A more efficient use of natural resources leads industry to increase the efficiency of manufacturedproducts. Transportation Diesel vehicles are included in this scenario. In Diesel engines, oneimportant component of the injection system is the nozzle injector. A recent legislation in Brazilintroduced EURO5 emission limits. These new parameters had impact on nozzle design withtolerances being reduced, in special on nozzle body seat area, changing the functional parameter ofnozzle opening pressure. In this scenario, the grinded conic surface impacts on this functionalparameter, thus on the Diesel engine performance. This study aims to relate the grinding process,surface generated in the process and the Diesel nozzle opening pressure variation; the goal is toreduce waste in manufacturing, generating less surface roughness dispersion in grinding process.Planned process tests were performed according to Taguchi methods and it was determined signalnoise ratio to surface roughness parameters. The seat surface will be analyzed using 2D and 3Droughness measuring equipments. Study results includes grinding process parameters optimizationfor smaller surface roughness, roughness relation to nozzle opening pressure and scrap costreduction in the manufacturing process. These points will be achieved through determination ofsurface roughness parameters that could detect surface quality problems after grinding processwhich means knowing the problem without the need of functional measuring at the end of theprocess chain.
The evolution of manufacturing goods, their applications and manufacturing process has increaseddramatically due to the necessity of more efficient products. Therefore the minimal amount of
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natural resources should be consumed in order to emit the smaller possible amount of rejects suchas pollution gases, for example. Transportation vehicles are included in this scenario, in special thosewhich uses Diesel engines, due to its large application in cargo and people transportation. In Dieselengines, the injection system is very important to assure smaller fuel consumption and pollution aswell as fewer gases emissions. In the injection system one very important component is the nozzleinjector (Huang et al, 2013), it realizes an important role in the fuel injection according to itsgeometry, affecting air fuel mixing, which impacts on efficiency and pollution gases formation(Benajes, 2008). In Brazil, a passed recent legislation introduced EURO5 emissions limits. Thereforethe nozzle designs of old families had to be reviewed to attend the new restricted requisitions. Inthis way the tolerances should be reduced, in special on nozzle body seat and blind hole dimensionsand shape, which are important regions of the nozzle injector (Jääskeläinen, 2011), as illustrated inFigure 1, which shows a typical EURO5 nozzle seat configuration. In this scenario with reduceddimensions and tolerances, there is an increasing requirement of more precise manufacturing andmeasuring processes, further on the correct correlation between geometry, surfaces and functionsbecame more important.
Figure 1: Important areas of the nozzle injector: 1 Injection hole, 2 Blind hole, 3 Body Seat, 4Nozzle body 5 Nozzle needle and 6 Sealing area. Source: Mahr, 2012 with modifications.
In the Diesel nozzle injector, the most important functional parameter is the nozzle openingpressure: the minimum necessary pressure to the nozzle needle lift and the injection occur. To theopening pressure, the most important area is the nozzle body seat (Figure 1); the body seat has amajor influence on this parameter, affecting the injection system and engine performance during itslifetime (Jääskeläine, 2011). The nozzle body seat is an internal conic grinded surface inside a hole ofapproximately 3.5 mm diameter and 45 mm depth, with high finishing quality, i.e. roughness smallerthan 2.5 μm (roughness parameter Pt).
In this situation, the seat and its surface generated through grinding become important because oftheir impact on nozzle opening pressure which impacts on the Diesel engine performance, in thisscenario it was verified an opportunity, due to the lack of a scientific study to the case so far, torelate the parameters of the internal grinding process of the EURO 5 generation Diesel nozzle body
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seat with the surface generated in this process, in special unknown causes for the bad grindingbehavior in the specific seat area 3 (Figure 1), this generate seat surfaces with a material peak in thisregion.
In addition, there is no detailed study of the relationship between the seat surface roughness, still inthe grinding stage, to the further opening pressure of behavior, evaluated only at the end of theproduction line.
The goal of this study is to reduce waste in grinding through the reduction of surface roughnessdispersion. Using the schema proposed by Jiang (2012) presented in Figure 2, the central range ofthe project remains between blocks one and two from left to right side.
Figure 2: Characterization of manufacture, surface and function schema. Source: Jiang, 2012.
The achievement of the project objectives will increase the knowledge regarding the nozzle bodyseat surface grinding process parameters related to the surface generated, especially in relation toroughness profile in the sealing area. This will be a valuable contribution to the understanding of therelationship between the seat surface roughness parameters and the opening pressure behavior.Other important contribution should be the cost saving with the reduction of scrap in the productionprocess, which presents an ambient and capital profit nowadays. The study of the bad grindingphenomenon described will also contribute for grinding process knowledge in general, once it hasmany characteristics found in other similar processes.
