LUBRICANTS FOR HIGH-PERFORMANCE GRINDING OPERATIONS

Ing. Ernst Klapp, Rödermark

List of Content:

1.   Introduction
2.   Coolants used for grinding of hard material
3.   Definition of cooling lubrication
4.   Grinding Oils for High-Performance Grinding
5.   Selection of grinding oil viscosity
6.   Assigned Honing Oils
7.   Feed and Filtration of Grinding Lubricants

In former days hard and brittle materials, as i. e. hard metals and cermets, were generally processed with rubber or resinoid bonded diamond grinding wheels where as result used cutting velocities (vc=18-22 m/s) and material removal rates (Q’w = 0.5-2.0 mm³/s) had been very low. Aqueous solutions or emulsions had been used as coolants. Such uneconomic operation methods are still used up today.

HEDG, High-Efficiency-Deep-Grinding processes, resulted in economical processes for grinding of brittle-hard materials. Simultaneously the development of high performance grinding and particularly high-performance grinding machines and grinding tools had been developed and consequently also corresponding adequate oil-based coolants and new coolant feeding systems had been introduced successfully.

Without the solution of the corresponding coolant problems, the development of high performance grinding (for steel and as well as for hard metal processing) could probably not take place. From the beginning MKU-Chemie was involved in developing the basic requirements and therefore has great knowledge and experience for the application of lubricating coolants in all areas of modern high-performance grinding technology, particularly for the HEDG processing of brittle-hard materials.

Since hard metals and cermets are still today processed on conventional machines with rubber or resinoid bonded tools, conventional water-based coolants must be still provided for these operations. These coolants have to provide good cooling properties and must have a good inhibition against dissociation of cobalt ions. Because of very low extension of the contact zone and the very low cutting speed, lubrication is in this case of minor importance.

For this conventional area of hard metal processing MKU-Chemie recommends the following synthetic solution concentrates which have optimum cobalt inhibition and can be adapted to the prevailing grinding operation by using variable solutions:

• Rotex^® KS V 1767-2
• Rotex^® KS V 1527 BF
• Rotex^® KS V 1527
• Rotex^® KS V 1804-4

Rotex^® KS V 1767-2, Rotex^® KS V 1527 BF and Rotex^® KS V 1527 are exceptionally developed for operations with minor removal rates on hard metals and hard metal-steel combinations and should be only used in this field.  Rotex^® KS V 1804-4 is also suited for operations on hard metals but particularly for ceramic materials (cermets, cutting ceramics).

Generally deep grinding or creep grinding is defined by low cutting and low feeding rates combined with very high axial (flank- or radial) infeed. Thus it becomes possible to process for example shank cutter work pieces consisting of hard metal with rubber bonded grinding tools using the following parameters marking the transition to high-speed grinding:

Cutting speed vc = 18 m/s; * Feed vf = 1,6 mm/s;  Axial infeed rate  ae = 13,0 mm.

This results in a relatively high material removal rate of Q’w = 20,8 mm³/s and a contact zone length of lc@ 22mm, using a grinding wheel diameter of dw = 400 mm. From these figures result some characteristics for the process which are important for the selection of the coolant.

a)  Caused by the high axial infeed rate and the relatively high removal rate a very high total grinding force results. Therefore a stiff and powerful grinding machine must be used. The viscosity of the cooling lubricant should be not too low, that means aqueous solutions or emulsions are not suitable.

b) Caused by the extended contact zone and the low feed velocity the thermal load in the contact zones is relatively low, while the mechanical load (friction, specific cutting force) ranges in a medium area. Therefore the coolant should provide good lubrication and sufficient cooling property. This can be managed by using a grinding neat oil of medium viscosity.

c) Because of the minor cutting speeds, there is no need for a special oil feeding system, but it is necessary to provide excellent wetting of tool and workpiece surface with the lubricant. This can be obtained with an additive containing lubricant, providing an optimal balance between viscosity and adhesion.

For deep grinding operations on hard materials MKU-Chemie recommends the application of high performance grinding oils with medium viscosity of Rotex^® Spezial V 1064 series. These grinding oils are suitable for all high-performance grinding operations (see chapter 4).

For HEDG-grinding of hard materials with high cutting speed rates and high axial infeed rates, MKU-Chemie recommends therefore a special additive doped grinding oil which lowers friction in the contact zone and lead in addition to a stable lubricant film in the contact zone, generated by a medium viscous oil. This lubricant is the high-performance grinding oil

High performance grinding was originally developed for the economical processing of steel- and cast iron materials. Today also brittle-hard materials as hard metals and cermets are successfully processed with this method. High to very high removal rates (Q’w= 30-200 mm³/s) are obtained at highest cutting speeds (vc = 150-200 m/s). Thus results, in any case, in high grinding forces and driving power, so that the following conditions must be met when using this production process.

