Continuous dress creep feed (CDCF) grinding is enjoying a comeback for processing complex profiles for aerospace and other difficult-to-machine workpieces. CDCF technology uses a constantly infeeding rotary diamond dresser that dresses the wheel continuously during the grinding cycle at a preprogrammed infeed rate. Introduced in the 1980s, CDCF quickly became the process of choice for major turbine engine manufacturers for production of high-accuracy nickel material parts. However, in the 1990s, CDCF faded in popularity as superabrasives, notably CBN, began to dominate aerospace grinding applications. At the time, CBN in the form of vitrified CBN and plated CBN wheels offered better grinding economics through lower G ratios that translated into lower wheel consumption, fewer wheel changes, and lower cost per part.
CDCF’s return to favor can be directly attributed to improvements in CNC control, machine accuracy, and grinding wheel technology, according to Larry Marchand, vice president-Profile Division, United Grinding Technologies (UGT; Miamisburg, OH). "Aerospace materials are ultrasensitive to any heat input from friction from the wheel or rubbing the surface with a dull tool. Heat generated by the machining process can actually distort the metal workpiece and create an unacceptable result," Marchand explains. "These materials use greater amounts of nickel and cobalt, and are combined with advanced material structures including single crystal alloys, mono alloys and dual alloys for producing jet engine parts and blades, vanes, and shrouds."
The unique element of CDCF is that the rotary-driven dresser unit is located above the wheel with a separate CNC axis dressing into the wheel simultaneous to the grinding process. "Special grinding macro programs maintain programmed infeed dress rates at a constant rotation speed that is matched to the ever-changing wheel diameter," Marchand explains. "As a result, the grinding wheel is always sharp and free cutting. The grinding wheel is clean and open allowing for chip removal and coolant carry-in, and the grinding wheel shape is constant and accurate from the constant dressing, minimizing wheel breakdown and loss of form. As the wheel is constantly getting smaller, the CNC control is calculating and compensating the wheel diameter to produce a flat or radial surface."
Here’s how Marchand details improvements in the three key process parameters of CNC, machine accuracy, and grinding wheel improvements:
"CNC control programming power has led to advances in grinding software such as In-Process Dressing [IPD] that allows the machine to monitor several critical machine variables and then increase or decrease the dressing infeed to stabilize the process. By monitoring the machine spindle power consumption, the grinding process can increase the dresser infeed automatically when the spindle load surpasses the programmed limits to increase the sharpening and cleaning of the wheel. Or, it can slow the dressing process feed to conserve wheel consumption for better economics. The dressing can be activated by several machine variables including spindle load, grind position, and dressing power.
"Improvements in machine accuracy have made it possible to continuously dress the wheel at infeed rates as low as 1 mil/wheel revolution, 0.000001" [0.00003 mm], a result of 0.1-mm resolution scale measuring systems. Compare that with the typical 40 mils that were common in the 1980s, and you see a reduction of 90% in grinding wheel consumption per part."
"Aerospace materials are ultrasensitive to any heat input from friction from the wheel or rubbing the
surface with a dull tool."
According to Marchand, the development of grinding wheels with high-strength wheel bonds and ceramic abrasives have complemented the attributes of CDCF grinding. "New wheel technology allows for dress infeed rates to be reduced to levels that were unthinkable 10 years ago, thus resulting in greatly improved economics for the process.
"One big advantage of CDCF is the process stability and its forgiving nature for variations in the stock of the material that is being ground. An example would be a cast part that you anticipate grinding a layer of 0.040" [1 mm] off. Because these parts are investment castings that result in inconsistent surfaces, the stock removal may range from 0.030 to 0.060" [0.76–1.5 mm] from part to part. Now, you’ve got roughly 50% more material than you planned on. If you have a conventional creep-feed or CBN process that was developed for 0.040" [1-mm] stock, the excess material will create a situation where you overload the wheel and create excessive heat and force into the work part. However, with CDCF this can be easily compensated for by simply increasing the dress ratio a little higher so you’re dressing a little faster, making the wheel sharper and cleaning out the material. It’s a very forgiving process both for stock variation and material sensitivity."
