IRJET- Development of Ejection System to Improve Productivity
Avoidance of Thermal Damage During Grinding.
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Avoidance of Thermal Damage DuringGrinding.
Shaun Edwards
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School ofEngineering,Technologyand Maritime Operations
James Parsons Building, Byrom Street, Liverpool, L3 3AF, UK.
Avoidance of Thermal
Damage During
Grinding.
6155ENG Engineering Project
Final Report
Name: Shaun Michael Edwards
Supervisor: Mr Andy Pettit
Programme: Mechanical Engineering BEng
Date: 02/03/2015
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Abstract.
“Grinding burn is the mostcommon anomaly in the grinding operation. It is important to detect such
anomalies to avoid quality deterioration.” (Chen & Griffin, Grinding Burn and Chatter Classification
Using Genetic Prgramming., 2008). It is vital for the advancement of the grinding process that the
methodstoavoid thermal damage to the workpiece are fully understood and optimised. There are
several different thermal models being used to complete this task and in this work, two will be
examinedandset againstexperimentaldatagatheredpreviously. Of the two models examined, the
firstwill be the one initially proposedbyRowe,Pettit,Boyleetal.intheirpaper (Rowe,Pettit, Boyle,
& Moruzzi, 1988) and the second will be the paper proposed 3 years later by Rowe, Morgan and
Allanson in (Rowe, Morgan, & Allanson, An Advance in the Modelling of Thermal Effects in the
GrindingProcess,1991). The majorfindingsfromthispaperare the difference inresultsbetweenthe
models, as one is a highly conservative model yet one is a much more aggressive and they vary
between each other considerably. This means that one of the models could be implemented into
industrystraight away whereas the other needs refinement and further exploration to be industry
ready.
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Acknowledgements.
I would firstly like to thank my mum, dad, girlfriend and the rest of my family and friends for their
continued support and enthusiasm; they have given me the motivation to complete this project. I
would also like to thank Andy Pettit, Dr David Allanson and Dr Michael Morgan and the rest of the
staff at the School of Engineering,TechnologyandMaritime OperationsatJohnMooresUniversityas
well as the rest of staff at Liverpool John Moores University. They helped guide me with their
expertise and gave up their time as well as giving me a chance to study at this fantastic university,
without that this project would never have gotten off the ground.
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Nomenclature.
Symbols. Definition. Units. Equation.
VW Workpiece Velocity. mm * s-1
-
Pn No Load Power. W -
PL Maximum Grinding Power. W -
ds Grind Wheel Diameter. mm -
dW Initial Diameter of the
Workpiece.
mm -
Vf In Feed Rate. mm * s-1
-
Vs Grindwheel velocity. mm * s-1
-
lW Workpiece Length. mm -
Pg Specific Grinding Power. W*mm-1 𝑃𝐿 − 𝑃𝑛
𝑙 𝑊
ZW Specific Metal Removal Rate. Mm2
*s-1 𝜋
2
× 𝑑 𝑤 × 𝑉𝑓
Us Experimental Critical Specific
Energy.
J * mm3 𝑃𝑔
𝑍 𝑊
a Depth of Cut. mm
𝜋 × 𝑑 𝑊 ×
𝑉𝑓
𝑉 𝑊
de Equivalent Diameter. mm 𝑑 𝑊 × 𝑑 𝑠
𝑑 𝑊 + 𝑑 𝑠
lg Geometric Contact Length. mm √𝑎 × 𝑑 𝑒
le Actual Contact Length. mm 3 × 𝑙 𝑔
αW Thermal Diffusivity of the
Workpiece.
m2
* s-1
-
αs Thermal Diffusivity
of the Grind wheel.
m2
* s-1
-
λW Thermal Conductivity of the
Workpiece.
W * m-1
* K-1
-
λs Thermal Conductivity of the
Grind wheel.
W * m-1
* K-1
-
R Energy Partition Ratio
Between the Workpiece and
the Grind wheel.
