Decoding Difficult-to-Cut Materials

Machining today’s aerospace parts correctly means using advanced materials, cutting strategies, tooling

October 15, 2012

Materials used in the aerospace industry need a tailored approach to tooling selection.

Titanium use in aircraft production has grown continuously over the past two decades. Today titanium alloys such as titanium 6Al-4V, along with INCONEL® 718, NIMONIC® and Waspaloy® alloys, and 316 stainless steel are being used extensively in both commercial and military aircraft parts. These parts include wing structures, landing gear components, fasteners, springs, and hydraulic tubing.

As the aerospace industry employs new alloys and manufacturing methods, their use is expected to grow rapidly. And the ongoing demand for these materials has called for the development of new machining technologies, aimed at assisting shops to be more competitive and productive. New tools, grades, geometries, and chip breakers have been developed to aid in the production of complex shapes while maintaining the tight tolerances demanded by the industry. 

“The need for parts and components machined from these materials is on the rise, but they are very difficult to machine and wear out tools fast,” said Peter Matysiak, president of Emuge Corp., West Boylston, Mass.

This prompted the company to develop new end mills to meet the specific challenges of milling these difficult-to-cut materials.

A new flute and cutting edge design has been combined with a heat-resistant, multilayered titanium nitride/titanium aluminum nitride (TiN/TiAlN) coating to minimize friction and vibration and efficiently evacuate shorter chips, which is especially important in tough, long-chipping materials. 

“[Tools with a] variable flute spacing and a serrated chip breaker profile along the cutting edge can provide unmatched material removal rates and impressive RA values in both roughing and semifinishing, while producing minimal tool wear,” said Matysiak.

Why These Materials?

The use of these materials in aerospace applications results from the specific properties associated with the metals. These include a high strength-to-weight ratio, corrosion resistance, and low thermal expansion.

Of all the materials used in aerospace production, one stands apart from the others in popularity. Titanium 6Al-4V is the most common material for aerospace production because of its light weight and high strength.

Machining titanium alloys requires cutting forces that are higher than those required for machining steels. The alloys also have metallurgical characteristics that make machining them more difficult than steels of equivalent hardness.

Titanium work-hardens very easily, which can create a thin chip that contacts a relatively small area of the cutting tool’s face. The friction created as the chip travels across the face results in more heat in a localized portion of the cutting tool. Heat generated by cutting titanium does not dissipate quickly into the air due to poor conductivity, so a substantial amount of heat gets locked between the cutting edge and the tool face.

This combination of high bearing forces and heat produces crater wear within the proximity of the cutting edge, resulting in rapid tool breakdown. To make matters worse, titanium alloys have a strong tendency to react chemically with the materials in cutting tools, as well as to gall as chips weld to the cutting edges of tools.

Second, chip load per tooth typically is uneven; it is high at the point where the cutter has advanced farthest into the workpiece, and lower in other areas.

Last, when the cutter fills most of the slot width, little room remains for chip evacuation, so the chance of recutting chips is high.

Trochoidal Milling

Figure 1 Trochoidal milling makes it possible to apply an end mill with a diameter that is smaller than the pocket’s width to allow room for chip evacuation.

The advice from Iscar Cutting Tools, Oakville, Ont., is to employ trochoidal milling, when possible, for high metal removal when machining titanium.

Iscar’s R&D center in Tefen, Israel, recognized the potential of trochoidal milling to cut these materials and in recent years developed new solid-carbide end mill cutters, as well as extended-flute milling cutters to be used with indexable inserts.

Trochoidal, or spiral, milling moves the cutter in a circular pattern, with each circle advancing into the cut. One key advantage of trochoidal milling is that only a small area of the cutting tool is engaged at one time. The feed rate is also always constant.

In addition, trochoidal milling makes it possible to apply an end mill with a diameter that is smaller than the pocket’s width to allow room for chip evacuation (see Figure 1).

Despite its potential, trochoidal milling also presents challenges. The cutter must undergo a complicated motion that is beyond the capabilities of some CNC software. In addition, the machine tool must be rigid and fast enough to accommodate trochoidal cutting. The cutter likewise must be able to operate at high speeds. It is machine rigidity, however, that determines how aggressive the trochoidal cut can be. Other factors include the cutting tool’s size, workpiece material, and depth of cut.

The basic idea of trochoidal machining involves substantially increasing the cutting speed and feed rates. Chips are cut to their maximum thickness at the initial engagement of the cutter’s teeth with the workpiece and decrease in thickness at the end of engagement. The toolpath is optimized based on the results of previous machine cycles, eliminating air cutting and minimizing retract movements.

Tests have shown that trochoidal method is faster than conventional slot milling method because much higher cutting conditions can be achieved.

Producing UNJ Threads to Meet Aerospace Requirements

By Mark Hatch,
Product Director – Taps and Thread Mills,
Emuge Corp.

The UNJ profile originally was developed to meet aerospace requirements for a screw thread to provide maximum fatigue strength for fastening safety-critical components subjected to high stress loads.

The basic profile includes a controlled root radius on the external thread which is larger than specified for UNR threads. The minor diameter is increased on both the external and internal thread to accommodate the larger external root radius. The pitch diameter is controlled to a close tolerance with Class 3A for external and Class 3B for internal.

Producing UNJ threads, particularly the internal form, can be very challenging because not only are the thread limit dimensions held to a close tolerance, but aerospace components often have blind holes that require the effective thread length to be within one pitch of the hole’s bottom. And to add to the overall difficulty of internal “J” series threads, aerospace components are commonly made from materials such as INCONEL 718, Ti-6Al-4V, and 17-4 PH/15-5 PH stainless steel.

With the ever-increasing use of CNC machines to produce aerospace components, manufacturers may realize numerous production benefits of thread milling.

10 Benefits of Thread Milling

1. Chips can be evacuated efficiently because of the interrupted cutting process.
2. Lower cutting torque can be used without the need for a high-lubricity coolant or hand-applied tapping paste/oil.
3. High process security and control can be maintained.
4. Threads are produced with excellent form, finish, and dimensional accuracy.
5. One tool can machine all difficult aerospace materials up to 58 HRC.
6. Broken taps are no longer a problem.
7. Full-bottom threading can be achieved.
8. Pitch diameter can be easily controlled with a CNC offset.
9. Fewer number of tools need to be stocked.
10. A variety of threading applications can be performed.