Taking on titanium

Due to production rates ramping up for new aircraft, particularly in the commercial market, developers of cutting tools are constantly under pressure to deliver faster and more efficient metal removal rates. In an industry that often uses difficult to machine materials such as titanium this throws up some demanding challenges.

Kennametal has developed a new carbide grade KCSM40, said to greatly enhance cutting of titanium Ti6Al4V and other high tensile strength alloys.

Werner Penkert, Kennametal, manager aerospace solutions engineering EMEA, says: This new grade is a development from our carbide grades, KC725M and X500 which are well-known for their reliable performance and long tool life.

“This new grade enables us and our customers to mill titanium and other high tensile strength alloys, stainless steels, and Inconel with higher cutting parameters. It increases speed by around 30% but still with excellent tool life.”

The KCSM40 grade has an advance cobalt binder that provides very high thermal fatigue resistance. In addition to the new material substrate, Kennametal’s proprietary AlTiN/TiN coating enhances wear resistance at the cutting edge.

Optimising speed

The target cutting speed for KCSM40 in titanium Ti6Al4V was 175 SFM (53m/minute) while achieving greater than 20 cubic inches per minute (327cm³/minute) metal removal rate for 60 minutes. Not only has KCSM40 achieved this, but results of milling titanium at speeds up to 270 SFM (85m/minute) at a lower radial depth of cut have been achieved.

Scott Etling, director of global product management for indexable milling at Kennametal, says the radial engagement of the milling cutter to the workpiece is one key component when optimising the cutting speed.

He explains: “At higher radial engagements, the insert is cutting the material for a longer time which creates more heat at the cutting zone. The heat will not transfer into the titanium chips as is the case when milling steels so controlling the heat is critical. Lower cutter speeds at higher radial engagements is good practice. At lower radial engagements, higher cutting speeds can be achieved.”

Kennametal has carried out extensive trials using a new Harvi Ultra helical milling platform to mount the new inserts. It achieved over 100 minutes of tool life running at 155 SFM (47m/minute) with a 0.0067 ipt (0.12mm/tooth) chip load. The radial depth of cut was 25mm and the axial depth of cut was 76mm.

Mr Etling states: “The edge condition on the KCSM40 grade insert still looked great and we are getting many other fantastic results. In one face milling operation on a hardened steel wear plate the number of passes was reduced from 234 to just 22. I am very excited how KCSM40 will benefit our customers.”

Demand for higher metal removal rates was the main driver for Kennametal to develop the new grade.

“Cycle time is very important because all aircraft builders are under pressure to deliver higher volumes,” Mr Penkert continues. “When companies invest in new machine tools, they want to process the parts much faster. However, customers also require secure processes with other developments such as increased automation.

“These days it is rare that a machine operator will be constantly monitoring a machine. Machine tools also use high pressure coolant so once you apply that it’s very difficult to see what is happening at the cutting edge. A secure manufacturing process is essential because of the value of the parts being produced and the value of the machines they are being made on.”

The heat is on

Titanium poses particular challenges when being cut because of the very high temperatures generated at the shear zone. Coolant can vaporise even before it makes contact with the workpiece.

“You need high pressure coolant and emulsions because of the superheated steam in the shear zone. It’s very important to protect the carbide tools from heat because there can be a softening effect where the cobalt binder weakens and your cutting process is no longer secure,” Mr Penkert says.

Kennametal carries out many trials with its customers to help them come up with efficient cutting strategies.

“We have good data about our cutters’ tool life but you always have to judge what is happening with an application in the real world,” Mr Penkert explains. “We have to consider what material is being cut? What is the stability of the machine tool? The workpiece clamping? What cutting lubricant is being used? What sorts of pressures are being applied? All these variations have to be taken into account. It may also be the part being made itself. Is it thick and stable or thin and unstable?

“We use our data as a starting point but then we have to prove out strategies in a manufacturing cell. Here we can monitor the wear, so after a number of minutes of cutting we have an accurate idea of the wear rate which occurs in that application.”

Along with its metalcutting products used for machining high grade alloys, Kennametal has also developed a range of cutting tools for the increasing amounts of composite materials used in aerospace.

“We anticipate that composites will continue to grow in aerospace. Aircraft such as the Boeing 787 and Airbus A350 use it as a high proportion of their aircraft weight. The automotive sector is also beginning to discover that this is one method through which the vehicle can be made much lighter. This used to be only high-end cars and motorsport, but now it is moving to larger volume models.

“We have several products for composite machining. These include trimming tools, which mill the outside edge of a part or tools that drill and then mill from the inside of the part. We also have a few applications where we carry out contour milling of CFRP.

“We are also involved in the assembly of the aircraft with rivet hole drilling. This can involve drilling through CFRP/aluminium or CFRP/titanium stacks. It can be difficult because you have to prevent burrs on the entry and exit of holes, avoid delamination and fibre breakout and we have to provide a geometry that can deal with these different materials in one operation. Another factor is that these composite materials are highly abrasive. In these circumstances we find PCD is a very good cutting grade or diamond film coated drills.”

Application advances

The aerospace sector is an important market for Kennametal and its tool development as the advanced engineering involved always poses new machining challenges. An example of this is the tooling specialist’s membership of the Advanced Manufacturing Research Centre (AMRC) in Sheffield, where along with aerospace OEMs and other industry stakeholders, machining and materials research is carried out for aerospace and other high-value manufacturing sectors.

“We work very intensely with the OEMs and tier 1 companies on projects, along with our application engineers who help all our customers in the industry,” Mr Penkert says. “Also, there is always the demand created by new and improved machine tools. Manufacturers and their customers want turnkey projects which includes us providing solutions. In these situations, there is a lot of collaboration between the cutting tool supplier, the machine tool builder, the fixture manufacturer and the end user. Our specialists work with all involved to support each other.”

And Mr Penkert believes there are still possibilities for cutting tool performance to be enhanced.

“Improvements can be made to the carbide grade, the substrate and the coatings. And there are ongoing developments with machine tools which means there is always a research partnership needed between us and the manufacturers. When a machine tool comes along with new performance capabilities, this allows the cutting tool developers to create products that match the improved performance of the machine tool.”

Kennametal www.kennametal.com