Vitrified superabrasive wheels use either diamond or CBN in a glassy or ceramic bond. As with all grinding wheels, the bond imparts certain characteristics, making it better suited to some applications than others.
Vitrified CBN grinding wheels are ideal for high volume applications where productivity is paramount such as Automotive applications like Cam & Crank shaft grinding, and Aerospace engine component grinding. Recent advances in bonds and wheel specifications has led to the development of wheel structures for use on older less suitable grinding equipment and applications like surface and cylindrical grinding in mold and die shops.
Vitrified diamond grinding wheels are most suited for grinding of very hard Ceramic and Carbide materials. Applications include grinding of PCD and PCBN cutting toolings, structural ceramic components, and Carbides.
Vitrified Diamond and CBN wheels compositions continue to evolve through the use of new chemistries and processing variables to enhance grinding wheel structures and performance. When it comes to grinding hard to grind materials, there are no better abrasives than Superabrasives; Diamond and CBN. Vitrified structures combined with Superabrasives often times allows for the best compromise between wheel life and grinding performance, thus giving the lowest overall costs in grinding.
Any review of vitrified grinding wheel development should cover both the abrasive materials and the bond medium. Part 1 of this series addressed materials; here we turn to the bond.
Grinding wheels probably originated in ancient Egypt where they were likely cut from sandstone. They also appear in sketches by Leonardo da Vinci and it's thought Belgian gem-polishers were using a cast iron wheel impregnated with diamond powder during the late 1700's, but little else is known until the early nineteenth century.
Around this time the first solid-bonded abrasive wheels appeared. These were made in India for hand-grinding gems and used emery or corundum abrasive in a gum resin shellac binder.
Rubber bond grinding wheels were introduced around 1860, again using corundum.
The first Vitrified (glass) bond wheels were commercialized about ten years later. What are today known as Resin (plastic) bonded wheels didn't appear until 1923. Metal bonds for diamond grinding wheels weren't introduced until the early 1940's.
The term “Vitrified” in its simplest form means “glassy” or “glass bond”. Abrasive Materials such as Aluminum Oxide, Silicon Carbide, Diamond, and CBN are mixed with glass frit (ground glass) and other ceramic materials (clays, feldspars, fluxes, etc…). More recent advances in “Vitrified” bonds has led to the development of “Ceramic” bonds. In general terms, Ceramic bonds are those in which some or all of the glass phase has been converted to a crystalline phase to enhance certain material properties (i.e. higher strengths). Blended grinding wheel compositions are then formed either by “Hot Pressing” or “Cold Pressing” processes.
In Hot Pressing, the blended wheel materials are placed into a suitable mold and simultaneously pressed and sintered (baked). In sintering, the glass & ceramic components are fused together forming the hard Vitrified Bond that holds the abrasive materials in place. Because the grinding wheel is simultaneously pressed and sintered at high temperatures, mold material selection has to withstand the processing temperature. Hot Pressed Vitrified wheels usually have low porosity levels and are very different in their use applications when compared with Cold Pressed Vitrified grinding wheels.
In Cold Pressed Vitrified Products, an extra material called a binder is blended into the wheel composition. The purpose of the binder is to provide handling strength to grinding wheels that are pressed in molds at room temperature and then removed for subsequent and separate sintering in kilns (high temperature ovens). In the sintering process, the temporary binder is removed and the glass is fused together forming the rigid hard Vitrified bond. Cold Pressed Vitrified grinding wheels generally have high levels of porosity in their structures.
Grinding wheel properties are adjusted by varying the percentage of abrasives, size of abrasives, blends of abrasives, different bonds and bond types, manufacturing process, etc… Varying the amount of the abrasive in Superabrasive Grinding wheels is referred to as the “Concentration”. Higher Concentrations contain more material and in general provide longer grinding wheel life as there are more Diamond or CBN cutting points to remove material.
Porosity carries coolant through the grinding zone and provides space for chip clearance. By providing spaced to remove grinding debris (used grinding wheel and removed work piece material), the grinding wheels are able to grind faster, provide better surface finishes (especially if the grinding fluid is filtered to remove the debris). Because open spacing in a wheel structure brings liquid coolants to the grinding zone, they are also known to grind cooler. In so doing, Vitrified Grinding wheels can remove the heat generated during grinding much more efficiently thus allowing faster grinding cycle times and less thermal damage to sensitive materials like Steels and Aerospace Alloys.
