The High Speed Aluminum Challenge

When shops crank up the speed, they have to be prepared for the consequences, even if they’re milling "easy-to-machine" aluminum.

There was a time when only aerospace manufacturers felt the need to push the limits of their machines and tools when milling aluminum. But not any more. The use of aluminum has become common in a wide variety of industries. The automotive industry, in particular, is forecasting a dramatic increase in the use of aluminum for major powertrain components. Aluminum pistons already power nearly every car on the road, and the use of aluminum heads and blocks is becoming common among automakers.

At the same time, competitive pressures are forcing shops to look for ways to become more productive. As part of this effort to move more parts through the shop in less time, operators are dialing up the speeds and feeds on their milling machines. Shops working with aluminum are just as interested in high productivity as shops working with ferrous materials. As a result, more and more operators are milling aluminum at high speeds.

Few operators would consider milling aluminum at high speeds a challenge. When aluminum is machined, the process doesn’t produce the intense heat that is produced when ferrous materials are cut, and it takes less force to cut the material than it takes to cut iron or steel. Because aluminum is easier to machine than many other metals, it is often the first material that student machinists work with. This has contributed to aluminum’s image as an easy-to-machine material. Aluminum’s reputation has led many to conclude that it can be cut on any machine tool with enough power to turn the spindle, and with virtually any tool material.

The truth is that milling aluminum at high speeds does take special machine tools and cutting tools. As the speed of the operation increases, the dynamics of the process change, and any engineer who isn’t prepared for these changes is putting the operator, the tools, and the productivity of the job at risk.

Technological advances have raised the boundary between standard and high-speed milling over the years. Given today’s technology, users and manufacturers generally agree that "high-speed" milling is milling with a surface speed between 3,300 and 33,000 sfm. When comparing similar operations with similar machines, aluminum will be cut at a faster speed than ferrous materials. Milling either aluminum or ferrous materials at speeds above this range is considered ultra-high-speed work, and it is rarely performed outside of the lab.

The use of larger cutter diameters is one reason an increasing number of jobs are being performed in the high-speed range. As shops replace their older machines with equipment that is more powerful and more rigid, users are finding they can use tools, such as facemills, that have a larger diameter than the endmills they were using. And as the diameter increases, so does the surface speed, even if the spindle speed remains the same.

Is It Safe?

Users may not realize how much this change in the surface speed alters the forces acting on the tool or the workpiece and the impact these changes have on the operation. When an engineer suggests increasing the surface speed of an operation, either by increasing the spindle speed or increasing the diameter of the tool, one of his first concerns should be the safety of the operator. Even at the low end of the high-speed range, tools, tool components, and inserts store large amounts of kinetic energy. If the tool or toolholding system fails, this energy is released unpredictably as components hurtle out of the machining area. Any screw, wedge, anvil, or insert can become a dangerous projectile when it is propelled away from the machine at a high speed.

Shops that are milling at high speeds must select their tools carefully and maintain them to prevent catastrophic failures that turn tool components into deadly missiles. Using a conventional tool in a high-speed operation is unsafe. Only tools designed and balanced for high-speed milling should be used to cut aluminum at speeds above 3,300 sfm. Such tools have low mass to avoid balance problems and easily maintained replaceable components that will allow users to keep them in as-new condition at all times.

At present, there are no standardized designs for high-speed milling tools; each manufacturer has its own design. Some tools may have only a few features that distinguish them from conventional tools. Inexperienced users may not even realize they are using a high-speed tool, or they may not realize how important these features are to the safe and proper operation of the tool at high speeds. If they are not made aware of these differences, they may compromise the safety of the tool by replacing these specialized components or inserts when they become worn or damaged with conventional parts or substandard parts from a different manufacturer. Even after educating machinists about the importance of using the right components for high-speed milling, a shop ought to consider purchasing high-speed tools with parts that are not interchangeable with conventional tools. This will prevent potentially dangerous errors.

