Understanding the Elements: The Coordination of Materials, Coatings and Geometry
Key To High-Speed Aluminum Machining
Technological developments such as the Makino MAG-Series machines have made tooling vendors rethink the core concepts of their tooling designs. To get the most out of any state-of-the-art machine technology, it is vital to apply the right tooling and programming concepts.
The primary tooling concerns when machining aluminum are: minimizing the tendency of aluminum to stick to the tool cutting edges; ensuring there is good chip evacuation from the cutting edge; and ensuring the core strength of the tool is sufﬁcient to withstand the cutting forces without breaking.
Materials, coatings and geometry are the three elements in tool design that interrelate to minimize these concerns. If these three elements do not work together, successful high-speed milling is not possible. It is imperative to understand all three of these elements in order to be successful in the high-speed machining of aluminum.
Minimizing the Built-Up Edge
When machining aluminum, one of the major failure modes of cutting tools is the material being machined adheres to the tool cutting edge. This condition rapidly degrades the cutting ability of the tool. The built-up edge that is generated by the adhering aluminum dulls the tool so it can no longer cut through the material. Tool material selection and tool coating selection are the two primary techniques used by tool designers to reduce the occurrence of the built-up edge.
Two different carbide materials used in high-speed machining tools are sub-micron grain and course grain. Sub-micron grain carbide material has generally been accepted as the preferred material of choice because it is very hard and maintains a sharp cutting edge. When machining aluminum at very high speeds, however, this conventional wisdom is incorrect.
The sub-micron grain carbide material requires a high cobalt concentration to achieve the fine grain structure and the material’s strength properties. Cobalt reacts with aluminum at elevated temperatures, which causes the aluminum to chemically bond to the exposed cobalt of the tool material. Once the aluminum starts to adhere to the tool, it quickly forms a built-up edge on the tool rendering it ineffective.
The secret is to find the right balance of cobalt to provide adequate material strength, while minimizing the exposed cobalt in the tool for aluminum adherence during the cutting process. This balance is achieved using coarse-grained carbide that provides a tool of sufficient hardness so as to not dull quickly when machining aluminum while minimizing adherence.
The second tool design element that must be considered when trying to minimize the built-up edge is the tool coating. Tool coating choices include TiN, TiCN, TiAIN, AlTiN, chrome nitrides, zirconium nitrides, diamond and diamond-like coatings (DLC). With so many choices, aerospace milling shops need to know which one works best in an aluminum high-speed machining application.
The Physical Vapor Deposition (PVD) coating application process on TiN, TiCN, TiAIN, and AlTiN tools makes them unsuitable for an aluminum application. The PVD coating process creates two modes for aluminum to bond to the tool—the surface roughness and the chemical reactivity between the aluminum and the tool coating.
The PVD process results in a surface that is rougher than the substrate material to which it is applied. The surface “peaks and valleys” created by this process causes aluminum to rapidly collect in the valleys on the tool. In addition, the PVD coating is chemically reactive to the aluminum due to its metallic crystal and ionic crystal features. A TiAIN coating actually contains aluminum, which easily bonds with a cutting surface of the same material. The surface roughness and chemical reactivity attributes will cause the tool and work piece to stick together, thus creating the built-up edge.
In testing performed by OSG Tap and Die, it was discovered that when machining aluminum at very high speeds, the performance of an uncoated coarse-grained carbide tool was superior to that of one coated with Tin, Ticn, TiAlN, or ALTiN.
This testing does not mean that all tool coatings will reduce the tool performance. The diamond and DLC coatings result in a very smooth chemically inert surface. These coatings have been found to significantly improve tool life when cutting aluminum materials.
The diamond coatings were found to be the best performing coatings, but there is a considerable cost related to this type of coating. The DLC coatings provide the best cost for performance value, adding about 20 percent to 25 percent to the total tool cost. But, this coating extends the tool life significantly as compared to an uncoated coarse-grained carbide tool.
The rule of thumb for high-speed aluminum machining tooling designs is to maximize space for chip evacuation. This is because aluminum is a very soft material, and the feedrate is usually increased which creates more and bigger chips.
The Makino MAG-Series aerospace milling machines, such as the MAG4, require an additional consideration for tool geometry—tool strength. The MAG-Series machines with their powerful 80-HP spindles will snap the tools if they are not designed with sufficient core strength.
On previous technology, the number of flutes on the tool had to be increased to provide the proper chip load at speeds required to achieve high quality cuts in aluminum. With the 30,000-RPM and the 80-HP spindle technology, the number of tool flutes must be reduced and the core strength of the tool increased.
The high RPM capability of the spindle will ensure the proper chip load and the strong core tool strength. This enables the entire 80-HP to cut metal without fear of tool breakage. In detailed testing outlined later, it was discovered that a two-flute tool provided the best geometry for chip evacuation and tool strength.
