Advanced Abrasives
Examining diamond dressing in grinding applications
November 21, 2011
Proper dressing is essential in any grinding operation.
Dressing restores a dull grinding wheel to its original sharpness. During a dressing operation, swarf is removed, as are the dulled abrasive grains and any surplus bonding agent. Dressing also can be performed to customize a grinding wheel's face to the correct profile.
Diamond dressing tools use the inherent hardness of a diamond's point to restore a grinding wheel's face. For years single-point diamond tools were the only choice, but as natural diamonds became rarer, multipoint diamond tools were developed.
In multipoint diamond dressers, a number of small diamonds are held in a matrix. During dressing, diamonds on the surface of the tool wear away and new, sharp diamond points are exposed.
Certain discs lend themselves better than others to certain dressing operations, depending on the type of dressing that is being performed. For example, in plunge dressing, rolls contain the workpiece's geometry and transfer this geometry to the grinding wheel in a single motion.
Dressing CBN Wheels
Rotary diamond discs typically are the standard choice for dressing cubic boron nitride wheels. This is because CBN is the second-hardest material, behind only diamond. Four bonding types typically are available for this type of dressing:
- Electroplated bond. The electroplating process deposits a single layer of diamond onto a steel body. This dresser is very sharp but wears quickly, requiring frequent replacement.
- Metal bond. This is the most common dressing tool bonding agent. It is durable and can be used on both CBN as well as diamond wheels.
- Vitrified bond. This technology is suitable for applications that require a very sharp grinding wheel surface. Wear rate is slightly higher than a metal bond.
- Hybrid bond. Meister Abrasives has a new, proprietary bond that combines the free-cutting structure of vitrified bond with the durability of metal bond, the company states.
According to the manufacturer, the advantages of this new hybrid bond include more efficient dressing cycles that lead to more consistent CBN grinding wheel performance; longer intervals between dresses, resulting in longer grinding wheel life; improved longevity of the dressing tool itself; and elimination of the need to change out the dressing tool for reconditioning.
Diamond particles are both mechanically and chemically bonded to the tool. As a result, the bridges that attach the diamond crystals to the bonding matrix are slender, allowing for large pores to be naturally distributed between the crystals. The improved porosity means that these rotary dressing tools can carry away more material and heat from the wheel with every revolution.
More efficient dressing means that CBN grinding wheels last longer, because less of the abrasive needs to be removed, less often, to return the wheel to optimal condition.
"The new [hybrid] technology will come as welcome news, especially to high-volume shops that keep tight statistical control over their processes," explained Bruce Northrup, general manager of Meister Abrasives USA. "The hybrid dressing technology maintains the CBN wheel's shape and cutting condition without ever allowing it to become dull. This greatly reduces, and in many cases eliminates, sudden changes in part size, surface finish, [and] roundness that often occur before and after a dressing cycle."
From abrasive manufacturer Dr. Kaiser, RIG tools are plated diamond dressing tools with a steel carrier body to which one layer of diamond is attached with a nickel bond. According to the manufacturer, vitrified-bonded CBN wheels dressed with these tools have an excellent stock removal capability. However, as there is only one layer of diamond, the life of the dressing tool is relatively short.
Another option is sintered RI tools. These dressers feature a thick layer that is impregnated with diamond grit, which improves tool life. This diamond layer can be shaped very accurately for dressing of profiled wheels.
Gear Grinding
Rotary diamond dressing tools also are used for profiling gear grinding wheels, and the latest generation incorporates polycrystalline diamond (PCD) reinforcing of the dresser tip on a direct-plated diamond dresser. These tools generally can be relapped and replated to lengthen their lifespan.
Monocrystalline diamond dressers are stationary tools for dressing complex profiles and radii on grinding wheels. They provide long life, stability, and repeatability, especially when dressing ceramic abrasive wheels.
Diamond Types
Generally speaking, all diamonds used in the dressing process fall into one of four categories:
- Natural diamond, randomly set
- Natural diamond, hand set
- Natural diamond, with reinforcing
- Synthetic diamond, hand set
"If you are doing a lot of dressing, natural diamond is the way to go," explained Lyman Munson, president of S.L. Munson & Co., a distributor for Dr. Kaiser. "With a natural diamond you get only one or two relaps. With synthetic you get more. It's critical that the correct diamond shape and size is used based on the type of dressing that you are employing."
Grinding Connecting Rods
Noncoplanar connecting rod grinding system improves productivity, flexibility
One of the processes at Eston Manufacturing, a division of Linamar Corp., Guelph, Ont., is grinding a 1.8L connecting rod with a symmetrical reduction of the pin end to 3.8 mm. The challenge is producing this part from a coplanar connecting rod in one operation.
Stock removal from the pin end of the connecting rod is 5 mm, and stock removal from the crank end is 1 mm. Previously this connecting rod required three grinding operations to complete, and the crank and pin end were ground on two separate machines.
C & B Machinery, Livonia, Mich., suggested Eston try the company's double disc noncoplanar connecting rod grinding system, first introduced in 2009 for the Chevrolet Volt® 1.4L auxiliary engine.
The DG-2H-30/SA connecting rod grinding system has the ability to grind parallel parts, noncoplanar parts, and pin end only or crank end only configurations with very simple changeovers.
For the Volt's part, the grinding system produced a stepped connecting rod from a parallel (or coplanar) rod. This 1.4L connecting rod had a 1.2-mm, symmetrically smaller crankshaft end width, and both the pin and crank ends of the rod were ground in one sequence.
For the new 1.8L connecting rod, C & B's application engineers took the challenge to task by developing a special tooling package and unique two-stage grinding cycle that first rough-grinds the pin end to a stock condition equal to the crank end. It then finish-grinds the crank and pin end of the connecting rod simultaneously. Grinding the crank and pin ends at the same time produces much better symmetry between the two, the company reports. It also exceeds Eston's 1.67 Ppk requirements for size, parallelism, and flatness.
The grinding system includes a postprocess gauging system with automatic feedback to the grinding wheel infeed servos, providing automatic compensation for wheel wear. The gauge also inspects the step relationship between the crank and pin end faces.
With grinding system models utilizing up to 42-in.-diameter grinding wheels, large diesel connecting rods can also be produced in this manner.
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