Holemaking with high-speed steel drills

The HX Jobber drill series combines the strength of HSS with a 135-degree split point to allow lower thrust requirements, improved cutting action, and reduced walking. Photo courtesy of Dormer Pramet.

Carbide drills have substantially taken over high-speed steel (HSS) drills in the industry. Before 2000 the market split between carbide and HSS drills was at about 50-50. The market share for HSS slowly shrank and today it sits roughly at 10 to 15 per cent of all drill purchases. However, millions of holes still are being made with HSS drills.

CHARACTERISTICS

"HSS drills have higher toughness and lower hardness when compared to their carbide counterparts," said Adam Dimitroff, application engineer, OSG Canada.

Under certain conditions, particularly when the operator with a hand drill begins to suffer fatigue, a carbide option has the potential to break, whereas the HSS option will stand up better to that flex.

"The ability to flex has becomes a big advantage in unstable conditions—older machines with poor spindle runouts, loose slideways or fixtures, and in many other situations," said Sarang Garud, product manager, marketing, Walter USA LLC.

When HSS drills were first entering the market, they often came with a chisel point. The centre of the drill didn’t turn very fast but pushed and extruded the material until the lips could come and cut it.

"Today’s HSS drills have been configured to include split points that are able to grab the material and enter quickly, making accurate holemaking easier," said Dan Cormier, regional sales manager, Dormer Pramet.

Typically, HSS drills have a 118-degree angle on the drill point, although in recent years many manufacturers have introduced drills with 135-degree and 140-degree angles. A 135-degree drill is flatter than a 118-degree option, allowing the cutting lips to engage with the material sooner and begin full metal cutting action quickly. However, the 118-degree option remains the standard configuration.

These characteristics help improve tool life as HSS drills tend to spend less time in the cut because of their material softness.

APPLICATIONS

HSS drills commonly are found in machine shops with low-volume runs in which some holes are being drilled but not too many per shift. They are also suited for shops where the equipment’s cutting speeds are limited, applications with heavy interruptions, and weak setups. In these situations, HSS drills prove to be an economical option.

"These drills are suitable for non-ferrous materials because they typically have a sharp edge," said Dimitroff, "Whereas carbide drills typically have a honed edge to combat the brittleness of carbide material. Non-ferrous metals are also very soft and do not cause tools to wear as much as steel alloys. They are also suitable for interrupted cuts due to their higher toughness."

The Walter Titex A1249XPL HSS drill’s wider flute design allows for better chip evacuation and helps with flood (external) coolant as well as pecking. Photo courtesy of Walter USA.

Our experts agreed that when a machine does not have internal coolant capabilities, HSS drills are the first choice for drilling stainless steel materials.

According to Garud, if the machine doesn’t have a coolant-through option, these drills can be used with the pecking cycle. The overall process is slow, but this allows the machine to be used. He added that some shops also opt for HSS drills when producing holes in expensive parts.

"If the HSS drill breaks inside the part, it can be drilled out," said Garud. "If a carbide drill breaks inside an expensive part, it can be very hard to remove. Also, smaller shops have always liked HSS for cost if not performance. Particularly with small and medium series production, with unstable machining conditions and always when toughness is required, users continue to rely on drilling tools in high- speed steel."

Cormier said that HSS drills are also used in hand drilling applications, which are numerous. Millions upon millions of holes are drilled by hand. He gives the examples of aircraft skins and maintenance, repair, and overhaul (MRO). Using a HSS drill with split points enables quick entry into the material with no walking. If the drill wants to walk or move to the side, the hole won’t start in the right position, which could be extremely detrimental in precision positions on an aircraft. When a hole is being drilled to hold the skin on the wing or fuselage, you certainly want it in its exact location.

The experts said that HSS drills are commonly used in the aerospace industry where deep reach but shallow hole depth is needed. Cormier added that drilling is a multimillion-dollar business in Canada’s aerospace industry with players like Bombardier, MHI, IMP, KF Aerospace, Asco, Avcorp, and more producing millions of holes.

"In aerospace manufacturing, it’s common to see longer-length drills," said Cormier. "Whether it’s a 6-in.- or 12-in.-long drill, the fluting part is only at the front because they’re only drilling through a little material, but have to reach into places to drill. We also see drills with adaptive shanks, which allow the drill to reach around the corner to drill the hole, like in the side beam of a fuselage. This style of drill drills at a 90-degree angle; the operator is holding it straight but it’s drilling on the side with a threaded shank that’s on the HSS drill, which actually makes the hole."

It really depends on a shop’s holemaking frequency and overall manufacturing strategy. A small or medium-size job shop that produces only one part that has eight holes in it may not need to spend a significant amount of money on a drill. It can choose the HSS option, and although it may produce holes slower, the overall cost per hole is also reduced.

"A lot of job shops, even though they might have a machine that runs coolant-through, will tend to use HSS drills because of the cost," said Cormier. "They’re not making a million of anything so they don’t need to worry about the run time in the same way as someone like Linamar, where they are making millions of holes. They would opt for a solid-carbide drill because, for them, savings of seconds per hole can equate to weeks’ worth of production savings on a single shift."

IMPROVING PRODUCTIVITY

One of the major improvements in HSS drills has come from an evolving design. According to Garud, a wider flute design allows for better chip evacuation and helps with flood (external) coolant as well as pecking.

Cormier added that another improvement was reinforcing the center web of a HSS drill, particularly when it came to hand drilling operations.

Dormer’s A012 TiN tip drill resists flank and crater wear and has low friction, promoting higher cutting speeds. Photo courtesy of Dormer Pramet.

"For the longest time that web was consistently straight, meaning the gullets were consistent the whole way back on the drill," he explained. "We made the gullets less deep as you work your way back to the shank and then that centre web tapered bigger, making the drill more robust to withstand the rigors of all the hand drilling."

The experts agreed that adding enrichments and coatings is another way to improve the drilling process. For example, adding an oxide treatment will enable the HSS to withstand heat better, allowing the drill to be used at faster RPMs.

"Cobalt enrichment improves the drill productivity," said Garud. "Typically, 5 to 8 per cent enrichment adds strength and wear resistance. Vanadium adds heat resistance, providing some ability to run at slightly higher speeds."

These different alloying elements can be made through powdered metallurgy. Beyond this, the addition of tip coatings such as TiAlN, TiCN, and CrN can improve heat resistance and the ability to run at faster speeds.

According to Dimitroff, these methods add cost but are aimed at prolonging tool life.

"Coatings generally increase a tool’s hardness and its lubricity," he said. "Hardness allows for the tool to resist wear. Lubricity allows for the tool to move through material while avoiding built-up edge, which is material galling or sticking to the drill. It also lowers temperatures the tool is exposed to while cutting, further increasing tool life."

Walter USA, www.walter-tools.com