Common complaints among people new to machining Aluminum alloys include poor surface finish, gummy deposits on cut faces or tool edges, and a smeared appearance. All materials have different properties that require adjustments and care to achieve the optimal finishes.
Aluminum is a soft and ductile metal that has high thermal conductivity. The former means it cuts easily but creates long chips. The latter means it is susceptible to heat build-up. Following these tips when machining aluminum can help mitigate both these concerns.
Use the Right Cutting Tool
Although aluminum is soft and ductile, it requires a good cutting tool for best results. Don’t use high-speed steel or cobalt tools for this job; use carbide cutting tools instead. Also, it is sometimes better to use a tool with fewer flutes. Since Aluminum produces long chips, a tool with fewer flutes will allow chips to escape more easily. Using the right tool with the correct number of flutes will allow you to employ a broader range of spindle speeds.
Keep it Cool
As mentioned above, aluminum has a high thermal conductivity. It gets hot when cut, and that heat can build up fast. This, in turn, can result in a finish with a smeared appearance, workpiece warpage, and leading edge build up (that gummy problem). Proper coolant flow will move chips away from the cutting zone and keep the cutting surfaces properly lubricated.
Horsepower is Your Friend
While aluminum may be softer than other metals, it is still a metal and machining it requires a great deal of power. If your machine can’t keep up with the power requirements for a cut, it will result in chatter and deflection. It may be necessary to use horsepower “derating” in feed and speed calculations when using smaller, lightweight machines for aluminum milling.
A Note on Chip Management
Aluminum’s high ductility results in long unbroken chips that can quickly build up around a tool. This can cause tool breakage, leading edge buildup (more gummy problems) and heat buildup (that smeared finish). When cutting aluminum, be vigilant about cleaning chips. Whether it is with a fixed air blast system, high coolant flow rates or chip conveyors, chip removal should be integrated into your process.
Aluminum is ductile, lightweight, thermally, and electrically conductive with excellent strength characteristics. With a few adjustments to machining techniques, it can be a great material for many parts and projects.
The finishing cut is the last step before a part reaches inspection, and after all the time and effort that goes into making a precision part, it is critical that this final step produces a quality surface finish. Incorporating a few key practices can help prevent the need to scrap parts in this last stage and ensure your surface finishes are always up to snuff.
Feeds and Speeds
It may seem like an obvious point, but dialing in the correct feed and speed is essential for achieving a high-quality surface finish. Increasing the tool speed reduces the built-up edge (BUE), while reducing the feed decreases flank wear. Both can reduce the possibility of tool failure and prolong the life of your tools. A word of caution, however: don’t dial the depth of the cut back so far that the tool rubs against the surface instead of cutting it This will result in a smeared finish.
Although seeing a long-coiled chip come off a tool tip can look satisfying, those long chips may mean the cutting pressure is too high. This can result in accelerated tool wear and create a mess of metal strips that damage the part surface. Adding a chip breaker improves chip removal and control. Factors such as part ductility, tool shape, milling setup and coolant usage all influence chip breaker selection, but once the right one is implemented, it’s easy to clean away chips before they contact the finished surface.
Positive Rake Angle
In addition to reducing the BUE and breaking up chips with a chip breaker, increasing the rake angle helps produce more manageable chips as well, thereby improving cutting efficiency and decreasing tool wear for a more precise surface finish.
It is important that the toolholder remains rigid throughout the cutting process. Any pockets of movement in the setup can cause chatter and result in a scalloped surface. In addition, using best practices like a short tool reach and proper feeds and speeds to minimize tool deflection will also minimize chatter.
Use a Finishing Tool
Roughing tools and finishing tools are distinctly different beasts. Although it may seem like a cost saving measure to use the same tool for both, it can result in greater volumes of scrap after the finishing stage. Additionally, the desired surface finish may require different cutter and insert geometries than those of roughing cuts. A different nose radius, a finer pitch or a wiper insert might be considered for the finishing cut.
If your finding your surface finishes lacking, try implementing some of these practices to improve your final products.