Slow Down to Speed Up: Understanding Optimized Roughing

by Jay Ball, Product Manager Solid Milling - North America, Seco Tools, LLC

Modern machine tools provide an ideal platform for achieving the benefits of optimized roughing, but many shops continue to miss out on its advantages. This strategy's up side includes longer tool life, better surface finishes, shorter cycle times and reduced machine wear, but it tends to intimidate many machinists because it takes them outside their comfort zones.

Knowledge is power, and it's tough to implement any productivity method unless you understand it. That's the big barrier for many shops with high-speed machine tools: the mistaken belief that if your equipment can go fast, it always should run at its highest possible speed. If that were true, we'd drive to the grocery store at 110 mph because that’s the top speed of most automobiles.

For machine tools, the flaw in the "always as fast as possible" philosophy is simple, once you think about it: Depending on the part design, the equipment actually can't sustain its highest speeds and feeds throughout a cut path. When spindle speeds are kept high, the cutting tool’s angle of engagement increases through complex features and in corners. But those high RPMs force feed rates and stepovers both to drop. This combination of high spindle speed and slow feeds produces the classic scenario in which a cutting tool/end mill rubs rather than cuts, producing heat and friction that induce premature tool wear – and that scenario impedes proper chip development as well.

Tell machinists to slow their machine RPMs so they can speed up their overall cycle times, and they most likely won't believe it. Yes, it sounds counterintuitive, but when RPMs match sustainable feeds and stepovers, shops achieve better results – including more than 50% longer tool life in many cases. Simply put, matching speeds with feeds maintains a constant chip load.

Along with the "need for speed" and its impact on machining results, some shops also fail to grasp the logic of the optimized roughing process. They understand that producing good parts typically means making good chips, but they're not clear on the connection between optimized roughing and its results.

What shops often do know, however, is that their part surface finishes are subpar and their tool wear is high. Typically, they're engaging the full diameter of a cutter, creating a large angle of engagement, but they're not using the full length of the flutes. Changing the cutter engagement solves both problems: surface finish and tool wear. In fact, the best way to see how optimized roughing works is to implement it and look at its results.

Just as it's important to remember the advantages of optimized roughing, it's critical to know when to apply the method in the first place. Not every part qualifies as a good candidate. Try engaging full flute lengths to machine a complex mold cavity, for example, and you quickly see that you'll never achieve a good chip load on this job with this approach. However, if you train yourself to think of optimized roughing when you see a part with straight prismatic walls, you'll be on the right path.

As you look at your workflow, ask yourself whether you're matching speeds and feeds at sustainable levels. Look at the data coming from your equipment and see if your machines can run at the speeds you program, or if your hardware is trying to protect itself with reduced feeds and stepovers. This method works with any suitable workpiece and any type of tool, from the tiniest of diameters on up, provided that the part’s geometries and features allow it.

Along with the information you're getting from your equipment, take a moment to talk through your needs with your machine tool supplier. Here at Seco Tools, we've developed reference tables that show you workable baseline values for speeds, feeds, angles of engagement and stepovers, values you can achieve with ease if you use the right approach to your machine tools. We're always happy to help our customers maximize their productivity. After all, what good is great hardware if it can’t be used effectively? Productivity and efficiency are the keys to profitability, and we want to see our customers succeed.