| September 2009 / Volume 61 / Issue 9
Bending the rules
By Alan Richter
|Because high-speed steel inserts can bend without breaking, they are an effective choice for select applications.
To cut a long story short, the advent of carbide as an effective cutting tool material caused HSS to become continuously less popular as a tool material. When appropriate, tool designers developed tool bodies to accept throwaway carbide inserts and enable machining with carbide cutting edges while reducing tool costs. But HSS’ bend-strength toughness compared to carbide’s harder and more brittle characteristics still offers advantages in some applications, and rather than always applying solid-HSS tools, the same bodies for carbide inserts accept HSS ones as well.
Interrupted cutting is one area where HSS offers an advantage, according to Ken King, CEO of THINBIT/Kaiser Tool Co. Inc., Ft. Wayne, Ind. The toolmaker offers standard HSS inserts for grooving, face grooving, threading and parting, as well as specials. Compared to carbide, “high-speed steel will take a lot more abuse as far as flexing and interrupted cuts,” he said. “If you do crash high-speed steel, sometimes you’ll see the inserts are actually bent or flexed. You can use that to your advantage if the application is so bad that you require the tool to bend a little bit.”
Courtesy of THINBIT/Kaiser Tool
Harder tool materials, such as carbide, don’t like to bend. King used the analogy of glass and wood to compare and contrast the material properties of carbide and HSS. “You’re not going to bend a piece of glass; it’s going to break and shatter,” he said. “Whereas wood will shatter eventually, it will bend more than glass.”
Cutters tooled with HSS inserts are also suitable when machining with less-than-optimal rigidity. “High-speed steel inserts are for machining at a lower cutting speed where high toughness is required for application under unfavorable machining conditions,” said Hellmut R. Ross, president of Arno Rouse USA LLC, Harvard, Ill., which supplies HSS inserts among other tools. “I wouldn’t say the machine, but the setup might not be that rigid.”Speed Limit
Once upon a time, HSS tools were applied for “high-speed machining,” but that’s no longer the case. In applications where carbide inserts are able to run at 300 sfm, the recommended cutting speed for HSS inserts is about 60 sfm, according to Mike Warner, president of Arthur R. Warner Co., Latrobe, Pa., which specializes in producing HSS inserts. “High-speed steel should be called slow-speed steel, but it’s amazing some of the speeds you can run,” he said. “We have some high-speed steel in applications running up to 125 sfm with coatings.”
Warner added that when a machine tool, particularly an older one, does not have an adequate spindle speed to achieve the required surface footage to cut efficiently with carbide, HSS is the material of choice. “You have to slow down a little bit but your tool is going to last a lot longer and you’re going to get more parts per tool,” he said, adding that the chip load can be larger when cutting with HSS inserts vs. carbide inserts.
King concurred that HSS inserts cut slower relative to the same workpiece material. For example, HSS inserts can machine Teflon, or polytetrafluoroethylene, at 250 sfm with a 0.003-ipr feed rate compared to 400 sfm with a 0.003 ipr for carbide inserts, according to THINBIT. For one Teflon application, THINBIT engineered a special HSS insert design with a 60° top rake. “That gives a huge upsharp angle to help peel the Teflon away,” King said.
Ross noted that the lower machining parameters for HSS inserts, which he estimated are about half of the recommended parameters for carbide, make them suitable for machining plastics. Other suitable workpiece materials include aluminum, wrought and cast alloys, nickel alloys, copper, brass, bronze “and the outside skin on a forged part, which is really difficult to machine,” he said.
Courtesy of Bill Kennedy Keen Cutting
Plastics are difficult to cut because they are typically gummy and tend to melt, burn and adhere to the cutting tool. HSS’ toughness enables toolmakers to produce inserts with a keen edge that has minimal edge preparation to reduce heat and friction at the tool/workpiece interface. “It’s a razor sharp cutting edge,” Ross said.
Warner explained that the depth of an edge preparation, such as a T- or K-land, on a carbide insert equals the minimum DOC the insert can effectively take. “If it’s an 0.008 " hone on an insert, you can’t take a 0.006 " depth of cut,” he said. “It would generate so much heat that your tool life would deteriorate.”
Although edge preps to minimize chipping aren’t required on HSS inserts, the inserts can have chipbreakers to enhance chip control. “Our standard chipbreaker is a 10°, 0.090 " effective-width chipbreaker, which 90 percent of the time will work sufficiently,” Warner said.
He added that, in addition to controlling chip size and the direction chips travel, the chipbreaker creates a double-positive cutting configuration to reduce heat at the tool/workpiece interface and impart a finer surface finish. As previously noted, tool bodies that accept carbide inserts also hold HSS ones, but not all bodies will work equally well. “A lot of people will buy a high-speed steel insert and put it into a negative carbide holder and it won’t work,” Warner said. “We watch that very closely whenever we’re matching high-speed steel to holders for carbide inserts.”
