Coolant Performance Variables for Optimum Glass Grinding
Matching the material being fabricated and the machining operation with the ideal coolant helps maximize the efficiency of swarf settling and improve individual part quality.
Grinding or cutting is a process whereby an abrasive particle embedded in a rotating wheel or knife edge removes material from a workpiece by mechanical means. The grinding wheel can be attached to a horizontal or vertical spindle, and the specimen can be mounted on a table surface that travels in either a rotary or reciprocating direction.
It is important that the tensile strength of the material being processed is less than that of the abrasive. Other process-related variables that are important for this operation include the depth and speed of the cut, coolant flow, wheel speed, and particle removal/filtration. A non-process-related variable that is often overlooked, yet can drive the overall grinding performance, is coolant chemistry.
Flocculents in Heat Dissipation
The mechanical and fluid frictional forces created by the machining operation require a cooling/lubricating fluid. This operation can be expressed as energy between the workpiece and the tooling while also producing heat at the point of contact (frictional forces). Understanding the machining mechanism for brittle materials is beyond the scope of this article. However, the simplest way to think of material removal in a generic grinding operation is to consider that softer, ductile materials tend to shear or undergo plastic deformation (positive, oblique knife angle). On the other hand, in order to reach the desired ductile-mode removal, brittle materials must be cut with an applied negative rake angle1,2 until fracture.
The distinction becomes relevant when considering the shape, concentration and overall volume of swarf created. Diamond turning or milling of glass are operations that can precisely control the amount of material being removed, creating a nanometer-scale finish for sophisticated optics. The depth of cut, wheel speed and speed of cut are critical for a smooth finish. Although these fines are microscopic, the interaction between the volume of entrained air (foam) in the coolant system and the swarf created play a key role in keeping the heat generated at the workpiece interface to a minimum.
Data collected from a flat glass operation illustrate this assertion. Figure 1 shows time series plots of peak power measured for runs with and without a flocculent, and points out the importance of particle removal and the potential for the effective use of a flocculent. As seen in Figure 1a, with or without a flocculent, the energy generated from this operation remains fairly constant for a period of approximately 50 parts, at which point the energy undergoes a severe jump at the grinding interface. This response remains high until a wheel dressing is performed. Subsequent to wheel dressing, the amount of work measured at the workpiece interface is reduced to its relative baseline.
For the run shown in Figure 1b, the concentration of flocculent was twice that which was used for Figure 1a. (This implies that the effectiveness in settling out particles is greater for the test runs in Figure 1b than in Figure 1a.) Process energy variation is reduced but, more importantly, there is measurably less work expenditure with the more concentrated flocculent. As a result, less energy is allowed to reside in the surface of the glass substrate, allowing for increased surface and edge quality.
The following hypothesis is suggested: under pressure, small fines get continually forced into the coolant stream at the point of contact for the grind and dramatically impact the surface quality of the finishing process. This disruption adds energy and heat to the grind interface, and ultimately transforms the fluid film between the workpiece and the grinding wheel from a uniform thin film to a deviated layer. Reducing the amount of friction as much as possible will reduce the amount of energy driven into the surface of the workpiece and increase the efficiency and quality of the finishing process.
The fines created in a flat glass operation or single-point diamond turning (e.g., lens generation) are quite different than those created by a metal-working operation. Metal fines are less likely to get reintroduced into the recirculating fluid, and the heat associated with these fines (along with the trapped air in the circulating fluid) is different than that of a lens generation or flat glass operation. The coolant chemistry should reflect this and be chosen accordingly. It is important to match the material being fabricated and the machining operation with a coolant that will maximize the efficiency of settling out the swarf and improve individual part quality.
Coolant Formulation and Energy Management
In addition to cooling and lubricating, coolant’s roles in corrosion control and microbial growth prevention should not be overlooked in the overall grinding system. The complexity of the coolant is appreciated when considering its role in maintaining the fundamental functions while remaining biologically and corrosively stable. An “alkaline reserve” is required to keep the hydrogen ion concentration stable. Passivating the ferrous and nonferrous metal surfaces to prevent corrosion is a mandatory requirement. The coolant must also reduce foaming and misting, provide sufficient wettability, and leave no residue. The overall health and safety of the machine operator is also of key importance and must be considered when choosing an effective coolant.
