Grinding Toward Perfection
Multiaxis grinding machines get creative with their axes, moveable workholding systems, software and measuring tools
By Sarah A. Webster
Editor in Chief
Precision grinding is evolving incrementally to better handle old challenges and new. These include manufacturing, regrinding and finishing tools and eccentric parts—such as nonround or nonpolygon shapes—made of a wide range of materials.
To meet these challenges, a variety of machine builders are offering faster, more accurate multiaxis precision grinding machines. Many come with other new enhancements such as improved in-process gaging, systems that easily or automatically dress, change and balance a growing number of wheels—and even new mechanical approaches that promise to reduce machine wear and maintenance or move the workpiece in creative ways.
And that doesn’t even include the changes in software, which are moving toward working (almost) automatically with accessories to deliver a near-perfect part with little operator involvement.
Combining Grinding and Erosion
One shift that has had a big impact on grinding is the increasing use of new materials, such as carbon fiber reinforced plastics (CFRPs) and other composites, in industries such as aerospace and automotive. That, in turn, is driving an increase in demand for tools made out of super-hard materials.
Tools made out of polycrystalline diamond (PCD), the hardest man-made material in the world, are targeted for the machining of nonferrous materials, such as CFRPs, aluminum, titanium alloys, ceramics and carbides. Meanwhile, polycrystalline cubic boron nitride (PCBN), the second hardest material, is targeted for cutting iron-based alloys, such as cast irons, hard steels and iron-based composites. Making these super-hard tools is a challenge that is leading the development of new approaches.
Take the new Anca EDGe, launched at EMO Hannover 2013 in September. The EDGe machine features a double-ended wheel spindle that has the ability to both erode PCD and grind carbide and high-speed steel (HSS).
The ability to manufacture a variety of tools (up to 220 mm in diameter and up to 20 kg in weight) makes the EDGe ideal for customers looking to reduce the risk associated with moving into the growing PCD side of the business.
While erosion-grinding machines aren’t new in and of themselves, the double-ended HSK wheel spindle in the EDGe can erode and grind in one setup and it has other new features as well.
Russell Riddiford, president, Anca Inc. (Wixom, MI) said the proprietary EDGe-Spark generator monitors and controls the energy level of every spark for the resistivity of the substrate, be it tungsten carbide, diamond or cobalt binder. It automatically detects the substrate and adaptively adjusts the gap distance for optimum surface finish, material removal and cycle time. What’s more, it provides exceptional operator feedback in real time. “We take the precision and high-accuracy demands of our customers very seriously,” Riddiford said.
Testing of tools eroded on the EDGe machine resulted in better figures for cutting force, exit burrs on CFRP and drill life than ground tools, Anca said. The process achieves surface finish on tools of less than 0.2 µm Ra.
Anca, an established grinding company that is well known for its ToolRoom design software, also boasts that the EDGe has a fast, easy-to-use interface for inputting tool geometry. The Anca software, which includes integrated 3D graphics, is used to adjust power generator settings and is designed to create programs for a wide range of tool types.
In order to deliver the EDGe, Anca partnered with Precorp, a leading manufacturer of PCD cutting tools and a key supplier to companies such as Boeing, as well as the Royal Melbourne Institute of Technology, which assisted the project with research and data analysis.
The EDGe, with a base made of polymer concrete, weighs 4500 kg and is 2160-mm wide, 1530-mm deep and 1990-mm high. It accommodates wheel sizes up to 202 mm. The machine offers programming resolution down to 0.001 mm in the X, Y and Z axes, with programming down to 0.001° in the C and A axes. The standard in-process probe system is from Renishaw.
A Solution for Very Long Tools
Meanwhile, the Walter brand from United Grinding (Miamisburg, OH) has developed a unique approach to grinding long, rotationally symmetrical tools up to 700 mm in length made of carbide, HSS, ceramic, cermet and cubic boron nitride (CBN). Walter sees a market in the automotive sector, which is replacing many gundrill and deep-hole indexable applications with very long solid-shank drills for engine blocks and crank housings. Spiral-flute tools offer faster feed rates, due to their higher chip removal rates.
Originally introduced in 2012 as the Helitronic Vision Long, the recently renamed Helitronic Vision 700 L from Walter specializes in long milling cutters, drills, step drills, woodworking tools and profiling tools from 3 to 200 mm in diameter.
The five-axis CNC Vision 700 L is based on the popular high-precision Helitronic Vision, which has linear or torque motors in all axes, and a unique kinematics concept. In the Vision, a closed 3D gantry supports the grinding spindle and a large rotary C axis supports the tool, which allows grinding in all four quadrants. But the Vision 700 L features a much larger working envelope than the Vision.
Edward Sinkora, senior product manager for United Grinding’s Tool Division, said that finite element analysis showed that “we could open up the back of the gantry” and reconfigure several parts, including the automated worktable, grinding head and wheel changer to accommodate longer tool lengths.
Sinkora said the Vision 700 L “does everything the regular Vision does” but “in order to grind very long tools, we moved the rotary C axis to the grinding head. So in this approach the tool is carried on a strictly linear axis while the grinding spindle swivels.” And by opening up the rear of the gantry, the machine is able to travel farther on the X axis during very long grinds.
Two independent carriages support the workpiece and allow it to be moved in a manner that allows for graceful grinding down the length of the tool, as shown in a video at http://tinyurl.com/visionlong.
