When planning a milling application, there are several factors you must consider for optimum results. First and foremost, you want to have the right cutting tool for the job. But before diving too deep into a tooling supplier catalog, it’s important to understand the variables that impact cutting tool performance, with mechanical loads being one of them.
A mechanical load, not to be confused with cutting force, can be thought of in terms of pressure (force per unit of surface area). This pressure has significant influence on tool life and failure. Consider this: a high cutting force spread over a large tool area produces a relatively insignificant load, while a low cutting force concentrated in a small section of the tool can create a problematic load.
Milling exposes multiple cutting edges to continuously changing loads that go from small to large and back again. And, no matter what milling cutter type you use, its cutting edges will repeatedly enter and exit the workpiece material. Loads on the milling teeth will go from zero before entry to peak values in the cut and back to zero at exit.
Therefore, you want to moderate these intermittent loads so you can achieve the best possible tool life, reliability and productivity in your application. Elements such as cutter positioning, entry and exit strategies and chip thickness are key to controlling mechanical loads and ensuring your success.
When approaching a workpiece, you must consider what milling direction will best meet your goals. In conventional “up” milling, the cutter rotates against the direction of the workpiece feed, while climb “down” milling moves in the same direction as the feed.
Whether you go “up” or “down,” you’ll want to position the cutter to one side or the other of the workpiece centerline. Central positioning mixes the forces of conventional and climb milling, which can result in unstable machining and vibration.
Entry and Exit Strategies
The way the cutter and its cutting edges enter the workpiece largely determine mechanical loads in milling. More times than not climb milling will offer the best point of entry over conventional milling, but there are pros and cons to both.
- Climb milling pros: Full-thickness entry into the workpiece allows for proper heat transfer into the chips, protecting both the part and the tool. Chips flow behind the cutter, minimizing recuts and yielding better part surface finishes.
- Climb milling cons: Full-thickness entry into the workpiece can subject the tool to heavy mechanical loads (which is not a problem for most cutting tool materials). Face milling via the climb method creates a downward force that can cause backlash on older manual machines.
- Conventional milling pros: Gradual entry into the workpiece protects brittle, super hard cutting tools from damage when machining rough-surfaced or thin-walled materials. It also handles heavy cuts on less stable machines.
- Conventional milling cons: Shallow-thickness entry into the workpiece creates excessive friction and heat that can have detrimental effects on a tool. Chips drop in front of the cutter, increasing recuts and lowering part surface finish quality.
Furthermore, how your cutter exits the workpiece is just as important as how it enters. If your cutter’s exit is too sudden or uneven, the cutting edges will chip or break. When handled properly, however, you stand to benefit from up to 10 times more tool life. The exit angle, defined as the angle between the milling cutter radius line and the exit point of the cutting edge, should be the primary focus of your exit strategy. Keep in mind your exit angle can be negative (above the cutter radius line) or positive (below the radius line).
Chip thickness is the thickness of the non-deformed chip at the right angles of the cutting edge, and it’s influenced by the radial engagement, edge preparation of the insert and feed per tooth.
When chips are too thick, they tend to generate heavy loads that can chip or break a tool’s cutting edges. When chips are too thin, cutting takes place on a smaller portion of the cutting edge, creating friction and increased heat that results in rapid tool wear.
Cutting tool manufacturers typically have the average chip thickness data for their milling products, so be sure to ask your supplier for this important information. When the average chip thickness data for your cutting tool is applied and maintained, you benefit from maximum tool life and productivity.
Milling cutters have significantly evolved over the years, allowing us to achieve levels of productivity and profitability never before possible. However, many fail to take full advantage of this technical progress. Don’t be one of them. By taking the time to understand the variables that influence cutting tool performance and planning out a proper milling strategy, you’ll have it made.