Creative tool geometries, better edge prep and coatings, and amazing new materials expedite faster, deeper cuts
Industry veterans often say the makers of machine tools, cutting tools, CAD/CAM software, and other components push each other in an endless feedback loop to deliver ever faster cutting speeds in ever harder materials. Lately it’s the cutting tool manufacturers who seem to be leading the charge. Let’s see what they’re up to.
The idea of spacing an end mill’s flutes unevenly (unequal indexing) and/or changing the helix from flute to flute to reduce chatter is old news. It has also long been possible to vary the helix and the rake within a flute (i.e. from front to back). What’s new, according to Oliver Rapp, R&D manager for round tools at Ceratizit (Balzheim, Germany and Warren, MI), is the degree to which cutting tool manufacturers are combining these features in a single tool and the degree to which the market is willing to pay for this technology.
At the same time, newer milling techniques like trochoidal milling and high dynamic cutting (HDC) are driving the demand for longer flute lengths. Rapp explained that varying the helix from 20° to 40° (for example) over the length of the flute can help with chip evacuation when the axial depth of cut (DOC) is two or more times the diameter of the tool (2 × D or more). “But it’s tricky. It’s not always a help and can be a problem. If the helix angle is too low, the tool gets noisy and you don’t have enough contact area with the workpiece,” he said. Ceratizit offers this technology in tools that cut at up to 5 x D, greatly reducing machining time. But Rapp added that doing so also requires the assistance of CAM software. “You can’t program these cuts manually,” he said.
Horn USA Inc. (Franklin, TN) also plays this game of deep cuts. Steve Boss, product manager, solid round tools, said he’s seen smaller vertical machines literally fill with chips because they’re removing 20 in.3 (328 cm3) of material a minute taking a big axial DOC with tools that lack a critical feature: chipbreakers. Such chipbreakers (Horn prefers to call them “chip splitters”) are notches staggered along the tool’s cutting edges. “The problem with a chipbreaker,” said Boss, “is you can add too many to get a smaller chip. You should add only as many as necessary to make a manageable chip with the machine tool you’re dealing with.” That’s because done properly, the shape of each chipbreaker must be chamfered to eliminate any sharp corners, and tool manufacturers can’t grind these features in one simple pass.
“You can’t just come in with a notching wheel and notch the flute, because it leaves that corner where you’re restarting the chip again exposed,” explained Boss. In effect, “every chipbreaker acts just like the end of the tool and you have to take the same care you would in creating the endface geometry. Horn has done a really good job of designing the shape of that notch.” Boss added that the additional benefit to taking a large axial DOC with a small radial cut is the ability to use smaller diameter tools with “smaller step-overs and greater success at a lower cost.”
The latest advance from OSG USA Inc. (Bensenville, OH) in this area is the A-Mill, which will be introduced in the US around IMTS. (It is now available in Japan as the AE-VMS.) The A-Mill combines unequal indexing and a different helix on each flute, but “the flute shape is a little deeper than normal,” explained Tim Holmer, aerospace product manager. “It’s also produced with a special flute grind that varies the rake along the length of cut. The design of the cutting edge as it enters the workpiece creates a better, more curved chip shape, helping with chip evacuation. You’ll often see chipping, wear, or built-up edge on the cutting edge in more typical designs. The improvement in chip shape and evacuation in this design decreases these factors to improve tool life.” A-Mill will have a maximum length of cut of 2½ × D or so.
Another key trend in modern end mill design is the multiplicity of cutting edges, even nine or more flutes. Boss calls it the “shiny new thing” that users are drawn to because it offers the possibility of machining faster. But he cautioned that people are missing other variables that need to be taken into consideration. “For example, the speed of the machine tool control must be able to handle increased feed rates to apply these tools. If the control isn’t fast enough it will decrease the feed rate on its own, and the chip thickness becomes almost nothing. You rub, build up heat, and burn the tool,” he said.
Holmer echoed this experience and added that, in general, cutting tool manufacturers and users need to adapt together. “Customers sometimes buy a tool with the new geometries and use it the way they’ve always done. They get some improvement, but not what they expected or could have gotten. For example, they’d probably see better removal rates and longer tool life if they used trochoidal machining.”