LITERATURE FOCUSED REVIEW
Taguchi methods
Taguchi methods use the concept of energy which can be used effectively to perform a givenoperation performing value generation to generate quality or be lost as noise (variance, friction,heat, etc.) or as the method names: function loss. It allows, by using an appropriate orthogonalarray, to determine the effect of many parameters efficiently, besides that it’s an importanttechnique in robust Engineering (Davim, 2002). The main output value of Taguchi methods is thesignal to noise ratio, identified by S/N; this ratio is used in manufacturing engineering as a value ofhow much robust a process is (Moura, 2001).
Manufacture, surface and function
Authors such as Whitehouse (2001) and Jiang (2012) defined the importance and method foranalyzing manufacture, surface generation and function behavior of some product or component. Agood solution, which fits well for sealing issues, is the surface roughness characterization in order topredict the future of any product behavior; however the quality of this prediction will depend on a
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good roughness analysis, parameters selection and manufacturing process study, this may be doneusing experts experience, design of experiments or both (Deltombe et al, 2013). A quality surfaceroughness measuring process and a well know production processes are the basis for this procedure.
The grinding process is one of the most important operations used in manufacturing engineering toremove unwanted material or introduce a desired geometry and surface properties (Shaw, 1996).The process involves the removal of material, usually for finishing; it aims to "surface generation",the value of this surface can be described in terms of the function performed by it and how fast thesurface can be generated (Oliveira et al, 2009). The most important aspects of the grinding processinvolve the tool, which is the grinding wheel, a body with abrasive particles and a binder.Furthermore the dressing process of the grinding wheel, which is the grinding wheel cleaning andsharpening and what determines material removal rate, grinding forces, grinded surface quality andsubsurface material properties (Wegener et al, 2011). The most common grinding processes are theexternal plane cylindrical, external cylindrical and internal cylindrical grinding, which processes havetheir particularities, however the process principle is always the same: remove material in a smallrate, with high specific energy in order to achieve a surface with small roughness and high finishingquality.
The internal cylindrical grinding is a very important machining process. It is widely used for smalldiameter holes finishing, for internal grinding of complex surfaces and inner race of bearings (Shaw,1996), inside the internal grinding process group, there is an important process that is the internalconic grinding. The internal conic grinding presents characteristics and challenges similar to theinternal cylindrical grinding process, however its intrinsic features introduces additional difficultiesand concerns. The major ones are listed:
i. Process vibrations: due to the restrict access to the grinding surface (conic surface inside ablind hole with high relation depth/ diameter), it’s normally used a shaft with small diameterto support the grinding wheel. This situation may ease the vibration to appear;
ii. Process lubrication and cooling: due to the grinded part, grinding wheel and fixing devices,the access to necessary oil might be difficult, it can overheat the grinding zone, which can beharmful to the process (part, grinding wheel, cutting power, etc.);
iii. Parts fixing and removed material: parts must be very well fixed to the machine spindle toavoid process variation, besides that the part stopper position must standardized to avoidmaterial removal amount variation, which could lead to problems in the process;
iv. Correct and constant dressing necessity: depending on the part surface requirements,grinding wheel dressing must be performed in order to clean and restore the cuttingcapacity of the grinding wheel. More frequent dressing occur due to poor lubrication andcooling of the grinding region, what turns difficult the chips and detached grinding wheelabrasive grits to flow outside the cutting area. This causes more damages quickly on grindingwheel surface, besides the overheating of the grinding area, what is not good either to thegrinding wheel. Other important factors concerning with the dressing process: dressermaterial, dresser to grinding wheel angle, correct rotation speed and absence of vibrations
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from the electrical motor. This last point is quite important, once motors, by the end of theirlifetimes, may cause vibrations that could compromise the dressing process.
In order to have a better understanding of the internal conic process, Figure 3 shows the processelements configuration in the grinding machine.
Figure 3: Process agents of the grinding process of the project. Source: Project partner company.
Surface roughness analysis: 2D and 3D
The surface roughness reflects irregularities of a solid surface, i.e. the border of given material inrelation to another. The properties of solid surfaces are crucial to the interaction between them;these properties affect the contact area, friction, wear and lubrication (Bhushan, 2010). The analysisof a surface aims to reveal its texture. This includes roughness, waviness, direction and flaws(Bhushan, 2010).
Actually, due to the measuring velocity, roughness measuring equipments that uses using a small tipdevice, normally with only some micrometers of diameter, is the most used technique to provide anasperities distribution relation to a measuring length, resulting in a 2D surface roughness profile.
