CNC intelligence enables grinders to deal with the most challenging materials
By Jim Lorincz
The challenge for manufacturing today is to meet the demand for tighter tolerances, better finishes, and shorter leadtimes for workpieces—all at a lower cost. Grinding results have always seemed to depend on the skill of operators who were privy to the mysteries described as some mythical “black art.” Today, intelligence has been designed and programmed into CNC grinders to enable them to deal with the most challenging materials, the tightest tolerance requirements, and a broader range of industry applications. Advanced production grinding technology is marked by machines that are built for rigidity, energy efficiency, and careful use of valuable floorspace. These CNC grinders have payback times that easily fall within the favored one-and-a-half to two-year window so attractive and necessary to forward-thinking manufacturers in a wide variety of industries.
For Fives Cinetic Corp. (Hagerstown, MD), the LANDISflex line of traverse/contour/plunge shaft grinder line represents a significant entrée to the precision requirements of nonautomotive markets. “The LANDISflex line of precision grinders represents diversification of our capabilities and technologies into the nonautomotive applications found in the construction, mining, and agricultural equipment markets,” said Dwight Myers, president and CEO of Fives Cinetic. “These applications share the same precision requirements and quality characteristics that we have delivered to the automotive market so successfully and that have guided design of our grinding machine technology.”
Typical applications in the construction/agriculture/mining markets for the LANDISflex include large cylinders and struts, large drive shafts, landing gears, compressor rotors, axles, and large components that have high-performance coatings that require polishing as well as precision grinding. “We’ve incorporated grinding and polishing onto one platform for applications that require both and perform them in a single operation,” said Tim Hykes, director of technology. “For one customer, we supplied a fully automated process that significantly improved quality levels. We are able to deliver the precision and stability of the machines because we have single-digit micron accuracies in all axes of motion by virtue of our use of linear motors, precision roller guiderails, and precision spindles,” said Hykes.
“We’re seeing many of these large components with coatings replacing chrome,” said Hykes. “These hard coatings are typically ground with a vitrified diamond wheel because of the hardness of the coating and finished with a polishing film. A lot of typical grinding is done with an aluminum oxide, silicon carbide, or vitrified CBN wheels. We can use conventional aluminum oxide wheels which the industry has used historically, and we’re bringing in a lot of CBN and vitrified diamond wheels that allow us to do all these features including grinding ODs and interpolating radiuses in different forms and faces also in the same operations,” said Hykes.
“The LANDISflex traverse/contour/plunge grinding system is designed to provide sustainable productivity, precision, and flexibility in a reduced carbon footprint of reduced power consumption,” said Myers. LANDISflex machines are capable of grinding shaft-type parts up to 4.0 m in length. The base machine, available in single and dual-wheelhead configurations, can accommodate cylindrical parts up to 400-mm diameter, with a 750-mm part swing optional. Standard models include dedicated 30° plunge machine, dedicated straight machine; single and dual-spindle swivel machine; and twin wheelhead machine. LANDISflex grinders can accommodate either conventional aluminum oxide or superbrasive CNB or diamond grinding wheels. A B axis allows using multiple spindles and programmable angles to handle combined operations in a single fixture.
“Advanced coolant management design isolates the grinding coolant from the machine structure and the thermal effects of the energy being put into the grinding process. Historically, that would have required a lot of compensation in the machine. With our coolant management system we keep the coolant off the machine structure with layers of insulation and panels so that warm coolant falls into a channel away from the machine,” said Myers.
“One of the prime design goals for the LANDISflex line was energy efficiency. We’ve seen about 25% reduction in energy consumption, which lowers the total cost of ownership of the machine,” said Hykes. He goes on to point out another important target for the grinder production optimization.“We’ve basically taken the black art out of grinding by incorporating the skills required of the operator into the machine itself and have reduced takt times from hours to fractions of hours,” adds Hykes, as he ties together long-term savings an immediate value.
