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Productive Aluminum Machining - Part2
16 Nov 2022
Information provided by
Global Industry Project Manager - Aerospace, Power Generation and Medical
In last month’s article (See: Productive Aluminum Machining – Part 1) we discussed the popular misconception that machining aluminum alloys is “easy”, we discussed in some detail the naming convention used for aluminum alloys and how to differentiate them, some of their key characteristics, and the industries where they are most commonly used. In this month’s article, the focus is more on the unique demands machining aluminum alloys places on the machines, the tools, and the processes. It is important to have a clear understanding of the challenges and strategies to be competitive and produce parts profitably.
Due to the nature of machining aluminum, there are some common challenges that need to be considered. The volume of chips being produced per hour can create handling issues for the chip conveyors on many types and configurations of machines for example. Having adequate coolant capacity and pressure are important. Coolant through the spindle can be the difference between a good part and a broken tool in applications such as drilling and threading.
An important consideration is that despite the inherently low cutting forces required to machine aluminum, the high cutting speeds require spindles to provide high power at sustained high RPMs, and place very different demands on the machine as compared to machining the more common steel alloys for example. Since cutting speeds can vary a lot depending on the alloy, the lower limit is typically defined by the tendency of built-up edge formation at low cutting speeds (Vc) and when machining near the high limits is limited by the melting temperature of the aluminum. There are always challenges in designing the tools to create tool geometry sharp enough to cut cleanly and smoothly, while also considering the need to have a strong edge that can resist being easily broken at such high speeds.
< Showing an example of a rake face surface to illustrate creating a tool with a sharp edge. >
Because of the possibility to machine at higher RPM the balancing of the tools becomes important. Tools which are balanced by design such as the Alu-Power HPC carbide endmills for non-ferrous materials from YG-1 allow higher performance and a more secure process. These tools are balanced by design and have all 3 flutes ground to the center of the tool which allows for the highest possible RPM to be achieved without creating harmful imbalance in the tool assembly which can cause severe damage to the spindle and bearings of the machine over time, as well as critical safety considerations due to possible tool failures at very high RPMs. Regarding the volume of chips being produced during heavy machining operations, YG-1 offers the Alu-Power and Alu-Power HPC tooling in both chipbreaker and non-chipbreaker designs to optimize the operation being performed. A chipbreaker serration in the tool can reduce the amount of horsepower required during cutting as well as reduce the amount of residual stresses being introduced into the part being machined, while also creating smaller and lighter chips which are easier for the coolant stream to evacuate from the cutting zone more quickly.
Proper Tool Selection
Due to the differences in the alloys, there are important advantages to using coatings in some applications or even PCD (Polycrystalline Diamond) tooling. Coatings such as DLC (Diamond-Like Carbon) can provide a very good balance between tool wear and cost, and usually provide higher value in the more demanding alloys with high silicon or in the lithium alloys. In these applications it can be very important to consider all of the process and the requirements it will place on the machine to be able to fully utilize all the tool features and produce profitably. In our Alu-Power and Alu-Power HPC line of tools we can offer both the uncoated carbide tools, or DLC coating as standards, with the ability to produce customized solutions through our Technical Centers when working with PCD tooling to help to optimize the process for our customers.
< Alu-Power HPC Endmill >
Another very important consideration is the use of coolant. Coolant-through holders which flush coolant down along the tool aid in the evacuation of chips and help to make a secure process vs using flood coolant where the coolant flow can be blocked due to part geometry as the tool moves around the part. A tool running at such high speeds does not need to be starved of coolant very long before the heat being produced will cause problems. High pressure coolant is an advantage as well due to the higher pressure helping to get coolant closer to the cutting edge and to help push the chips being produced away more quickly. Drilling operations see tremendous gains in hole quality, tool life and process security when used with coolant through the tool. YG-1 Dream Drills-Alu has optimized tool geometry to allow for sharp, clean entries into the holes with large coolant holes to allow for maximized coolant volume to be pushed through the tools. In addition, the flute geometry allows the chips to exit quickly and with less chance of clogging or creating built up edge. These are important factors to consider to be as successful as possible, and the deeper and/or smaller the holes are, the more important these become.
< Dream Drills-Alu >
While oil-based coolants can provide good lubricity to the cutting action of the tools, there is a large amount of heat generated during the high-speed machining of aluminum, and it is very important that the cutting edge of the tool is kept cool and does not begin to build up chips that are sticking to the cutting edges, or building up in the flutes of the tool. If the chips begin to stick or “chip weld” to the tool it will seriously affect the surface finish of the workpiece and the service life of the tool. Heat can also deform the workpiece and seriously affect the dimensional accuracy of the part being machined. It is also possible that the tool could reach the flash point of the oil-based coolant and create heavy smoke, and the potential for fire.
For water-based cutting fluid, it is more important to consider rust preventatives. Some of the commonly used water-based aluminum antirust agents include silicate and phosphate grease. For workpieces stored between processes, it’s important to select cutting fluid with phosphate grease antirust agent rather than silicon because silicon will corrode and produce black corrosion spots if allowed to stay in contact with the raw aluminum long enough. The pH value of cutting fluid is mostly kept at 8 ~ 10 to help prevent alkaline condition. Therefore, water-soluble cutting fluid must have good aluminum antirust performance.
In summary;
Because of the very high feed rates achievable, and the high RPMs possible to run the tools, it is critical to be sure to use good principles in the set-up and programming processes. It’s important to use tools designed for aluminum that have high rake angles, large flutes for chip evacuation, with fine, sharp edges. It is important to use modern holder systems, and to balance the tools when running at high RPMs, as well as being sure to consider adequate fixturing and the overall safety of the workspace. Aluminum machining generates very high lateral forces on the tools and the parts being machined during roughing cycles.
So, you can see how thinking “aluminum is easy to machine” can lead to problems when it comes time to do it efficiently.
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