How To Machine Light Metals

Here are some recommendations that can help you make the move to machining aluminum and magnesium.

 

While die casting experts are working on producing die cast parts to net shape, machine tool and machining system builders are busy producing machines that remove the not-so-net excess more efficiently. These machines are different from the ones designed for heavier metals.

First of all, the amount of power needed for working the lighter metals is much less than with cast irons or steels. Secondly, metal removal rates can increase substantially. Surface feet per minute (sfm) values are much higher than with heavy metals (up to 1200 SFM with uncoated carbide). The exception to this rule is magnesium. "You have to submerge the part, and go much slower," explains John S. Klimach, president & CEO at Ex-Cell-O Machine Tools, Inc. (Sterling Heights, MI). "If you don't the part will catch fire."


 

Mark Tomlinson, vice president of engineering at Lamb Technicon Machining Systems (Warren, MI), feels that there's also a basic change in tooling philosophy between cast iron machining and aluminum machining. "The tooling technologies that are used and the types of tools and inserts that can be used are different," he explains. "What that corresponds with is actual tool life. In cast iron, you're looking to get one or two shifts worth of production out of a tool. With aluminum, you're looking at a whole year's worth." But to actually achieve this kind of productivity, the machine's spindle has to be much more robust. Why?

For starters, spindles require much higher speeds, and less low-end torque than when machining steel or other materials. And they have to be stiffer, to hold tolerance under the faster conditions. Miles Arnone, president and COO at Boston Digital Corp. (Milford, MA), recommends using HSK tapers for added rigidity and accuracy. He also recommends that if very fast tool changes are required, closed-loop motorized spindles should be used to reduce the rotating inertia.

The higher spindle speed also lends to increased frictional heat in the bearings, so a cooling system has to be added. And since light metal machining is not a dry operation, the spindle has to be well sealed against coolant infiltration.

But the spindle isn't the only factor to take into consideration. Here are a few more:

  • Coated-carbide tools, such as TiAlN, should be employed to extend tool life and to increase SFM levels up to 5,000 surface-feet-per-minute.
  • Everyone is "Wowed" by high feed rates, but the ability to quickly accelerate or decelerate is more important than maximum feedrates when machining 3D contoured surfaces. CNC features such as gain switching (changes servo parameters on the fly) or contour optimization (lets you set a contour tolerance and automatically manages feed rate) are also important as axis feed rates rise when machining light metals.
  • Because speeds and feeds are so high, thermal growth can be a major problem when working with light metals. Seek machines with good thermal control systems, and glass scale feedback on all axes. A high-performance machine should provide thermal control of the spindle and major structural elements, such as the column in a VMC. For high precision applications (tolerances better than ±0.0005"), temperature controlled coolant should be considered as well.
  • Chip removal can be a big issue when working in light metals such as aluminum. Because so much material can be removed in a short period of time, it is important that the machine tool be well designed for chip evacuation.
  • Even though the cutting forces to machine aluminum are less than when working with steel, for example, it is still very important to have a stiff, well-damped machine. While cutting forces are lower, dynamic forces due to accelerations and decelerations are very high.

But with Magnesium…

 

These suggestions and observations apply to aluminum machining very well, but if you tried to do the same things with magnesium, you'd be in big trouble (remember Klimach's warnings of fire?). Safety is the big issue here, and there are a few ways you can safely machine magnesium. 
•SLOW DOWN. The high speeds you enjoy with aluminum create heat, and heat is not good here. 
•Water-based cutting fluids are okay, but use a cutting fluid with a high pH-value. This will reduce the amount of hydrogen gas produced as the water and magnesium react. 
•Since hydrogen gas explodes under pressure, maintain good ventilation to dissipate any gas that may be produced. 
•If a fire does occur, DO NOT throw water on it, the chemical reaction will actually cause the fire to get bigger. Smothering the fire is the better option. Klimach mentioned one company that kept a barrel of cast iron chips near by when machining magnesium. If fire broke out, they'd shovel the chips on top of it. 
•The National Fire Protection Association (NFPA) adds these safety guidelines for handling chips: separate the fluid and chips using a centrifuge system, and do so as soon as possible after machining. Letting the chips sit around lets the fluid and magnesium interact further, increasing the potential for fire. Also, make sure the storage containers are nonflammable, ventilated containers. If not properly ventilated, the possibility of dangerous hydrogen concentrations increases. Store the containers in isolated areas so that if a fire or explosion does occur, it is away from employees, machines, and other flammable materials.