Energy at the work interface affects both quality and cost. During the grinding operation, energy at the spindle workpiece interface changes with the depth and speed of cut, as well as wheel speed. In addition, the strength of the material being processed plays a role in the heat being created and ultimately the surface finish. Coolant flow and pressure must be carefully controlled to adequately manage heat and friction at the work interface. When properly chosen, synthetics provide consistent viscosity and film strength for efficient material removal and a superior finish. The fluid chemistry in some of the newest synthetic coolants provides hydrodynamic film strength and viscosities two to three times that of petroleum-based fluids.
Figure 2 plots 12 different coolants against the grinding energy (J/cc) produced at the interface of a flat glass operation, along with G-ratio (ratio of material removed from the fabricated specimen to the amount removed from the grinding apparatus). Machine parameters and the material being processed were specific and constant.
It is evident that a significant drop in energy can be realized by changing the coolant chemistry alone. In addition, G-ratios can be moderated by coolant selection. Coolant L exhibits the lowest grinding energy, along with a very high G-value. Provided that one can create parts with a superior surface finish, this coolant would be the fluid of choice.
Quality and Cost Impact of Energy
The quality and cost of a machined part is related to the operation being performed and the substrate or material being fabricated. Hole drilling, sharp radius turns and deep grooves are examples of high heat- and energy-generating processes. The volume or amount of material being removed is critical in heat generation. Increasing the force acting on the substrate increases the removal rate of the material, but also creates surface and sub-surface damage. Lowering the energy at the point of contact may, in fact, reduce stored energy at the surface of the part, as well as lower chipping, subsurface micro-cracking, and overall breakage. The quality of the fabricated parts can be considered best when the optimum force is applied and subsurface damage held to a minimum. Process knowledge is key to understanding the optimum force and material removal required for high-quality parts.
The coolant concentration can help mitigate heat generated at the work interface (see Figure 3). At 0% coolant (water only), the grinding energy is at its highest point. After about 25-30:1 coolant concentration, the coolant’s effectiveness levels off; it is maximized at around 15-20:1. It can be assumed that the hydrodynamic film strength is optimum at this concentration and is stable. It can also be assumed that the lowest levels of entrained air are present at this dilution. Overall, grinding conditions at this point are shown to reduce wear and help extend wheel life. Extending diamond life has a significant impact in cost savings, while optimizing coolant chemistry and concentration play an important role in controlling the specific grinding energy of the operation. (It should not be neglected that amine chemistry is extremely important and vital in extending wheel life.)
Figure 4 plots the volumetric removal rate (cm3/min) and G-ratio for different coolant chemistries of a precision glass process, as in Figure 2. The data show that the removal rate can be increased from 15 to 22.5 cm3/min simply via adjusting coolant chemistry, a 50% increase that any manufacturing engineer would certainly accept. Of course, surface finish cannot be neglected. As in Figure 2, coolant L exhibits the strongest performance, providing minimum surface damage.
Achieving Optimum Performance
The functions of a coolant are numerous, and optimum grinding performance is measured by yield efficiency and low processing costs. In addition to being operator friendly, a good coolant features superior wettability, exhibits no foaming or misting, leaves no residue, and—above all—provides a superior finish with limited sub-surface damage so that subsequent polishing steps have low cycle times.
Broadly speaking, cooling and lubricating is the fundamental purpose of a coolant, regardless of the material being fabricated. In addition, ferrous and nonferrous metal corrosion protection and bio-stability (regarding operator health and safety) are also important functions. A flocculent can effectively reduce energy at the workpiece interface. Optimizing coolant chemistry can create a hydrodynamic film strength resilient enough to sustain a harsh grinding environment. It is expected that these factors help lead to extended diamond and tool life.
The use of flocculent also aids in managing fines settling, particularly for brittle materials. Efficiently flushing chips away from the workpiece interface of a flat glass process and minimizing entrained air/foam in the fluid flow play an important role in minimizing heat and optimizing grinding performance.
1. F.Z. Fang, et. al., International Academy for Production Engineering, CIRP Annals, 62 (2013) 823-846.
2. F.Z. Fang, et. al., Official Journal of Indian Academy of Sciences, Sadhana, Vol. 28, Part 5, October 2003, pp. 945-955.