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Tight-Tolerance Grinding

 

The precision package


By Robert B. Aronson
Senior Editor 


Grinding is coming on strong as the preferred, and sometimes only way to meet today’s standards for accuracy. The price of CBN is dropping as it becomes more a commodity than a specialty item.

Grinding machines and processes have improved to take full advantage of the newer grits. For a time, manufacturers tried to use older machines with CBN and diamond wheels, often with disappointing results. Now, more precise grinding machines are dominating the market. But machines are not the only piece of the precision formula.

The grinding wheels and grits, the coolant and how it’s delivered, the truing and dressing system, the software, and operator intelligence all work together to deliver high-precision products. Here’s a look at how the suppliers of these elements make their contributions.

Grinding research at The National Institute of Science and Technology (NIST) (Gaithersburg, MD) is chiefly concerned with high-performance grinding. “There still is a lot of basic information we need to discover with the grinding process,” says Rob Ivester, mechanical engineer, Manufacturing Metrology Div. Most recently they have been looking at high-speed grinding with single-layer abrasive (SLA) wheels. In one case they investigated grinding with a 10" (254-mm) diameter SLA wheel at up to14,000 rpm with surface speeds up to 614 ft/sec (186 m/sec).

“With SLA wheels you get progressive grain dulling without much grain fracture or pull out,” says Ivester. “It becomes harder and harder to cut, and the process consumes more power, and puts more heat into the work.” The amount of exposed grit and wear-flat area increases with time, leading to increased grinding temperatures and forces. “We wanted to understand how feeds and speeds affect wear in SLA wheels to model potential thermal damage driven by the growth of the wear flat areas, so effective adjustments to process variables could be predicted.”

The researchers made topographic charts of the wheel surface showing changes in grain size, shape, and distribution with increasing wheel wear and different feeds and speeds. “Our initial conclusions showed that some wheels had more grains than needed. You don’t want the wheel ‘encrusted,’ the grains should be spread out.”

The NIST research indicates: “You should not grind with an SLA wheel at the same feed and speed for each part because the factors that make an ideal cut are constantly changing with wheel wear. By treating each part individually and adjusting variables appropriately, you can lower cost per part dramatically, and you will not throw away as much unused grain,” says Ivester.

The problem is, how do you get enough information to be predictive? The shape and size of abrasive grains in SLA wheels have to be regular. If the users know wheel topography and that the wheel surface is consistent, then they know how to program the cutting variables to compensate for wear.

"There are few sweeping changes in truing and dressing,” explains Ted Giese, of the Abrasive Engineering Society (Butler, PA). Perhaps the most significant, but not necessarily recent, are CNC control of dressing and truing devices, and the use of acoustic emission sensors to reduce cycle times. “Most recent technology advances have been applied to relatively specific machines, wheel designs, or manufacturing objectives,” he explains. “For example, advances like ELID dressing have limited application such as precision finishing of ceramics where a fine-grained, metal-bond diamond wheel is used.” Similarly, new vitrified bonds for diamond and CBN superabrasives have brought with them the need for greater attention to dressing and truing in specific applications for those grinding wheels.

When it comes to grinding wheels, customers have very distinct needs. “The main features customers ask for are better dimensional tolerances, taper, and roundness, particularly for automotive and bearing applications. They want to achieve these tolerances while maintaining or increasing productivity. So the material removal rates may also increase to 1.0 in3/min/in. while holding a Cpk of 1.33 or better,” explains Patrick Redington, corporate engineering manager, St. Gobain Abrasives (Worcester, MA). Better roundness in bearing component and bearing surfaces has become an important issue. It reduces friction, noise, and chatter and therefore increases the life of bearing components.

“Many of the wheels, both conventional abrasives and CBN, used in precision automotive grinding applications are made with vitrified glass bonds. This allows the product to anchor the abrasive better and give us higher G Ratio, that is volume of work removed/volume of wheel used,” says Redington. “We are looking for ever higher strength bonds and better ways to control our porosity.

Porosity allows the wheel to carry more grinding fluid to the work area, it also allows more space for grinding swarf, thus reducing the tribological components in the grinding zone.

Grinding wheels use structure (conventional abrasive) or concentration (diamond or CBN) to describe the volume of abrasive in the wheel. Normally a higher structure (lower volume) or a lower concentration (lower volume) wheel is used on applications where you have high contact area and sensitivity to metallurgical damage. Typical applications are creep-feed grinding, double disk, or material like high alloy steels.

“Vortex, our new conventional abrasive technology is used for creep-feed, centerless, and double-disk grinding applications,” explains Redington. “It allows us to get the higher wheel structure we want without induced porosity. This technology excels in applications where metallurgical damage is a concern but we need high material removal rates and tighter tolerances.

Truing Systems (Troy, MI ) manufactures diamond rolls and dressing blocks for grinding wheels, specializing in upper-end work with tolerances down to 0.5 µm and very tight sphericity. “Our old standard used to be Ra 16, now it’s Ra 4,” says Dave Stempin, vice president.

“In the 1990s hard turning was taking jobs away from grinding. Now it’s coming the other way. This shift is due, in part, to dropping CBN prices and working with harder materials that only grinding can handle.”

Truing Systems owes their ability to reduce cycle time to cutting the number of dressing cycles, greater wheel consistency, and the fact that the tougher materials put more wear and tear on the wheels, requiring an even tougher dressing system.