In general, sharp cutting edges should always be used to avoid aluminum elongation. A sharp cutting edge will create high shearing and also high surface clearance, creating a better surface finish and minimizing chatter or surface vibration. The issue is that it is possible to achieve a sharper cutting edge with the fine-grained carbide material than the coarse grained material. But due to aluminum adherence to the fine-grained material, it is not possible to maintain that edge for very long.
The coarse grained material appears to be the best compromise. It is a strong material that can have a reasonable cutting edge. Test results show it is able to achieve a very long tool life with good surface finish. The maintenance of the cutting edge is improved using an oil mist coolant through the tool. Misting gradually cools down the tools, eliminating thermal shock problems.
The helix angle is an additional tool geometry consideration. Traditionally when machining aluminum a tool with a high helix angle has been used. A high helix angle lifts the chip away from the part more quickly, but increases the friction and heat generated as a result of the cutting action. A high helix angle is typically used on a tool with a higher number of flutes to quickly evacuate the chip from the part.
When machining aluminum at very high speeds the heat created by the increased friction may cause the chips to weld to the tool. In addition, a cutting surface with a high helix angle will chip more rapidly than a tool with a low helix angle.
A tool design that utilizes only two flutes enables both a low helix angle and sufficient chip evacuation area. This is the approach that has proven to be the most successful in extensive testing performed by OSG when developing the new tooling line, the MAX AL.
Tale of the Test
OSG has extensively tested the new tooling line, MAX AL on the Makino MAG4, which was developed and tested concurrently with the release of the MAG4. The MAX AL tool is designed for higher spindle speeds and a higher feedrate.
OSG created a corner radius, two-flute design with a K-grade or course-grain carbide. This creates high rigidity and a thick core without sacrificing chip capacity. And it works under the most severe conditions.
This tool achieved impressive performance with respect to metal removal rate and tool life cutting of a wing rib part. The wing rib has general dimensions of 2000 mm-x 500 mm x 2000 mm. A 0.750-inch diameter MAX AL tool with through-coolant mist was used at 21,500 rpm and a feedrate of 394 inches per minute (ipm), and a .68-inch depth of cut, 90 percent of the tool diameter.
Typically OSG’s standard tools prior to the MAX AL last up to 15 hours. The MAX AL tool lasted nearly 90 hours during this test cut with an amazing cycle time of 2 hours and 30 minutes per part.
While designing the MAX AL tooling line, OSG tested 15 different one-inch diameter prototype tools at Makino’s facility in Japan. The axial depth was 15 mm, and radial depth 20 mm on aluminum material 7075-T651. Water-soluble coolant was used at 30,000 rpm, with a feedrate of 20,000-mm/min achieving chip removal rates of 366 cubic inches (6,000 cc). Ebara Mist Oil No. 6 (Ebara EcoMist) was used at 80 cc/h with a mist conveyance pressure of 0.6Mpa/o.3Mpa.
Since the primary test concern was the material removal rate, the measurement of success for this test was the spindle load, and maintenance of spindle RPM, at designated metal removal rates.
Each cutting tool was run with 30,000 RPM, and spindle RPM after 500mm cutting length was measured. The less variance of spindle speed the better.
Tool #9 with mist coolant proved to be the best under this cutting condition. There was no significant difference in spindle load between DLC and non-coating tools.
Cutting Under Severe Conditions
Many of the tool concepts for the OSG MAX AL did not survive the rigors of the tests, and were not included in the test data that has been presented. The MAX AL tool design was tested in a three-flute configuration with less chip room, causing immediate breakage.
It had been thought the three-flute tool might be an appropriate solution because this configuration works well on machines with 15,000 to 20,000 rpm spindle. In addition to the two carbide materials, a high-speed steel, two-flute tool was tested at 30,000 rpm and 20,000 mm/min cutting conditions. The chip removal rate of 4.8-liters/min also led to breakage.
High-speed steel materials are not strong enough to hold up under such cutting conditions. The 80-HP, 30,000-rpm spindle of the MAG 4 simply overpowered the traditional tool design concepts.
In aluminum high-speed machining applications, use coarse-grain carbide materials for endmills, which should be either DLC or non-coated with wide chip room and solid rigidity. When running at 30,000 rpm and 80-HP, the core strength of the tool is critical.
The tools do not need to have excessively sharp cutting edges for high-speed aluminum application. No margin is desired, as that also causes a breakage problem as it provides more torque on a cutting edge. Tools with lower helix angles perform better in this application.
Most importantly, for new high-quality products like the MAG-Series and the MAG4 from Makino, understand that the rules are being re-written. The methods traditionally employed will not necessarily yield the best result.
No other machine tool on the market today has the ability to handle the punishment of such tests as does Makino equipment. A key factor is that the spindles of many other machine tool companies will fail, as they can not handle the stresses incurred when tools are broken off as done in our testing.
Such machine tools can save a company 65 percent to 75 percent versus purchasing a gantry-type system, and can make an operator up to four-times as efficient.
Speak to Makino’s engineers and learn what they already know about their machines as well as tool companies like OSG. An expertise in high-speed aluminum milling and an understanding of the three elements of tool design can make you successful in high-speed aerospace machining.