Another option for surface roughness assessment is the use of optical interferometry, widely used tobuild 3D roughness profiles that could measures surfaces with a high resolution technique withoutcontact (Laopornpichayanuwat et al, 2012). In this technique a light beam is emitted on the samplesurface, and the refracted and reflected light is captured by the equipment. After that, a datatreatment is carried out and this signal provides the profile of the surface, furnishing this way thesurface roughness parameters to be assessed. Currently, electronic optical interferometers becomea powerful tool because measurements can be made with relative ease and high accuracy(Joenathan et al, 2001).
METHODOLOGY
The subject of this study still going on, because some of the planned steps are not closed; howeverthe available data by now already allow some important discussions and conclusions. Neverthelessnot closed points will be clearly identified during the preliminary tests.
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The necessary try outs of the project were performed using series EURO 5 nozzles of partnercompany with DIN18CrNi8 steel, it was used the nozzle body grinding machine from manufacturerUVA model U88 and it was utilized the actual normal tools of the process: Cubic Boron Nitride (CBN)grinding wheel and a vitrified diamond dresser (VDD), both with vitrified binder. For the necessarymeasurements of 2D roughness was used the Mahr Perthometer Concept Version 7.1_9 roughnessmeasuring equipment and for 3D roughness measuring will be used, once these measurements willbe done in next project steps, the white light beam interferometer Taylor Hobson CCI with Talymapsoftware.
The process try outs were planned, realized and analyzed using the Taguchi method; method whichcombines the analysis of the mean and standard deviation in only one measurement (Maghsoodlooet al, 2004); it was used the “smaller is better” analysis procedure once the goal is reduce the seatsurface roughness. The orthogonal array L9 34 (9 experiments with 4 control parameters and 3 levelsof variation) was used, the software MiniTab 15 was selected for the related statistics tasks. Theorthogonal array with the grinding process parameters and levels of variation is showed in Table 1(values are omitted in compliance with the project partner company internal procedures).
Table 1: Try out planning according Taguchi method using orthogonal array L9 34
2 20 1 Low level 2 Medium level 2 Medium level 2 Medium level
3 20 1 Low level 3 High level 3 High level 3 High level
4 20 2 Medium level 1 Low level 2 Medium level 3 High level
5 20 2 Medium level 2 Medium level 3 High level 1 Low level
6 20 2 Medium level 3 High level 1 Low level 2 Medium level
7 20 3 High level 1 Low level 3 High level 2 Medium level
8 20 3 High level 2 Medium level 1 Low level 3 High level
9 20 3 High level 3 High level 2 Medium level 1 Low level
The dresser rotational actuator device was used as noise parameter following the Taguchi method,i.e. the special electrical motor used for the dresser, so it was performed two rounds of nine try outsteps, one with an old (near to end of life time) dresser motor and another with a new one. For eachtry out step were grinded 20 parts, which means that 3 dressing intervals occurred, once thedressing frequency in the process is 1 in each 8 grinded parts; this is important to take in a accountpossible dressing influences in the process.
After the tests with the produced samples, the parts were assessed regarding seat roughness,parameter Pt, which is, according to Mahr GMBH (1999), the distance between the highest peak andthe deepest valley in the profile P length evaluated. The surface profile graph was also generated foreach part. When roughness measurements are finished, the nozzles manufactured will be submitted
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to the opening pressure functional parameter measuring using a special test bench for this purposein the partner company, after that, this functional parameter will be associated to the roughnessparameters already calculated.
For the process, tools involved and surface generated characterization, it was performed visualanalysis with Scanning Electron Microscope (SEM) using the secondary electrons to revealtopography aspects of surface.
FINDINGS
Process try outs according Taguchi method
After the grinding tests a 2D roughness measurements of the samples and seat were done, and thecalculations were performed using a MiniTab 15 statistics software using a Signal noise ratioaccording Taguchi method and analysis of variance for the mean value (ANOVA) regarding theroughness parameter Pt, which is actually used for the process control in the grinding process step.The results are shown in Figure 4 and 5.
Figure 4: Signal noise ratio graph from Taguchi method “smaller is better” for seat roughnessparameter Pt. Source: MiniTab 15.
Figure 5: Analysis of variance in terms of mean for seat roughness parameter Pt. Source: MiniTab 15.
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According to the values showed in Figures 5 and 6, the optimized parameters levels for smallerroughness parameter Pt in nozzle seat is the shown in Table 2:
Table 2: Optimized process parameters levels for smaller roughness Pt
There is an optimized group of process parameters for smallest roughness Pt value as showed in theprevious Figures and Table 2. It is important to point out that the best parameters were the same forthe Signal noise ratio (always the biggest value, i.e., more good signal and less noise signal) and forthe analysis of variance by mean (smallest value indicates less variation). These values are not thesame as the used actually for the nozzles production series. This could cause concerns in the processexperts, once the series parameters should be the optimized once they have been tested manytimes during process release and production. The most surprising result was considered for theparameter spark out time, which best level resulted for 0 seconds. Normally, for grinding process, itis necessary a minimum spark out time in order to clean and better finish the grinded surface. In thevalidation test these parameters will be performed and tested once again. However, it will benecessary much more tests in order to validate the results.
