By: Scott Walker, President, Mitsui-Seiki U.S.A. Inc. and Mark Huston, Vice President, Global Engineered Solutions, Kennametal Inc.
When it comes to sourcing capital equipment for machining specialty materials, many organizations would do well to remember the 10-speed bicycle analogy. People shop and buy a 10-speed bike for recreational or workout goals, but when riding rarely go beyond two or three of the most comfortable gears. Tour de France riders and other finely tuned athletes both size and operate all their components (frame, pedals, shifters, wheels, etc.) to get the absolute most out of their equipment, whether climbing mountains or racing in the flats.
Machining high-strength, high-temperature alloys like titanium is a significant mountain to climb for many shops. Machine tool builders have responded with milling and turning centers that feature improved stiffness and damping on spindles and sizable machine structures and motors, all to provide the significant cutting forces required while minimizing undesirable vibrations that deteriorate part quality and tool life.
Achieving the ultimate system for machining titanium for maximum metal removal means paying close attention to the machine tool that provides the force, the cutting tool characteristics where the cutting edge meets the workpiece, and the spindle connection – the handshake between the machine tool and the cutter.
In April 2012, machine-tool builder Mitsui Seiki, Franklin Lakes, NJ, in connection with tooling and tooling systems provider Kennametal Inc., Latrobe, PA, conducted a test cut on a titanium (Ti-6Al-4V) workpiece on the Mitsui-Seiki HPX63 CNC horizontal machining center equipped with four Kennametal tools, each using the KM4X 100 spindle connection.
Key design criteria of the HPX63 include a large work capacity featuring a swing diameter up to 1,050mm and available work height (Y-axis) up to 1,050mm. Axis stroke is 1,000mm in X and 900mm in Z. Pallet size is 630mm. The B-axis rotary table offers 12rpm and high-torque, high-acceleration availability. Rapid travel rates are 32m/min with 0.5G acceleration/deceleration, and the cutting feed rate is 12m/min.
Made for precision work, metallurgically configured castings deliver the utmost stiffness, its box way axis slides are hardened, ground, and hand-scraped. Positioning accuracy and repeatability is 0.001mm. The spindle, Mitsui’s own, automatically compensates for thermal changes and does not require a warm up period. The company offers several spindle options to meet user needs for direct or gear drives and the amount of torque and rpm requirements.
Overall, the ruggedness, rigidity, and precision of the HPX-63 make it ideal for machining titanium, Inconnel, tool steels, stainless steels, and aluminum for the aerospace, energy, compressor, mold and die, fixtures and tooling, automotive prototyping, and general precision machining industries.
A spindle connection that makes the best utilization of available power possible is an important consideration to achieving the ultimate system. Most tools in the market are solid and the spindles have relatively low clamping force. Connection stiffness is limited, as radial interference needs to be kept to a minimum. The required tolerances to achieve consistent face contact are thus very tight, leading to high manufacturing costs.
The Spindle Connection
KM4X from Kennametal represents the next generation of KM. Some systems may be able to transmit a considerable amount of torque, but cutting forces also generate bending moments that will exceed the interface’s limits prior to reaching torque limits. By using three-surface contact for improved stability and optimized clamping force distribution and interference fit, KM4X engineering results in three times the bending moment resistant capacity compared to other tool systems.
In the test cut, the HPX63 was equipped with a high-torque, high-power spindle with maximum 26/22kW power and 1081 Nm torque. The KM4X100 spindle connection generated 85kNm clamping force, more than twice an HSK100 and three times that of a BT50 (40kNm and 25kNm, respectively).
The Cutting Tools
The four different cutting tools employed in the test were:
- A 203.2mm diam. face mill with seven square indexable inserts;
- The same diameter face mill with seven round inserts;
- A 76.21mm diam., 228.6mm long helical (HARVI Ultra) cutter with five helical rows of 11 inserts each;
- A flat-bottom indexable (FBI) drill unit at 125mm diameter with six indexable inserts.
With the power of the machine tool and spindle and superior clamping force of the spindle connection, test cut results were phenomenal across the board. For the square-insert face mill, the material-removal rate reached 88.74cc/min. at 64min-1 spindle speed, 12mm depth of cut, and 45mm cutting width, feeding at 164.3mm/min.
At 73min-1 spindle speed, 177.8mm cutting width, 3mm depth of cut, and 88.9mm/min cutting feed, the round insert face mill achieved a 47.42cc/min. material-removal rate.
The HARVI Ultra helical cutter, cutting in X and Y directions simultaneously, topped 309cc/min. material-removal rate at a spindle speed of 171min-1, 101.6mm/min. cutting feed, 40mm depth of cut and 76.21mm cutting width.
The FBI drill made a flat-bottomed hole in the titanium workpiece at 102min-1 spindle speed, 10.2mm/min. feed, and 125mm hole diameter at 125.2cc/min. material-removal rate. Following the first hole, the workpiece was rotated 45° away from the spindle in order to drill an angled hole pushing through the edge of the previous hole. In spite of the heavy interrupted cuts, both machine tool and drill performed well.
To the Max
Compared to commodity or general-purpose machine tools, the HPX63 is able to reach between one-and-a-half and two times the material-removal rate in machining titanium alloy. In addition, the KM4X spindle connection had enough clamping force and interference fit to allow a customer to use the higher RPMs and torque levels the machine tool and spindle can provide. Gains from the machine tools are more productivity potential while gains from the tools are additional cutting performance.
Moreover, a KM4X100 spindle connection will reach performance levels of an HSK125, but makes unnecessary the longer spindle, bigger tool-changer arm, larger tool magazine, and all the related increases a larger-footprint machine would require. Sizing the right machine tool with the right tools and connection can result in an ultimate productivity system for cutting titanium and other difficult-to-machine alloys. The connection can stay viable right up to the machine tool’s performance potential, which will drive the most out of the cutting edge, Milling, drilling, and even turning just got more productive.