Grinding oils are far more than pure coolants. They shape the quality of the workpiece, the tool life and the cost-efficiency of the whole process. This overview shows which properties define a good grinding oil, which requirements apply depending on the machining process and what to watch for in safe handling.
Grinding oils keep the friction heat throughout the grinding process at an optimal level, matched to the material. A wide range of process factors plays a role here.
A wide range of process factors plays a role here.
Productivity - fast machining of the workpiece - matters just as much as surface quality (no hairline cracks, no grinding burn) and low tool wear. To deliver this, the grinding oil chosen for the process needs specific properties. The key characteristics are the additive package for the application, the foaming behaviour, the air release capacity, viscosity and flash point, and good filtration behaviour.
When grinding at low to medium wheel peripheral speeds, cooling is the main focus. As the material removal rate rises, cooling lubricants with particular friction reduction come into demand. The choice of the right grinding oil also strongly influences operating-cost reduction and therefore the economic efficiency of the process. Every individual case calls for a complex assessment along the parameters of lubricating, cooling, cleaning, flushing and corrosion reduction.
Grinding oils adapt to the requirements of each process. From surface grinding through finish grinding, cylindrical grinding and profile grinding to creep-feed and slot grinding, special oil grades are available. Their use also depends on the machine design. The machine must suit the use of grinding oils. It needs a fire extinguisher and pressure-relief flaps, for example, and the components must resist oil.
Several base oils are available for grinding oils in principle. Oils on a hydrocracked base (HC) stand alongside the higher-quality GTL (Gas to Liquid) oils and polyalphaolefin products (PAO).
The foaming behaviour and the air release capacity of the grinding oil are always decisive for high productivity. The air trapped and compressed in the oil (air bubbles) practically explodes as it exits the nozzle. A good process result is therefore only possible with low-foam grinding oils that release air well.
Health safety has top priority when using grinding oils. To protect the user fully, operators must meet legal requirements, updated in the new rule for hazardous substances (see Important information for carbide grinders - TRGS 561). Where this rule addresses the inhalation of hazardous substances such as cobalt in particular, other regulations cover further aspects - dermatological protection and precautions for the skin, for example.
Grinding burn ranks among the most expensive faults in the grinding process. It impairs workpiece quality, raises scrap and costs valuable production time. This overview shows which test methods make it visible, which causes lie behind it and how the right grinding oil prevents grinding burn deliberately.
On heavily damaged workpieces, marked discolouration (blue to black) and "grinding commas" can appear. In most cases, however, grinding burn stays invisible. This is called "hidden" grinding burn. Negative compressive stresses then occur only in deeper layers, which makes detection considerably harder and calls for special test methods.
Three methods have established themselves for detecting grinding burn reliably. Nital etching (with nitric acid) is one of the oldest and most common test methods. It follows ANSI/AGMA standard 2007 and ISO standard 14104 through etching in an immersion bath. This non-destructive method is extremely reliable, because after the test a dark colouring of the affected workpiece areas shows up clearly.
A second test method is Barkhausen noise. Put simply, this magnetises the workpiece. The impulses generated in the material create a kind of noise, whose intensity is measured. The third method is surface crack testing by the magnetic particle method. This also magnetises the workpiece. The powder reveals magnetic stray fields that point to transverse cracks in the material.
The damage from grinding burn has economic consequences too. Higher scrap causes more time and cost, and productivity suffers significantly. So the question of causes becomes more pressing.
Given the complexity of the grinding process, the causes of grinding burn are many. Feed rate, the condition of the grinding wheel, poor cooling and lubrication performance (which in turn can stem from a faulty specification, quantity, pressure or the wrong shape or arrangement of the nozzles), and the workpiece geometry are parameters that can cause grinding burn on their own or in combination.
The question is how to avoid grinding burn along with the sometimes considerable costs and the loss of productivity it brings. One effective means is the use of high-performance grinding oils defined specifically for this area, such as esgeGrind NUS 1500, esgeGrind ZRS 1500 or esgeGrind ZRS-G 1500. These grinding oils are colourless, low-viscosity, chlorine-free and low-mist metalworking oils, developed specifically for high-speed grinding - exactly the kind of application where the risk of grinding burn is especially high. Thanks to their excellent cooling and lubricating properties, these lubricants are an effective measure for preventing grinding burn.
Carbide is not a naturally occurring metal but an alloy of a hard material and a binder metal, produced by sintering. Its enormous hardness and wear resistance make it indispensable in modern tool manufacturing today. This overview shows which components make up carbide, where it is used and why machining it makes special demands.
Carbide is not a naturally occurring metal but an alloy consisting of a metallic hard material and a binder metal. Unlike conventional metal alloys, carbide has its components pressed together as a powder into a so-called green compact and sintered afterwards.
In 1923, Karl Schröter and Heinrich Baumhauer filed the first patent for sintered carbide, which they then sold to the Osram company. Lohman and Voigtländer had already filed a patent for cast carbide in 1914, but only the sintering process brought carbide its breakthrough.
Sintering is a process that bonds the components of carbide together under exclusion of oxygen at high temperatures up to 1,500 °C and under pressure. The hard material and the respective binder metal join into a solid metal matrix. This makes carbides considerably harder and more wear-resistant than ordinary metals and alloys, but also more prone to fracture.
Carbide always combines a hard material, also called a carbide, with a binder metal. The hard material is usually tungsten carbide (WC). Other hard materials can be titanium (TiC), tantalum (TaC) or chromium (CrC), for example. The most common binder material is cobalt (Co). Depending on the desired properties - such as higher corrosion resistance or a non-magnetic carbide - nickel (Ni), iron (Fe) and nickel-chromium (NiCr) can also come into use.
