A surefire way to overcome microdefects

Tests with improved-design carbide drills open up new possibilities for hole quality.
“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 leading global manufacturer in aerospace sought to eliminate an entire second stage from its drilling processes while also improving the hole quality in its aerospace components, it turned to the global leader in metal cutting. Here, James Thorpe, global product manager at Sandvik Coromant, explains how a drill’s design is key to producing better quality holes.

Hole making is the most common of all machining processes, but it is also the one most often taken for granted. Many machine shops have seen little reason to change or upgrade their existing holemaking set-up and have been using the same tools and cutting parameters for years. However, starting with the unpredictable effects of the COVID-19 pandemic, this has all been set to change. Manufacturers have encountered unpredictable and enduring shifts in customer markets and now see a need to adopt their production.
Most manufacturers are exploring new vendor bases and products, for example. Thus, machine shops that once specialized in a certain area of production 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 very much 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 standards of manufacturing and part quality. 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.
The term was coined by a leading global manufacturer in aerospace—also a customer of Sandvik Coromant. It refers to a thin, ultra-fine grain structure that is 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 disks, 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.
It’s important to note that the tests were not only motivated by quality management. At senior management level, the customer wanted to reduce its overall operational time and increase profits, and do so by eliminating an entire secondary machining process.

Hole surface integrity is a real concern for aerospace manufacturers or those in general engineering diversifying into aerospace. Here, boss drilling of an HRSA casing with CoroDrill 860 with -SM geometry

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Second act

The secondary process happens after a hole has been created with the carbide drill, and can involve reaming, plunge milling or end milling to finish the component. The secondary stage occurs mainly to meet surface integrity demands and reduce issues such as 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 Sandvik Coromant’s customer was interested in doing away with the process altogether. A supplier with a product that produces a conforming hole to size, without any secondary processes, is in a strong business position to significantly reduce cost per part.
The investigation into causes and possible ways to prevent the white layer involved four tests on 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 customer.
The tests assessed drilling with two solid carbide drills from Sandvik Coromant: the CoroDrill® R840 and the CoroDrill® R846. Each was run with 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. The cutting force and torque data were measured throughout the tests, as was the white layer thickness.
Since these tests, the CoroDrill R840 has been superseded by the CoroDrill® 860 with -GM geometry and the CoroDrill R846 by the CoroDrill® 860 with -SM geometry. Each of these next-generation tools is designed to further enhance tool life without compromising on hole quality.
The results provided valuable insights into what causes white layer thickness. Notably, the CoroDrill R846 generated less white layer due to the preparation of its curved and radial cutting edges. Meanwhile, the straight cutting edges and chamfer on the cutting edge of CoroDrill R840 result in an increase in the 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.
Not only did the global aerospace manufacturer’s tests with Sandvik Coromant reveal a thing or two about the white layer, but the company has also been able to eliminate some secondary processes, like reaming and plunge milling, which resulted in time and cost benefits. In addition, the results have also validated the design of Sandvik Coromant’s CoroDrill 860 range of carbide drills.

Better by design

The range includes the aforementioned CoroDrill 860 with -GM geometry, designed as a good all-rounder for drilling challenging ISO P, M, K and H materials across all industry sectors. The CoroDrill 860 with -SM geometry is optimized for machining ISO S grades such as heat-resistant super alloys (HRSAs), titanium and Inconel. The latter has proven especially popular in the aerospace sector.
With the CoroDrill 860 with -GM geometry and CoroDrill 860 with -SM geometry, Sandvik Coromant’s engineers applied the principle that longer tool life and better hole quality depends on the design of the drill. The CoroDrill 860 with -GM geometry has an innovative polished flute design that improves the evacuation of chips, offering high core strength and reduced cutting forces while drilling.
The CoroDrill 860 with -SM geometry, 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 higher hole quality.

With the CoroDrill 860 with -SM geometry, longer tool life and better hole quality depend on the design of the drill.

The CoroDrill 860 has already been proven in pre-market tests in a range of sectors. A mechanical engineering company in France put the CoroDrill 860 with -GM geometry to work on AISI 4140 structural steel. It was able to produce high-quality holes with both concave and convex entries of the drill, with good straightness and tolerance. The company has since formed a new business relationship with Sandvik Coromant.
Another Sandvik Coromant customer, an Italian general engineering manufacturer, achieved a productivity increase of more than 45% using the CoroDrill 860 with -GM geometry when machining the strong steel alloy 34CrNiMo6, when compared to a competitor drill. It also doubled the tool life. Elsewhere, the CoroDrill 860 with -SM geometry has yielded impressive results when machining Inconel 718. In particular, during testing in Katowice, Poland, the CoroDrill 860 with -SM geometry increased the tool life by 180% compared to the CoroDrill R840.
Whatever the sector—aerospace, general engineering or other areas—high tool performance can be achieved if the drill is designed properly for its application. Meanwhile, additional online tools like Sandvik Coromant’s CoroPlus® Tool Guide can provide further support. By accessing the tool via a web browser and entering the desired workpiece material, hole diameter and depth, users can find the best solid round tool and cutting data for their requirements.
While “measure twice and cut once” drilling hasn’t been achieved yet, the performance of the CoroDrill 860 with -GM geometry and the CoroDrill 860 with -SM geometry is helping manufacturers rethink how they make holes.