As with the Vision, the machine base of polymer or mineral cast, with the patented gantry, promises extreme rigidity. The maximum control resolution for the linear axes is 0.0001 mm (0.1 µm) and for the rotary axes, 0.0001º. The spindle power is 35 kW with 10,000 rpm. Grinding wheels up to a maximum diamater of 254 mm can be changed by means of a 6-, 12- or 24-station wheel changer, with the latter two being options. A Fanuc six-axis CNC robot is used for the automatic loading and unloading of the tools.
The Vision 700 L uses Helitronic Tool Studio, Version 2.0, grinding software with complete 3D simulation for secure off-line setup. Tool accuracy can be even further enhanced and certified with the addition of a Helicheck Pro measuring machine, which uses several CCD cameras and segmented LED lights to measure virtually all geometries. Repeatability is under a micron. An optional 1000× magnification camera measures the edge prep (hone) down to a 5-µm radius. For form tools, the software can compare the measured tool profile to the nominal profile and calculate the necessary offsets for the grinder to make the tool, as Sinkora said, “more or less perfect.” At least under 2 µm.
In making continual improvements to its GrindSmart lineup (528XS-6000XL), Rollomatic (Mundelein, IL) has redesigned its GrindSmart 620XS to make it even more flexible and reliable.
The resulting GrindSmart 628XS is a six-axis CNC super-precision tool grinder for the manufacturing and regrinding of round and nonround cutting tools, from Ø 0.1 to 16 mm. Its features include an automatic loading/unloading system, precision clamping system, and innovative V-block and steadyrest support. The machine can grind specialty tools, including form tools, indexable and nonindexable inserts, replaceable-tip tools, reamers, taps, step tools, torx drivers, surgical and dental tools, among others.
Aside from improved ergonomics, such as a larger control monitor, the 628XS features a new Fanuc control 30iB. The workhead has been redesigned to accommodate quick-change features to switch from round tools to inserts. The guiderails of the three major linear axes have also been reinforced.
“The linear scales are not glass scales anymore, but magnetic, which increases the life of the scales,” said Eric Schwarzenbach, president of Rollomatic. “The design of the guiderails and other moving parts has been deliberately focused on longevity with minimal maintenance intervention.”
Previously, the machine featured a belt drive, with a servomotor controlled by a rotary encoder. “On the 628XS, we have fitted a rotary glass scale that is directly coupled to the workhead spindle, thereby taking the belt drive out of the equation. This allows us to control the rotary tool axis movement much tighter. Its benefits include better flute finish and higher feed rates.”
The flexibility of the 628XS lies in several areas. The workhead is configured so one can switch from round tools to inserts simply by swapping two plates. “The switch over is remarkably rapid,” Schwarzenbach said. The other is the proprietary software, VirtualGrindPro, which features a large library of different types of cutting tools and inserts ready for grinding. The machine also has an optional wheel and nozzle changer, which includes six wheel packs with the coolant nozzles prepared and ready for production grinding.
Unique Hydrostatic Guideways
While it is marketed as a universal, high-precision grinder, the recently updated Kellenberger Kel-Varia from Hardinge (Elmira, NY) is targeted at low-volume, high-value, large and heavy parts, especially those that are nonround, such as polygons, free contours, and eccentric shapes—a consequence of the C-axis option of interpolating the X and C axes.
This spring, Hardinge began offering a new direct-drive workhead and hydrostatic B axis on the Kel-Varia. The precision hydrostatic guideways in combination with the direct-drive workhead technology provide extreme accuracy in concentricity and radial run-out: Part roundness of <0.2 μm; nonround contour accuracy of <6 μm; internal eccentric bore of <4 μm; straightness (slots) of <2 μm.
Jeff Hilliard, North American sales manager for Grinding, Hardinge, said the hydrostatic guideway system is unique. Hydrostatic guideways, combined with a strict separation of the machine base from the assemblies that generate heat or vibration, provide superb precision and productivity.
The hydrostatic guides for the longitudinal slide movement (Z axis) and for wheelslide infeed (X axis) provide the basis for the machine’s extreme accuracy. The X- and Z-axes movements are practically frictionless at all speeds. There is no stick slip; even the smallest increments of 0.1 μm can be traveled without a problem. Because there’s no metal-to-metal contact, there are also no wear characteristics compared to the traditional Vee and Flat system. “There’s a film of oil for shock absorption,” Hillard explains. “You don’t have to worry about bearings and guideways being damaged.”
The direct-drive workhead has an infinitely variable spindle speed from 1 to 500 rpm (with Heidenhain control) with a maximum weight capacity of 750 Nm / 300 kg (660 lbs) with an option of ±60 second micro-correction for easy adjustment of cylindricity.
The new workhead allows heavy parts to be quickly and precisely accelerated and braked with zero backlash during bidirectional movements for increased productivity. A large spectrum of parts and a wide range of geometries can be processed using combinations of cylindrical grinding, OD and ID out-of-round grinding, thread grinding and jig grinding all in one setup. Advanced C-axis (ultra-high resolution) operation allows precise grinding of grooves and satellite and centering bores.
Hardinge finds many of the end users of the Kellenberger Kel-Varia are in the container mold business. The Kel-Varia can now deliver parts up to 24" (610 mm) in diameter. Previously, 18" (457 mm) was the limit. The weight limit also has doubled to 550 lb (250 kg). Previously, these bigger molds would have endured “days upon days of finish polishing,” Hillard said. But now, “We can eliminate a lot of steps and tons of man hours of hand-polishing the mating parts.” ME
This article was first published in the November 2013 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 11/1/2013