How to keep holemaking from becoming a horror show

Coolant starvation and chip packing. Drill walk, binding, and breakage. These are just a few of the problems encountered while performing the most common of all metalworking operations: drilling. When using the wrong tool or applying incorrect cutting parameters, the result is poor productivity, scrapped parts, and sometimes even a damaged machine tool.  

It doesn’t have to be this way. By following a few holemaking best practices and selecting the right drill, this routine machining task becomes no more challenging than any other operation. It might not be as fun as a trip to the lake or streaming your favorite TV show, but as you’ll see, drilling is nothing to be frightened of. 

Modular, solid carbide, indexable tools provide greater hole quality. IMAGE: SANDVIK COROMANT

The Right Tool

Randy McEachern certainly isn’t afraid. A product and application specialist for Sandvik Coromant Canada Inc., he has plenty of recommendations on making holes that are smooth, round, and straight, and top on the list is guidance that applies to every machining operation imaginable: use the shortest tool possible. 

“Oftentimes, people will use whatever they have available,” he says. “Let’s say a shop needs to drill a one-inch diameter hole [25 mm] that’s five inches deep [127 mm], so they buy a 5xD drill. One year down the road, another job comes along with the same size hole but is only two inches deep [51 mm]. They’re going to take that 5xD drill and get the job done, right?”

Of course they will, and while this might be okay for very low-volume work and less demanding materials, it’s generally a good idea to invest in the right tool for the job. “For the best performance, you want to get a 2xD drill instead of a 5xD,” McEachern adds. “That will reduce the chance of vibration, drill walk, and a host of other problems.”

This “best tool for the job” maxim also applies to the drill geometry, the grade and coating, and the style of drill. Like many cutting tool manufacturers, Sandvik Coromant no longer supplies high-speed steel (HSS) drill bits, preferring instead to offer modular, solid carbide, and indexable tools for their greater productivity and hole quality. 

Which of these to use, however, depends on hole size and available budget. Choosing the right drill for your needs will have an impact on cost per component. With solid tools seeing the greatest use in holes ranging from 0.0025 – 20 mm (0.001 – 0.787 in.), followed by indexables for larger diameters from 12 – 63 mm (0.47 – 2.48 in.) and above, with modular drills straddling (and often overlapping) the two types. 

As noted, grades and coating selection is another important talking point, a discussion that McEachern encourages shops to have with him or another cutting tool authority. There are far too many choices to delve into here, although he suggests that a high-quality CVD-coated carbide is a good place to start for general-purpose work. 

He also strongly encourages customers to perform test cuts to optimize chip control and cutting data before going into production. “It’s critical as well to keep the insert pockets and seating surfaces clean, and use the highest coolant pressure available,” McEachern says. “And once in production, machinists should routinely inspect for the five main wear mechanisms that most affect tool life and tooling cost—plastic deformation, flank wear, built-up edge, crater wear, and chipping. That’s true for any cutting tool, drills included.” 

Shops are looking for eight hours straight without a tool change, swapping out the drill head before the next shift. IMAGE: KENNAMETAL

Chasing the Goal

Scott Etling of Kennametal offers similar advice. The vice president of global product management might recommend a different brand than McEachern, and also suggests that PVD-coated grades are typically the first choice for all-around versatility and wear resistance in holemaking. “PVD coatings are good for most stainless steels, superalloys, and aluminum,” he says. “And yet, in other metalworking applications, many experienced machining people will tell you they prefer CVD-coated carbide for its greater resistance to cratering and superior thermal properties. As with most things in machining, though, which grade and geometry to go with depends on a number of factors. These include the workpiece material, machine power and rigidity, desired hole quality, available budget, and the number of parts you need to make.” 

The target might also be one of hours rather than part quantity, especially as an increasing number of shops turn to automation as the skilled labour shortage solution. “More and more, people are telling me they just want to get to eight hours without a tool change,” says Etling. “That lets them swap out the drill head or the inserts at the end of the shift in preparation for the next shift, which is increasingly unattended or at least lightly attended. At the same time, some shops are all about the shortest cycle time possible, while still others want one grade and geometry that works for everything, regardless of performance. It’s a mixed bag.” 

Somewhat surprisingly, one of Etling’s strongest recommendations has nothing to do with drill bits, milling cutters, taps, or turning tools. “Read The Goal by Eliyahu Goldratt,” he says. “For a manufacturing person, it’s one of the greatest books ever written.”

What does the Theory of Constraints have to do with holemaking? Nothing and everything. Goldratt’s well-known and easy-to-read novel won’t offer guidance on cutting tool selection or machining parameters—that’s the job of Etling, McEachern, and other industry experts. What it will do is make the reader look at the big picture. They’ll come away knowing that shaving 20 seconds off a part’s cycle time might just be a waste of time, and that their continuous improvement efforts should be part of a broader, company-wide plan.

That’s not to say that optimizing a machining process or investing in the highest-quality cutting tools and tooling available has no merit—just the opposite. But here again, it should be done as part of a comprehensive strategy that goes well beyond job-specific wins. Says Etling:  “There’s no sense increasing throughput on one machine if the parts are just going to pile up at the next one. All you’ve done in this case is increased your work-in-process and moved the bottleneck.” SMT

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