Carbides are mainly used to manufacture tools such as drills or milling cutters. Thanks to the exceptional hardness of the material and its high temperature resistance, carbide is particularly well suited for use as a cutting tool. To cut costs and increase tool effectiveness, carbide indexable inserts are also often used. Indexable inserts are small inserts that usually have several cutting edges and attach to a tool holder. Once the edges wear out, the inserts can be turned or replaced.
Carbide can have different levels of hardness depending on its composition. This is why we generally distinguish between soft grades and highly wear-resistant grades. The hardness of the material is measured in Vickers and determined using the Vickers indentation method. In every case, though, the exceptional hardness of the material requires a special form of machining. This is why carbide is in most cases ground with diamond tools or processed by electrical discharge machining. When grinding carbide, the binder cobalt can be washed out of the material. Since cobalt is a hazardous substance that the body cannot break down, you should always use a special coolant lubricant that prevents this cobalt leaching by adding special ingredients.
Carbide always combines a hard material, also called a carbide, with a binder metal. The hard material is usually tungsten carbide (WC). Other hard materials can be titanium (TiC), tantalum (TaC) or chromium (CrC), for example. The most common binder material is cobalt (Co). Depending on the desired properties - such as higher corrosion resistance or a non-magnetic carbide - nickel (Ni), iron (Fe) and nickel-chromium (NiCr) can also come into use.
Carbides are mainly used to manufacture tools such as drills or milling cutters. Thanks to the exceptional hardness of the material and its high temperature resistance, carbide is particularly well suited for use as a cutting tool. To cut costs and increase tool effectiveness, carbide indexable inserts are also often used. Indexable inserts are small inserts that usually have several cutting edges and attach to a tool holder. Once the edges wear out, the inserts can be turned or replaced.
Carbide can have different levels of hardness depending on its composition. This is why we generally distinguish between soft grades and highly wear-resistant grades. The hardness of the material is measured in Vickers and determined using the Vickers indentation method. In every case, though, the exceptional hardness of the material requires a special form of machining. This is why carbide is in most cases ground with diamond tools or processed by electrical discharge machining. When grinding carbide, the binder cobalt can be washed out of the material. Since cobalt is a hazardous substance that the body cannot break down, you should always use a special coolant lubricant that prevents this cobalt leaching by adding special ingredients.
Carbide is the most important base material in high-performance machining - whether for complete tools or indexable inserts. Because pressed and sintered carbide parts shrink during sintering, only high-precision finishing achieves exact dimensions. Given the enormous hardness, this mainly means grinding, honing, lapping and EDM.
Grinding is currently the most common process for machining carbide. DIN 8589 classifies the different grinding processes by the type of surface produced, for example:
Given the high hardness of carbide, grinding today uses almost exclusively diamond tools. In rare cases, conventional grinding wheels made of silicon carbide can also serve.
Diamond grinding wheels usually consist of two components: a core with an abrasive layer applied to it. Through its structure, the core determines properties such as rigidity, heat conduction or vibration damping. A steel or aluminium core tends to offer very good rigidity and good heat conduction, while resin bond systems provide good damping properties.
In the abrasive layer, grit size and bond mainly decide the cutting properties. A high grit value (e.g. >D126) means a coarse grit. Coarse or large grits deliver a high removal rate. A small grit size (e.g. ≤D64) achieves better surface quality and profile accuracy. Among the bonds, the main distinction is between resin bonds, sintered metal bonds, electroplated metal bonds and vitrified bonds.
– suit low grinding forces and allow a wide scope for adjustment. They count as soft, fast and cool-grinding.
– are suitable for grinding wheels with sharp-edged profiles. They are used when low wheel wear and high profile stability are the priorities.
– are mostly single-layer and, thanks to their large chip spaces, are especially suited to high removal volumes and cool grinding. In this bond, the grit is embedded on the surface of the metal body so that the grit tips protrude beyond the bond level by 30 to 50 percent of the grit diameter.
– can be produced with different hardnesses and pore volumes. They enable high cutting speeds and large material removal rates, cool grinding and free-cutting behavior, and offer good dressing and profiling properties.
– suit low grinding forces and allow a wide scope for adjustment. They count as soft, fast and cool-grinding.
– are suitable for grinding wheels with sharp-edged profiles. They are used when low wheel wear and high profile stability are the priorities.
– are mostly single-layer and, thanks to their large chip spaces, are especially suited to high removal volumes and cool grinding. In this bond, the grit is embedded on the surface of the metal body so that the grit tips protrude beyond the bond level by 30 to 50 percent of the grit diameter.
– can be produced with different hardnesses and pore volumes. They enable high cutting speeds and large material removal rates, cool grinding and free-cutting behavior, and offer good dressing and profiling properties.
For the grinding wheel to do its job in ideal form, the machining process also needs the right cooling lubricant. The best results come from grinding oils on a fully synthetic base. Alongside removal behaviour, important properties of a cooling lubricant include dirt-carrying capacity, foaming behaviour, flushing capacity, skin compatibility, filterability and air release capacity. In carbide machining, however, one more key factor comes into play: the binder used most often to produce carbide is cobalt. Grinding can wash this hazardous substance out of the workpiece and carry it into the user's body. (See also our article "Important information for carbide grinders - TRGS 561".)
Special polyalphaolefin-based lubricants for carbide machining, such as the grinding oil esgeGrind VOSO 500, therefore contain special ingredients that prevent so-called cobalt leaching - the dissolving of this substance - and so protect the user from harm.
Under the esgeCool name, esgemo GmbH & Co. KG also offers water-soluble cooling lubricants inhibited against cobalt leaching.