- 1
(
𝛼 𝑊 × 𝑉𝑠
𝛼 𝑠 × 𝑉 𝑊
)
0.5
× (
𝜆 𝑠
𝜆 𝑤
) + 1
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Table of Contents
Abstract......................................................................................................................................2
Acknowledgements....................................................................................................................3
Nomenclature. ...........................................................................................................................4
1. Introduction. ..........................................................................................................................7
1.1. The Importance of the Grinding Process................................................................................... 7
1.2. The Aims and Objectives of the Project. ................................................................................... 7
1.3. The Grinding Process............................................................................................................... 8
1.4 Types of Grinding................................................................................................................... 12
1.5. Dressing the Grind Wheel...................................................................................................... 13
2. Research...............................................................................................................................15
2.1. What is Thermal Damage?..................................................................................................... 15
2.2 The Concept of Thermal Modelling. ........................................................................................ 16
2.3 A Review of Research Concerning the Avoidance of Thermal Damage....................................... 17
3. Methodology........................................................................................................................21
4. Results. .................................................................................................................................22
5. Discussion.............................................................................................................................31
6. Conclusion............................................................................................................................36
Appendix. .................................................................................................................................37
Appendix I................................................................................................................................... 37
Appendix II.................................................................................................................................. 38
Appendix III................................................................................................................................. 42
References................................................................................................................................46
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1. Introduction.
1.1. The Importance of the Grinding Process.
The grinding process is commonly considered to be a finishing process. There are several different
typesof grinding processesthatare all usedfordifferent applications,forexample toachieve a good
surface finish or to establish a geometric accuracy etc. Generally grinding is a process in which the
removal rate of the metal,normallyhardened steel,isverysmall whencomparedtoother processes
such as turningor milling. If the removal rate isincreasedtospeedup the process, a problem arises;
thisproblemiscalledthermal damage,althoughinindustryitismore commonlyknownas burn, and
happenstothe surface of the workpiece.The problemis to try and speed up the process as much as
possible but without thermal damage being caused to the workpiece.
Grinding is also a process that is hard to automate as opposed to other similar processes; it is
therefore a slow process, and due to the labour intensity, a costly process too. It is therefore
importantthatthe skill level of the grindingoperatorisveryhigh;which reliesheavily on knowledge
of the machine itself andexperiencefromworkingwiththe material. It is especially important as by
the time a workpiece reaches the operator many other processes will have been carried out on it.
Each stage of processing adds a value on to the workpiece; therefore by the time it reaches the
grindingstage a significantvaluewill have been added onto it and it will not want to be scrapped at
such a late stage because of a problem, such as thermal damage. It is this problem, the one of
thermal damage, which this research project is looking to overcome.
1.2. The Aims and Objectives of the Project.
The aim of thisprojectis to come up withan improved thermal model for the grinding process; one
that can be implementedtoasmanygrindingmachinesaspossible.Itwill buildonpre-existing work
and usingpreviouslyrecordeddataandadvances inthe field,improvementswill be suggested other
models too.
There are several different objectives that want to be achieved during the course of this research
project. This entails conducting research into the grinding process, thermal damage and different
thermal models.Once sufficientresearchhasbeenattained, two thermal models are to be selected
and examined in further detail. Using previously recorded data, calculate the experimental critical
specificenergy,theninputthe dataintothe thermal modelsandgraphthe resultsof the calculations
to give a visual representationandhelpportraythe findings. The resultsof the calculations will then
be discussed as to what they illustrate as well as comparing and contrasting the results of each
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model.The truenessandprecision (BritishStandards,1994) etc. of the results will be commented on
as well as the sourcesof errorand any assumptionsmade.Thenusing the results, establish which is
mostuseful andsuitable for a process control purpose and to help avoid the workpiece from being
thermally damage. And finally using what has been established through research, propose some
improvements to the thermal models chosen.
