C. Guo

Temperatures and Energy Partition for Grinding with Vitrified CBN Wheels

An investigation is reported of the temperatures and energy partition for grinding with vitrified CBN wheels. Temperature distributions were measured in the subsurface of hardened bearing steel workpieces using an embedded thermocouple during grinding with a water soluble fluid at specific removal rates from 5 to 60 mm2/s. The energy partition to the workpiece and heat flux distribution within the grinding zone were estimated using temperature matching and inverse heat transfer analyses. In all cases, the maximum grinding zone temperature rise was less than 120°C. The energy partition to the workpiece was found to be only 4.0 to 8.5%. Such low energy partitions are consistent with a thermal model which takes into account conduction to the workpiece, conduction to the abrasive grains, and cooling of the workpiece by the fluid at the grinding zone.

Analysis of Energy Partition in Grinding

An analysis is presented for the fraction of the energy transported as heat to the workpiece during grinding. The abrasive grains and grinding fluid in the wheel pores are considered as a thermal composite which moves relative to the grinding zone at the wheel speed. The energy partition fraction to the workpiece is modeled by setting the temperature of the workpiece surface equal to that of the composite surface at every point along the grinding zone, which allows variation of the energy partition along the grinding zone. Analytical results indicate that the energy partition fraction to the workpiece is approximately constant along the grinding zone for regular down grinding, but varies greatly along the grinding zone for regular up grinding and both up and down creep-feed grinding. The resulting temperature distributions have important implications for selecting up versus down grinding especially for creep-feed operations

Energy Partition and Cooling During Grinding

Abstract High temperatures in grinding can cause thermal damage to the workpiece. This paper presents an overview of quantitative methods to calculate grinding temperatures and the energy partition to the workpiece. It is shown that the energy partition, and consequently the grinding zone temperature, depends on the type of abrasives, fluid application conditions, and grinding process parameters. For regular grinding with conventional aluminum-oxide abrasive wheels, the energy partition typically ranges from 60% to 85%. However, for creep-feed grinding with slow work speeds and large depths of cut, cooling by the fluid at the grinding zone reduces the energy partition to less than 5%. For grinding with cubic boron nitride (CBN) superabrasive wheels, the energy partition is about 20% due to the high thermal conductivity of the CBN abrasive. However, this may be reduced from 8% to 5% for grinding with porous vitrified CBN wheels at high removal rates due to the combined effect of the high thermal conductivity of CBN abrasive and cooling by the fluid at the grinding zone.

Analysis of Transient Temperatures in Grinding

Temperatures generated in the workpiece during straight surface plunge grinding follow a transient behavior as the grinding wheel engages with and disengages from the workpiece, and throughout the entire grinding pass for workpieces which are shorter than needed to reach a quasi-steady state condition. In the present paper, a thermal model is developed for the transient temperature distribution under regular and creep-feed grinding conditions. Numerical results obtained using a finite difference method indicate that the workpiece temperature rises rapidly during initial wheel-workpiece engagement (cut in), subsequently reaches a quasi-steady state value if the workpiece is sufficiently long, and increases still further during final wheel-workpiece disengagement (cut out) as workpiece material is suddenly unavailable to dissipate heat. Cooling by a nozzle directed at the end face of the workpiece should significantly reduce the temperature rise during cut out.

Development of a Sensor-Integrated “Intelligent” Grinding Wheel for In-Process Monitoring

Abstract A sensor integrated ‘intelligent’ grinding wheel was developed for in-process monitoring of wheel preparation and grinding processes for machining of ceramics. The ‘intelligent’ grinding wheel consists of an aluminum core embedded with piezoceramic sensors and fitted with diamond abrasive segments on its periphery. Multiple sensors equally spaced around the core periphery are used to measure the instantaneous normal force and its variation during each wheel revolution, and additional sensors near the wheel bore monitor the acoustic emission. A DSP-based telemetric data acquisition module attached to the wheel face is used to acquire, process, and transmit data from the rotating wheel to a remote receiver. Experimental results are presented which demonstrate the ability of the ‘intelligent’ wheel to monitor wheel truing and grinding processes.

thermocouple fixation method for grinding temperature measurement

A new thermocouple fixation method for grinding temperature measurement is presented. Unlike the conventional method using a welded thermocouple, this new method uses epoxy for affixing the embedded thermocouple within a blind hole in the workpiece subsurface. During grinding, the thermocouple junction is exposed and bonded to provide direct contact with the ground surface by the smearing of the workpiece material. Experiments were conducted to evaluate this simplified thermocouple fixation method including the effect of thermocouple junction size. Heat transfer models were applied to calculate the energy partition for grinding under dry, wet, and minimum quantity lubrication (MQL) conditions. For shallow-cut grinding of cast iron using a vitreous bond aluminum oxide wheel, the energy partition using a small wheel depth of cut of 10 μm was estimated as 84% for dry grinding, 84% for MQL grinding, but only 24% for wet grinding. Such a small energy partition with wet grinding can be attributed to cooling by the fluid at the grinding zone. Increasing the wheel depth of cut to 25 μm for wet grinding resulted in a much bigger energy partition of 92%, which can be attributed to fluid film boiling and loss of cooling at the grinding zone.

Turning of Hardened Steels

Abstract An investigation is reported of the face turning process for machining a hardened bearing steel using CBN cutting tools. Studies were made of the specific energy, the chips generated, the tool inserts used, and the machined surfaces. The specific cutting energy was found to be directly related to the maximum undeformed chip thickness, which is attributed to the plowing at the engaged cutting edge. Most of the chips generated were saw-toothed, which can account for the relatively large chip area ratios measured. The measured surface roughness and the ratio of 10 point to arithmetic average roughness, Rz/Ra, were comparable to their predicted ideal values for rougher surfaces with larger feeds, but the values tended to become progressively bigger for smoother surfaces with smaller feeds. The incidence of white layer generation decreased with a sharp tool at faster cutting speeds.

ENERGY PARTITION FOR GRINDING OF NODULAR CAST IRON WITH VITRIFIED CBN WHEELS

Abstract An investigation was undertaken to determine the energy partition to the workpiece for grinding of an automotive nodular cast iron with a vitrified CBN wheel. The energy partition is a critical parameter which is needed for calculating grinding temperatures and predicting the occurrence of thermal damage to the workpiece. Straight surface grinding experiments were conducted over a wide range of removal rates to measure the workpiece temperature response and spindle power. Temperature matching analyses were then applied in order to obtain the energy partition. Values for the energy partition were found to range from 3 to 6% when the temperature at the grinding zone was maintained below the fluid burnout limit. Such low energy partitions indicate cooling of the workpiece by the fluid at the grinding zone. Higher energy partitions up to 60% were obtained by reducing or eliminating the fluid, which also raised the temperature above the burnout limit.

COMPUTER SIMULATION OF BELOW-CENTER AND ABOVE-CENTER CENTERLESS GRINDING

Abstract Centerless grinding of ceramic components in the conventional above-center mode under aggressive conditions often results in workpiece spinning. One way to avoid workpiece spinning is to use below-center grinding, but this can lead to problems with part rounding and lobing. The present investigation was undertaken to develop a practical simulation to assist in the selection of acceptable set-up conditions for centerless grinding. From the initial workpiece profile and set-up conditions as input, the simulation predicts workpiece spinning and the progressive change in the workpiece profile for both above-center and below-center grinding. Simulation results for workpiece lobing were found to be consistent with experimental results obtained for below-center grinding of zirconia specimens.