- April 25, 2012
New generation of tougher cutting tools for hard metals
by Tim Wilson
In the world of cutting tools, hard metals don’t have to be hard-to-cut metals.
Whether we are discussing superhard alloys, stainless steels, or titanium, with the right application and approach, it’s possible to have a successful outcome for just about any job.
“Titanium is not hard when compared to 68 Rockwell,” says John Palmer, a UK-based technical engineer at ATI Stellram with over 35 years experience in precision engineering. “The biggest problem in cutting titanium is when you don’t approach the job correctly in the first place.”
The best cut occurs only after extensive knowledge is applied to the entire cutting process, and that comes from experience. For example, ATI Stellram faced some initial challenges with Ti 5553, but the company now has the knowledge and experience in place to get the job done reliably and effectively, says Palmer.
“We get tremendous results with Ti 5553. But you need a view to the entire process. You need to take into account many factors including the toolholder, the spindle, the rigidity of the machine, and component stability. It is only when the cutting tool touches the workpiece that you complete the circle.”
It takes some know-how to minimize material abuse, degradation due to heat issues, and surface problems. For example, titanium can suffer from adhesion to the back of the cutting tool. If you run too fast, there is more heat, and if you slow it down, there is more build-up. But with the right tool geometry, you can reduce the build-up on the edge behind the flank.
“You can increase the arc of contact, which will add to tool life,” says Palmer. “Once you have radial clearance, you can control the temperature through the arc of contact and increase the speed so that the chip is being evacuated quickly, and the tool remains cool when it is into the cut.”
With hard metals, there are so many factors to consider in an application and so many variables in play, that it is important to pay close attention to vendor recommendations.
This is true even for smaller shops. In North America there are a huge number of small and medium shops, and they can’t afford to waste time with trial and error.
“We receive a great deal of phone calls from customers experiencing trouble with their applications. At points, it is a challenge to attack the root of the problem over the phone; it takes time,” explains John Mueller, the milling product engineer with Sumitomo Electric Carbide. “Retrieving information from the end user to resolve the issue is not always easy. Every application is unique, like a snowflake. Yet, 99 per cent of the time those unique applications and scenarios can be solved during a customer’s first call, and we prove that through our guaranteed tests.”
Approaching the same material and the same part, but with a new tool, can result in a completely different scenario. This is due to the wider range of unique, application-specific coatings and edge preps available today.
“Our New Super ZX coating for the ACP and ACK milling series features multiple layers of alumina to increase heat resistance by 40 per cent,” says Mueller. “The alumina upgrade also provides at least 20 per cent better wear resistance at higher speeds.”
Sumitomo’s Super ZX Coat also has nano-scale layers of titanium, alumina and chromium nitride. These elements are laminated alternately and reach over a thousand layers.
“The Super ZX Coating is a dynamic coating that provides a boost to all heat resistant alloys. The coating also works well with stainless steel, including the precipitation hardened types (PH),” says Mueller.
Compared to Sumitomo’s original ZX Coating, the Super ZX Coating has increased amounts of titanium and alumina. The titanium and alumina mix with chromium to improve hardness and oxidation resistance. Tool life is then extended when cutting hard metals because of the enhanced
coating strength, with improved fracture resistance on the cutting edge.
Finding out if the part is in a horizontal or vertical machining centre can help guide you to the right type of cutting tool. Pinpointing whether the part is secured through casting or forging produces another clue in troubleshooting. Knowing if it is a mill or a lathe, combined with the above, can make a big difference in how the cutting tool performs or in untangling a specific problem.
People tend to fall back on, “it’s a new machine” or “tool life has been great until today.” The tooling from every supplier is always new, and under strict quality control, but the machine, fixturing and holders are always getting used and worn down. Cutting tools aren’t perfect and can be the source of a problem, but they are only one variable in the machining equation. One has to examine the application further to find the true cause.
Mueller from Sumitomo notes that new coatings are making a difference because of material innovation, such as the cobalt-chrome metal alloy. This alloy, which has a high specific strength, is popular in power generation and biomedical engineering, along with aerospace—all high-growth industries.
Seco Tools, for example, has its eye on these industries with the TS2000 and TS2500 inserts for turning heat resistant alloys.
“The TH1000 grade was initially intended for hardened, heat-treated materials like steel,” says Tim Aydt, Seco’s product manager, turning. “Then we tried some of that grade on higher temperature alloys, and had great results.”
When cutting hard metals, the combination of grade and PVDcoating can extend tool life and improve speeds. Getting the right mix of hardness and toughness is usually the goal. And when dealing with hard-to-cut metals, a 360° view of all factors, from workpiece, to spindle, to toolholder, is necessary to take into account all the fluctuating variables.
Variable geometry helps in machining high-temp alloys
A symmetrical end mill, with an even number of flutes evenly distanced, makes sense in some applications. But the problem is,without having cadences that prevent the resonance of natural harmonics created during cutting, you can encounter severe vibration and chatter that can create serious problems and impact productivity.
“It is like when soldiers march across a bridge," says Jason Wells, product manager at SGS Tools. “They break cadence, to avoid creating a frequency that would match that of the natural frequency of the bridge. If the frequency of the forced vibration of soldiers marching in time becomes equal to one of the natural frequency of the bridge, resonance may occur. This may make the bridge oscillate or vibrate with great amplitude causing the bridge to collapse. The same can be said of a tuning fork—if you hit one side, the sound wave will travel and cause the other side to start vibrating.”
Sound waves aren’t just audible, they also act as pressure waves that vibrate against a workpiece, causing surface finish and cutting tool damage. And when cutting into difficult-to-machine metals, the more aggressive the approach, the bigger the sound, and the bigger the problem, with the harmonic resonance causing the tool to chatter, resulting in chipping along the cutting edge and poor surface finish.
Sound travels by transferring vibration or energy across molecules. The molecules must be close enough to vibrate against each other to transmit sound. The closer and more aligned the molecules, the more efficiently the sound travels.
“The vibrations from sound waves travel more efficiently along a straight line,” says Wells. “By varying the geometry, speeds and feeds can be increased, and cut quality greatly improved compared to symmetrical tooling.”
Another analogy is a bent pipe. Sound waves can move immense distances along a straight pipe, but if you bend or put a knuckle in the pipe the molecular transfer of vibration will change, causing the sound waves to suppress.
One industry example of what suppliers are doing with vibration dampening is ATI Stellram’s variable cutting geometry, called RSM, a solid carbide endmill for machining walls, pockets and surfaces. The RSM design overcomes the vibration and material tarnishing often associated with
machining titanium and other challenging materials.
“The RSM is a multi-flute technology with differential pitch and differential helix,” says John Palmer, a technical engineer at ATI Stellram. “Variability pitch creates stability on the tool. This is not the same as in the past. It offers a much better surface finish because it generates less
noise and chatter for a smooth cut. Consequently, you can get extremely good tool life.” SMT
Tim Wilson is a freelance writer based in Peterborough, ON.