1.3. The Grinding Process.
Figure 1.1. A basic example of the grinding zone in a typical surface grinding process. (Marinescu,
Rowe, Dimitrov, & Inasaki, 2004)
Figure 1.1. illustratesatypical reciprocating grindingoperationtakingplace, aswell as illustratingthe
5 keyareas in the process:the fluid,the grindingwheel,the atmosphere, the grinding swarf and the
workpiece. The fluid has 3 main jobs, as a: heat sync, lubricating mechanism and transport medium
for swarf.The coolingfluid coversthe workpiece andthe grindingwheel,and the heat energy that is
inthemis thenusedtoevaporate the coolantfromthe surface.Thisis similar to how when a person
sweats; the heat energy in them is used to evaporate the sweat that has formed on their skin. This
helpstoreduce the temperature inthe workpiece from increasing too much and therefore reduces
the risk of thermal damage and other thermal effects from occurring. The fluid also acts as a
lubricant between the abrasive grains in the grinding wheel and the workpiece, thus leading to a
reduction in friction occurring amongst them. This helps keep the abrasive grains relatively sharp,
and so reducing the wear of the grinding wheel as well as making the process a smoother and
cleaner one. Due to the reduction in friction, the lubricating effect also helps to reduce the heat
energybeingproducedandtransferredtothe workpiece, sotherefore the likelihood of any thermal
damage occurringreducestoo. As the liquid is moving through the parts, it helps to rinse the pours
of the grinding wheel, allowing for new swarf to fill those voids. This makes the process more
efficientandhelpsreduce the needforthe wheel tobe redressedasoften.Italso helps to move any
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swarf away from the workpiece; which helps to keep the machine free from any sizable pieces of
sharp metal that could injure the labourer.
The grindingwheel isthe part of the machine that grinds off the metal from the workpiece. It has 3
main constituents, as shown in figure 1.2. They are the grains, the bonds and the pores. The grains
are the part of the wheel that does the work on the product; they carry out three processes on the
metal as shown infigure 1.3. However,rubbingandploughingaren’tthe desired processes; rubbing
occurs when the grit is too blunt and gently rubs along the surface of the workpiece, therefore not
reallyaffectingitmuch.Ploughingiswhere the gritpenetratesthe surface of the workpiece butdoes
not remove the material it has moved, leaving undesirable ridges along the workpiece. Although
rubbingandploughingaren’t desirable on their own, it is part of grinding and as long as the cutting
and the chipbeingremovedfollowthem, itisn’tmuch to worry about. The grains can be made from
several different materials,summarisedintable 1.1.Bondshold the grains in place; these bonds are
strongenoughto withstandthe initialforcesonthe grit,however overaperiodof time they can fail,
as showninfigure 1.4. The bondsalso fail whenthe grindingwheel is dressed, however this is what
theyare designedtodo; expose new grainsetc. Whenthe chipcomesawayfromthe workpiece, if it
is small enough it will potentially occupy the void in the pore of the grinding wheel. This can have
negative effectson the grindingprocessasitappearsto smoothoff the grindingwheel. However,itis
makingitso there isno difference with the cutting edge of the grain and the chip so nothing can be
ground. Table 1.2. shows the ISO marking system for grinding wheels; this system incorporates 7
categories.The categoriesare explainedbelow inthese helpful guidelines, taken from (Boothroyd &
Knight, 1989), and are to assist with the selection of a grinding wheel:
1. “Choose aluminiumoxideforsteels,andsiliconcarbide for carbides and nonferrous metals.
2. Choose a hard-grade wheel for soft materials, and a soft-grade wheel for hard materials.
3. Choose large gritfor softand ductile materials,and small grit for a hard and brittle material.
4. Choose small gritfora goodfinish,andchoose large gritfor a maximum metal removal rate.
5. Choose a resinoid, rubber or shellac bond for a good finish, and a vitrified bond for a
maximum metal removal rate.
6. For surface velocity greater than 32 m/s do not choose a vitrified bond.”
(Boothroyd & Knight, 1989)
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Figure 1.2. Structure of the grinding wheel, made up of grains or grit, the bonds and the pores.
(Marinescu, Rowe, Dimitrov, & Inasaki, 2004).
Figure 1.3. The grain rubbing, ploughing and cutting at different positions in the arc of contact.
