RSX Series High Friability Resin Bond Diamond
By: Roy A. Wagner, Technical Director, SAT SuperAbrasive Techniques, Inc.
Copyright SAT Inc. 1999, All rights reserved.
1.0 Application Engineering Guide.
The increased performance capabilities of the New RSX-Series Resin Bond Grinding Diamond has opened new opportunities for achieving superior surface finishes and increased productivity on Ceramics as well as the old traditional Cemented Carbide (WC) work-pieces. This paper will explore some of the different properties and characteristics of these diamond products and recommend the applications where these diamonds are especially well suited for meeting the grinding challenges of today.
Friable Diamond Particles Cut Best !!
Grinding Ceramics and Cemented Carbides require diamond properties that allow the crystals to remain sharp throughout their entire cutting life. Unfortunately, the abrasive process when grinding super hard ceramics and cemented carbides tends to polish the crystals cutting edges, reducing the cutting effectiveness. This is where the friable crystal characteristics of RSX diamond come into play. Rather than becoming polished and requiring frequent dressing, the friable crystals tend to micro-fracture as their primary wear mode, continually presenting sharp new cutting edges to the workpiece throughout the life of the wheel.
There are several ways to approach the need for a friable diamond crystal, but they ultimately involve:
The Intrinsic Crystal strength is determined by the type, the level, and the location of inclusions and the overall crystal defect structure. Ideally, for Resin Bond Diamond to work effectively, the inclusions will be spread throughout the crystal to allow 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.
This ideal crystal "defect structure" is easily recognized when microscopically examining the RS4 crystals against a white background with reflected light. Alternately, 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-Makers viewpoint, this blocky shaped friable crystal enhances the possibility 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 platelets contained within the crystal mix. However, while the angular shape does encourage some progressive breakdown of the crystals as they begin to dull, this shape factor is typically much less predictable than the blocky crystals with the intrinsic crystal structure as described above.
An extreme example of attempting to achieve friability with shape is seen when transparent metal bond diamond or even natural diamond crystals are highly angular in an attempt to achieve the micro-fracture wear mechanism. The limitations of this practice become obvious when considering the probability that the angular crystals will be oriented perfectly within the bond to use the shapes friable cutting edges. Improperly oriented crystals are less useable with the results being inconsistent wear, inconsistent surface finishes, and generally shorter wheel life.
In the most extreme case, if a tough Blocky Cubo-Octahedral crystal were being used in a resin bond diamond grinding wheel, the force necessary to Micro-fracture or to Macro-fracture the crystal would be so high that the crystal would quickly pop out of the bond rather than to fracture. Wheel life would be low, surface finishes would be poor, and the thoughtful engineers would quickly change to a more friable diamond crystal.
Coated Crystals Perform.
No paper on resin bond diamond would be complete without mentioning the well known advantages of Coating the Friable Diamond crystals with Nickel-Alloy or with Copper. These coatings have been shown to significantly improve the performance relative to un-coated resin bond crystals.
Typically 80+% of all resin bond diamond is used with a coating, most frequently Nickel at 56% by weight. The Advantages of coating include increased resin bond wheel life, frequently by 150+%, superior surface finishes, superior corner holding capability, etc. On the downside, we find that the power and force requirements are increased and we observe that heat generation is higher.
Un-Coated Crystals: The un-coated resin diamond wears by a combination of Micro-fracturing and Macro-Fracturing. Careful examination of the grinding wheel face will show that the working un-coated crystals protrude further from the bond surface and the actual number of working crystals/cm2 will be lower than what would be observed with coated crystals under the same conditions.
Coated Crystals: The Coated resin diamond crystals are able to 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 noted that there are more working crystals at any one time.
Heat Control: Diamond is a super thermal conductor, and even though the resin diamond material has inclusions within the crystals, the diamond will conduct the heat rapidly away from the cut zone and into the resin bond.
The presence of a Ni or a Cu coating is actually a two edged sword when it comes to heat control.
On the positive side: The Ni or Cu coating acts as a relative thermal insulator which slows the transfer of heat from the diamond 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 good 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.
These factors produce the results as mentioned above. One of the early methods for confirming these effects, Diamond Swarf Analysis, has demonstrated that the recovered diamond particles from Nickel Coated Friable Resin Bond Diamond were significantly smaller than the diamond swarf recovered from the un-coated resin bond diamonds. This would 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.