A Marposs Protomar absolute gage with a 300-mm measuring range has been incorporated into LANDISflex machines so that a range of diameters up to 300 mm can be handled in grinding shafts with a variety of features. These parts are up to 4-m long and the features that can be ground include OD and faces, multiple operations that in the past had to be ground on multiple machines.
Take One Deep Pass with Creep-Feed Grinding
“Creep-feed grinding is especially valued in applications where high stock removal and very accurate grinding are required. Typical parts are automotive, aerospace, and medical applications, among many others,” said Steve Lefebvre, process and applications engineer for the Micron line of machines at Peter Wolters of America Inc. (Des Plaines, IL).
“The Micron Macro creep-feed grinding machines are based on our innovative Moving-Wheel-Head design concept in which all linear movements when positioning and grinding are performed by the grinding head. Compared with conventional moving column and moving table concepts, this design arranges the axes in a very compact way, which results in optimum force distribution and minimal thermal variation, properties that result in best possible process control over a long lifespan,” said Lefebvre.
Creep-feed grinding takes material out in one pass with a deep cut whenever possible. “Micron Macro machines are designed with extreme rigidity in both static and dynamic modes. Our ballscrews are oversized to prevent interrupted action or stopping when it’s plowing through the cut. Our Siemens integral spindles are 5′ [1.5 m] long with five main bearings for stability and precision machining,” said Lefebvre.
“We can grind a wide variety of materials, including aluminum, carbide, diamond, ceramic, different steels, Inconels, Hastelloys on the Macro standard with wheels to 16″ [406-mm] diameter and up to 6″ [152-mm] wide. The challenge is to choose the right kind of wheel and dresser for the material being ground,” said Lefebvre. “Aircraft part materials, for example, tend to be very gummy so most of the time you would spec-out an aluminum oxide wheel with an overhead dresser on it. Also different rotary axes can be put on the machine to grind compound forms. Vitrified CBN wheels, which are dressed with diamond rolls or single point dressing or disks, are used for fast stock removal with very little wheel wear in the automotive industry which tends to use a lot of steels that are case hardened,” said Lefebvre.
“For materials that get gummy and that tend to stick to the grinding wheel, we have five different diamond dressers—everything from overhead dressing, intermittent dressing to continuous dressing with table-mounted dressers and roll dressers. For every application, it’s important to spec-out the proper options for the job and design the machine, tooling, and fixturing to optimize the application,” said Lefebvre. “Inconel aerospace parts with thin walls where heat is a big issue, for example, require choosing the right wheel and the right configuration of coolant nozzles.”
Micron Macro machines are available in sizes from the smallest footprint Macro S with X-axis travel of 12″ (305 mm), the Macro standard with a 24″ (610-mm) X-axis travel and the Macro L with a 36″ (914-mm) travel in X axis. All machines have 14″ (356 mm) in the Y axis and 12″ (305 mm) in the Z axis. “About 70% of our Micron Macro creep-feed grinding machines are fully automated with robots and loaders, especially for cellular configuration for automotive applications,” said Lefebvre. “Typical automotive parts are connecting rods, valve train parts, and piston rings.”
PCD Cutting Tools Need Grinding, EDM, Laser Combos
It’s well known and often reported that automobile and commercial aircraft manufacturers are under the same pressure to improve the fuel economy and performance of their products. Reducing powertrain, engine, and vehicle weight is a key tactic. Automakers, for example, have lightened engine blocks that are machined from tough, high-silicon aluminum alloys. Aircraft manufacturers are selecting difficult-to-machine nickel alloys for engine designs that must rotate faster, run hotter, and burn cleaner. Lightweight carbon fiber reinforced plastic (CFRP) composites are replacing certain metal parts in cars and have ushered in a whole new era in combination with titanium for building aircraft structural components.
To keep pace with high-performance manufacturing materials, cutting tool manufacturers have developed machining techniques for polycrystalline diamond (PCD) tooling that includes grinding, EDM, and lasers.
