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University of Bremen (UoB) investigates the grind-hardening process for surface grinding with conventional grinding wheels. Grind hardening is an innovative method to combine the machining of workpieces by grinding and the heat treatment of these parts. The heat treatment step is achieved by utilizing the heat produced during grinding to generate the metallurgical transformations necessary to harden the material. The quenching of the workpiece takes place by the component itself respectively by the use of coolant lubricant. The introduction of this hardening method enables energy and cost savings by a distinct reduction of the process chain for the manufacture of high valuable steel parts.
UoB performed a wide range of test parameters in order to develop suitable machining strategies (including the use of coolant lubricants) and to determine effective grinding tools. Results refer to:
- suitable machining parameters in term of cutting speed, infeed and feed rate which enable a hardness penetration depth of > 0.5mm,
- strategies for the application of coolants,
- set-up of a test rig (grinding tapered workpieces) for shortening the definition of machining parameters for the grind-hardening process by.
During finish-grinding-investigations IFW had to focus on reducing the coolant supply with regards to electric energy savings as well as waste reduction. In order to do so, a monitoring system was developed.
In grinding, thermally induced phase transitions are believed to emit acoustic emission signals. Any significant change for the worse in the coolant conditions will therefore increase the AE level, since the thermal infliction on the part¿s subsurface will increase.
IFW has developed a testing strategy and given guidelines for the filter settings, to identify the optimum coolant condition within a set of given alternatives. By this technology it is possible to evaluate between several coolant nozzles as well as to set the optimum coolant flow rate.
This technology was currently applied to external plunge grinding processes but is believed to be transferable to peripheral surface grinding processes as well.
Experimental findings correlate to the guidelines based on measured data given by UoB.
Since the efforts of SAMPUTENSILI to try to reduce the energy consumption during the grinding process, an holistic approach has been taken in consideration. The total energy during the working cycle of the ring gear provided by Fiat has been evaluated and split in different component as the cutting oil filtering & cooling device, pure grinding process, dressing cycle, oil mist filtering device.
This has been evaluated for the state of art cycle based on the actual cycle used in Fiat workshop with a different brand machine and later on for the various modification applied on the tools (number of starts and technical specifications), on the cooling nozzle design and on the cutting parameters. Important energy consumption reductions have been gained in all the different items studied succeeding a total saving of 38%.
On the one hand the most industrial grinding machines have no possibility to measure important process data so that the optimisation potential is limited. On the other hand there is nearly no easy-to-handle-equipment, which allows a measure of important process data directly on the production machines. Especially with regard to the goals of this project the knowledge of this important process data is required.
In order to meet the requirements of this project it is necessary to have a detailed knowledge about the process behaviour, which can be evaluated by detecting the grinding forces, spindle power or feed-time-diagram. Moreover, a direct comparison of fundamental tests with real industrial grinding operations is necessary in order to find the fastest and most practical way of optimisation.
With the developed TYROLIT mobile measurement system this gap can be filled. The mobile measurement system allows a portable and flexible recording and evaluation of the mentioned process properties during grinding on nearly every grinding system. Further advantages are an easy handling and transportation by using an adapted attache case.
Important hardware elements are a portable computer notebook with an integrated A/D converter, an amplifier, an electrical current probe (clamp-on ampmeter), a LVTD (displace measurement), a main adaptor and a magnetic tripod. All components are controlled by a special designed software program.
UoB together with IFW is working on the development of an in process or post-process control system of the grind-hardening process. For the development of a process control system for grind-hardening the knowledge of the relationship between quality properties and process signals, as there are grinding forces, acoustic emission, power and control signals is required.
The grind-hardening machine tool of UoB was equipped with a rotating Dittel M-sensor for evaluation of correlation of acoustic emission to the grind-hardening results in order to provide IFW with data.
While IFW is more focused on the processing of AE-Signals UoB is engaged in correlating power and force measurements to the achieved quality in term of the hardness penetration depth. The investigations proved correlations of grinding power and grinding forces to the grind-hardening quality
University of Patras (UPATRAS) has concentrated the research effort in modelling and simulating grind hardening and grinding technologies. A detailed simulation model will allow the reduction of time and energy consuming set-up phases, the development of optimised cooling strategies and the selection of process parameters that will enable the best quality of the final part.
The main results that have been achieved during the project's duration:
1) Literature research on previous analytical conventional grinding simulation models.
2) Development of a simulation model for the prediction of hardness penetration depth (HPD) achieved from both dry and coolant fluid assisted grind hardening.
3) Realization of basic dry and wet grind hardening experiments and comparison with model's predictions. The investigated process parameters included the cutting depth and the grinding wheel peripheral speed.
4) Theoretical assessment of grinding wheel effect and comparison of the predictions with the experiments performed with TYROLIT's newly developed grinding wheels
5) Theoretical modelling of the development of residual stresses during the finish grinding processes that follow the grind hardening process
Normally in Industry the number of dressing strokes to achieve a good grinding wheel surface and topography is not known. The machine user apply a fixed number of dressing strokes due to their experience and some additional strokes for process safety reasons because he does not have a quality in-situ measurement for detecting the minimum number of required strokes.
