“Measure twice and cut once” is a common expression in manufacturing, but it’s easier said than done when machining difficult materials. That’s why, when a global aerospace manufacturer sought to eliminate an entire second stage from its drilling processes while also improving hole quality, it turned to metal cutting specialists. Here, James Thorpe, global product manager at Sandvik Coromant, explains how a drill’s design is integral to producing holes with better quality.
Most manufacturers are exploring new vendor bases and products. Thus, machine shops that once specialized in a certain production area are now opening their CNC lathes and mills to a wider variety of tough and challenging materials. At the same time, manufacturers must explore new ways to increase profits and reduce cycle times without sacrificing product quality.
In other words, it’s time for manufacturers to rethink how they go about making holes.
The white stuff
Hole surface integrity is a real concern for aerospace manufacturers or general engineering companies that want to diversify into aerospace. Better hole quality is vital for preventing component failure and is determined by the manufacturing processes used for machining or finishing the holes.
Tooling solutions and cutting-edge geometries in drills are continually evolving to meet the highest manufacturing and part quality standards. Coolant is also being used more effectively for reducing heat buildup in the tool. And tests have found that each of these factors can control the so-called “white layer” effect on workpiece materials.
An aerospace manufacturer coined the term. It refers to a thin, ultra-fine grain structure observed after component drilling, caused by the heat of the drill. Not only can the white layer change the surface properties of the material, but it was also deemed unacceptable in the customer’s quality management processes.
The manufacturer applies a strict hole-finishing process to drilled holes in aerospace components, including turbine discs, compressors, drums, and shafts. That’s why it chose to partner with Sandvik Coromant to investigate why the white layer forms and how to control it.
The secondary process happens after a hole has been created with the carbide drill, and it can involve reaming, plunge, or end milling to finish the component. The secondary stage occurs mainly to meet surface integrity demands and reduce issues like the white layer, rather than for dimensional accuracy, except when machining holes with tight tolerances.
From an overall cost perspective, the secondary process is even more expensive than maintaining low cutting data, which is the other way to preserve surface integrity. That is why the user wanted to investigate doing away with the process altogether. A supplier with a product that produces a conforming hole to size without any secondary processes can greatly reduce the cost per part.
The investigation into causes and possible ways to prevent the white layer involved four tests of drilling the high-strength, nickel-chromium material Inconel 718, a popular aerospace material. It was the first time any such investigation had been carried out by the user.
The tests assessed drilling with two solid carbide drills, the CoroDrill R840 and CoroDrill R846. Each was run at two different sets of cutting parameters, 58 mm/min and 98 mm/min, respectively, and spin speeds of 829 rev/min and 757 rev/min, respectively. Cutting force and torque data were measured throughout the tests, as was the white layer thickness.
R840 has been superseded by the CoroDrill 860 with -GM geometry, and R846 has been superseded by the CoroDrill 860 with -SM geometry. Each of these tools are designed to enhance tool life without compromising hole quality.
The results provided insights into what causes white layer thickness. Particularly of note was that the R846 generated less of a white layer due to the preparation of its curved and radial cutting edges. Meanwhile, the straight cutting edges and chamfer imposed on the cutting edge of the R840 are believed to be linked with the increase in cutting force, torque, and white layer thickness. Therefore, the drill’s design determines whether high hole quality with a reduced white layer can be achieved without sacrificing cutting data. The company has also been able to eliminate some secondary processes, like reaming and plunge milling.
The range includes the CoroDrill 860 with -GM (CD860-GM) geometry, designed to be a good all-rounder for drilling challenging ISO P, M, K, and H materials across all industry sectors. The CoroDrill 860 with -SM geometry (CD860-SM) is designed for machining ISO S grades like superalloys (HRSAs), titanium, and Inconel. The latter drill has proven especially popular in aerospace.
With the CD860-GM and CD860-SM, Sandvik Coromant’s engineers applied the ethos that longer tool life and better hole quality come down to the drill design. The CD860-GM has a polished flute design that improves the evacuation of chips and yields high core strength and reduced cutting forces while drilling.
The CD860-SM, meanwhile, has a new grade and optimized and refined point geometry, which further enhances tool life when working with difficult-to-machine HRSA materials. The result is greater hole quality.
Filed Under: Aerospace + defense