Catching up to creep-feed grinding.
Unfortunately, the US machine-tool industry has not kept abreast of this evolving technology and has lost market share to off-shore competition. US industry, until recently, has depended on foreign machine-tool builders to produce creep-feed grinding machines and modern grinding systems. US builders have had to play catch-up, with few companies fully embracing the new technology, because of the significant investment commitment required-both financially and technically in the machine-tool designs suitable for creep-feed and super-abrasive grinding.
CFG is a high-precision, highstock-removal abrasive process. Even the most difficult-to-machine materials can be machined relatively burr-free with excellent surface integrity. Metallics are being machined at rates much faster than milling and in the hardened state. The savings are not just the result of fast stock removal. Add to this the elimination of costly deburring operations, straightening after heat-treatment, inventorying of raw material and consumable tooling, risk of thermal or metallurgical damage to part surfaces, or the need for expensive near-net-shape technologies. This is why CFG is the choice of the aerospace industry.
New machine requirements
CFG has been in use for almost 30 years. The 1990's will be an opportune time to reassess the CFG process and develop a completely new concept in machine-tool design, a new generation addressing these present and future industry needs: Higher speeds: Industry recognizes the need for higher peripheral wheel speeds, particularly with superabrasives. This expertise lies predominately in Europe, and US safety standards for use of high wheel speeds lag those in Europe. Major high-speed advantages are being lost here due to inadequate machine designs and lack of initiative to improve wheel-safety standards.
Higher precision: Precision is directly affected by machine-tool design. Although ILNCs and pseudo-adaptive controls allow poor machine designs to perform somewhat satisfactorily, the only path to an advanced, more precise machine is through its basic design. A significant contributor is the epoxy/concrete machine base, such as the Granitan patent held by Studer in Switzerland. US builders are relying heavily on the Swiss for machine-base technology, as well as its fabrication and manufacture. Wheel technology: The latest abrasive technologies require superior machine tools from the standpoint of thermal and vibrational stability, as well as truing and dressing methods. Whether the raw grains are manufactured by GE or DeBeers, the majority of superabrasive wheels are made by foreign sources. The latest grinding-wheel technologies are vitrified superabrasive wheels and high induced-porosity conventional wheels. Japan leads the way in superabrasive wheel technology, followed closely by the Europeans. Domestic wheels have improved dramatically in recent years with some wheel specifications equal to European. Although the dollar decline has made foreign products less attractive, European wheel vendors remain highly competitive. Materials technology: Machining the latest materials-high-temperature alloys, ceramics, and nonmetals-requires machine tools with high stiffness and superior control and resolution. As this technology accelerates, it is leaving behind those grinding machines that have been on the shop floor for years and are now either technically or economically unable to machine these latest materials.
Continuous-dress capability: Continuous-dress creep-feed grinding (CDCF) is an additional need for US industry, beyond CNC creepfeed and CNC surface grinding. Other than companies such as Brown & Sharpe, Roberts, and Gallmeyer & Livingston in the US; CFG and CDCF machine-tool expertise lies in Europe with companies such as Elb, Maegerle, and Hauni-Blohm. The Japanese are closely following the creep-feed process with Niigata and Okamoto offering machines capable of CFG and ceramic grinding.
Controls and automation
Control systems need to be developed to perform complex multiaxis contouring of shapes on CFG machines. Very high resolution is required. Contouring has opened a new market for CFG, but needs the development of user-friendly controls. For CFG to machine a wide variety of materials and profiles, it will need machines designed for faster and easier setups and economical small-lot production. Today's just-in-time concepts are completely different from those for the CFG machines of even a short time ago.
Even in Europe, makers of CFG machines still rely on the traditional surface-grinder approach to their machine designs; i.e., a century-old concept of manual operation. A major disadvantage of CFG today is that cut time is such a small portion of floor-to-floor time.
Automation of part loading unloading, and wheel and dresser changing is of paramount importance. A newer surface grinder concept is needed that is uncompromising in capitalizing on the potential of the creep-feed process. The move is away from dedicated automated grinding cells (ably suited for producing turbine blades in high volume) to more flexible systems that allow economical production of medium to small batches of a wide variety of workpiece shapes and materials.
Research in creep-feed machineability is presently being conducted in the US, and accelerated to encompass wider fields of materials, grinding wheels, dressing systems, and cutting fluids. With so little CFG expertise in the US, the process desperately needs a source of definitive machineability data, and users need confidence in what is very much a foreign process-in both senses of the word. Process adaptive control is not presently a reality for CFG. Although machining to a predetermined algorithm is possible, true adaptive control is beyond the realm of present-day technologies.