Several possible geometric tool errors have to be considered to classify a grinding wheel as free of geometric errors. Some of them were investigated in Task 2.2:
- Edge wear: A very sensitive system of AE signal evaluation was built up based on curve fitting and optimisation algorithms to get a reliable detection of the dressing stroke time. It cannot easily be disturbed by the noisy AE signal.
- Notch detection: Normally, AE mapping technology only considers the actual surface of the grinding wheel, applying a colour map to the AE level. Mapping the depth of the grinding wheel regarding all the previous dressing strokes, offer a prognosis of the remaining depth of damage.
Within the scope of the Engy project Danobat investigates technologies to decrease the energy consume of grinding machine or to use new eco-efficient technologies of grinding.
--Investigations about eco-efficient lubrication in guides. The first step has been to analyse the commercial solutions of guides system without lubrication. The second step is to analyse the possibility to use the technology of coated tools actually used for milling in guides. The object is the same than in milling, decrease friction and decrease wear, but the application is different: guides system with high damping for grinder machines.
- Machine adaptation for industrial grind - hardening. Danobat has built a system to use coolant to cool the wheel but at the same time to dry it. The result is that the wheel is cool but at the contact point between de wheel and work-piece the heat is very high. A termography to demonstrate what has been done, and a test to demonstrate the improvement of the system in the grind hardening final results.
In the field of grinding and dressing monitoring and control, IFW did various grinding and dressing experiments to investigate the potential to achieve faster processes with a predicted surface quality.
Using a state-of-the-art rotating dresser with spread or set diamonds, applying a constant infeed every dressing stroke; it needs several dressing strokes for the grinding wheel removal to remain at a constant level. During this time, the grinding wheel topography is not the final and desired one. Due to less removal, the diamonds tend to flatten instead of breaking out, so especially for rough grinding, the wheel has less material removal potential.
At the final and desired dressing strokes, the envelope shapes of the dresser and the grinding wheel are penetrating themselves. The penetration depth has to reach a certain level before getting constant dressing conditions. A compensation strategy of this penetration depth was developed to get a constant dressing process within the first two dressing strokes by an increased infeed at the second dressing stroke. A few reference measurements have to be done using an AE sensor before knowing the final penetration depth. The compensation strategy saves grinding abrasives and also dressing time and therefore energy.
DAN's Investigations have been done focused on the development of a control integrated monitoring strategy for the on-line characterisation of the grinding wheel status, in order to detect when the wheel requires dressing.
CNC integrated monitoring system has been developed based on the acquisition and processing of internal data provided by the CNC. Experiments have been carried out under different cutting conditions in a tangential grinding machine. The grinding wheel wear, part roughness, specific power consumption and process forces have been measured. Increasing energy consumption during successive grinding strokes is a suitable performance index to detect when the wheel requires dressing.
In the same way, the effects of vibrations during cutting on process signals, roughness and wheel wear evolution have been studied.
Within the scope of the ENGY project the University of Bremen (UoB) investigates technologies for the effective and resource saving application of coolant. As approaches to the reduction of the use of coolants advanced strategies for the coolant supply by different special nozzles were investigated. The more effective supply of coolant promises the distinct reduction of the coolant flow rate (l/h) and therefore of energy needed.
UoB performed a wide range of tests. It was shown that the coolant supply conditions and nozzles are specific for the particular process. Therefore, an equipment was developed which enables the quick change of coolant nozzles and to retrieve the specific coolant nozzle position adapted to the grinding process. On the sector of energy savings when applying coolants UoB showed that bypass-systems are more effective for the control of the coolant flow rate than the control by valves directly in front of the coolant nozzle.
Within the scope of the ENGY project the University of Patras undertook the role of providing guidelines for the energy balance assessment. Life cycle analysis methodologies were used for the assessment of the energy balance.
The life cycle analysis took into consideration:
- the energy consumption
- the material consumption
- the coolant fluid consumption and
- the grinding wheel wear
induced either by the grinding or by the grind-hardening processes. Furthermore, the various handling procedures were considered for the assessment of each process setup.
For each pilot case, the achievements presented during the ENGY project were compared with the benchmark criteria that had been defined at the beginning of the project.
Indicatively, it was shown that:
- the substitution of conventional heat treatment processes with grind-hardening in the bearing industry reduces the seriousness of the environmental damage effect by 45%
- the substitution of conventional heat treatment processes with grind-hardening in the automotive components industry can reduce the seriousness of the environmental damage effect by 35%
- the utilization of optimised grinding process parameters such as multi pass grinding and use of newly developed corundum grinding wheels for the grinding of hardened hollow shafts in the defence industry can reduce effect to the environment from 30 to 60%
- For the case of grinding of hardened gears the changes imposed on the grinding machine and the grinding process resulted in an average energy reduction of 38% and a subsequent environmental damage reduction of ca. 43%
Significant improvement of vitreous bond wheel performance, in terms of life and total energy consumption, for finish grinding has been achieved by development of improved grinding wheel structures and use of new CBN materials.