Porosity in wheels structures can be created by a variety of methods. In cold press and sinter manufacturing, the wheels are pressed to limited densities and sintering is controlled to limit shrinkage and densification. Another method for creating porosity in grinding wheel structure is through the use of additives in the wheel compositions that are then removed in the sintering stage, thus leaving voids (porosity) where the material once resided. Additional porosity can be generated by adding constituents to wheel compositions that remain within the wheel through manufacturing but are initiated when used. For example, some wheel compositions include material that doesn’t melt in processing but is dissolved in coolant upon use (i.e. Salt). Another example of induced porosity would be the use of hollow microspheres which upon use are broken open, leaving the open void as a pore.
Part 1, Stay tuned for part 3 of this series.
The vitrified bond grinding wheel first appeared nearly 150 years ago, and continues to become ever more valuable. While some products reach a kind of development plateau where there's no potential for further improvement, research continues apace on ways of making vitrified wheels cut faster and last longer.
When it comes to shaping hard materials quickly, accurately, and cheaply the vitrified superabrasive grinding wheel has few rivals. Low wear, high heat stability, combined with a free-cutting nature and excellent ‘dressability’ mean very high material removal rates and less downtime.
Understanding how this tool evolved yields some fascinating insights into its use in manufacturing. This begins with an overview of grinding wheels and abrasive materials, covers the production process, and explores where the technology might be heading.
It's thought sandstone was the first abrasive material. Probably used for putting a sharp edge on axes, compacted quartz embedded in the rock grains proved an effective way of removing material, (much like sandpaper today.) However, as a natural material sandstone has the disadvantage that the quartz particles vary in size and shape, resulting in unpredictable performance.
An alternative material, emery, was known to the Greeks and Romans as an abrasive and is still mined on what is today the Greek island of Naxos. Emery is a form of corundum, the second hardest naturally occurring material, is the crystalline form of Aluminum Oxide containing traces of Iron, Titanium, and Chromium.
As an abrasive emery had two problems: it was expensive to extract and ship to the manufacturing centers in the UK and USA, and its performance was unpredictable due to variance in raw materials that are mined. Spotting an opportunity, entrepreneurs set about developing alternatives. The results included synthetic or manmade Silicon Carbide and synthetic Aluminum Oxide (Corundum). These materials are generally known as “Conventional Abrasives”.
Another alternative, natural Diamond, has been used for grinding since at least the seventeenth century when Belgian gem-polishers used Diamond powder embedded in cast iron. Cost and variability of natural Diamond held back wider use until the creation of synthetic Diamond in the 1950's changed the equation. As the hardest known substance, Diamond makes an excellent abrasive, except for grinding Ferrous, Cast Iron, and Aerospace alloy materials. In the case of grinding these materials with Diamond, a chemical reaction takes place between the material and Diamond, causing it to wear rapidly and to alter the properties of the material being ground. To address this problem, in 1969 General Electric introduced a crystalline material they had developed with a hardness approaching that of Diamond: Cubic Boron Nitride (CBN). Despite having a lower hardness than Diamond, CBN doesn’t react with Ferrous and Aerospace alloys when grinding and has an even higher temperature stability, thus resulting in better grinding performance when grinding these materials. Diamond and CBN are generally known as “Superabrasives”.
Stay tuned for part 2 & part 3 of this series coming later this month.
Materials like carbon fiber present some interesting machining challenges. While not particularly hard, they are often very abrasive. With conventional cutting inserts that means rapid wear, frequent tool changes, and low productivity. Switching to PCD tooling overcomes those problems and can lower costs.
As the hardest material known, diamond makes an excellent cutting tool. In grinding it’s used in grit form, dispersed in a binder, but it can also be used for other machining operations. Milling, turning, boring and drilling can all be performed with cutting edges coated in a thin layer of diamond.
While diamond is hard, it’s not necessarily tough. Indeed, if struck in the right direction a single diamond crystal will cleave quite easily, (which is how gems are ‘cut.’). To overcome this weakness, diamond inserts are manufactured from fine diamond grit in a metal binder. As the individual grains are randomly oriented micro-cracks are unable to propagate, which reduces and evens-out wear.
