Carbide drill opens new possibilities for hole quality in aerospace

Aerospace manufacturing 09062021
Aerospace manufacturing 09062021

Hole quality is of prime importance when machining aerospace components. But what happens when a manufacturer seeks to eliminate an entire second stage from its drilling processes, without sacrificing quality? In this case, it turned to a global leader in metalcutting, Sandvik Coromant. Here, James Thorpe, global product manager explains why an innovative drill design offered the necessary solution.

Is it time for manufacturers to rethink how they go about making holes? Despite being the most common of all machining processes, holemaking is also the one most often taken for granted. Most machine shops see little reason to change or upgrade their existing holemaking setup. Once certain tools or cutting parameters are allocated to manufacturing a certain feature or component, it can stay that way for years.

But this could all be set to change in the wake of COVID-19. According to McKinsey & Company’s COVID-19 briefing note (https://mck.co/3f5DNyo), “resilience will be needed if manufacturers are to navigate an economically and socially viable path.” The report refers to the ‘next normal’, where manufacturers face unpredictable and lasting shifts in the markets.

This is why a lot of manufacturers are already diversifying their lines to explore new products and vendor bases. Machine shops that once specialised in a specific area have opened their CNC lathes and mills to a wider variety of tough and challenging materials. For aerospace manufacturers, these changes must be made without sacrificing process security, quality of components or cost-per-part.

Diversify to survive


For aerospace manufacturers, or general engineering companies that want to diversify into aerospace, hole surface integrity remains of paramount importance. Better hole quality prevents component failure and depends very much on the manufacturing processes that are used for machining or finishing the holes.

In particular, a drill’s design is integral to producing better quality holes. That’s why tooling solutions, including cutting edge-geometries in drills, are continually evolving to meet the highest standards of manufacturing and part quality. The use of coolant, used to reduce heat build-up in the tool, is also improving. However, manufacturers should seek to control the so-called ‘white layer’ effect on workpiece materials.

The term white layer refers to a thin, ultra-fine grain structure that is observed after component drilling and is caused by the heat of the drill. The term was invented by a leading global aerospace manufacturer, which also happens to be a Sandvik Coromant customer, after the white layer was found to be problematic for two reasons. First, it can change the surface properties of a material. Second, it was deemed unacceptable within the customer’s quality management processes.

Indeed, the manufacturer applies a strict hole-finishing process to drilled holes in aerospace components – from turbine discs and compressors to drums and shafts. After identifying the white layer, the manufacturer sought to investigate why the layer forms and how best to control it. For this, it chose to partner with Sandvik Coromant.

The white layer


The investigation into causes, and possible preventions, of 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 customer.

The tests assessed drilling with two solid carbide drills by Sandvik Coromant, the CoroDrill R840 and CoroDrill R846. Each was run at two different sets of cutting parameters where cutting thrust and torque were measured throughout the tests, as was the white layer thickness.

Since these tests, 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 next generation tools is designed to further enhance the tool life, without compromising hole quality.

The results gave some valuable insights into what causes white layer thickness. Particularly of note was that the R846 generated less 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. So, 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 – the company has also been able to eliminate some secondary processes, like reaming and plunge milling, which has resulted in time and cost benefits. What’s more, the results have also validated the design of Sandvik Coromant’s CoroDrill 860 range of carbide drills.

New design, better quality


The range includes the aforementioned CoroDrill 860 with -GM geometry (CD860-GM), designed to be a good all-rounder for drilling challenging ISO P, M, K and H materials across all industry sectors. There is also the CoroDrill 860 with -SM geometry (CD860-SM) for machining ISO-S grades like super alloys (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 is down to the design of the drill. The CD860-GM has an innovative 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 optimised and refined point geometry, which further enhances tool life when working with difficult-to-machine HRSA materials. The result is greater hole quality.

The CoroDrill 860 has already been proven in pre-market tests in a range of sectors. A mechanical engineering company in France put the CD860-GM to work on AISI 4140 structural steel. It was able to achieve quality holemaking with both a concave and convex entry 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% by using the CD860-GM when machining the strong, steel alloy 34CrNiMo6, versus use of a competitor’s drill.

It also achieved a 100% greater tool life. Elsewhere, the CD860-SM has yielded impressive results in the machining of Inconel 718. In particular, testing undertaken in Katowice, Poland, was able to achieve a 180% improved tool life with the CD860-SM versus the use of the CoroDrill R840.

Whatever the sector – whether it’s aerospace, general engineering or other areas – high tool performance can be achieved if the drill is designed properly for that purpose. 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.

No second act


Although hole quality is majorly important in aerospace component manufacturing, quality wasn’t the only motivator behind the Sandvik Coromant customer’s tests. A key reason was that the customer wanted to reduce its overall operational time and increase profits. To do so, it sought to eliminate an entire secondary machining process.

The secondary operation happens after a hole has been created with the carbide drill and typically involves reaming or plunge end milling to finish the feature. The main reason for the secondary stage is predominantly to meet surface integrity demands, which includes reducing the white layer.

By reducing the secondary process’ cycle times, tooling inventory can be reduced which results in cost savings for the customer. That’s why Sandvik Coromant’s customer wants to eliminate the process altogether.

At the moment, to our knowledge, there isn’t a drill that can handle the whole process and produce a conforming hole to size, without the need for the secondary operation. A supplier with a product that can would be in a strong business position to significantly reduce the cost per part.

That’s why, having helped our aerospace customer examine the white layer, Sandvik Coromant is now doing more investigations into the effect of cutting data. Specifically, we’re looking at the effects of speed and feed on surface integrity to achieve the parameters necessary to produce a hole that conforms to our customer’s exacting quality standards.

If successful, the possibility is for a drill that can achieve optimum surface integrity, bringing savings and process security for the customer. This could be particularly advantageous if, as hoped, aerospace manufacturing ramps up as the industry strives to recover from COVID-19.

For now, tools like the CD860-GM and CD860-SM can play a vital role in helping manufacturers in aerospace, automotive and other industries achieve a better cost-per-hole and cost-per-component – and even rethink how they make holes altogether.

Sandvik Coromant www.sandvik.coromant.com

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