Even tools designed for high-speed milling can fail catastrophically, however. In these situations, the operators may have to depend on the machines’ guarding to protect them from shrapnel flying through the work area. Conventional machine guards and shields may not be enough, however. Often these are designed to contain chips, fluids, and atmospheric contaminants rather than flying tool parts. Machines designed for high-speed milling should have guard walls and windows made from thicker, tougher materials.

Machine guards and shields should be seen as the last line of defense against catastrophic tool failure. If tools are properly maintained, these beefed-up guards may never be put to the test. Cutters should be regularly inspected. High-speed milling subjects the cutter to higher rotational loads and greater erosion from contact with chips. This damage can reduce the holding power of the tool’s fasteners or cartridges. Regular tool inspections should be stressed, especially with the use of diamond cutting tools. Operators may not realize the need for frequent cutter inspections when using diamond tools because the cutting tool itself lasts such a long time.

Taking extra safety precautions when milling at high speeds may sound like common sense to most operators and engineers. Such safeguards are necessary regardless of the material being machined. The need for other measures to ensure the success and productivity of a high-speed aluminum milling operation may seem less obvious to operators, especially if they are laboring under misconceptions about aluminum’s machinability.

More Power

One common myth holds that it doesn’t take much power to machine aluminum at high speeds. This myth has persisted because, until quite recently, most high-speed milling of aluminum required machines that were little more than drilling machines. Even aerospace manufacturers milling giant spars used tools that were not much different from ordinary endmills. Endmills do not place significant axial and radial loads on the spindle, and they don’t require the horsepower that facemills might require.

When operators compared their aluminum operations to cutting steel, it reinforced the idea that high-speed aluminum milling is a low-power operation. While it may be true that milling aluminum requires less horsepower, the process still requires a powerful machine to raise speeds into the high-speed range. It can take significant amounts of power just to rotate the spindle at 20,000 rpm or faster. As a rule of thumb, a machine needs 1 horsepower for every 1,000 rpm of spindle speed. This is the minimum needed to rotate the spindle. Cutting the workpiece will require additional power. For cutting aluminum at high speeds, the machine will need a spindle that can handle the radial loads as well as a powerful spindle motor.

Given these basic requirements, a shop may need a 50-hp machine with a 50-taper spindle and a 15,000 rpm continuous-speed rating to mill aluminum at high speeds. That is not a small machine tool in anybody’s book. Clearly an older, low-power machine with inadequate spindle support will not suffice.

But spindle speed isn’t the only concern. True productivity gains can best be achieved by raising the feed rate. All else being equal, an increase in the feed rate results in a more efficient metal-removal rate than the rate an equal change in any other cutting parameter will yield. These increases cost less in terms of horsepower than changes in other parameters cost, because the relationship between horsepower and feed rate is nonlinear. In other words, once the machine is expending enough force to cut the material, it takes a decreasing amount of power to cut a thicker chip.

Machines that have enough horsepower to cut with higher feeds still need structural strength. To mill aluminum at elevated feed rates, a shop will want a beefy, rigid machine tool with powerful axis drives and large precision ballscrews or linear drives. Generally speaking, the present generation of machine tools can cut aluminum with feeds ranging from 300 to 600 ipm. On the horizon are machines with double the feed capability.

Diamond Needed

A second misconception among operators is that any cutting tool material can cut aluminum. Technically, this may be true, but it would be a mistake to think that all cutting tool materials cut aluminum with equal efficiency in all situations. The fact is that facemilling a long run of aluminum parts with anything other than polycrystalline-diamond (PCD) tools is a waste of time and money. On a dollar-for-dollar basis, a PCD tool will produce more high-quality cuts than a tool of any other material. Typically, the cutting edge of a PCD insert chips because of a loose, broken, or improperly loaded part before it wears out. A shop should consider less expensive cutting tools, such as carbide tools, only if parts are often misloaded or if it is cutting poor-quality castings. The silicon or sand inclusions in these parts can rapidly damage PCD tools.

