MicroPyramidä CBNXä Series – Cubic Boron Nitride
The Right Tool for Precision Steel Grinding.
By: Roy A. Wagner, Technical Director, SAT SuperAbrasive Techniques, Inc.
ç Copyright SAT Inc. 1999, All rights reserved.
Tougher CBN Crystals Micro-Cleave to present sharp new cutting edges.
Over 70% of the CBN Grinding Wheel is Worn away by Dressing and Truing.
MicroPyramid CBNX Series Cubic Boron Nitride
ä = Trademark, SAT Inc. All Rights Reserved.
SAT SuperAbrasive Techniques, Inc., 886 East College Ave., Westerville, OH 43081-2505 USA
Tel: 1-614-882-0499 Fax 1-614-899-0993 E-Mail:firstname.lastname@example.org
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CBNXä Series –
By: Roy A. Wagner, Technical Director, SAT SuperAbrasive Techniques, Inc.
ç Copyright SAT Inc. 1999, All rights reserved. ä = Trademark, SAT Inc.
Much of the recent development of CBN has been focused on higher levels of purity and thermal stability of the CBN crystals in hopes of conquering higher and higher Material Removal Rate applications. While these tougher grades have significantly increased the range of super aggressive grinding applications of ferrous materials, they have failed to completely address the efficient employment of CBN in the vast majority of Precision Steel Grinding Applications.
Many efforts to optimize the CBN grinding performance on hardened Steel workpieces have been hampered by the inability to effectively hold onto the CBN crystal long enough to really harvest its true potential. Crystal Pop-outs, whether caused by the crystal coming out of the Nickel Coating or by the complete coated crystal popping out of the Resin bond matrix, have been a major source of premature wheel wear.
Holding onto the crystal is only half of the battle. In addition to strong bonding properties, the most effective crystals for precision steel and superalloy grinding would retain and regenerate sharp, free-cutting characteristics for extended periods of time during the grinding process. A retained crystal that refuses to re-sharpen itself by periodically micro-fracturing is eventually worse than no crystal at all. Such a crystal, when combined with its equally stubborn neighbors, would need to be forcibly evicted by means of excessive Trueing and Dressing operations.
In his recent book "Principles of Abrasive Processing" Dr. Milton C. Shaw(1), notes that:
"The inherent smoothness of CBN faces makes bonding difficult."
In the June 1997 Issue of "Cutting Tool Engineering" magazine, another well known industry expert explained the need for coatings to "add texture" to the surface of the crystal by explaining that: "Bond Strength can be undermined by the smooth surface of the (CBN) crystals."
Both of these experts are agreeing that the world of CBN does not just need a stronger crystal, but one which allows bonds to firmly grasp the crystal, allowing it to truly wear by Microfracturing as had always been the theory of grinding with CBN. Not Tougher, just Better!
The most desireable scenario would require the allowance of the crystals to remain sharp throughout their entire cutting life. Unfortunately, the abrasive process when grinding hard steel materials tends to polish the crystal’s cutting edges over time, reducing the cutting effectiveness. Although more obvious with diamond, this type of crystal polishing happens with CBN as well. It just takes a rather long time to happen. The operator sees this manifested as a need to Re-True and Dress the grinding wheel so that it can become "free Cutting" again.
Experts agree: Over 70% of the wheel wear occurs due to Wheel Dressing rather than being due to actual grinding wear!
Now, the unprecedented performance capability of our recently developed CBN crystal, CBNX , with its MicroPyramid surface structure, has opened new opportunities for truly superior crystal bonding. The new stronger bonds lead to more effective use of each and every crystal. Now, rather than being a myth, wear by micro fracturing actually happens!
For the CBN grinding wheel user, this translates into predictable consistent wheel wear when precision grinding hardened steel and Superalloy workpieces.
The end result is increased productivity when evaluated by one or more of the following key measures:
This paper explores some of the different properties and characteristics of these CBNX products and recommends the applications where these new MicroPyramid CBN5 and CBN2 products are especially well suited for meeting the grinding challenges of today.
As mentioned above, the first and perhaps the most obvious feature of the CBNX Series CBN mesh powders are the Micro Pyramid formations on the surfaces of the individual crystals. These MicroPyramids enhance the bonding capability of the CBN5 and CBN2 crystals when compared to the more conventional smooth faced CBN crystals.
The Increased Bond gripping capability only partially explains the grinding performance when compared with other CBN crystals even though the competitive crystals may actually show higher apparent crystal "strength".