Diamond tooling manufactured from grit in this way is termed “Polycrystalline Diamond’, abbreviated as PCD. In the same way, CBN can also be sintered to give an edge that’s both hard and tough on an insert. In this case it’s termed ‘polycrystalline cubic boron nitride, or PCBN.
PCD and PCBN tools give long life, even on abrasive materials like carbon fiber. This means fewer stops to rotate or replace inserts and so higher productivity. Another benefit of the polycrystalline structure is more uniform edge quality, which results in excellent workpiece surface finish.
Edge uniformity and low wear, combined with the inherent high thermal conductivity of diamond, mean PCD and PCBN tooling is ideal for tight tolerance work. However, diamond should not be used to cut ferrous materials as this will lead to rapid tool wear.
PCBN overcomes those problems, and so should be used for machining ferrous materials. High-performance PCBN is available in several grades with varying grain content. This allows PCBN inserts to be tailored for specific applications. For example, a low grain content insert would be suitable for finishing operations on tool steels whereas an insert with high grain content would perform better on cast iron.
CDT will manufacture high-performance PCD and PCBN inserts to suit our customer’s needs. A discussion is advised as a deeper understanding of the application results in higher quality, longer lasting, tools.
Modern jet engines depend on superalloys to handle combustion temperatures of 1,4000C or more. These high-performance nickel-based materials are exceptionally hard, which makes machining components like turbine vanes extremely challenging. The solution is to use electroplated CBN wheels. Here’s why.
The aerospace industry asks a lot from their high-precision grinding tools. Specifically, aerospace grinding wheels must:
Remain hard at high temperatures
Be chemically inert to resist coolant
Provide excellent size control throughout a production run
Be available in complex forms
Not damage the workpiece – no burning or microcracking
Minimize residual stresses in the finished workpiece
Missing from this list is wheel life. This is less of a consideration as aerospace production runs are generally short as compared with an industry like automotive. This means less pressure to eliminate downtime for wheel dressing or replacement.
As the hardest known substance, diamond might seem the obvious choice for grinding exceptionally hard materials. However, it doesn't perform so well at temperatures over 8000C as it tends to lose hardness. It's also vulnerable to attack by some of the chemicals found in grinding coolants, which reduces wheel life.
The alternative to diamond is Cubic Boron Nitride (CBN). This is a man-made or synthetic abrasive material that's not as hard as diamond but is harder than anything else. Unlike diamond, it retains its hardness at elevated temperatures and offers superior resistance to chemical attack. (It also doesn't exhibit the affinity for iron that makes diamond so poor for machining steel.)
An electroplated grinding wheel has a thin layer of superabrasive bonded by a metallic layer to a solid core. This has three advantages:
Plated CBN grinding wheels are used extensively in aerospace. They perform well on exceptionally hard superalloys, providing high material removal rates coupled with excellent dimensional control throughout a production run.
How a grinding wheel performs is dictated as much by the bond as the abrasive. Superabrasives, which can be either diamond or cubic boron nitride (CBN) have four types of bond: metal, plated, resin and vitrified. Of these, vitrified may be the least well-known, yet it’s attracting attention, thanks to the improved performance and lower costs that it offers.
In a vitrified wheel superabrasive grit is mixed with a type of clay before firing in a kiln. This turns the clay to a porous, glass-like structure that holds the grit in a rigid matrix.
Unlike a plated wheel where the abrasive is just a thin layer, in a vitrified wheel it can have considerable depth. This means the wheel can wear and go through repeated dressing. Wear rates are very low as the vitrified bonds must fracture to release worn grit and expose fresh edges. It’s also possible to put a form into the wheel to allow grinding of complex profiles, (although this does complicate dressing.)
The open, porous structure ensures the grit stands proud of the bond surface, creating a wheel that’s described as ‘free cutting.’ This means good space for chip clearance, which:
As an additional benefit, these last three points result in lower temperatures at the interface and reduced risk of workpiece burning.
Lower temperatures help maintain control over final size, as does the very rigid nature of the wheel. With minimal wheel deflection due to grinding forces and low thermal growth very high tolerances can be maintained throughout a production run.
High volume production is an ideal application for vitrified grinding wheels. Low wear rates, combined with a free-cutting nature and ‘dressability’ mean very high material removal rates and less downtime for wheel changes.
