A soft spot for aluminum: as a soft metal, aluminum has many machining advantages, but its very nature can lead to unique challenges. Here are some tips and tricks to keep the chips flying and the scrap to a minimum.
Everyone loves to run aluminum. It is soft, clean, and easy to machine, but it poses machining challenges. Done properly, aluminum can make a beautiful part where the chips fly away. But, machining a material so soft can also lead to poor finishes, torn threads, and problem burrs. My shop's tackled problems on different aluminum jobs and the lessons we've learned we've applied to almost every piece of aluminum we run.
While running a small cast aluminum housing, we had problems making small-diameter threads. At the beginning of the threading process, the threads were fine, but as the threading progressed, material would build up on the taps and cause tearing. We tried a variety of fixes.
Tapping fluids helped, but were not practical on a production basis.
Thread-forming taps had many of the same issues as cutting taps: material would build up on them, leading to breakage.
Our solution was chrome-plated taps. The chrome coating kept material off the taps without the use of special tapping fluids. Chrome-plated taps are available in many styles and even come in forming taps.
If the size allows, thread milling is a way to handle aluminum threads. Aluminum is responsive to thread milling as it produces manageable chips and does not deform the material like a tap can. This was so helpful in machining some large diameter pipe-threads that we used smaller solid-carbide thread mills for even a 3/8 NPT.
Rough milling aluminum can be a joy to watch when done properly. Conventional tools will machine aluminum adequately, but tools specially-designed for aluminum can speed up cycle times and produce better quality results.
Many manufacturers make aluminum-specific tools that come in variety of types, but they all share some common themes: Inserts on these cutters have a sharp edge, a high shear angle, and generally are polished to a bright mirror finish.
Left in the Cut
On a recent job, we were so pleased with the depth of cuts and feedrates of a 1" diameter inserted cutter, we used it to do the work of a face mill and avoided a tool change.
The key to programming these cutters is to leave the tool in the cut as long as possible to achieve maximum feedrates and stable cutting conditions to avoid excessive entrances and exits.
Another advance in aluminum machining is titanium dibromide tool coating. The coating promises to provide protection to an edge that a brightly polished tool will not. It will not stick to the material such as titanium nitride coatings do. It goes under a few trade names but it is available from many sources.
The problem with trying to deburr aluminum is that it is so soft that the slip of a hand can turn a part into scrap in a matter of seconds. There are a number of ways to avoid this problem.
Try to include deburring in the machining operations themselves. Include chamfering of holes, or perhaps run a chamfer mill along a problem area under the precise control of your machine tool.
If this is not possible, there are some other options, such as switching to bristle discs, rather than abrasives, for some deburring. Abrasive discs can damage the finishes and edges of aluminum if used improperly, but the bristle discs handle the burrs and are much more forgiving.
In the typical job shop, all sorts of materials may come through the door, but aluminum is one of my favorites.
Dave Sterling, Application Engineer Ansco Machine Co.
Dave is responsible for programming, tool selection, and fixture design for Ansco Machine's 20 CNC machines in Peninsula, OH. He's been working in a machine shop since age 12, starting by drilling holes on a turret lathe and making simple parts on a Bridgeport.