Chasing a precision sphere with a spindle probe as the sphere moves with the machine’s pallet throughout its five-axis machining range is the key to the new method.
One of the toughest challenges in five-axis machining of large parts is maintaining accuracy throughout the entire work zone. Ideally, a five-axis machine can produce results with the same accuracy when cutting in the far corners of the work zone or at its center. Grob Systems has developed a method of checking and compensating for possible distortions to a machine’s structure caused by the weight of a massive workpiece when machined in five axes.
Grob, a manufacturer of five-axis universal machines and production systems, says its new method improves accuracy of five-axis machines, especially those called upon to produce large, complex parts and components for critical applications. This is a summary of the company’s explanation of how this new system works:
The traditional approach to improving machine accuracy was to use laser interferometry to compensate the position accuracy of the individual axis, and the ball bar measurement to improve the perpendicularity between the axes. This is a simple technique and useful only for the three linear axes of the machine. When the spindle and table start moving around the work envelope, infinitely more deviations (both translational and rotary) come into play. That is the fundamental problem, and it becomes exponentially more complex when two additional rotational axes are introduced.
To improve the process for increasingly complex machines, Siemens pioneered the use of a six-dimensional pictorial compensation map. A 3D laser is used to simultaneously measure yaw and axis alignment, feeding data back to the machine’s control. The resulting picture is comprised of many, many points throughout the work zone. This created a much better measurement of geometry and positioning, allowing the CNC to compensate for distortion and improve volumetric accuracy anywhere in the work zone.
The limitation also of this approach is that the checks and compensation take place on the factory floor during final assembly, and it is available only for linear axes. In the real world, once you put a part on the pallet everything changes. Consider the capacity of some larger five-axis machines to accommodate loads up to 3,000 pounds. When a mass of that size is put into rotation on a five-axis machine, possibly suspended at 90 degrees, it creates a powerful force for distortion on any machine.
Simultaneous to the software developments at Siemens, Grob developed a function called Automatic Kinematics Adjustment. This process locates the true center of rotation, which is critical to accuracy in a five-axis work zone. It works like this: A spindle probe is paired with a precision sphere mounted on the pallet. A kinematic cycle touches probe to sphere, and then rotates to reposition the rotary axis. Repeat. Chasing the sphere around the five-axis envelope in this manner identifies the true center of rotation. The process takes about 10 minutes.
Grob has now integrated its kinematic process with the Siemens software to create what it calls the Volumetric Compensation System (VCS) for shop floor use. VCS applies the same basic software package used for Automatic Kinematic Adjustment, but takes it beyond the center of rotation. A probe stored in the tool magazine chases the sphere to map positional accuracy all around the work zone. By spinning and rotating the pallet, the probe identifies any rotational or translational deviation—from top to bottom of the Y stroke, maximum X to maximum Z and everything in between. The measured deviations are sent to the compensation software, which then perfects positioning accuracy. The process can applied to any critical component assigned to the Grob machine.
An Airbus research and technology leader was quoted in this article describing what he sees as low-hanging fruit in the supply chain—that is, opportunities to get greater output from existing CNC machines. One opportunity he sees is increasing metal removal rate in milling by selecting spindle speeds with an understanding of the machine’s and the overall system’s tendencies to chatter. The practice has been known for decades, and Dr. Scott Smith of BlueSwarf explained in a recent Webinar just how to obtain this productivity gain.
The webinar is worth your time, because the steps for increasing productivity by mastering a machine tool’s dynamic characteristics can be counterintuitive. For example, while a shorter milling tool is likely to produce a more rigid system, that shorter tool is not necessarily better when it comes to chatter’s effect on how deep the cut can go. Dr. Smith explains why a longer tool more prone to deflection might actually be the tool able to take the deepest cut. Animations in the webinar illustrate the various frequencies your milling process is subject to, and why your cut is behaving the way it is.
With a reading of 53.6, the June metalworking index showed that the industry continues to grow at a strong rate. The index was higher than 53.0 every month in the first six months of 2014, and the industry grew eight of the last nine months. The June index also was 17.0 percent higher than it was one year earlier. For the fourth month in a row, it grew by more than 10 percent, which was the fastest rate of growth since July 2010. Annually, the industry has grown at an accelerating rate the last four months and is growing at its fastest rate since April 2011.
Both new orders and production increased for the ninth month in a row, however these rates of increase have decelerated since March. Backlogs contracted at a very moderate rate for the third month in a row, but they are expanding at a rapid rate compared to one year earlier. The annual rate of change in backlogs continues to accelerate, which is a positive sign for future capacity utilization levels and capital spending. Employment has expanded since last October, and in June grew at its fastest rate since September 2011. Exports contracted for the third straight month, and supplier deliveries continued to lengthen at a slightly increased rate, as they have since June 2013.
Material prices have increased at a faster rate this year and are at their fastest rate of growth since March 2012. Prices received by the metalworking industry increased for the second month in a row, and while future business expectations remain near the peak levels of this expansionary cycle, they have dipped slightly since February.
Future capital spending plans reached their second highest level of 2014 and have grown month-over-month for four straight months. In June, future spending plans increased 13.1 percent compared to one year earlier. Annually, future spending plans are growing at a rate of 4.0 percent, the fastest rate of growth this year.
CNC retrofits are ideal for large machines in good overall mechanical condition such as this G&L 350T boring mill with 5-inch spindle diameter installed at KD Machine in New Castle, Pennsylvania.
There are a number of reasons why shops serving the oilfield industry see value in upgrading existing machine tools with a new control package. If price were no concern, these shops would more than likely choose to buy new machines. However, the cyclical nature of this industry combined with the size of the equipment required to machine some oilfield components has a number of shops eyeing more affordable CNC retrofits. This story cites examples.
The VMX24i CNC machining center is one of the machines the winner might choose in the entrepreneurial contest to be decided at IMTS.
Hurco’s “Chipmaker Challenge” contest will allow a manufacturing entrepreneur to win a free CNC machining center or CNC lathe (winner’s choice). The contest is currently underway, accepting applications until August 8.
The contest is modeled after the TV show Shark Tank, says Hurco North America General Manager Joe Braun. Entrepreneurs in manufacturing who have been in business five years or less can pitch their business plans, including what they expect to achieve with the free machine. The top five finalists will appear in front of a panel of judges at the Hurco booth (S-8319) at IMTS September 9.
One of the aims of the contest is to “get publicity for the entrepreneurs in our industry who do remarkable things each and every day,” Mr. Braun says. “We decided to create an exciting, competitive, entertaining event that showcases these entrepreneurs and highlights high-tech manufacturing.”
To learn more about the Chipmaker Challenge, and to enter, go here.