(Rowe, Principles of Modern Grinding Technology, 2009)
Table 1.1. Knoop hardness for various materials and abrasives. (Boothroyd & Knight, 1989)
Common glass 300 – 500 Titanium carbide 1800-3200
Hardened steels 700 - 1300 Silicon carbide* 2100 - 3000
Tungsten carbide 1800 - 2400 Boron carbide 2800
Aluminium oxide* 2000 - 3000 Cubic boron nitrite* (CBN) 4000 - 5000
Titanium nitride 2000 Diamond* 7000 - 8000
* Usedcommonlyasgrinding abrasives
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Figure 1.4. Types of wheel wear. (a) freshly dressed grit (b) attritious wear (c) grit fracture (d) bond
fracture. (Andrew, Howes, & Pearce, 1985)
Table 1.2. ISO marking system for grinding wheels. (Boothroyd & Knight, 1989)
The most unobvious element of the grinding process is the atmosphere and the important role it
plays. When a metal is put through a machining process, most of them become slightly chemically
reactive. Thisis due to two reasons: one, the new layer that has been formed on the surface of the
workpiece ishighlyreactive, asopposedtothe already oxidised surface that has been ground. Two,
the hightemperaturesinvolvedatthe site of contact betweenthe workpiece and the grit speeds up
any slow-moving reactions taking place on the surface. The result of which is that an oxide, or any
othersuch compounds,is formedrapidlyonthe newlyexposedworkpiece surface as well as slightly
on the grit. Oxides can be helpful; they can act as a lubricant if the shear strength of them is low
enough,helpingtoreduce the frictional force.Howeverasthe grindwheel speedsup,the lubrication
effect diminishes.
The grindingswarf ismade of several components;these are: the cuts from the workpiece, drops of
the lubricatingfluidandanyworn gritsthat have beenbrokenoff, orparts of them.Swarf isa useless
product from the grinding process and although it is not value-less, as the metal itself will hold a
value, it is not useful or does it have an impact on the process itself.
In summary,asstatedby (Rowe,Principlesof ModernGrindingTechnology,2009),grinding relies on
a set of factors and their characteristics. These are:
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The workpiece material and thus its properties, i.e. its chemical, mechanical, thermal,
physical properties etc. as well as its shape and dimensions.
The type of grindingmachine andthe accuracy it works to, as well as the control system it is
using, the vibration it is creating and its ability to control the temperature effectively.
The kinematics of both the grind wheel and the workpiece; which include the speeds, the
motion and the in-feed rate.
The grind wheel and its constituent parts, including the grain size, type of bonds and their
make up whichinfluencesthe abrasivenessof the grind wheel;the hardness,stiffnessandits
properties.
The lubricating fluids flow rate, its velocity, pressure and its properties.
(Rowe, Principles of Modern Grinding Technology, 2009)
1.4 Types of Grinding.
There are several differenttypesof grinding,whichcanall be brokendownintotwomaincategories:
stock removal grinding(SRO) andformandfinishinggrinding(FFG). Bothof these typesof operations
are usually for mainly machining flat or cylindrical surfaces and will vary mostly depending on the
grindwheel, itsproperties and the kinematic motion of the workpiece. The most common types of
grinding are:
Internal grinding
Cylindrical grinding
Centerless grinding
Creep-feed grinding
Belt grinding
Surface grinding
High energy deep grinding
(Shaw, 1996)
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Figure 1.5. Diagrams of the operation of grinding machines: (a) cylindrical grinder, (b) internal
grinder, (c) internal grinder with planetary motion, (d) centerless grinder, (e) centerless internal
grinder, (f) surface grinder using the periphery of the wheel, (g) surface grinder using the end of the
wheel; (1) grinding wheel, (2) clamp, (3) workpiece, (4) chuck, (5) regulating wheel, (6) workrest
blade. (Farlex, 2010)