Vitrified cubic boron nitride grinding wheels are preferred for grinding hard ferrous workpieces like crankshafts and camshafts as well as tool steels. Vitrified diamond grinding wheels are good in non-ferrous applications such as shaping ceramics.
Of the four main bond types, the vitrified wheel is best suited for high volume, extended production run applications. The rigid, porous structure delivers high material removal rates and extended life, helping lower manufacturing costs.
Shop for sandpaper in the hardware store and you'll see a numbering system indicating roughness. A coarse 80 grit grade removes material quickly but leaves the surface rough while a fine 400 grit won't take much off but leaves a smooth finish.
Superabrasive grinding wheels are no different. The size or coarseness, of the grit is indicated numerically in the wheel identification code, and guides the user in how that particular wheel should be used. Diamond is available in grit sizes from 40 to 1,200 while CBN comes in the range of 50 to 600 grit. As with sandpaper, a smaller number signifies the abrasive particles are larger.
Grit numbers are derived from mesh sizes, mesh being the standard way of grading powder. The general principle is that of sieving: the finer the mesh the smaller a particle must be to pass through the gaps between the wires. Mesh size is indicated in gaps per inch, so a mesh of 16 has 16 gaps per inch.
When referring to powders mesh size is actually shown by two numbers, such as 50/60. The first number is the sieve through which most of the powder grains would pass, and the second is the mesh size that traps most of the grains. For simplicity, in the superabrasives industry the grit number refers to the larger size mesh, the one through which most particles would pass, so powder with a mesh size of 50/60 is referred to as 50 grit.
On average, in a 50 grit powder the particles are around 300 microns or 0.011 inches in diameter. That's roughly the size of grains of beach sand, although it's important to note that sieving result s in a distribution of sizes and shapes.
As with sandpaper, larger abrasive particles, (a smaller grit number,) remove material faster but leave a coarser finish. With a finer grit, (larger number,) each particle removes less material. The removal rate will be lower but the surface finish will be smoother.
Grit size only approximately correlates to surface finish: machine condition and the type of workpiece make a big difference. As a guide though, a diamond grinding wheel with a grit of 100, used on carbide, should produce a finish in the region of 24 to 32 micro inches. A CBN wheel with the same grit size used on High Speed Steel should achieve a finish in the region of 40 micro inches.
Any time a grinding wheel is mounted on a machine it must be trued and dressed to produce satisfactory work. These two terms are sometimes lumped together as “conditioning” but they mean different things. Inexpensive conventional abrasive wheels are relatively robust: dressing and truing can often be performed at the same time and repeated periodically. The same is not true for CBN and diamond grinding wheels.
These superabrasive grinding wheels represent a considerable investment, which should be repaid in longer life and higher productivity. However, achieving that performance demands close attention be paid to truing and dressing.
No matter how precisely manufactured, once a grinding wheel is mounted on a spindle there will be some eccentricity. Even if it's less than 0.001” it's going to affect the final size and finish of the workpiece, so to produce high-quality work the wheel must be trued.
One method of finding high spots is with a child's wax crayon. Spin up the wheel and bring in the crayon until it just touches: high spots will quickly gain a colored layer.
Conventional grinding wheels are easily trued with a diamond cutter that's harder than the wheel matrix. Diamond and CBN wheels can't be cut, and instead are effectively ground to size. While this can be done by traversing a conventional grinding wheel or sintered diamond roller across the wheel face, many machinists prefer to use a brake-controlled truing device (BCTD.)
Like the other methods, the BTCD presents an abrasive surface to the wheel, but turns more slowly. This speed differential results in a precise and controllable truing operation.
Truing creates a smooth surface on the wheel periphery, and with no exposed grit the wheel won't cut. Dressing sharpens the wheel by removing bond material and fracturing the superabrasive grit to expose fresh edges, so dressing always follows truing.
A superabrasive grinding wheel is largely self-sharpening, although the workpiece material can cause it to dull or load-up. In such situations the grit starts to either rub against the surface or plow, (pushing material aside). Both affect surface finish, increase cutting forces and create heat, which can damage the workpiece. The solution is periodic dressing.