CAM for Mind-Bending Designs
Circle segment tools from Emuge Corp. (West Boylston, MA) are perhaps the most remarkable new end mill concept. The name refers to the fact that part of the tool’s side or nose profile corresponds to a segment of a circle with a much larger diameter than the cutting tool. One such tool bulges at the side like a barrel. Another combines a barrel-like bulge with a sharp taper to a ballnose. It’s called an “oval form.” The “taper” form follows a large radius all the way to the ballnose. The “lens” form has a large radius across the nose. In each case, the center of the circle that defines the radius is well outside the center of the tool.
What is the point of such strange designs? As Dan Doiron, Emuge milling product manager, explained, “You typically would machine a deep-cavity mold wall surface with the tip of a ballnose end mill, which has a small contact point, and you’d have to take small step-downs to get a good surface finish. A circle segment end mill with a 1500-mm radius on the side profile effectively presents a 3000-mm diameter ball. You can take larger step-downs for greatly reduced cycle times with equal or better surface finishes.”
It’s like being able to magically present an extraordinarily large ballnose tool in a small cavity. But pulling off this trick also requires the ability to engage the tool at every possible angle (five-axis motion) and sophisticated CAM software that can calculate the correct tangent point on the tool throughout the cut. Only two packages suffice, Open Mind’s hyperMILL and the latest versions of CNC Software’s Mastercam. It seems a small price to pay, since there’s a member of the “circle segment” family that can deliver more tangential machining than you could otherwise achieve in a wide variety of situations. “The barrel shape is mostly for undercut areas, though it can be used in other applications,” explained Doiron. “The oval form has a longer flute length and a longer contact length than the barrel form, depending on the depth of cut. It has more flexibility in getting into tight corners as it can be tilted more to avoid collisions between the part and the toolholder. The taper form has a programmed angle at which you must approach the part to make contact with the cutting edge. The lens form is for shallow areas, like the bottom of a tire tread mold. Instead of having to pinpoint in a groove, going back and forth, the lens form covers more area, reducing cycle times.”
Doiron added that Emuge has seen cases in which switching to circle segment endmills reduced cycle times by up to 80% and it continues to find new applications for the tools. “But it takes some creativity. There are areas you wouldn’t necessarily think you could apply these tools, like the parting line on a mold if it had some kind of curvature. You would typically machine this with a ball mill in a zig-zag pattern. Now, if you have a five-axis machine you could tip that whole part of its side and use the side of a circle segment mill to machine a much bigger area at a significantly reduced cycle time.” Why not? The biggest hindrance to their adoption appears to be reluctance to try a new, advanced CAM package.
Edge Prep is Critical
One of the ironies of cutting tool manufacturing is the need to add a hone to the cutting edge—dulling your sharp tool ever so slightly so it lasts longer. “Many tools are also edge prepped before they are coated to ensure good adhesion of the coating,” explained Ceratizit’s Rapp. “But if you can’t control your edge prep you run the risk that the coating increases the radius on the edge too much. It requires the right know-how to succeed or fail with such a tool.”
Rapp added that some of the biggest improvements Ceratizit has made in this area are due to new measurement technology that enables tool manufacturers to see the hone they’ve created, even on highly reflective tools, and to do so quickly. “Being able to measure the hone greatly improves our ability to control the process of creating the correct hone for consistent tool performance.”
So far we’ve concentrated on solid round tools. But lest you think indexable inserts are always simpler and immune from these concerns, Jan Andersson, global manager for TechTeam and marketing for Greenleaf Corp. (Saegertown, PA) pointed out that Greenleaf offers more than 30 different micro-geometries on the edge of its ceramic inserts. “For example, the RNMG 0.5″ round insert can be ground with a small, rounded hone, a land with sharp edges, or an edge that combines the two. It’s important to suit the cutting tool material and the micro-geometry to the application. And when you change ceramic grades, they behave differently and therefore the edge conditions should be different, even for the same application.”
Drill Baby Drill
The news in drilling “larger chipping materials like steel” is the wide availability of three-flute tools “with features that still enable adequate chip evacuation,” explained Rapp. “Everyone” now offers such tools in standard sizes (4–16 mm in diameter and lengths of 5 × D to 12 × D). “Three cutting edges versus two is obviously one-third more, which enables higher feed rates. And you gain much more due to higher feed rates than higher speeds.”