1.5. Dressing the Grind Wheel.
As mentioned earlier, a grinding wheel must be dressed. This is a maintenance process that takes
place to ensure efficient,accurate andhighquality grinding is consistently produced. It is necessary
that thisoccurs as afterthe grind wheel hasbeeninuse fora longtime,it will notbe able to perform
the functiontothe same level andwill losethe accuracydue to a change in theirprofile.The dressing
processdoestwojobs:one; it producesthe requiredaccuracyof formand profile andtrue runningof
the grinding profile, which is known as ‘truing’. Two; it must generate chip space and sharpness
suitable forthe grindingprocesstotake,alsoknownas ‘cleaningup’(Tawakoli,1990). Inmodern day
grinding,the processof truingandcleaningup takesplace usingone tool at a single time,asopposed
to when separate tools were needed for each job, this singular practice is known as dressing
(Slamon,1992; Chen& Griffin,Grindingburnand chatter classification using genetic programming.,
2009). To dress a grinding wheel by traditional methods requires a specialist-dressing tool. This
dressingtool contains an active cutting surface created by an ultra hard material, which is normally
diamonds. There are several different types of tool, including single and multigrain (Marinescu,
Rowe, Dimitrov, & Inasaki, 2004). It is their job to grind off a fine layer of dull, blunt chips by
fracturing the bonds that hold them together, which unveils new sharper chips along with new
emptypores.Whilstdoingthis, ithelpsshape the grindingwheel by levelling off any unevenness in
the geometry of the wheel.
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In relation to thermal damage, when a grind wheel has recently been dressed, it increases the
likelihood of thermal damage occurring. This is due to the fact that if the process has been
automatedusingacomputernumericcontrol (CNC) system, thenthe necessaryprecautions are very
rarelytaken to account forthe newlydressedgrindwheel owing to the considerable time between
dressings. The grit is sharp and the pores are open so it cuts through workpiece easily, this isn’t
accountedforand so the grindwheel is set as if it was blunt and clogged up. This causes as massive
surge in the power consumption by the grind wheel due to the fact it’s easier than it should be to
grind.Thisenergyconsumptionleads to the workpiece being thermally damaged. However regular
dressingof the grindwheel intime willhelpthe grinding process by optimising the efficiency of the
processand allowsforconsistent,repeatable results.Overtime the wheel wearsandthe consistency
starts to waver, then the wheel must be dressed again to ensure the efficiency is maximised.
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2. Research.
2.1. What is Thermal Damage?
The grindingprocesshasone of the highestenergyinputsto removal rate of metal in the machining
industry.Nearlyall of the energyinputtedintothe processistransferredtothermal energy and kept
withinthe grindingzone.Thiswill cause adramaticrise intemperature inthe workpiece leading to a
phase change in the material, which leads to multiple different types of thermal damage to the
workpiece. Some examples of thermal damage are: burning, tempering, residual tensile stresses,
cracks and reducedfatigue strength. Tempercoloursappearingonthe surface of the workpiece after
grindingindicate the majorpossibilitythatthe workpiece hasbeen thermally damaged and that tiny
cracks have formed on surface of the workpiece. These cracks, although minute, can have
catastrophic consequences later on in their working life. For example, a part that goes in an
aeroplane wing; the final manufacturing process it under goes before being put in service is a
grinding one, and whilst the part is being ground it is thermally damaged but is put in service
regardless.Thatpart isthenplacedinthe plane wing coveredintens,possiblyhundreds,of tinylittle
cracks and is subjected to cyclic loading and unloading. This cyclic action causes the cracks and
stressesinthe part to grow,until itfailswhilstin mid-flightcausingthe whole wingto fail, which will
consequentlylead the plane tofall fromthe sky.Regardless of whether it is a tiny two man plane or
the new superjumbo jets, which can carry at the moment 853 (Airbus, 2014) and is set to keep
growing, nobody should die because of the failure that could have easily been avoided. That is an
example asto whyit iscrucial that thermal damage isunderstoodandthermal models that are used
in order to avoid this occurring are as accurate as possible to avoid any loss of life.
A visual examination of a workpiece that has been burnt will show the workpiece has adopted a
bluish colour.Thisisdue to the temperingcolourchange of the workpiece inthe orderof lightbrown
to dark brown to violet to blue and how far it goes depends on how badly it has been burnt. The
most accurate way to detect grinding burn is by optical microscope examination of the workpiece
surface once it has beenground,orby havinga metallographicetchtaken. If an etch is taken, then a
white phase occurrence isshowninpatches.Howeverthere is aproblemwiththis methodof testing
to establishwhetherthe workpiece has been burnt or not; it is a destructive technique. This means
that the workpiece hastobe cut downa plane andsimplydestroyedtoestablishwhether ithasbeen
burnt or not. Therefore, even if the workpiece isn’t burnt, it has been destroyed so badly for the
meansof testingthatit cannot be used.Thismeansthata passive methodof determining whether a
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workpiece is damaged or not during the process is needed (Nathan, Vijayaraghavan, &
Krishnamurthy , 1999).