• The use of stiff and powerful grinding machines with stable and homogeneous guiding behavior.
• The use of metal bonded high-performance/high velocity diamond grinding wheels.
• The use of high-performance oil-based cooling lubricants, as well as adequate lubricant feeding systems

Dependent on the determined process parameters generally two different technological process variants have to be taken into account. As these facts are practically not often sufficiently taken into account, particularly when selecting the corresponding cooling lubricant, serious mistakes occur.

Grinding of brittle-hard materials (but also steels) with high cutting speed rates, high axial infeed rates and medium high feeding rates, is historically seen, the High-Efficiency-Deep-Grinding process. It involves high axial infeed rates (deep grinding) with high velocities (high-speed-grinding) and is particularly suited for the high-performance processing of brittle-hard materials.

This situation of parameters results in very high values for the removal rates and consequently in high overall cutting forces. This leads, together with high cutting speed rates to high values of generated friction energy, so that excellent lubricity of the coolant is an absolute must. On the other hand a proportionally overlong contact zone results (lc 0 20-60 mm), because of the high axial infeed rates and relatively low feeding velocities. This result, despite of high values for the overall energetic level of the process, to low values for the specific energy flow in the contact zone. Therefore the cooling effect of the lubricant is of minor importance than the need of high lubricity.

For HEDG-grinding of hard materials with high cutting speed rates and high axial infeed rates, MKU-Chemie recommend therefore a special additive doped grinding oil which lowers friction in the contact zone and lead in addition to a stable lubricant film in the contact zone, generated by a medium viscous oil. This lubricant is the high-performance grinding oil series Rotex^® Spezial V 1064 AN

Rotex^® Spezial V 1064 AN has a viscosity of circa 17,4 mm²/s at 40°C and is blended with adequate additives to suite the highest demands. Machinery industry and university research institutes were involved in the development of this product. It was successfully tested in grinding operations on high-tempered steel with material removal rates of more than Q’w = 500 mm³/s.

When cooling lubricants are used at high cutting speeds and/or high axial infeed rates (large length of contact zone), the operation must always carried out by using increased lubricant feeding pressures and special feeding equipment in order to avoid insufficient lubricating, that causes process faults. As described in paragraph 5.

.This is a process variant of high-performance grinding as it is realized in the Quickpoint- cylindrical grinding process that is also called Speed-Feed Grinding. With medium axial infeed rates (per motion or work piece revolution) of ae = 0.2 – 0.5 mm combined with very high workpiece feeding velocities of vf = 150 – 400 mm/s only relatively short contact zones are formed with lengths of lc = 2 – 6 mm. The assigned cutting velocities of metal bonded diamond grinding wheel come to vc = 150 – 200 m/s in the case of HEDG.

In the speed-feed grinding process high to very high removal rates (Q’w = 30 – 200 mm³/s) are achieved that lead to lower overall cutting forces than in the HEDG process. Because the specific energy turnover in the reduced contact zone is definitely higher as in the HEDG process, the selection of an optimal cooling lubricant requires, besides of good lubricity, also an increased cooling effect.

MKU-Chemie recommends for this process the high-performance grinding oil:

series Rotex^® Spezial V 1064 and series Rotex^® Spezial V 1217.

These grinding oils have a viscosity of 8,5 mm²/s resp. 11mm²/s at 40°C and is blended with the same additive content to suit the highest process requirements. An important need is the use of an appropriate lubricant feeding system.

The previous described grinding lubricants are based on solvent refined mineral oil core fractions of different viscosities, blended with extreme-pressure (EP) additive in order to meet the needed lubrication properties. The lowest viscosity attainable is obtained with

Rotex^® Spezial V 1734-2 AN

With a relatively low additive content and a viscosity of 7,6 mm²/s at 40°C. It is used for deep grinding of brittle-hard materials with a medium sized axial infeed and medium removal rates with low till medium cutting speed.

The indication AN in the product name refers to a special (Anti-Mist) additive component which is necessary to reduce mist formation of low viscosity oils. The previous mentioned, Rotex^® Spezial V 1064 and Rotex^® Spezial 1217 are regularly also blended with AN-additive and is superior to Rotex^® Spezial V1734-2, with regard to attainable material removal and cutting speed rates.