In fact, it will be observed that this combination of the MicroPyramid crystal surface, and the slightly more friable crystal nature, lead to special highly desirable grinding performance characteristics.
Rather than becoming polished and requiring frequent dressing to restore the wheel’s cutting ability, the CBN5 and CBN2 crystals tend to micro fracture as their primary wear mode, presenting sharp new cutting edges to the workpiece throughout the life of the wheel. Meanwhile, in the background, the MicroPyramids make sure that the crystals stay in the wheel!
There are several ways to approach the need for a strong, but friable CBN crystal, but they ultimately involve:
CBNX Crystal Structure:
Intrinsic Crystal strength is determined by the type, the level, and the location of inclusions and the overall CBN crystal defect structure. For CBN to work efficiently, the crystal must be Strong, but not too strong ; Tough, but not too tough! The optimal crystal strength allows the crystal to wear by micro fracturing when the initial crystal wear starts to cause a build-up of grinding forces.
Relative Comparisons of CBN:
¬ Grinding Strength: Test # OCT2 MT 79-2.88: Crystals are placed between two rigid plates , with lower plate being stationary and upper plate rotating. The upper plate is fed toward the lower plate to crush the crystals. The higher the value, the higher the crystals’ relative strength.
Static Pressure or Crushing Test in Newtons: Single Crystal Crushing Tests in which 50 Crystals are selected at random, placed between a rigid plate and an anvil to which Force (in Newtons) is applied until the crystal crushes. The values for the series of 50 crystals are averaged to get the "N" value for the crystals.
® Micro Pyramid CBN growth on crystal faces:
Due to our unique crystal growth process, a common characteristic of the CBN5 crystals is the outgrowth of micro pyramid shapes on the CBN5 crystal faces. (Figure 2). The advantage of this unique property of CBN5 and CBN2 is the enhanced bond retention which allows more of the CBN crystal to be used through the re-sharpening which occurs during the Micro-Fracture wear process.
¯ "Barnacled Pyramid" CBN growth: occurs on the crystal faces of a small portion of the CBN5 and CBN2 crystals. ( Figure 1).
For optimal results, the CBNX crystal must be
engineered to allow wear to occur by a progressive micro fracturing action. As a cutting edge begins to become polished it leads to a slight build-up of cutting force that causes a small portion of the crystal to break off or Micro Fracture, presenting a new sharp cutting edge that will then continue the grinding operation. The lower the force required to achieve this micro fracture, the more friable the crystal is said to be.
Unlike diamond, the ideal crystal for the CBN5 and CBN2 is rather difficult to see microscopically due to the crystal’s black color and other features such as the MicroPyramids, which become much more interesting under the microscope. To the extent that they are visible, perhaps the best viewing will be when microscopically examining the CBN5 crystals with transmitted rather than reflected light.
Transmitted light will also demonstrate the crystal inclusions, which lead to this highly desirable wear mechanism even though the overall Crystal Shape is relatively blocky. From the Wheel-Maker’s viewpoint, the CBN5 blocky shaped crystal enhances the probability of grinding wheel Performance Duplication.
Crystal Shape Factor:
The Crystal shapes are said to be "more Friable" when they are more angular shapes. The most angular product would be one with a high proportion of needles or elongated particles contained within the crystal mix. However, while the angular shape may encourage some progressive breakdown of the crystals as they begin to dull, this shape factor, when taken to extreme, is less predictable than the blocky crystals with the intrinsic crystal structure which promotes wear by micro-fracture as described above.
The difference between the CBN5 and CBN2 crystals is primarily a difference in the Crystal Shape Factor. The CBN5 crystals are relatively Blocky Black MicroPyramid CBN crystals. The CBN2 Crystals are somewhat less blocky Black MicroPyramid CBN crystals which has been demonstrated in some bond systems to be a perfect crystal for Vitrified CBN applications.
In addition to the inherent properties of CBNX products, to insure positive performance duplication, CBNX crystal characteristics are graded based on the "Uniform Crystal Characterization" system. The "Uniform Crystal Characterization" value, (UCC values ranging from 1 to 5), is designed to improve the definition of the uniformity of the component crystals of a SuperAbrasive (diamond or CBN) product.
A product with a UCC Value of 1 would mean that the constituent particles are primarily uniform, while a UCC of 5 would mean that the crystals are widely disparate in terms of shape, strength, and / or other characteristics.
For the most predictable grinding performance, all of our MicroPyramid CBN2 and CBN5 products are graded to a UCC values of 1 to 2.