To vary the grade of PCD tools, manufacturers use different sizes of diamond grits. For milling aluminum at high speeds, shops should stock both a roughing and a finishing grade of PCD. Generally, operators shouldn’t plan on using worn finishing tools for roughing applications. Diamond tools seldom wear out in normal use; they typically produce good cuts until the cutting edge is chipped. Therefore, if the tool is producing poor-quality finishing cuts, it will produce poor-quality roughing cuts as well. Even if an undamaged finishing-grade insert is used for roughing, the results may be unsatisfactory, because manufacturers have carefully designed their insert grades to produce a certain kind of cut. Course grades are more suitable for milling high-silicon aluminum and roughing; finer grades are designed to produce glass-like finishes.

Shops milling aluminum at high speeds should also consider using cutting tools with the diamond cutting edge brazed directly to the cartridge. This arrangement holds the cutting edge in position more securely than an indexable insert mounted in a cartridge does. A facemill rotating at 20,000 rpm generates extreme forces. The cutter has to resist these forces in order to hold the cutting edge in position and produce a good finish. A cutter loaded with cartridges on which the inserts have been brazed has fewer mechanical connections between the cutter body and the cutting edge. With fewer connections, there are fewer locations where movement can occur. Tests also have shown that using lubricants on connectors, such as the screws holding the insert or cartridge in place, can increase their holding power.

Ideally, operators should know how fast they can rotate their tools without causing components to move out of place. One manufacturer tests the speed potential of its high-speed cutters by rotating them at increasingly faster rates. The test is halted at the point where centrifugal forces cause a properly installed insert to move 0.0005". The maximum operational speed of the cutter is then calculated as the speed it takes to generate 33% of this force. This maximum speed is permanently etched into each cutter body. The reason the maximum operational speed is so much less than the maximum tested speed is that the test speed is gaged with a tool under no load. In an actual cut, cutters are subjected to impacts and vibrations that increase the forces trying to move the insert out of place.

Don’t Forget the Chips

Operators cranking up the speeds and feeds of their aluminum milling operations may be surprised at the volume of chips they will produce. Because aluminum is a workhardening material, these chips are harder than the workpiece itself. Therefore, they can abrade and damage the surface finish of the workpiece if they come in contact with it. To reduce the potential for damage, the chips must be controlled, but the shear volume of chips makes control and removal difficult.

Cutting fluid can be used to flush away the chips, but it must be applied correctly. When cutting fluid is applied to a cutter rotating at a high speed, it will probably be atomized before it can flush away the chips. Not only is the atomized coolant ineffective at controlling chips, it also poses a health hazard as the misting fluid hangs in the air.

Using machines and tools with through-coolant capability is more desirable. The coolant exiting the spinning cutting tool has much greater inertia than coolant applied externally. This additional force gives the coolant the power to carry the chips away from the cut. In tests, users were able to use feed rates with through-coolant tools that were twice as fast as the rates they could use with tools with externally applied coolant, and the operation produced better surface finishes.

Because coolant isn’t needed to control cutting temperatures, some users have switched to compressed air to evacuate chips. The method does blow the chips out of the cutting zone, but operators may not be able to tolerate the noise it generates, and the flying chips may pose a hazard. Other users employ a vacuum system that sucks the chips out of the cutting zone. This eliminates the problem of flying chips, but the approach can be as noisy as compressed air, and it makes cutter maintenance difficult.

Aluminum is becoming an important material in manufacturing, and its use is constantly increasing. Shops that mill aluminum can realize truly impressive gains in productivity by milling their parts at higher speeds and feeds. But they must thoroughly understand the dynamics of the high-speed milling process to achieve these gains. Only then will they be sure they are using the right machines, and the right tools, under the right conditions.

About the Author

Tom Howes is product marketing manager, Valenite Inc., Madison Heights, MI.