These same improved wheel structures have been applied to the grind hardening application for the first time and the ability to achieve grind hardened surfaces with CBN has been demonstrated along with the potential to deliver the high grinding ratio required to make the process viable.
However, during the course of the project it was not possible to bring the degree of control required to convert this potential into practical use due to currently inadequate control procedures.
The structure development completed as a result of this project has already been used very successfully in other grinding applications (both high and low stock removal) typically yielding 3 x the parts per dress of standard CBN products whilst maintaining burn free parts and good geometry.
Hydraulic systems are used for many different applications in machine tools like hydraulic drives for main and infeed movements or clamping devices for slides, workpieces or workpiece handling. The low part volume and the high power to weight ratio compared to electric drives are main reasons for the use of hydraulic components. On the other hand hydraulic systems are generally characterised as systems with a low degree of efficiency, sensitive in case of dirt and high heat generating.
The efficiency of the often-used hydraulic systems is quite low. In some operations, a large flow rate is required; at other operations, a high pressure with a low flow rate is needed. The design of the hydraulic system considers the maximum of both.
Two possibilities were found to reduce the energy consumption of the hydraulic system:
- Use of two pumps: One Motor can drive two pumps, one for high pressure and another for high flow rates at low pressure. These two pumps can be coupled by a valve to avoid the installation of two hydraulic circuits
- Use of storage of high pressure: High flow rates required only at short times. Storages offer the design of a hydraulic system with a pump of a smaller flow rate. The pump will work in an interval mode.
In grinding the process time is mainly depending on the chosen material removal rates. During the Engy project it was shown that in an overall balance, high material removal rates are favourable in terms of eco-efficiency. Hereby economic and ecological goals can be perfectly combined.
Nevertheless, too high material removal rates negatively inflict the subsurface integrity of ground parts due to thermal loads. Therefore a monitoring tool would be desirable which allows distinguishing between damaged and undamaged workpieces in-process.
To achieve this goal, IFW has used acoustic emission technology and developed new filtering and signal processing methods. Hereby it is possible to use a very small-scale test-matrix to identify grinding burn or tensile residual stress in the subsurface area. By this strategy a quick and cost efficient process set-up can be realized, using a common measurement technology without any need for further measurement equipment or evaluation time.
During grinding operations a large proportion of the electric energy is consumed by stationary operated sub-systems of the machine tool, regardless of the actual process state. Due to this problem any change of grinding parameters will not lead to significant energy consumption reductions without but shortening the process time.
Regarding the grinding process itself rough and finish grinding choosing this can be done be choosing adapted process parameters can reduce the power consumption of process related parts of the machine tool, like wheel or part spindle. Nevertheless, the energy consumption of all sub-systems stays the same, even during spark out operations regardless of the grinding operation, where grinding spindle power is much lower than elsewhere.
Since spark out operations are thought to improve the part's quality, especially roundness and surface roughness, the spark out time is rather process specific than parameters specific. In order to ensure optimum part quality, the time has to match the worst possible process input. In consequence, spark out time for most parts is much longer than needed.
This offers an interesting field of application for an adaptive control. If a sensing device would be possible to identify y such quality parameters as roundness error, an adaptive spark out time could be installed.
IFW has developed a sensing technology based on AE signal recording to maintain such information on the actual part geometry within the process.
The reliability of grinding processes can be increased by process control using Acoustic Emission Sensors. AE signals correlate to force signals and therefore also to the heat induced into the workpiece. In Task 2.5, it was discovered that grind-hardening processes cannot be controlled in this way due to following reasons:
- AE monitoring is not possible in continuous dressing processes. Dressing AE signals have much higher vibration amplitude than the grind hardening process itself
- Force and AE signals do not correlate, especially at the beginning of the grind-hardening process.
- Neither AE nor force signals stay constant at a process with constant parameter and grind-hardening result
- There is no clear correlation between the increase of the AE signal and the grinding wheel wear
- Thermal material structure changes cause different signal level
- No correlation between the AE signals and the process result was found
- Friction coefficient and feed speed do not correlate to feed speed, so they cannot be used for control purposes
Coiflet wavelet transformation of third order and four levels of detail might offer a post process quality monitoring possibility. The increase of the detail frequency bands could be used as a feature for input to a neuronal network. A certain uncertainty probably will remain.
In general quality, performance and efficiency of grinding operations are strongly dependant on the behaviour of the used grinding tools. Nearly every grinding process is based on special defined specifications. In order to further reduce the energy consumption during grinding and to realize a stable grind-hardening process optimised grinding wheel concepts are required.
TYROLIT, responsible partner for new corundum grinding wheels, has developed new corundum wheel compositions which enable optimised process behaviour for both finish grinding operations and grind-hardening operations.
On the basis of an analysis of suitable raw materials, the potential of special designed grain-bond-combinations in different structures has been verified during defined grinding tests. Dependant on the ground parts and the two tasks of finish grinding and grind-hardening suitable grinding wheel specifications can be selected
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