Recommended starting-point speeds and feeds for grooving with HSS (grade M-2).
Source: THINBIT/Kaiser Tool
Because many carbide inserts have a negative clearance on the insert’s periphery, Warner noted that when end users position the insert 7° forward to provide relief on the insert edge, but without a chipbreaker, it creates a negative cutting configuration on the top. That generates a lot of heat and—like carbide—heat is no friend to HSS.
Sometimes, it’s beneficial to put carbide and HSS inserts into the same tool body, such as a combination cutter for turning a 12 "-dia. part that requires its center to be bored. “On older machines, you can’t change the machine’s rpm during a cut, so the OD of that part has the correct surface feet for carbide but the middle of the part is hardly turning,” Warner said. “They will use high-speed steel in the middle and carbide on the outside.”
When coated, the coating must be deposited at a temperature that’s not too high to temper HSS, which causes it to loose its hardness. Therefore, HSS inserts must be PVD coated, where the deposition process takes place at 350° to 900° F compared to 1,800° to 2,000° F for the CVD process. The same coatings found on carbide inserts, such as TiN, TiCN and AlTiN, are available on HSS inserts. Although coated HSS inserts can cut at higher speeds and feeds than uncoated ones, Warner noted that only a small percentage of the HSS inserts his company sells are coated. “Shops don’t try high-speed steel unless they’re having a particular problem with carbide and usually 10 uncoated inserts gets them out of the problem,” he said. “Eighty percent of my customers buy fewer than 50 inserts.”
Warner added that if a customer needs more HSS inserts beyond the initial order, the subsequent ones are typically coated. For example, one of his customers was having a problem milling plastics with carbide and found that a HSS insert worked well. “In the next go-around, we coated it and it worked extremely well, so now that company regularly buys coated high-speed steel inserts,” he said.Material Matters
Warner pointed out that HSS inserts are effective for cutting gummy and workhardening materials because “HSS actually cuts the material where carbide is meant to fracture it in front of the cut.” He added that, in some applications, HSS is able to impart a finer surface finish than carbide. “High-speed steel cuts it; it doesn’t tear it.”
Courtesy of THINBIT/Kaiser Tool
To enhance HSS’ toughness while increasing its hardness, HSS producer Erasteel Inc., Boonton, N.J., is producing P/M HSS with higher alloy content, according to Oscar Lundvall, the company’s business developer. That includes alloying the material with niobium, such as for Erasteel’s ASP 2055-grade P/M HSS with about 2 percent niobium. “The powder steel is very homogenous in its structure, but with the niobium alloying we achieve even finer dispersion of carbides, which gives us a material that is even tougher than a similar steel without niobium,” he said. “It is easier to grind and produce a surface finish that is much finer than ordinary P/M steel.”
Other alloying elements in HSS include chromium, vanadium, tungsten, molybdenum and cobalt. Unlike cemented tungsten carbide, cobalt is not used as a binder in HSS but rather as an alloying element that’s melted with the base steel and other elements and then atomized into the finished composition. “You’re not mixing powders like with tungsten carbide,” Lundvall said. “It’s a totally different process.”
Lundvall noted that applications exist where HSS, in general, outperforms carbide because HSS’ higher toughness is more forgiving and its sharper cutting edges prove more effective. “One good area is milling high-strength titanium, such as the new titanium 555-3 alloy where we’ve seen some spectacular results with a coated high-cobalt ASP 2060 special endmill compared to carbides,” he said.
HSS also has the advantage of costing less than carbide, but that doesn’t necessarily translate into HSS inserts costing less than comparable carbide ones. Arno Rouse offers a TiN-coated carbide TCFT insert for $10 and the same one made of HSS is $37.35. “You have to get a lot more parts to justify that,” Ross said.
THINBIT, however, charges the same price for its standard HSS and carbide inserts, according to King, even though producing HSS ones requires a longer cycle time. He noted that all THINBIT HSS inserts are made of M-2 conventional HSS.
Courtesy of Erasteel
Arthur R. Warner doesn’t offer carbide inserts, but Warner said its HSS inserts range from $4 to about $15, depending on type and quantity. As a comparison, he noted that a high-quality TNMC32NV carbide threading insert costs about $7. “Ours are $12, but there are 38 operations on that insert to make it and it will last a lot longer than carbide in quite a few applications,” Warner said. He added that at least until 2 years ago, his research indicated that 30 percent of milling in the U.S. is still performed with HSS tools, including inserts and round tools.
Recently, cutting tool sales have been down across the board, but King indicated there’s a fairly healthy demand for HSS inserts. “When we add new products, we consider whether to offer it in high-speed steel or not,” he said. “It’s a material we still consider active and not obsolete or discontinued. We know it solves problems for people.” CTEAbout the Author: Alan Richter is editor of Cutting Tool Engineering, having joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
Making HSS inserts
Courtesy of Bill Kennedy
Arno Rouse USA LLC
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