Pete Umlor, applications engineer, abrasive machining, Aerospace Division, Moeller Manufacturing (Wixom, MI) attests to CDCF’s capability. "The main advantage to CDCF is high stock removal while introducing low stress levels to the part. The process is highly accurate, and it has really reduced cycle times over any other machining process. We’ve seen great quality and throughput improvements. If done with proper coolant nozzling, there is very little heat introduced into the part and very little thermal distortion or build up. CDCF is a great process, especially when dealing with difficult-to-grind alloys."
For high stock removal of difficult-to-grind materials, CDCF requires high horsepower like Blohm and Mägerle continuous dress machines, which have spindle power ranging from 65 to 150 hp (48.5–112 kW), compared with conventional creep-feed machines that range from 20 to 50 hp (15–37 kW). ME
For more information on CDCF from United Grinding Technologies, go to www.grinding.com, or phone 937-859-1975.
Robot Travels Path to
Learn Fastest Cycle Time
R-1000iA Gakushu (Learning) Robot with the Learning Vibration Control (LVC) feature is able to optimize a specific application’s toolpath and improve cycle-time performance. The Gakushu Robot’s LVC software records path data characteristics, then runs the path a number of times, allowing the robot to optimize the path data to provide fast cycle times and smooth motion.
The R-1000iA robot, which is available with either an 80-kg payload or 100-kg payload, features a narrow footprint that allows users to reduce the overall size of their automation systems. Alternatively, more robots can be used in each workcell, or a workcell can be made significantly smaller with a single robot. Adjacent robots, fixtures, and workpieces are readily accessible. The R-1000iA can be floor or invert-mounted and has a large work envelope.
The most significant benefit of dual check safety (DCS) speed and position check is in applications where the travel of the robot needs to be restricted due to floor space or process limits that are less than the full reach of the robot. Restricting the robot motion in Cartesian space means that the robot can be restricted to exactly the area in which it works, something that isn’t possible with the current systems that limit robot motion externally using limit switches.
There are a number of benefits to moving some of the safety functions to within the robot itself. Fanuc Robotics identifies them as significant savings in floor space, flexibility in system layout, reduced hardware costs, and improved reliability. In addition, "safe zones" can be enabled and disabled from an external source such as a safety PLC (based on cell design).
Designing a system with multiple zones and appropriate guarding means an operator can safely enter and leave the workspace of the robot. This streamlines the design of robot cells because it prevents the robot from entering the load area when an operator is present. This application is possible with existing technology, but is typically difficult to set up, expensive to implement, and requires more floor space than a system using DCS, according to Fanuc Robotics. ME
For more information on Fanuc Robotics America Corp., go to www.fanucrobotics.com, or phone 248-377-7570.
Motor Spindle Gets
Operating conditions in high-speed milling create a number of factors that can lead to collisions in the machining area during the production process. In machine tool crashes, tool collisions with high feed rates cause high-impact peak force values on motor spindles. As a result, spindle damage is common and the cost to repair or replace a spindle as well as lost productivity and downtime can run into the tens of thousands of dollars. In addition, these collisions often cause damage to the cutting tool and machine axes.
The patented collision protection system introduced by Jakob Antriebstechnik (Kleinwallstadt, Germany) at IMTS 2012 and marketed in North America by GAM Enterprises (Mt. Prospect, IL) is based on a double-flange design. In the event of a collision, the system allows a mechanical decoupling of the motor spindle from the feed axis, thus enabling a controlled deceleration of the feed axis before an overload occurs.
The motor spindle is screwed to the inner ring of the prevention system at the spindle’s flange. The inner ring is in turn located within the outer ring which is screwed to the headstock of the machine tool. The positioning of the inner ring to the outer ring and thus the location of the motor spindle is ensured by a precisely manufactured geometry. The required mounting forces are generated by using permanent magnets and preloaded springs.
Depending on the spring pre-load, mounting forces up to 18 kN in axial direction and tilting moments up to 2300 N•m (in case of a radial load) can be reached. The integrated damping elements absorb the surplus collision energy. The relative movement between the inner and outer ring is recorded at three points on the circumference in the axial direction using displacement sensors integrated in the system.
The system is available for all motor spindle designs and dimensions, is maintenance free, requires no external power source, and includes an integrated sensor for highly accurate position control. ME
For more information from GAM go to www.GAMweb.com/safetyspindlesystem, or phone 847-649-2500.
This article was first published in the September 2012 edition of Manufacturing Engineering magazine. Click here for PDF.