bonds by creating a heat zone in front of the cutting edge. That zone softens the material just before it's cut so the tool doesn't do as much work because its cutting softer material. You want to avoid cooling. This is a case where heat is not the enemy, which goes against instinct. If the cut isn't hot enough, material will build up on the insert edge.