By Sean Holt, Aerospace Applications Manager at Sandvik Coromant US
When it comes to manufacturing, few industries involve higher demands, stricter safety standards and tougher-to-machine materials than aerospace and automotive. In these industries, subpar surface quality or part integrity can mean life or death—and are simply unacceptable.
Because these parts must withstand extreme conditions, they often require titanium, super alloys and other hard-to-machine materials. It is these same materials that guarantee the most solid, reliable components, however, that often become subject to extreme manufacturing methods that cause heat and stress that adversely affect the integrity of finished parts. Additionally, manufacturing processes for these parts are checked and accepted prior to machining; making it impossible to change or improve aspects of the machining process—insert style, grade, geometry, cutting speed/feed/depth — without costly recertification.
The importance of surface quality
“Surface integrity” describes the quality and condition of a surface region, and encompasses the surface topography and any sub-surface metallurgical alterations.
A combination of stress and elevated temperatures that occur during machining can lead to alterations of crystalline microstructure, cause micro-hardness changes, surface cracking, craters, folds, inclusions, plastic deformation and residual stresses in any finished part.
The extent of such defects depends upon the properties of any work-piece and its interaction with the mechanical and thermal energy during machining.
Machine and tooling leaders are constantly developing technological innovations to eliminate multiple challenges inherent in machining hard materials. When optimizing their processes, however, manufacturers and their customers must know and understand the effects of changing operating parameters before they accept new machining strategies.
Profit takes productivity, quality
When optimizing a machining process for a critical component, keep productivity and quality in mind. Any new method should offer a reliable process with the lowest total cost, while still producing parts with optimal surface quality for high performance and longevity. In titanium, for example, the minimum requirements are finished parts with a deformation depth no greater than 10 micrometers, with compressive residual stresses returning to normal within 200 micrometers of the surface.
With titanium and other difficult metals, tooling plays a large part in the process. First, because of the materials involved, these machining applications call for larger-volume insert use. The cost of these inserts — and the resulting time operators must spend indexing them — can be a huge drain on profit. To combat this, manufacturers can use uncoated inserts with a ground sharp edge, which will maximize tool life. Previous tests in titanium have shown that coated tools offer no consistent surface-integrity advantage over uncoated tools, since coated tools cause a high level of chemical reaction and temperature extremes at the cutting edge.
An insert-holding system can also play a part in reducing setup and tool-change times, which occur more frequently when machining difficult materials. Quick-change systems can allow manufacturers to change inserts in a matter of seconds, rather than minutes. Finally, using a maximum cutting speed of 380 surface-feet per minute will slow down the tool-wear process.
Another productivity tip: Always use a round insert or the largest radius possible when machining critical parts, to allow an increased feed rate. This helps to achieve higher metal-removal rates and increase efficiency. Also, productive coolant systems can help with chip-breaking performance.
Partner up
Industries that require components with high surface quality can be difficult to navigate. It’s important to choose a partner that understands the nuances of challenging machining applications. Tooling partners should provide total solutions that encompass spindle interface, tool-holder selection, programming methods, insert grade and geometry. Be sure to choose a partner who has the training programs and resources to provide ongoing support if issues or questions arise.
While critical-component machining presents challenges, the growth opportunities are extraordinary. With a partner that understands the balance of productivity and part quality — plus the right knowledge and tools — the sky is the limit.