SEM analysis of grinded parts, grinding wheel, dresser and chips
Using the SEM technique and equipment it was visually analyzed 2 nozzle bodies that were reprovedin the roughness measurement after the grinding process (Pt > 2,5 μm) in the normal production,these parts were sectioned in half to have access to body seat area. The images obtained can beobserved in Figure 6.
Figure 6: SEM images of 2 nozzle bodies seat with roughness Pt > 2,5 μm after grinding process.Source: Project partner company.
The SEM images from the 2 parts grinded separated due to Pt > 2.5 μm corroborates with its 2Droughness profile measured, showed in Figure 7. If someone divide the seat image of Figure 6 into 4parts, approximately the area of the projection of the circumference of the number 1 on the sidesurface of the sectioned seat, on the third portion of the nozzle body seat there are some marks or
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dints which correspond to the roughness profile peak from profile of Figure 7. The causes of thispoor grinding behavior in this area are still open and the ongoing project will investigate them.
Figure 7: Seat roughness profile of part 2 from Figure 6. Source: Project partner company.
In this region it is observed a not well done plowing, which means a not performed material cuttingby the grinding wheel grits (Shaw, 1996), leaving irregular grooves produced by plastic deformationwith expressive material lifted on its edges. These edges will be harmful to the subsequentfunctional behavior of the nozzle; this situation was already noticed in another analysis produced bya partner company. The cause of this problem exactly in this region remains unknown and will be theobject of study in the next steps of this project in course.
The grinding wheel and the dresser were also visually inspected in order to characterize the processand bring information regarding the samples roughness variation. A new and an old nozzle pieceswere used for comparison (after their lifetime ended) and also each tool used. These images areshowed in Figures 8 and 9.
Figure 8: SEM images of the process grinding wheels. Source: Project partner company.
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Figure 9: SEM images of the process dressers. Source: Project partner company.
The old (worn out) and the new tools are compared in Figure 8. For the grinding wheel it is possibleto observe that the new tool has a higher surface roughness, which is expected because it was notdressed yet. Unlike the old grinding wheel has a better surface finishing and has even a smallerdiameter too, due to the many dressing cycles it has been submitted during life time. In bothdressing and grinding tools were not found visual problems, marks or damages in the third portioncorrespondent to the area explained in grinded parts SEM analysis discussion. Both tool presentshigh porosity too, which is considered normal for this kind of tool.
Regarding the dressers, the new one presents a normal appearance for this kind of tool, with nodiscontinuities, marks or anything that calls attention. The old dresser presents the same featuresexcepted by a visible crack even with a small magnification (40x), this crack could have emerged onlyby the end of the lifetime of the tool without impact in process or maybe in the middle of itslifetime, influencing for sure the dressing process, the grinding wheel surface roughness and by theend the nozzle surface body seat surface, one of the next steps of this project will be dedicated toinvestigate this crack.
Process chips were taken during the try outs from the machine lubricant oil flow during grindingprocess. These chips can be visualized in Figure 10.
Figure 10: SEM images of process chips. Source: Project partner company.
The visual inspection of the grinding chips can give important information regarding the processitself and the surface profile formation. Analyzing Figure 10 and its pictures at least 4 different typesof chips could be seen and it is possible to verify that the chips have a very small size (approximately
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50 μm of length and 12 μm of width for the biggest one). These chips are considered to be normal tothe internal grinding process analyzed; those have the shape and size consistent with a finishinggrinding process.
CONCLUSIONS
According to the results presented and analysis realized, some conclusions can be made regardingthe grinding process and the generated nozzle body seat surface:
i. There are room for improvements in the internal conic grinding process analyzed in thisproject regarding Pt roughness value reduction through process parameters optimization, itwas proved by the planned try out performed and results analyzed using the Taguchimethod; this will be validated in the next steps of this project with a final validation try outalso according to the Taguchi method;
ii. Notable marks or dints have been detected in the third portion of the nozzle body seat. Thecauses of this poor grinding behavior in this area will be investigated during the next steps ofthis project;
iii. Visual inspection using SEM of grinding wheel, dresser resulted normal, excepted by onecrack in the dresser which topic will be still investigated in this project. The visual inspectionof the process chip also was considered normal.
ACKNOWLEDGEMENTS
For this paper we would like to thank the Araucaria Foundation for the study presentation and thepartner company for the availability of all the equipments, personnel and costs necessary to theproject. We would like to thank also all colleagues who collaborated with the project, in special Mr.Sergio Pejas and Mrs. Alba Turin for the effort and time dispensed.
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