With these industry needs, there is enormous potential for the US machine-tool industry to build a new generation of CFG equipment. With little to be gained from equaling the competition, the effort should be to surpass it. A new approach would not only offer a competitive alternative, but boost export sales. This calls for a cooperation between user and machine builder, and input from independent sources to keep the design universal and not particular to one industry or application. A concept I have long proposed for a whole new generation of grinding machines is based on bringing the part to the wheel, instead of the wheel to the part. This, after all, is how our ancestors sharpened their knives and tools: they held them against a stable, spinning wheel. They never attempted to do it the other way around! My approach incorporates a wheelhead and dressing system more rigid and vibrationally stable than any existing production machine. Theoretical stiffness is in the order of 6 million lb/in. The principle of this patented design is a stationary, dual-supported grinding wheel. A special hydrostatic bearing allows the wheel and dresser to be changed easily automatically, and accurately without sacrificing the mechanical stiffness of the system. The result is a single grinding machine capable of flat, form, contour, cam, and OD cylindrical grinding. This concept of versatility, stiffness, and stability would take abrasive machining into the next generation.
Reviewing the latest in grinding technology
Long on the leading edge of grinding technology, Dr Stuart Salmon has some interesting insights on the latest rapid advances in grinding technology and their correspondingly slow reception in the US: What advances? Look in any textbook on the grinding process today, and you'll see hundred-year-old machines, he laments. "The tremendous advances in both the process and machine-tool technology are yet to be reflected in the textbooks of today. Too many people in grinding today are simply trying to do the same old thing a little better, and are unable to take a long-term view. They see no need to consider change or retraining their workforce."
Cylindrical grinding. Here, he faults our inability to achieve a smooth surface down the length of the workpiece. "It's unfortunate that there aren't many ballscrew or toothed-belt table drives on cylindrical grinding machines. The rule of thumb is for wheel traverse to be two thirds of wheel width. What we should be looking at is making pitch equally divisible into wheel width so that all the wheel idiosyncrasies overlap one another. Making part pitch equally divisible into wheel width will produce a much better finish without spiral marks and in a much quicker time. But, to do that, you need much better control of the table, and a more direct drive than the hydraulically driven tables prevalent today."
Centerless grinding. This machine's reaction force to grinding off a lump on one side of the part typically creates a mirror Image of that lump on the opposite side. That doesn't have to happen, Salmon says. "Research has shown that we can monitor the force on the workpiece, actually the reaction force pushing against the shoe. If that lump is sensed with a force sensor and a CNC-type axis on the shoe, we can get much faster round-up time. Unfortunately, this has yet to be offered on a machine tool, 'although well proven by adapting research machines. This is just another case where it's up to users to force machine builders to incorporate this into their designs to gain major productivity improvements."
Tool grinding. -In the past, all tool-and-cutter grinding was done dry," he explains, with a little finger Supporting the milling cutter as it rotated back and forth. Because that finger was always bending, you never got the same flute sizes. With today's CNC controls, much more consistent tools can be manufactured, and, because the grinder is totally enclosed, we can now use a flood of cutting fluid-a major key to successful grinding and a big step forward."
Jig grinding. In jig grinding, there have been a number of machine tools modified to become machining and grinding centers. "The Japanese have had major successes with their machining-center approach to grinding, and we are seeing jig-grinding operations performed on a milling-type machine, usually with superabrasives and small wheels." ID grinding. In internal grinding, the snag has always been trying to grind inside the workpiece bore with a limited quill support.
Salmon offers a neat trick here to stiffen that quill shaft by winding it with carbon-fiber composites. "In machine-tool design, the natural frequency of vibration of a shaft is proportional to its stiffness and inversely proportional to mass," he explains. "The heavier the machine, the lower the natural frequency of vibration. The stiffer the machine, the higher the natural frequency.
"In grinding operations, the major vibration frequency is caused by the wheel being out-of-balance or out-of-round. Since this occurs once every revolution, if our ID grinding speed is 60,000 rpm, the quill shaft will vibrate at 1000 Hz. By decreasing its mass and increasing stiffness, we can design the structure to make that frequency way out of our operating range. Thus, using composites to make the quill shaft stiffer without adding much mass is an ideal, inexpensive solution."
