Quality Scan: Using More Effective Coolant Application to Improve Grinding Quality
John A. Webster, PhD
Unlike machining processes such as milling and turning, grinding wheels do not present a defined cutting edge to the chip-forming contact zone. The abrasive grain can have a positive rake, negative rake, a variety of tip radii, and may possess wear flats. The cutting edges are also much smaller than with cutting tools, with a greater number around the periphery, giving a chip size much smaller than in machining. For the same volume of material removed by grinding as with machining, the multitude of small grinding chips requires significantly more cutting energy than much fewer but larger machining chips. This phenomena is defined by specific grinding (or cutting) energy, and partly explains why grinding is more prone to workpiece surface integrity issues than machining. Thermal damage of the finished workpiece surface includes visible burn, thermal softening, re-hardening, tensile residual stresses, and phase-transformations.
Achieving effective cooling during the grinding process requires the grinding fluid to be transported through the chip removal zone using the porosity within the wheel structure. Wheel porosity depends on the type of bond used and the structure of the wheel. High power grinding processes, such as creep-feed grinding, can only control thermal damage when very open wheel structures are used to transport the coolant through the process. Regardless of the degree of porosity, ensuring that the coolant fills as much of the pores as possible is paramount to achieving effective cooling of the process. Many grinding machines in use today supply excessive flowrate, low pressure coolant through large nozzle apertures, with an internal geometry which produces dispersed jets entrained with air.
The boundary layer of air that surrounds a rotating grinding wheel can become a barrier to filling the porosity of the wheel. A proven strategy for penetrating the air barrier is to increase the jet speed close to the speed of the grinding wheel surface. If the wheel speed is 6,000 feet per minute this only requires a pressure of 60 psi at the nozzle, but higher wheel speeds will require a greater pressure. The nozzle aperture has to be designed to create this pressure without excessive flowrate, and be within the performance curve of the pump. The plumbing that links the nozzle to the pump can also dramatically reduce the pressure at the nozzle, a 50% drop is not uncommon.
The flow-rate of coolant required to fill the porosity of the grinding wheel and also perform some bulk cooling of the workpiece only needs to be within the range of 1.5-2 GPM per grinding horsepower. Therefore a 5 horsepower spindle load will require a flow-rate between 7.5–10 GPM, depending on the wheel abrasive type and process used. If the grinding wheel is 1” wide, and 60 psi at 10 GPM is required, a nozzle aperture of just 0.04” x 1” is needed. If the nozzle is designed with a coherent-jet internal geometry the jet will hit the wheel as a ribbon as compared to a spray.
The ability to control thermal damage with more effective coolant application allows the process engineer to manipulate feeds, speeds, grit size, and dressing parameters to better achieve surface finish. Reducing the chip thickness in grinding will typically improve the surface finish of the finished part. To achieve this, the wheel speed can be raised, grit size decreased, or a finer dressing lead and/or compensation can be used. All three strategies will raise the grinding energy and the possibility of grinding burn. More effective coolant application will allow greater control of process parameters to control surface finish with reduced risk of damage.
The structure of a grinding wheel can become clogged (loaded) with workpiece material reducing the porosity. Whilst the loading can be reduced by more frequent wheel dressing, this is not an economical strategy since wheel life will be reduced. Coherent-jet nozzles will hit the grinding wheel surface with more impact energy than dispersed jets and help keep the wheel cleaner, allowing better cooling and control of the process. High-pressure cleaning jets at a pressure of over 500 psi are a more effective option but require an additional HP coolant pump and special nozzles to be integrated into the wheel-head.
As compared to the cost of the grinding machine, filter system, grinding wheel consumption, coolant disposal, etc, optimization of coolant application can cost less than 1% of the grinding system, but has the potential for huge economic and quality benefits. ME
This article was first published in the July 2014 issue of Manufacturing Engineering magazine. Click here for PDF.
Published Date : 7/1/2014