The RSX Series of Resin Synthetic Diamond offers the most application friendly Resin Bond Grinding Diamond available today. The RSX series ranges from the Semi-Friable RS6 product to the most friable RS2 diamond products, each with their unique characteristics to allow thoughtful application engineers to apply the best general purpose Blend product for grinding Tungsten Carbide (RS5-Ni56%) and to still have access to the more uniform crystals of RS4-Ni56% for all of those high precision Ceramics, Electronics, and the highest precision Tungsten Carbide grinding.
In order to better understand where each RSX diamond grade fits into the grinding applications, we will first discuss the general description of the products with a brief description of the potential applications. Then we will review the relevance of the Static Pressure or Crystal crushing test with the relative strength of each product.
The RSX diamond products range from the most Friable RS2 product which gives the appearance of being agglomerated micro-crystals in nature to the more blocky Semi-Friable RS6 diamond.
The above listed products are available in a wide range of Sizes as shown in the following Chart:
Uniform Diamond Characterization:
The "Uniform Diamond Characterization" value, (UDC values ranging from 1 to 5), is designed to improve understanding of the uniformity of the crystal make-up within the diamond (or CBN) product. A UDC of 1 would mean that the particles are completely uniform, while a UDC of 5 would mean that the crystals are widely different from each other.
Many of the diamond products which have come to be the "Standards of the Industry" are based on Blending Practices which worked "well enough" 25 or 40 years ago, and which have not been revisited often enough since then. In the extreme case, defining a diamond product by its "Friability" or by its "Toughness Index" could lead to the proportional blending of two widely disparate materials, neither one of which would fall within the "Toughness" specification. This could result in a product which technically meets the specification, but with a UDC value of 5.
So what does it mean to performance?
Many unexplained failures to achieve the required Grinding surface finishes on structural ceramics work-pieces or in the electronics industry could be traced to these blending practices. This would entail blending some minute or even not minute percentage (5%, 10%, 15%, 25%, or whatever) of tougher metal diamond into the friable resin bond grinding diamond product as a cost saving extender, a "Cost Improvement"!
Now after many years, these practices which worked without being noticed in 1975 are the reasons for the failures which are attributed to other causes such as "Oversized Particles" rather than being attributed to the real cause which is "Over-Strong particles". In todays vernacular, perhaps it could best be described as a 5% enriched product, with 5% of the crystals being on steroids!
In order to compete with these products that the conventional wisdom says "work" in the well established grinding applications, it is necessary to understand why they work when they work so that we can better predict and explain why they fail when, in fact, they do fail. To learn more about this, we have recently studied our SAT RSX-Series of Resin Bond Diamond where we are starting with the uniform friable diamond materials, without having any "high strength Diamond Blend-ins" added.
We were looking for the best way to offer Resin Bond Diamond products which were competitive for those applications where the "Old Blends" work effectively and to also offer products for those applications where the "UDC1" crystal products can help us to achieve a new level of Resin bond grinding performance.
To begin our study, we evaluated diamond products by evaluating the Single Crystal Crushing Strength in an effort to categorize the initial resin bond friable diamond particles prior to making a "Magic Blend" product. This would then enable us to prepare our own "Blend" products (UDC3) for those applications such as the general purpose wet grinding of Tungsten Carbide (WC) where finish is not critical. We would maintain our option to offer the Pure un-blended UDC1 Resin bond diamond powders for those more sophisticated applications where the blends have been hampering grinding innovation.
UDC Characterization Test:
1) Select the appropriate Tungsten Carbide Wear Plate for the specific Mesh Size of diamond which is to be tested. The Tungsten Carbide Wear plates are available for all mesh sizes of the RS-Series diamond. Each Plate has a row of 50 cavities into which the 50 crystals are placed.
2) Take the Diamond Crystals, 1 by 1, from their container to the Test Stand and place a single crystal in each of the 50 cavities.
3) Finer sizes must be tested while being viewed under a suitable microscope.
1) An Anvil is mounted directly above the first crystal.
2) Move the Anvil Straight Down at 80mm/min.
3) Observe the crystal through the microscope and record the Force reading (in Newtons "N") at the exact point of Crushing. The "Crush Point" is defined as that point where the Largest remaining piece of the crystal is less than ½ of the original diameter of the crystal.
4) Repeat for all 50 of the Crystals in the test plate.
5) Calculate the Average "N" reading.
6) Run a series of three (3) tests for the mesh size in question. Take the Average value for the 3 tests to get the "N" value.
Relative Crystal Strength in Newtons:
As can be seen from the Crystal Strength Data, the products are designated according to the Relative Crystal Strength of the 120/140 (D126) mesh size in each case.
As mentioned earlier, the RS5 product is actually a Blend of grown RS5 crystals plus RS4 and RS6 material designed to blend "nearly similar" materials (UDC3) (as opposed to milled saw diamond) to make a product which duplicates the performance of the generally accepted Diamond product for Resin bond grinding of Tungsten Carbide Tools.