Coated abrasives. Coated abrasives tend to be poorly regarded-just a piece of sandpaper on a crude machine that can't be very precise. Wrong," says Salmon. "Coated abrasives have grown by leaps and bounds. They are actually very sharp abrasives backed by cloth or paper, and can withstand extremely high forces. Machines up to 50 hp per 1" of drive-belt width have been designed, putting enormous power into the coated abrasive; and grains can be oriented so that their sharper points stick out. With a new approach-agglomerated grains the abrasive is crushed into smaller grains, and then reformed into larger grains. Instead of a polycrystalline single grain that wears flat, burnishing and burning the work material, agglomerated grains break down and expose new sharp edges and extend belt life. This means we can no longer ignore coated abrasives for doing flat-part grinding functions or even some form work."
Abrasive origins. With China now exporting synthetic diamond and CBN abrasive in large quantities, you must be aware that just because an abrasive is CBN doesn't mean it's necessarily good quality, Salmon warns. "You should be aware of where the basic abrasive material comes from. Is it Chinese, Russian, or made in the US by GE? Abrasive quality has a major effect on life and ultimate cost. The lowest bidder will probably be the poorest quality, whether made here or overseas.
"On the other hand, as wheel prices go up, you must evaluate if they are really worth what they promise-for example, do you really want to double the life of a wheel for three times the cost of the old wheel? You need to run a quick test to make sure the promise of a new wheel is not being exaggerated."
Wheel structure. This is essentially the air gap between grains. Traditionally, a very closed, dense structure is used for small depths of cut to withstand vibration. "Because deeper cuts with CFG are much more gentle on the wheel," Salmon points out, "we can open up its porosity and use a much more open structure. "However, this has been quite a challenge for wheel manufacturers. An open wheel becomes very I fragile, and it is difficult to keep the wheel together. Because of this, you must look at not just the wheel's stated porosity, but whether it has a homogeneous structure throughout. When wide wheels are pressed, for e am le, they can be much harder on the outside than in the middle-up to two grades softer in the middle."
Choosing superabrasives. Superabrasive trade names include Borazon CBN (cubic boron nitride) from GE and ABN (amber boron nitride) from DeBeers. Why choose them over other abrasives?
"In one comparison test," he relates, "we sent single abrasive grains around like a single-tooth milling cutter, slashing away at a workpiece to see how many rotations before each grain type wore out. Aluminum oxide wore attritiously-20 or so slashes, and. it was done. Diamond is much harder and retains its grain shape a little longer, but being carbon, chemically reacts with the carbon in the steel and gets only about 200 slashes before it wears out. CBN (inert to carbon) can make almost 1000 slashes before it wears out.
"Thus, although it appears that superabrasive grains never wear out, the reality is that CBN merely stays sharper longer. Because it seems to wear forever, people tend to not keep track of when the CBN wheel needs a dressing cycle. So the choice comes down to keeping soft aluminum-oxide or siliconcarbide wheels sharp by continuous or more frequent dressing, or to skip continuous dressing altogether and use a superabrasive wheel that stays sharper for much longer."
Higher speeds. With standard speed somewhere around 6000 sfm and high speed between 10,000 to 15,000 sfm, the new high-speed range will soon be 30,000 to 60,000 sfm, Salmon says. "At these higher speeds, we get much better wear of the abrasive. In theory, each grain does a lot less work. When we get to 60,000 sfm, close to the speed of sound, we're almost ballistically removing material.
"At these speeds, you cannot use vitrified, resin-bonded, or other weaker bond wheels; you must use plated wheels. At sonic speeds, we get into brittle fracture of the material. Wheels last longer, and hard and soft materials machine alike.
Normally, machining soft materials quickly gums up a plated wheel. An analogy with butter is to slowly scrape the top of it with a knife, and it gums up. But, a quick swipe of the knife cuts it cleanly. Similarly, when machining at high speed in the brittle-fracture regime, hard and soft materials machine alike. Instead of long curly swarf, you get concertina, dustlike particles.
"Thus, grinding at these speeds would represent a major threat to milling processes-if it ever really caught on. A minor snag here is that instead of the normal threat of wheel bursts, we would see entire shafts cracking and wheels coming off entirely at 60,000 sfm and cutting through anything they come in contact with. The machine would need a special enclosure or detection system that would sense lateral movement and use explosives to disintegrate or warp the wheel, making it buckle and clunk around inside the machine, rather than cutting a swath to infinity."
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|Title Annotation:||includes review of latest in grinding technology|
|Author:||Salmon, Stuart C.|
|Publication:||Tooling & Production|
|Article Type:||Cover Story|
|Date:||Apr 1, 1991|
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