Rapp and others added that drilling accuracy has been improved by changing the number, design, and position of guide lands. For example OSG’s new AD and ADO series drills (no coolant and coolant-through, respectively) feature an unusual double margin at 8 × D and above. The first margin is at the front of the cutting edge, but the second is in the middle. “This yields quicker engagement and better stability in deep hole drilling, as opposed to normal drills, which have margins on the front of the cutting edge and the back, or heel, of the drill,” said Holmer. OSG’s drills also feature a “very special cutting edge with a wavy point geometry” that Holmer said delivers superior results. The drills are offered in lengths up to 30 x D as a standard and even longer if needed.
Emuge’s product manager for drills, Marlon Blandon, pointed out that they feature double-margins on all their drills, regardless of length (its line goes up to 8 × D). “Our drill line complements our threading tools, taps and threadmills,” he explained. Another key feature is the use of sub-micro grain structure carbide. Blandon says such carbide provides a “higher transverse structure strength, allowing for more grinding since more grains are supporting the cutting edge. The smaller the grain, the sharper the edge you can grind.” He said sub-micro carbide, web design, and coating on Emuge’s drills have combined to offer high performance drilling, especially in tough materials like 718 Inconel, titanium, and 316 stainless.
Everyone agreed that through-coolant is critical for deep drilling. Or, as Boss put it: “The only thing that slows a drill from making its hole is chip evacuation. If it wasn’t for the need to clear chips up the flutes, the feed rates could be much faster. We’re optimizing the location of the holes in order to provide a constant, uninterrupted flow of coolant while drilling.”
Boss and others also said they’re starting to offer drills with material specific geometries. And as with endmills, edge prep is critical. For example, Holmer noted that while their Mega Muscle three flute drill can “crush” through cast iron at 678 IPM (1722 cm\min), such a tool would gum up at a high feed rate in stainless or titanium. “For a given material and a given diameter, you’re much more likely to be able to successfully evacuate the chips with a two flute design than a three flute.”
Micro Machining, Super Coatings
The exciting news in micro drills is the ability to create very small coolant channels. For example, Ceratizit has coolant channels as small as 0.05 mm in diameter so they can deliver internal coolant to drills under 1 mm in diameter. Emuge also offers micro drills down to 0.75 mm with through-tool coolant. Horn offers micro end mills down to 0.1 mm in diameter. OSG offers tools under 1 mm in diameter for machining medical components in cobalt chrome, titanium and some stainless steels.
According to Rapp of Ceratizit, one of the biggest advances that make such tiny tools possible are new coating techniques: HIPIMS (high-power magnetron impulse sputtering) and S3p (scalable pulsed power plasma). “These give us fewer droplets and smoother coatings, which is more or less a requirement for micro tools. It’s quite difficult to polish a 0.5-mm diameter tool after coating. So if your coating deposits fewer droplets, eliminating the need for polishing, it’s a big advantage.” Rapp added that for diamond coating it’s also important to leach cobalt from the tool in a controlled fashion to enable good adhesion. “You have to remove the cobalt to create an interface between solid carbide and the diamond. But done incorrectly, you’ll have a bad tool.”
New grades of solid carbide with higher toughness (flexural strength) have also contributed to the viability of micro tools. As Rapp put it, “Even flood coolant can break a micro tool.” This leads to the subject of new cutting tool materials.
Improved Cutting Tool Materials
All the players make and/or source a variety of carbide grades, depending on the application. Ceratizit offers its CTS20D sub-micron grade carbide for milling and drilling a wide range of materials. It boasts a hardness of 1600 HV30 (91.9 HRA). If that’s not enough, it has ultra-fine grades going up to 2200 HV30 (95.2 HRA). Ceratizit also offers solid ceramic tools for the energy and aerospace fields (e.g., blisks and turbine blades in titanium and Inconel). “These materials are difficult to cut and ceramic can really help, though as roughing tools. You would run these tools until you can’t even tell they’re a tool, and then use a finishing tool,” said Rapp. OSG is studying solid round ceramic tooling for machining ceramics and Holmer said the company will probably have a series of tools in the next three years.