Resin and vitrified superabrasive grinding wheels have some depth to the matrix. This allows a wheel to be dressed and trued several times. However, plated and metal bond wheels have a much thinner layer of grit and can only be trued to the depth available.
Having decided to leave conventional grinding wheels in favor of superabrasives, buyers are confronted with a difficult choice: diamond or CBN grit?
Both are extremely hard and offer the potential for greatly improved grinding productivity, but there are differences between the two. It's important to understand these and match the abrasive to the task if peak productivity, and lowest cost-per-piece are to be achieved.
Diamond is a crystalline form of carbon. Under heat and pressure carbon atoms link with adjacent atoms to create the hardest known material. This makes it an ideal cutting tool, or would do but for it's rarity and price. However, in the mid 20th century scientists figured out how to manufacture diamond, and today most diamond grit used in superabrasive grinding wheels is man-made.
Unlike diamond, cubic boron nitride (CBN) doesn't exist in nature and is synthesized from boron and nitrogen. When chemically bound together these two elements behave much like carbon in that they can create an immensely strong crystal lattice structure. Of the two, diamond is considerably harder at room temperature, (knoop hardness around 7,500 versus the 4,500 of CBN,) but CBN has better thermal and chemical stability, remaining inert at temperatures up to 1,000 oC, versus the 800 oC at which diamond begins to degrade.
As the harder of the two, diamond is preferred for shaping extremely hard workpiece materials such as ceramics, carbides, stone and glass. It is not however suitable for use with steels. This is because carbon and iron have a strong affinity for one another, especially at elevated temperatures. This results in rapid erosion of the diamond grit, quickly destroying the grinding wheel.
CBN is better suited to grinding applications that generate high temperatures, meaning it can be used at higher speeds. And it's unreactive nature makes it the preferred choice for grinding most steels, such as tool steel and HSS.
The relatively low thermal limit of diamond also affects the type of wheel construction it can be used in. Vitrified wheels are made by firing a clay mix at very high temperatures: if the mix includes a proportion of diamond grit the firing temperature must be kept below 800 oC otherwise the diamond will start to react. For this reason, CBN is more common in vitrified grinding wheels.
Use diamond on the hardest workpiece materials, but don't let it get too hot. Use CBN on ferrous workpiece materials to avoid the chemical reaction that will quickly wear diamond.
Selecting the best superabrasive grinding wheel for an application is never easy. Each type of bond, vitrified, resin, metal or plated, has distinct performance characteristics. Some provide higher material removal rates, others wear more slowly or give better control over final size. Then there's the choice of whether to use CBN or diamond abrasive.
This post reviews the choice between diamond and CBN as it relates to vitrified grinding wheels. A basic understanding of vitrified wheels is assumed, although readers may wish to refer to “Grind Quickly And Accurately With A Vitrified Superabrasive Wheel.”
Very hard, and possessing a porous, open structure promoting good chip clearance and coolant delivery, vitrified wheels are a popular choice in high productivity applications. Wear rates are low because the grit is locked rigidly in place, and little dressing is needed, resulting in a high ratio of stock removal to wheel use.
Unlike a resin bond wheel, there's no deflection as the grit starts to cut, so finished piece size control is excellent. Vitrified wheels can't run as fast as resin bond wheels, which means less heat generation and less workpiece burn, especially when coupled with their ability to carry coolant through the grinding zone. However, they are somewhat brittle so not suited to interrupted cut-type grinding operations.
With it's exceptional hardness, diamond seems the obvious choice for every grinding operation, but it does have limitations, most notably, a strong affinity for iron. This comes about because diamond is nothing more than a crystal of pure carbon, and carbon and iron atoms like to bond to one another. As a result, use a diamond wheel on steel and you'll see very rapid wear.
A second issue is that diamond becomes chemically reactive at temperatures over 800 oC. This makes it unsuitable for very high temperature applications, although this is less of a problem with vitrified wheels, for the reasons explained previously.
CBN's two big strengths are its ability to cut at temperatures as high as 1,000 oC, and its refusal to react with iron. This makes it ideal for use with tool and high-speed steels, especially when grinding dry. The downside is that it is not as hard as diamond. That means faster wear, therefore more wheel changes and lower productivity.
It's diamond for grinding ceramics, carbides, glass and stone, CBN for steels, and vitrified bond for high productivity.