These suggestions for machining magnesium may seem a bit alarmist and silly, industry-wide, magnesium is considered a "problematic" metal to machine. Recent developments in magnesium alloys have somewhat dampened its fiery nature without sacrificing its light weight charac-teristics, but users should still be cautious.

Of course, if the diecasters have anything to say about it, suppliers won't have to worry about the considerations and concerns of machining excess material from light metal parts. After all, you don't have to remove anything from parts that are "nothin' but net."

Not Quite Metal?

DSC aluminum

DSC aluminum from Chesapeake Composites (New Castle, DE) is an alloy, but isn't quite all metal. Instead of the traditional materials used to strengthen aluminum (like nickel), DSC contains ceramic compounds. Traditionally, adding ceramic to aluminum has created an alloy that is difficult to machine (very abrasive), and expensive to cast. The patent-pending process Chesapeake has developed more evenly disperses the ceramic particles, which are smaller than the ceramic grains used previously. The process also brings the price of billet down to around $3 per lb., as opposed to the $4 or $5 per lb. cost usually affiliated with ceramic alloyed aluminum.

Nothin' But Net

Casting parts to net size is something the automotive production world is eagerly trying to achieve. Not only would parts producers save lots and lots of time, but lots and lots of money, as well. The castings aren't quite there yet, but advances in technology and innovative new techniques are driving things closer to that so far elusive, but not impossible absolute net-shape part.

Even before metal is poured into a die, flow control is a factor. Selcom, Inc. (Southfield, MI), develops laser-based pouring systems for all sorts of molten metal applications. It's not something component suppliers typically think about, but the quality control system surrounding how the metal is poured into the die is something that effects the overall quality of the casting they have to work with. The LaserPour systems Selcom develops use sensors to keep an eye on how the metal flows into the die, bringing the resulting casting even closer to net shape.

They track flow of the metal into the mold, and mold levels, resulting in more accurate physical dimension and grain structure within the mold. With high-speed resolution at about 0.006 in., and a data sampling rate of 16 kHz, the sensors read data fast enough to stop the automated system from over pouring, pouring too slowly, or pouring too quickly, all of which has an effect on the part. The laser feedback loop also allows accurate measurement without being effected by plant conditions (temperature and light changes, etc.).

Putting On the Squeeze

Moving from process controls to processes, one that stands out is the squeeze-cast process for producing aluminum parts called Hi-Cast from Amcast Industrial Corp. (Dayton, OH). It uses a coaxial double piston pressure system for delivering molten aluminum to the die. The system—developed mainly for multiple-cavity die casting—consists of a shot sleeve that tilts to receive the aluminum from a ladling system.

After receiving material, it swings back into sync with the die, where the primary piston squeezes the metal into the die cavity. After the casting solidifies, the piston advances again to eliminate any cavities caused by shrinkage. At this point, the second piston advances to continue both filling the die with aluminum and applying pressure to eliminate shrinkage cavities. Quick cooling rates and consistent pressure provide higher fatigue strength to both thick and thin areas of the die. The higher consistency of die coverage also brings the part nearer to net shape, reducing the amount of machining needed after the die is cast.

 

Making It At Mazda

At Mazda, experimentation with net shape parts has resulted in the development of a process that introduces air pressure into the mold. Called "Air-Assisted Infiltration Process," it reduces the amount of pressure (from 1,000 atm to 10 atm) and energy needed to introduce metal into complex dies. This process has even been used successfully with dies with a divided core, a place where even squeeze casting proved inefficient.

According to some, casting magnesium is even easier than aluminum. The thixomolding process is said to produce net-shape parts in a single setup. The process, used at Thixomat, Inc. (Ann Arbor, MI), is similar to plastic injection molding. Magnesium feedstock is heated until the metal is semi-solid. The resulting slurry is injected into a preheated metal mold to make the part. The semi-solid molding process is particularly well-suited to magnesium since its creep properties do not lend well to pouring.

Several companies, including Mazda, are interested in the process. Especially beneficial is the resulting net-shape parts, which seem to retain desired structural and mechanical properties, and come out of the die without needing any post casting machining to bring parts to net shape. This is especially desirable since finish machining magnesium is a difficult and expensive process.