The big news in polycrystalline diamond (PCD) tooling is the use of lasers to create the tools. Older methods like grinding and erosion force the tool manufacturer to use smaller grain sizes because the quality of the edge is limited to the size of the PCD grains, even though the performance of the tool would be better with a bigger grain size. Laser can cut the diamond grain and the binder very precisely, creating a sharper edge even with bigger PCD grain sizes. Laser can also create tools made of monocrystalline diamond (MCD), which are even harder than PCD and can’t be eroded. Horn makes MCD indexable tools for high-polish machining of non-ferrous materials. The finish is so fine that it can’t be measured with a profilometer, which would scratch it.
Greenleaf’s new XSYTIN-1 ceramic is indeed an exciting development. Decades ago, Greenleaf blasted old notions about the applicability of ceramics with its silicon carbide whisker reinforced WG300 inserts. More recently, it challenged expectations by introducing a coated whisker reinforced ceramic that effectively cuts stainless steel (the WG600). But XSYTIN-1 is an entirely new type of phase-toughened ceramic. Greenleaf uses a secret process to achieve “natural grain growth in terms of whiskers.” The result, said Andersson, is a ceramic with a transfer rupture strength (flexural strength) of 1200 MPa. “That’s higher than any other ceramic, including whisker reinforced ceramics.” (WG300’s flexural strength is about 600 MPa.)
“Higher flexural strength is the critical factor in being able to run ceramics at slower speeds,” said Andersson. “If you can’t run fast enough to get plasticization of the workpiece material, and the tool doesn’t have the inherent strength, you start getting ruptures of the cutting tool material, which primarily shows up in flank slicing and top slicing, but also edge chipping. But XSYTIN-1 is twice as strong as any other ceramic we’ve made and doesn’t necessarily require plasticization. We can get a partial, or very low, degree of plasticization and run just fine without chipping the edge or flank slicing or top slicing the insert.”
As an example, Andersson pointed to 48 Rockwell Inconel 718. You’d run whisker reinforced ceramics at about 800 SFM (244 m/min), down to perhaps 600 SFM (183 m/min) in this material. “If the size or imbalance of the part didn’t allow you to do that, you’d be forced to switch to carbide and go all the way down to 150 to 180 SFM [45.7–54.9 m/min] in the roughing operation, maybe 200 in finishing. With XSYTIN-1 you can run predictably below 400 SFM [122 m/min].” Conversely, even if carbide had the flexural strength to run at 400 SFM, the heat would create of plasticization of the cutting tool and it would lose its hot hardness very quickly.
“XSYTIN is so unique we keep finding different applications where you’d never think about ceramics,” said Andersson. “We’re using XSYTIN today milling 20 Rockwell steel in a very predictable and productive fashion, running speeds two to four times higher than carbide with similar feed rates and comparable or better tool life.”
One big forging house used a 24″ (610 mm] cutter with XSYTIN-1 inserts to mill P20 steel (30–32 HRc) at 600 SFM, with a feed of 0.013″/revolution (0.33 mm/rev) and a DOC of 0.375″ (9.5 mm). It had been running this job with carbide inserts at 340 SFM (103.6 m/min), with a feed of 0.0086″/rev (0.218 mm/rev) and the same DOC. XSYTIN-1 increased the material removal rate from 40 to 100 in.3/min (655–1639 cm3/min and decreased the cycle time from 14 hours to five, for a 265% increase in productivity and an annual savings of $290,000. Using XSYTIN-1 also enabled the customer to eliminate the annealing process before milling and grinding the part after milling, both of which are typically bottlenecks.
Another intriguing application has been compacted graphite iron (CGI), a widely used material in automotive, truck, and heavy equipment engines. “The problem with CGI is it’s incredibly difficult to machine owing to the spheroidal graphite particles,” explained Andersson. “It’s like little grains of graphite sitting throughout the material.” Those little grains cause micro-interruptions in the cut, which quickly causes edge cratering in traditional ceramics. But Andersson said XSYTIN-1 is strong enough to cut CGI at speeds three to four times higher than carbide.