New machine has potential to increase milling productivity 100 per cent
by Ed Robertson
Rockford Engineering Associates LLC (REALLCo)’s mission is to provide unique solutions to complex manufacturing problems.
So when the US National Institute of Standards and Technology approached the company with the challenge of developing a test machine that would demonstrate machining of titanium and other high temperature alloys on a super-stiff vibration-free (SSVF) motion platform to enhance metal removal rates and tool life, the company was ready for it.
The Rockford, IL-based company’s three main partners Kanwar Singh, John Hurd, and John Osborn, each have more than 40 years’ experience in developing custom machinery and innovative manufacturing processes.
REALLCo’s approach to research has its basis in the decades of engineering accomplishments at Ingersoll, where REALLCo’s principals worked. “We didn’t work on standard products,” says Kanwar Singh, “we offered manufacturing engineering solutions to our customers. Our M.O. is the same today.”
Singh relates the circa-1978 story of an aerospace company wanting to triple the production rate of a very large titanium component. Many machine tool companies were invited to bid on the equipment and processes. Singh and Ingersoll took a different approach. “Our solution was to start with a clean slate to envision the process of cutting titanium from scratch,” he says. “Otherwise, to continue with the tools and processes they were currently using, they were perpetuating their obsolescence.”
Singh’s clean-sheet approach takes the exact opposite path. “Define your optimum cutting tool first, develop spindle specs to support the tool, then machine-tool specs to support the spindle,” he says. “We were determined to find out how titanium cuts optimally based on its mechanical and physical properties, cutting edge material, tool edge prep, optimum angles, and ideal speed and chip thickness.”
The aerospace company agreed with Singh’s approach and funded a six-month study. “We basically wrote a textbook for them on titanium cutting,” Singh relates. For example, the specific amount of energy consumption required to cut a unit of 4340 steel and titanium is about the same, say 1 for steel and 1.1 for titanium. “But the speed at which you can cut steel is about three times that of cutting titanium,” Singh explains. Steel has a low melting point, conducts heat well, and transfers about 80 per cent of the heat energy at the point of plastic deformation into the chip.
By comparison, titanium has a high melting point, low thermal conductivity, and transfers less than 20 per cent of the heat to the chip and the part, with the rest going to the cutting edge. With three times the cutting forces needed for cutting titanium as opposed to steel for the same metal removal rate, only massive machine structures could provide the necessary rigidity and torque. “There was no real innovation,” Singh says. “A machine with a cubic-meter work zone for milling titanium parts would look like a battleship.”
Support the Spindles
One of the findings of the 1978 study on cutting titanium became the basis for the SSVF machine – that cutting tools do not fail from wear as they typically do in iron and steel. Intense heat and pressure encountered in cutting titanium causes thermal micro-fissures along the rake face. “Any vibration at the tool/workpiece interface helps propagate these fissures and eventually cause the failure,” Singh says. “It was clear that a very low-compliance structure with high damping characteristics was required for machining titanium.”
REALLCo engineers hypothesized that machine tools are made up of passive and active elements. Passive elements are the structural metal segments; active elements are the bearings that facilitate motion between them. Mechanical bearings, regardless of the preload, always have some compliance and typically low damping ratio.
For the NIST study, REALLCo offered a refined and enhanced version of membrane dual-restrictor (MDR) hydrostatic bearings in which positive, infinite and/or negative stiffness could be achieved. “Besides the infinite stiffness, a squeezed film of oil is inherently damped,” Singh says. “We theorized negative stiffness at the bearings allows us the provision to compensate for any compliance in the structure.”
REALLCo then built a milling machine to validate the MDR hydrostatic bearings contribution to the stiffness and damping of the structure. Ultimate proof of the SSVF concept would not only be in ultra-high stiffness, but also in improved metal-removal rate or increased tool life or both.
The test machine’s spindle is designed with constant-flow hydrostatic conical bearings for high stiffness and damping, powered by a high-torque/low-speed torque motor. Calculated and measured stiffness of NIST machine’s spindle was approximately 12,000,000 lb/in., where conventional bearing spindles are between 5,000,000 and 6,000,000/in. Calculated and measured aggregate machine stiffness at the spindle face is approximately 2,000,000 lb/in, four times greater than the current state-of-the-art at 500,000 lb/in.
Major tooling suppliers were invited to submit their latest products for test cuts on the new machine. “All submitted tools were a success not only in validating the SSVF concept, but in also improving tool life an average of 75 per cent,” Singh says. “We believe that performance goals were met or exceeded largely through the machine’s extremely high static stiffness and damping characteristics. Indeed, the test tools selected more than stood up to the challenge as well.”
The test machine belongs to REALLCo and is available to demo the SSVF concept as well as new and emerging cutting tool technologies. For more information visit REALLCo oline.