PCD material is extremely hard and wear resistant, proving long tool life compared to carbide or high-speed-steel when machining abrasive CFRP materials and difficult-to-machine nickel-based alloys. “A typical PCD tool is composed of a carbide body with brazed-in PCD tips or cutting edges. CNC tool and cutter grinders impart final tool geometries and cutting edge finishes, and the hardness of both the PCD and the carbide tool body require the use of diamond abrasive grinding wheels. Vitrified bond wheels generally provide a balance of accuracy and cutting performance when grinding PCD; specific abrasive mesh sizes and bond materials depend on the end user’s needs in terms of productivity and wheel cost,” said Markus Stolmar, vice president universal CNC tool & cutter grinders, United Grinding North America (UGA; Miamisburg, OH).
Grinding machine technology in combination with EDM and more recently laser technologies are at the forefront in machining these critical PCD cutting tools. “Grinding machines must be rigid to prevent chipping of PCD material which is hard but somewhat brittle. In addition, the grinding wheel must be trued and dressed frequently to keep it sharp and cutting freely because a dull wheel may vibrate and chip the PCD tool being ground. Many grinders feature linear drives instead of ball lead screws, because an oscillating motion is needed for grinding diamond. Linear drives not only minimize the mechanical wear and maintain the maximum uptime, but also ensure smooth, fast and accurate positioning and grinding of PCD tools,” said Stolmar.
EWAG’s five-axis Compact Line, for example, features a direct-drive spindle and linear motor technology for grinding indexable inserts, including carbide, CBN, and PCD. The Compact Line is capable of untended operation over several shifts because of an integrated six-axis FANUC 200i-C robot and a “three-in-one” dressing, regeneration, and crushing unit that assures the grinding wheel is always near perfect. The compact design and linear motors also minimize non-grinding times and ensure the shortest possible cycle times. The plug-and-play HSK-E63 clamping system enables fast and precise changeover between different types of clamping systems needed to manufacture a wide range of inserts and similar tools. Automatic 3D probing measures all workpiece features in one clamping.
“Machines engineered with a focus on grinding hard materials have systems to measure and control grinding pressure to protect the tool’s cutting edge from sudden changes in grinding forces. On-machine measurement capability, usually in the form of touch probes, is essential to ascertain the location of the tool before grinding and to then confirm that the desired results have been achieved,” said United Grinding’s Stolmar.
Grinding a diamond tool with a diamond wheel pits two similarly hard materials against each other. When a lot of excess material has to be removed from the tool, wear on the diamond wheel can result in frequent wheel dressing and replacement. In some cases, an electrical discharge machining (EDM) or spark erosion process can be a cost-effective alternative.
For machining with the noncontact EDM process, the PCD tool’s metal binder provides the required electrical conductivity. EDM is accomplished using a wire electrode or a solid tungsten-copper disk (referred to as electrical discharge grinding or EDG). “EDM can be used to rough-cut the tool geometry, and then a diamond grinding wheel provides the final finish for the cutting edge. Conventional grinding may also be employed to contour a tool’s body or flutes. Especially modern geometry PCD tools, like drills using wafter inserts and solid PCD heads of spiral end mills with PCD sintered veins can only be manufactured with the EDG disk technology,” said Stolmar.
Grinding machines that combine conventional grinding with EDM/EDG capabilities may have a single spindle and a wheel changer to mount a conventional grinding wheel or an EDG disk. Other machines have two spindles, one for grinding and the other for grinding or electrical erosion. “These multitasking machines offer the important capability to complete a tool in a single chucking, saving time otherwise spent switching between machines and also assuring accurate relationships between the ground and electrically eroded features,” said Stolmar.
“Laser machining is a recent development that is well-suited for rough machining PCD blanks, creating chip breaking geometries on the face of a cutting tool, cutting chamfers and even marking a tool with identification information,” said Stolmar. “Laser machining can also be advantageous when finishing the cutting edge of a coarse-grain PCD tool. Conventional grinding can break individual large grains away from a cutting edge and leave it rough, while a laser will cut cleanly through the grain and produce an edge that is smooth. Because no cutting forces are involved, laser machining permits very accurate and consistent positioning of the tool during the material removal process.” ME
This article was first published in the June 2014 edition of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 6/1/2014