Mitsui Seiki has been an integral partner in an R&D project with Boeing for the past four years. The goal: To determine the optimum machine tool characteristics required to cut the new triple nickel titanium (Ti5A15V5Mo3Cr) materials productively and economically.
Much has been written as to the “why” of the research. For one, the number of parts made of the new triple nickel titanium materials, or “heavy metals,” will be increasing to make up 15% of the weight of the new generation of aircraft, such as the Boeing 787 Dreamliner. These include the skeletal structure as well as struts, floor ribs, window frames, door hinges, and engine mounts—each requiring structural integrity. The 21,000 lb of heavy metal parts used in the aircraft will be machined from 240,000 lb of raw stock.
Another reason for the long research period owes to the fact that there is not enough titanium machining capacity to meet all the needs of the various high-titanium-content planes being launched. The aircraft parts suppliers will have to take on more of this capacity, and these new materials, although light in weight compared to aluminum, are extremely tough to machine. The Ti5553 is about four times more difficult to machine than the well-established Ti6A14V in terms of tool life, stock-removal rate, and the required resiliency of the machine structure to push a cutting tool through the metal.
What follows is a description of the major areas of discovery and the machine tool characteristics required.
The machine tool must have the structural design to machine at low-amplitude ranges, in the less than 350-Hz (especially at the 20, 90, and 320 Hz) ranges. A one-in, four-flute end mill, for example, would be run at about 90 rpm. Each time a cutting edge strikes the material, it sends a shock wave into the machine. As each cutting edge “hits” the metal in a consistent, repetitive sequence, it creates a low-frequency wave into the machine. General-purpose machines have a tendency to chatter at low frequency. This negatively affects both the quality of the part and tool life.
A machine that is designed specifically for low-frequency machining dramatically reduces chatter at the necessary low rpm at which the cutting tools must run to cut these materials. Logic follows that eliminating chatter significantly increases tool life.
So how do we do it? By designing an application-specific machine that has structural modifications to reduce the amplitudes of the excitation frequencies in the low frequency ranges. We have also paid attention to requirements for handling the low-frequency stresses such as height-to-width ratios on columns and tables to accommodate high moment loads; optimal ballscrew locations for axes stability; and, of course, hand scraping throughout to provide the high accuracy needed for quality parts in these materials.
At a depth of cut 1-1/4 in diameter x 3/8 in into triple nickel titanium, the tool starts to separate from the taper at about 8500 lb·in of torque on general-purpose machines with a BT/Cat 50 taper. A 7-in-long, one-in-diameter, four-flute cutter will remove about 1.2 in³/min of material from 5553 titanium before the tool separates from the spindle taper. If you increase the radial depth of cut to remove more stock, you will exceed 8500 lb·in. Through trial and error and ultimately success, we’re building tool taper interfaces right now that can handle 35,000 in•lb and are able to remove 18 in³/min of 5553 material with a Mitsui Seiki horizontal machining center.
In “heavy metal” machines, all of the materials in the machine tool structure must stay within a specific range of stiffness and resiliency so that, during cutting, the “spring memory” of the machine structure is very repeatable. This repeatability is paramount for tightly controlling the cutting edge machining parameters as they pass through material. Titanium 5553 is a material that exhibits superior linear-elastic behavior. However, when machining these types of materials, this behavior dramatically increases cutting forces and generates tremendous heat directly at the cutting edge shear location.
In aerospace components, tool lengths are long, axial cuts are deeper, and many application engineering hours are dedicated to process development. Unlike conventional machines that are designed for a wide range of materials, these material-specific machine tools allow engineers a wider range of process opportunities in the specific cutting ranges needed for these materials. This dramatically contributes to longer tool life, chatter control, part finish quality, and predictable process control for FMS operations.
Stiffer machine tool materials cost more up front, so the machines cost more; however, the definition of “cost” as it relates to “profit” is worth consideration. If you would profit by cutting deeper, faster, and with better quality, then you may be losing money by not doing so. The extra cost, after scrutiny, could be miniscule by comparison to what you might be losing. Beware of false economy, especially when it comes to getting what you need to produce titanium and other heavy metal parts.
To cut heavy metals, the machine needs ample torque. The spindles should produce 2000 lb·ft at 100 rpm, and large servo motor drives are needed on fine-pitch lead ballscrews. Ballscrew location is also critical for axis stability. These elements will provide the advantages necessary to push the tool through these tough materials. But, these power mechanisms need to be designed so as not to influence the low frequency excitation conditions.
Of course, there are several other issues with chip control, coolant use (lots of it), and the types of cutters. But the crux of optimum heavy metal machining lies in these areas:
• Ability to perform low-frequency machining with minimal chatter
• Hold tools tighter with heavy-duty tool taper interface
• Increased machine stiffness a must
• Jet parts need jumbo power
We believe we are blazing the trail in titanium machining understanding, yet also concede that new knowledge in machine design, methodologies, techniques, and leaner approaches are unveiled to us (we are open to new ideas) continually as we and our customers unravel the intricacies of working with titanium and other heavy metals.
Scott Walker, President of Mitsui Seiki USA Inc., wrote this article for Aerospace Engineering.