For high-performance grinding of brittle-hard materials with low axial infeed rates and high material feed rates (for example: processing of hard metal with ceramic grinding tools) very low-viscous grinding oils (viscosity 4 – 6 mm²/s at 40°C) are in use  For this application, MKU-Chemie does not use mineral oils because of their unpleasant odor. Instead MKU-Chemie uses medical white oils with practically zero aromatic content (as i. e. used in cosmetic industry, corresponding with DAB 10). Because of their poor solubility properties, white oil could not be blended with additives and therefore unusable as grinding oil. MKU-Chemie developed a unique blending process to solve this problem. Thus white oil of low viscosities, blended with EP-additives, skin kindly and odorless can be offered by MKU-Chemie.

Dionol^® V 1519-3 AN (viscosity 5,6 mm²/s at 40°C)

Dionol^® V 1519-5 AN (viscosity 5,3 mm²/s at 40°C).

Grinding oils based on poly-alpha-olefins are more expensive and have the disadvantage of aging after some months in use and thus their skin and environment compatibility becomes lost as well as they damage sealings and cause highly mist production. For this reason, low viscous white oil based grinding oils have been accepted for thin hard materials and ceramic elements, also at increased cutting speeds.

Depending on the grinding machine and the assigned lubricant feeding system, all low viscous grinding oils can be blended with increased AN additive in order to minimize the formation of aerosols. This is always urgently to recommend when the temperature of the lubricant cannot be kept in a low range between 22 ° and 26 °C, (i. e. missing cooling equipment).

Often a honing process follows the high performance grinding operation of cutting- and component parts consisting of brittle-hard materials or of highly tempered tool steels with plain or profile surfaces. For this operation special high-performance honing machines with diamond- and CBN tools are in use having also the need for especially developed high-performance cooling lubricants in order to produce optimum results. Because honing, compared with grinding, is carried out at remarkable lower cutting speeds, honing oils have to be sufficiently blended with anti-wear additives in order to reduce tool wear at low local temperatures. At high tool pressures and/or small tool width and high operation velocities the honing oil must contain also extreme pressure (EP) additives to maintain lubrication at high removal rates and high local temperatures.

With respect to optimal viscosities two different products are offered by MKU-Chemie:

series Dionol^® HV 1770 (Viskosität 21,7 mm²/s bei 40 °C)

series Dionol^® HV 1412 (Viskosität   4,0 mm²/s bei 40 °C)

Dionol^® HV 1770 is a medium viscous high-performance honing oil optimal for wide diamond and CBN tools when high removal rates and best surface quality should be obtained, as i. e. on modern Sunnen-machines. Analogue to HEDG grinding, excellent lubrication is required. Dionol^® HV 1412 is a low viscosity high-performance honing oil ideal for slim diamond or CBN tools but also for conventional corundum tools when high removal rates with adequate surface quality should be achieved. A great field of application are high-performance honing operations on modern CNC-honing machines where this process is more dependent on cooling than on lubrication (analogue to Speed-Feed-Grinding).

It is generally valid for grinding operations that cooling lubricant can only be effective when it penetrate into the contact zone between grinding tool and work piece so that the whole contact zone is covered with a closed lubricant film. The higher the length of the contact zone and the higher the radial velocity of the grinding wheel, the more difficult is it, to obtain a closed lubricant surface. This is also valid when the viscosity of the lubricant is too high or too low, then a correct wetting of the contact zone is not possible.

At the previous described cases of HEDG-, Speed-Feed-Grinding and Deep Grinding with increased removal rates, as well as for all other grinding operations with higher cutting speeds and increased removal rates, it must be taken care that the feeding pressure of the lubricant is sufficiently high and that the feeding jet nozzle is correctly pointed towards that zone where the grinding wheel enters the contact zone. This seems to be simple but in practice it is not often considered. Decisive for the positioning of the main jet nozzle is the question whether the work is carried out in rotation or anti-rotation direction.