CBN Bond Systems:
Most of the market growth during the past twenty years has been with Vitrified Bond and Electroplated CBN grinding wheels, for very good reasons.
The vitrified CBN wheels have an extremely strong, frequently porous bond which matches the coefficient of expansion of the CBN crystals themselves. The MicroPyramid surfaces of the CBNX crystal serve as miniature grip points and become bond post sites to allow the bond to securely hold onto the CBNX crystal.
The vastly superior bond retention, afforded by the employment of our revolutionary MicroPyramid CBNX series are empowering thoughtful Superabrasive Engineers around the world to develop better, more easily used, and more productive vitrified CBN grinding wheels.
Electroplated and Braze Bond tools have a single layer of CBN abrasive, typically bonded by nickel or a proprietary metal Alloy. Since Electroplated bonds mechanically hold the crystals to the tool, there is only a small portion of the crystal that can be exposed above the nickel plating.
The conventional wisdom for CBN was to look for the stronger crystals in order to make sure that the tool achieved the longest possible life before the crystal stopped cutting or wore down to the bond level. In actual practice, once the "High Flier" crystals break off, the percentage of "working" crystals progressively increases, leading to a force or power build-up that ultimately becomes unacceptable for the workpiece or for the grinding operation.
MicroPyramid CBN5 crystals help Electroplated Tools in two ways:
Initially, the most popular bond system for CBN grinding was the Resin Bond. Carrying over from the industry’s experience in resin bond diamond wheels, the introduction of a Nickel Phosphide coating for CBN was "an automatic".
These coatings, typically 60% by weight, have been shown to significantly improve the performance of CBN crystals much as the coating has enhanced the performance of the RSX series of resin bond grinding diamond.
The Coated CBN crystals can be held in the bond for a significantly longer time so that a higher proportion of the wear is by micro fracture. Careful study of the grinding wheel face would show that the crystal protrusion distances are smaller than with the un-coated crystals. It would also be noteworthy that there are greater populations of working crystals at any one time.
Like diamond, CBN is a super thermal conductor, and even though the CBN material has inclusions within the crystals, the CBN would conduct the heat rapidly away from the cut zone and into the resin bond.
The presence of a Nickel coating is actually a two edged sword when it comes to heat control.
On the positive side: The Nickel coating acts as a relative thermal insulator which slows the transfer of heat from the CBN crystal into the bond and allows the heat to be transferred to the coolant or to the ambient air after the crystal exits the cut zone.
On the negative side of the heat issue: The coated crystals’ improved micro-fracturing characteristics cause the wheels with coated crystals to have a higher working concentration of slightly flatter crystals which generate more heat in the first place.
Numerous tests have demonstrated that the coating did indeed prevent gross crystal fracture and premature pullout. Hence the conclusion that more complete micro fracturing was occurring with the coated crystals.
Premature Pullout of CBN crystals in Resin Bond grinding wheels occurs for a number of reasons. The most commonly observed failure occurs when the crystal and coating pulls out of the bond. This failure mode is shown as "Type A" Pullout.
The second type of pullout occurs when the crystal fails by popping out of the Nickel coating. This is typically known as "Type B" pullout.
Figure 1: Type A Pullout (Left), Type B Pullout (right)
The miniature grip points afforded by the MicroPyramid surfaces of the CBN5 crystal enhance the "bond-ability" of the Nickel Coating to the crystal.
These same MicroPyramids further enhance the texture of the Ni-coated CBN crystal which enhances the bonding of the resin bond to the nickel coating, a feature which is especially critical for Polyimide bonds.
In summary, through the employment of CBN5 and CBN2, the Wheel maker is assured of excellent bond adhesion due to the unique crystal surfaces, whether uncoated for Vitrified and Electroplated bonds or as Coated Crystals for the Resin Bond applications.
And, although periodic Truing and Dressing remain an integral part of the Resin and Vitrified Bonded wheel precision grinding process, the true micro-fracturing capabilities of CBN5 and CBN2 will allow the grinding wheel to productively run for longer periods of time in between conditioning operations. The result is longer wheel life and superior performance overall.
Finally, because of the superb uniformity of the crystals, the excellent performance realized from utilizing CBNX crystals is predictable for every production run.
The CBNX products are available in the
For Further Information and possible special products to meet your special needs, please contact us at:
SAT SuperAbrasive Techniques, Inc.
886 East College Ave.
Westerville, OH 43081-2505 USA
The CBN5 MicroPyramid Advantage:
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