Whenpredictingthe burnthreshold,the mostimportant aspect to consider is the temperature that
onsetof burn occurs forthat particularmaterial.Whengrinding,withsteelsespecially,itisimportant
to look at the thermal properties of that individual compound as they can vary quite considerably
depending on the percentage alloy content. In ferrous metals, temper colours will appear at
temperature valuesaslowas 220°C indry grinding,so thermal damage could have occurred slightly
lower than that (Rowe, Pettit, Boyle, & Moruzzi, 1988). However if a cooling fluid is used in the
process the workpiece can be ground above the critical energy that corresponded to 220°C; this is
because the coolingfluidactsasanotherheatsink,thusallowingforanincrease inthe critical energy
but not the temperature so it can be ground more. Cooling fluid, as discussed before, has the
additional bonusof lubricatingthe grinding zone; which means that the required energy needed to
grind is lower.
2.2 The Concept of Thermal Modelling.
The thermal modelsare algorithmsthatpredictthe response of the grindingprocessaccordingtothe
conditions defined. (Jaeger, 1942) was applied to dry grinding and showed how the temperature
risesgoingfromthe leadingedge toa maximumatthe trailingedge.Howeverthiscannotbe applied
to fine grinding. During fine grinding, the miss-matches in sizes, i.e. the wheel-work contact being
tiny when compared with the diameter of wheel etc., cause problems. This is due to the limited
amountof time the betweencycles, as the energy cannot be dissipated and so the wheel and work
cannot be considered semi-infinite, meaning the model of (Jaeger, 1942) cannot be used there.
(Malkin & Cook, 1971) found that in practice, shear plane and wear flat energies are important and
onlyso muchenergycan be carriedaway bythe swarf. (Hahn,1962) didn’tuse a sliding heat source,
insteadhe consideredthatthe majorityof heatgeneratedcame fromisthe grain-workpiece surface.
This is because it can’t account for the larger energy dissipated in reality and so neglected shear
plane energy.Insteadstatingthatthe heatgenerationisbestdescribedasthe energyisdissipated at
the contact between the grain and the workpiece. According to (Rowe, Pettit, Boyle, & Moruzzi,
1988), grinding energy is dissipated from the grinding zone in 7 different ways, and they are:
1. The heat conducted away by the grinding wheel.
2. The heat conducted away by the workpiece.
3. The heat carried away by the grinding chips.
4. The heat dissipated to the coolant by means of convection.
5. The kinetic energy imparted to the chips.
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6. The energy required to generate a new surface.
7. The residual energyimpartedtothe groundsurface.” (Rowe,Pettit, Boyle, & Moruzzi, 1988)
They researched further and found that if you model the energy partitioning to all 4 sinks
analytically, you could come up with a lower bound prediction that give confidence limits on the
temperature method. The explanation for this is the energy partition between the wheel and the
workpiece allowingformore accurate real life valuesof heat fluctuation within the workpiece to be
applied. (Outwater & Shaw, 1952) concluded that from the list above; 5, 6 and 7 were so small and
insignificantwhencompared withthe previous4before them thattheydon’tneedtobe considered.