Uniform Crystal Strength è Optimal Finish
The RS2, RS3, RS4, and RS6 Diamond products are composed of statistically uniform strength crystals (UDC1). This is the key to optimizing Resin Bond Diamond Grinding wheels for Ceramics and other precision work-pieces.
In actual practice, RS4 is the number one choice for the resin bond grinding of ceramics since it has the most desired properties and it is available in the full range of sizes. The RS3 product offers superior performance in sizes 80/100, 100/120, and 120/140.
3.0 Grinding Performance Tests:
For the grinding tests on Ceramics we refer to the excellent work by Bailey, Garrard, and Juchem which describes the advantages of using the more friable resin bond diamond crystals for a wide range of Ceramic work-pieces .
Fig: 1 - Resin Bond Wheels grinding Aluminum Oxide / Zirconium Oxide.
Parameters: Type D6A2 wheel,
Wheel Speed: 4300 sfpm
Infeed: 0.040 in/min
Fig: 2 - Resin Bond Wheels grinding Hot Pressed Silicon Nitride.
Parameters: Same as Fig. 1.
These specific Examples, (Fig. 1 & 2), involve the Grinding of hard, brittle Aluminum-Oxide / Zirconium Oxide (Al2O3 / ZrO) and HPSN Ceramic cutting tool inserts with Resin Bond Diamond in Cup Grinding Wheels.
One of the characteristics of grinding these types of hard, brittle, short chipping materials is the high normal force which is required to initiate cutting. The high cutting force, while necessary, is detrimental in terms of maintaining tolerances, workpiece chipping, and heat related damage such as workpiece cracking.
Achieving the desired Surface Finish and Minimizing Edge Chipping are of critical importance on this hard but relatively brittle workpiece material. As can be seen from the Data in Fig. 1 and Fig. 2, the more Friable Diamond gave the superior results in terms of Normal Force Level (Fn), Surface Finish, and of overall Workpiece quality.
The Stronger, more blocky material (RS6) tends to form wear flats on the crystals and the tougher crystals are too strong to micro-fracture at an achievable threshold force level. The flat polished (RS6) crystals in Fig. 1, gave the highest G-Ratio but at the expense of generating poorer surface finishes and unacceptably high normal Force values. Most Grinding wheel fabricators select the RS4 Diamond for this type of application since RS4 offers the best overall performance in terms of Life, Surface Finish, and Normal forces which fall within the tolerance range.
Conclusions and Recommendations:
Whether grinding these Hard, Brittle Ceramic work-pieces or grinding the more standard Tungsten Carbide materials, it is important to find the right compromise between wear and the grind-ability of the tool / workpiece. In applications such as the grinding of Non Oxide Ceramics, it is common for the Normal Forces (Fn) to be 4 times (4X) the normal forces for grinding Steel work-pieces, so using Friable UDC1 diamond is critical for effective grinding.
Even for the grinding of Polycrystalline Diamond (PCD) materials in Vitrified Bonds, it is generally observed that a friable diamond such as RS4 or RS3 is the best diamond for the application.
4.0 RSX Diamond Product Selection:
4.1 For: General Purpose replacement of existing UDC4 and UDC5 Resin Bond Diamond:
.. RS5 and RS5-Ni56%
Over the last 40 years, the product, which has dominated the market, has been a general Blend of a range of friable and progressively tougher crystal types, UDC4/5. Although there are many advantages to using a pure UDC1 diamond product to achieve the best surface finishes, it is somewhat of a fact of life that many bonds and applications have been engineered around the existing industry standard.
In order to match the grinding performance of this product, we recommend our RS5 and RS5-Ni56% diamond products with UDC3 diamond.
4.2 For: Premium Ceramics and WC Grinding:
. RS4 and RS4-Ni56%
For those applications such as the Ceramic Grinding applications listed above, the RS4 series of products is the best diamond for achieving the best results on the widest variety of workpiece materials. The UDC1.5 crystals in this product will help to consistently achieve the Best Surface Finishes.
4.3 For: Specialty Ceramics:
RS3 and RS3-Ni56%
The RS3 Diamond product offers a slightly more friable UDC1 crystal in the coarser sizes and this difference can be especially helpful for the Hard, Brittle Ceramic materials.
Special Products: HELP !!!
Of course, for those special applications and special grinding problems, we are fully aware of the fact that special tasks call for special solutions. Our engineers are happy to make specific recommendations to help solve these problems on a case by case basis, utilizing our years of experience both here and abroad.
RSX Products and Applications:
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