Optimize cutting efficiency, optimize throughput: a machining process based on maximizing cutting efficiency, rather than speeds and feeds, offers aerospace component manufacturers an option to high speed machining when high material removal rates are required. The best strategy may be to apply both machining concepts

Modern Machine Shop, March, 2005 by Derek Korn

High speed machining has been, and will continue to be, an eminent machining strategy in the production of aerospace components. The combination of very high spindle speeds, fast feed rates and small depths of cut delivers the surface finishes and accuracies required for parts such as frame components that often have delicate thin walls and bases. However, if there is one overriding "speed" the aerospace industry is most focused on in terms of component production, then it is the rate at which parts are completely machined and ready for assembly. Sandvik Coromant (Fair Lawn, New Jersey) believes that, for cases where substantial material must be removed quickly, shops may be better served to shift out of HSM gear and into a slightly slower, more efficient cutting process. The company refers to this process as high efficiency machining (HEM).

As the name implies, HEM focuses on optimizing cutting efficiency to maximize material removal rate. Unlike HSM, HEM typically throttles back spindle speed to a level that offers both high machine torque and power, and allows deeper depths of cut. Unlike traditional hogging, in which the cutting tool typically is fully engaged with the material, HEM cuts using a fraction of a tool's effective diameter to allow faster feed rates and higher material removal rates.

HEM likely won't supplant HSM in the production of aerospace components. The light, fast cuts inherent to HSM will still be required to for semi-finishing and finishing of certain critical part features. However, the potential benefits HEM offers in terms of roughing aluminum and titanium without damaging parts or prematurely wearing cutting tools make it worthy of consideration. This is especially relevant considering that, in the case of some aerospace parts, up to 95 percent of a billet workpiece may end up as chips.

Brian Davis, aerospace development manager at Sandvik Coromant, explains in this Q&A session how a blending of HEM and HSM techniques may be the best approach to quickest throughput of aerospace components.

* What are basic differences between HSM and HEM? HSM is a function of the machine in terms of potential spindle speeds and feed rates, and is also a function of the cutting tool in terms of geometries designed for very shallow depths of cut. The HEM strategy focuses more on the overall process and the component, in an attempt to minimize cycle times.

One aspect that HEM has in common with HSM is the lack of a universal set of process parameters that can be applied for every application. If you ask ten machinists for a definition of HSM, you'll likely get ten different responses. Sandvik Coromant considers HSM to involve spindle speeds in excess of 18,000 rpm. Typical HSM depths of cut may range from 0.010 inch for finishing operations up to 0.100 inch for roughing. In contrast, HEM may be a fraction of HSM spindle speeds, but multiple times HSM depths of cut. These parameters will vary greatly depending on workpiece type.

* What is the best type of machine for HEM? The most appropriate machine for HEM is a 50-taper horizontal machining center (HMC). An HMC for HEM needs only have top spindle speeds in the 12,000- to 15,000-rpm range, but should be high in both available power and torque.

While feed rates and spindle speeds get primary focus in HSM, machine torque is as important as any other specification for HEM, as it is key to maximizing material removal rates. In settling on an initial spindle speed for an HEM process, it is helpful to understand the relationship between a machine's torque and power over the range of spindle speeds.

A machine tool will not deliver its maximum torque at maximum spindle speed, as torque begins to fall away at very high speeds. The goal is to determine the highest spindle speed at which torque and power are both sufficiently high for the cutting operation. This target speed varies per material and machine.

Aluminum, for example, is relatively soft and typically allows high cutting speeds. Considering the reduction in torque at very high spindle speeds, however, it is possible to stall a powerful machine tool while cutting aluminum. Machining titanium also requires high torque, but it is often done at much lower spindle speeds than aluminum.

Besides offering a rigid, stable platform to handle HEM's higher-than-HSM cutting loads, horizontal cutting allows the resulting large volume of chips to fall away from the part so as not to be re-cut, as can happen in vertical cutting. Obviously, very long aerospace components such as spars will require large vertical gantry machines. Through-spindle coolant is important not only to extend tool life, but also to assist in evacuating chips from the cutting zone. The machine's capacity to convey a large volume of chips must also be considered.

* What are the main tooling differences between HSM and HEM? Solid carbide tools are commonly used in HSM, and they often will have a polycrystalline diamond (PCD) coating. Indexable insert tools may also be used, but they are typically no larger than 2 inches in diameter. The cutting edge for tools designed for HSM's fast, light cuts are very sharp to allow good chip flow. These tools may not be appropriate for HEM, because they do not lend themselves to deep depths of cut. In some cases, a less-expensive, uncoated carbide tool may provide better material removal rates, as it offers sufficient depth of cut and higher edge strength to absorb greater chip loads. Indexable insert tools are more commonly used for HEM.