The cooling lubricant has to meet four requirements: lubrication, cooling, cleaning the tool surface and removal of abrasives. Therefore the related technologies and lubricant feeding equipment are complex. A certain advantage at processing of brittle-hard materials is the shape of abrasives which are not in the form of more or less long chips, but caused by the brittleness of the material having the shape of small globulite particles. Thus the tendency of blocking grinding wheel surfaces or feeding jets is relatively low. For highest demands as i. e. for HEDG grinding of a 10 mm deep profile groove in a hard metal work piece with a material removal rate of 100 mm²/s, three single operating feeding jet nozzles have to be mounted to guarantee the overall system of the grinding process.

a)  Main Feed Jet Nozzle

Because the grinding wheel has a recognizable higher velocity than the coolant outflow of the jet the problem of sufficient wetting of the grinding wheel arises. When the coolant stream impacts on the much faster grinding wheel surface it is turbulently whirled on the boundary layer and becomes immediately bounced off from the rough work piece surface. This is the matter of an increased pressure caused by turbulences on the boundary layer. The only useful measure to avoid this turbulence is a distinct increase of the coolant jet velocity by increasing the feeding pressure up to 15 – 20 bar (and often more). Thus the coolant jet velocity comes closer to the grinding wheel velocity, so lower whirling and better adhesive wetting of the work piece surface is obtained. The higher the viscosity of the lubricant the better is the wetting effect but also resulting in a lower jet velocity. Therefore an optimal medium viscosity has to be used as recommended by MKU-Chemie.

b)      Cleaning Jet Nozzle

When the grinding wheel steps out of the contact zone it transports a great amount of abrasive particles, caused by the high velocity of the rotating surface. The majority of this, partly red glowing particles are thrown off from the grinding wheel by their centrifugal force, forming spark emissions that are distinctly visible despite of strong cooling. Some of these particles are not released from the tool surface because they had not enough free space and therefore were pressed with high energy into the surface of the wheel tool. In this case the particles are impacted in an extent that they could again enter the contact zone after one rotation of the tool. Such an effect would cause rapidly a build-up of metallic material layers that would damage the lubricant film in the contact zone so that a metal-to-metal dry friction would occur. This has to be avoided by mounting one or more cleaning jet nozzles which lead the lubricant with high pressure to the surface of the grinding wheel. Selected pressures being 5 bar higher than those described in section b), can definitely avoid a build-up of metallic material layers.

c)  Extinguishing Jet Nozzle

If the spark emission is very intensive and/or many abrasive particles stick on the wheel surface and circulate the tool as a ring of fire, a broad coolant jet must be directed towards the grinding wheel surface shortly after exit of the contact zone. Thus, the spark emission is extinguished or reduced and sticking particles on the grinding wheel are cooled down. With the cooling effect the particles are somewhat shrinking and so could easier flushed out by the following cleaning jet. The extinguishing jet is used with considerably higher pressure as with the main jet due to the fact that it is also particularly used as a cleaning nozzle.

This relatively expensive lubricant feed equipment for high-performance grinding process is not a minor option, it is an absolutely must to guarantee a trouble-free processing.

High-performance grinding is not possible without the use of high performance grinding oils and this cannot be effective without suitable feeding equipment. These systems of components are an indispensable part of high-performance grinding machines and belong absolutely to the responsibility of the machine manufacturer.

An important aspect for the use of the cooling lubricant at high-performance grinding is the immediate and careful filtration of the grinded abrasive particles. At HEDG grinding with a removal rate  Q’w = 100mm³/s, a volume of 60 cm³  is machined. This volume corresponds to a particle number of approximately 30 billion with a medium particle size of 25 µm. If this abrasive volume would not be permanently filtered, the total lubricant volume would become useless within some hours. With increasing number particles would be pressed through the contact zone, endangering an increase of wheel blocking and impair the result and quality of grinding. Furthermore, the tribological properties of the lubricant would be strongly reduced with regard to additive level and viscosity and thus becoming useless.

Filtration is a technical continuous process which becomes more difficult the smaller and spherical the particles are and the greater their number per volume is. This causes a problem in high-performance grinding, because the produced abrasives are in the shape of globulitic fragments with a diameter of 3 – 50 µm. Therefore band pass filter, magnetic separators and cyclotron systems are not suitable for the filtration of grinding oils in this case. Dependent on the viscosity of the cooling lubricant the following filtration processes can be recommended.

a)  Low viscosity (4 – 12 mm²/s at 40°C

For this viscosity disc type edge-filters can be used which are also suitable for aqueous solutions.

b)  Medium viscosity (12 – 22 mm²/s at 40°C

In this case it is possible to use special band pass filters with vacuum technology or also special pre-coated filter systems, but the best suitable method is filtering with turbo separators which work efficiently, reliable and independent of the particle form.

In practice it must be noticed that the filtration process also causes a certain lubricant loss, connected with a reduction of the additive content. With refilling of lubricant and/or additive top-up the initial quality can be restored. When the cooling lubricant is maintained in such an optimal way it can be used over 2 – 3 years. During this time over all volumes of abrasive material can be up to 30 times greater than the volumes of the used cooling lubricant, but highest lubricant quality and optimal tribological properties are required.