The company’s diamond rolls are specifically designed for a given project. That includes the type, size, and concentration of the grit, as well as the bonding material that holds the grit or diamonds together, and number of stones on one roll. There may be as many as six different types of diamonds and concentrations on one roll.

“For positioning of the rollers and wheels we rely on machine accuracy,” says Stempin.

At Tru Tech Systems (Mt. Clemens, MI), they have a stringent set of standards for their high-precision CNC grinding machines, which are designed specifically for cylindrical grinding of round parts.

  • Spindle speed on the Tru Tech is adjustable from 2000 to 5000 rpm to produce a variety of surface finishes.
  • Stepper motors, rather than servomotors, are used to position the machine. These motors won’t oscillate or overshoot their destination as servos can do. The encoder resolution is so fine the machine can be edited in 10 µin. (0.00003 mm) increments.
  • Online wheel dressing allows the grinding wheel to be trued to the spindle, providing better surface finish and longer wheel life.

The company uses three-axis machines with a single grinding wheel to grind multiple shapes on a part. It’s possible to generate multiple steps, radii, angles, back taper, and drill points with a single 1A1 wheel using one program and one setup.

Several features allow the Tru Tech grinding machines to hold roundness to 15 µin. (0.0004 mm). All part diameters run concentric with each other within 30 µin. (0.0008 mm) on the standard model and up to 10 µin. (0.00003 mm) with the ultra-precision model, with no indicating required.

“The days of 0.001" [0.03-m] tolerances are gone,” says Steve Smarsh, vice president of operations.

The size of workpieces on the Tru Tech machine can range from 0.010 to 4.000” (0.3–102mm).

The software allows a company to put a new operator with little experience on the machine and program most jobs in under five minutes. The Tru Tech software is self-training and has built-in Help videos that guide operators through programming, setup, and preventive maintenance without leaving the machine. “This allows companies greater flexibility in cross-training employees and reduces training costs,” says Robby Faulkner of Tru Tech Systems.

Within the grinding industry, finding skilled operators can be an issue. “We’ve resolved that by building a highly accurate machine with very user-friendly software that anyone can learn to operate. We can turn a McDonald’s clerk into a precision grinding operator,” concludes Steve Smarsh.

Dynamic stiffness and thermal stability are two of the key issues influencing accuracy of Studer grinding machines from United Grinding Technology (Miamisburg, OH). “We don’t use cast iron but Granitan, an artificial stone, which is basically interlocking granite pieces bonded with epoxy. It reduces vibration and takes care of all thermal instability within the machine,” explains Hans Ueltschi, manager for the cylindrical group of UGT.

Studer has gone to higher precision bearings. In the past they used hydrodynamic bearings, but they now use high-precision, angular-contact ball bearings. They give high accuracy and are more flexible when there are speed variations.

However, hydrostatic bearings are used on machines for special applications such as fuel injectors. “When grinding a chuck we have roundness accuracy of between 8 and 16 µin. We can achieve less than 4 µin. The machine has specially designed linear motor drives that allow us to achieve an accuracy of 0.00001 micron (10 nm). The benefit of the linear motor is not so much for the cutting time, but time saved in positioning. Linear motors can move at 1200 ipm (30 m/min) with an acceleration of 3 m/sec2.”

Another precision machine from UGT is the Jung J 630D CNC unit. It has a 600 × 300-mm table range. Guideways are double V and give an accuracy of 1 µin. or less over the entire range of table motion.

“With our profile grinders, the accuracy of the dressing system is as important as the rest of the machine,” explains Reinhard Koppen, applications manager, profile grinding. “Ours is a three-axis system with high-end, continuous-dress capability. During dress you maintain contact and don’t have to get out of the cut. You dress and grind at the same time.”

A heat exchanger maintains the Jung machine at ambient temperature.

Software is a proprietary system that lets the operator go from art to part. An imported file goes to the proprietary Grips compiler, which contains all the variables of that particular machine. The dressing system is three-axis and automatically compensates for machine errors, runout, and dressing tool error, then generates the proper contour after factoring out all dynamic errors.

More grinding of bevel-gear sets by US auto and truck manufacturers is one trend seen by Eric Mundt, chief research and design engineer, grinding products, Gleason Corp. (Rochester, NY). “The old method was machine, heat treat, then lap. Now it’s, machine, heat treat, and grind.

There is also a production advantage. When you lap a gear set, the pinion and gear are mated for life. You can’t change one. With grinding, the pinions and gears can be easily mixed. Interchangeability is possible because grinding minimizes flank-form geometry and tooth spacing errors from part to part.

Also, notes Hermann J. Stadtfeld, vice president, Bevel Gear Technology, “There is a much higher scrap rate with lapping. Plus error on gears can be more easily corrected, so scrap rate for the grinding operations can be virtually zero.”

There has also been a similar move to more grinding for finishing the helical and spur gears in the front-wheel-drive elements of cars. This is driven by the increased torque through these gears plus a need for lower noise, particularly in the higher transmission-speed ranges.

Thermal issues are a major consideration. The Gleason philosophy is to design out thermal influences so that dimensional changes are self-canceling or don’t reach the work area. There is also conventional coolant-temperature control and sophisticated temperature compensation of major machine components.

“We are also working with aerospace companies to improve their gear-grinding operations,” explains Stadtfeld.

To help ensure each customer takes full advantage of their machines, they offer a “gear college” to buyers. This covers topics from basic gear design, through performance, metrology, and maintenance.

 

This article was first published in the June 2006 edition of Manufacturing Engineering magazine. 


Published Date : 6/1/2006

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