Theyappliedheattransfertheorytoa drygrindingprocessto establish a mean surface temperature
during grinding, though they didn’t consider the importance that convective cooling played. (Des
Reisseaux &Zerkle,1970) analysedthe effect that a cooling fluid would have on cooling the surface
overthe whole workpieceandfoundthatitwouldreduce the temperature of the workpiece outside
of the grindingzone quite dramatically.However, incertainsituations, this method is futile; current
models suggest that this extracts less than 10% in the grinding zone, so if the temperature in the
zone ishighenoughto thermally damage the workpiece, it is useless. (Makino, Suto, & Fokushima,
1966) used thermocouples and discovered that the length of the heat source in reality is roughly 3
times greater than the actual contact length between the wheel and the workpiece. In (Rowe,
Morgan, & Allanson,AnAdvance inthe Modellingof Thermal Effectsinthe GrindingProcess,1991) it
was found that the contact length could be predicted using the elastic contact length and the
geometricone. (Howes,Neaily,&Harrison,1987) foundthat if the temperature in the grinding zone
goesabove the temperature atwhichthe coolingfluid boils, it will lead to a drastic reduction in the
heattransferredtothe coolantand the lubricationeffectof the liquid.Thisisknownas the fluid film
boiling effect and due to the coolant not being an effective heat sink; it causes the temperature in
zone to rise even more, usually resulting in the workpiece being burnt. Consequently, it can be
concluded that the maximum energy that can be imparted into the coolant is limited by the
minimum amount energy required to boil the coolant. (Rowe, Black, Mills, Morgan, & Qi, 1997)
exploredthe critical temperaturesatwhichthe onsetof burn occurs and found that generally it falls
between 450 and 500°C for ferrous metals.
2.3 A Review of Research Concerning the Avoidance of Thermal Damage.
It isimportantto define whatthe ‘grinding zone temperature’ and the ‘local grinding temperature’
actually are so there is no confusion. The localised grinding temperature is the temperature
generated by the energy of the grinding actions of a single grain in isolation from the rest of the
grindingwheel. The grinding zone temperature is the rise in the temperature due to the energy of
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the collective grinding action of all of the grains. (Jaeger, 1942) developed this by considering the
grindingzone asa two-dimensional perfect insulator of length, l, moving along a semi infinite body
with a velocity, v. This is displayed in figure 2.1.
Figure 2.1. A two dimensionalperfectinsulatorwith length,l, moving overa semi-infinite body with a
velocity, v. (Shaw, 1996)
Most of the recentworkin this field isbasedon the work of (Jaeger, 1942) and the perfect insulator
moving across a semi-infinite body idea he used, shown in figure 2.1. This theory predicts the
temperature atthe contact of the workpiece duringgrinding andsohasbeenused and expanded on
in great detail. It is the basis of nearly all of the modern day models.
As discussed earlier, (Des Reisseaux & Zerkle, 1970) looked into the where the heat energy is
dissipated to. They built on the work of (Jaeger, 1942) and found that if the coolant could get into,
and affect, the localised grinding area it would work quite effectively.
Thermal modellingreliesonthe partitioningof the heatgeneratedthrough grinding. This is because
not all of the heat generate through grinding goes into the workpiece. Knowing how much of the
total heat energygeneratedflowsintothe workpiece enables the critical temperature at which the
onsetthermal damage occurs to be calculated. Many researchers have looked at the values of how
much energy goes into the workpiece and their approximations are shown in the table below as a
percentage:
Table 2.1 showing thepercentageof heatenergy flowing into the workpieceand the swarf and wheel
that different researchers found and how they established the percentage.
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Author Percentage entering
the workpiece
Percentage entering
the swarf & wheel
Comment
Saucer 30-70 70-30 Experimental
Lee 80 20 Experimental
Malkin 60-80 40-20 Experimental
Sato 84 16 Experimental
Outwater & Shaw 35 65 Theoretical
Eshghy 10 90 Theoretical
The reason for Sauer’sresultsvaryingsomuch isthat he foundthatincreasing the removal rate lead
to a decrease inthe energy entering the workpiece. (Malkin & Cook, 1971) research found how the
sharpness of the grind wheel affects the total energy entering the workpiece, so the blunter the
grind wheel gets, the more energy enters the workpiece.
As discussedearlier,the lengthof contactbetweenthe workpiece and the grind wheel is not always
as it seems. (Makino, Suto, & Fokushima, 1966) discovered, through their work with applying
thermocouplestothe process,thatthe real contact length could be as much as 3 times greater than
the geometriccontactlength. (Rowe, Morgan, & Allanson, An Advance in the Modelling of Thermal
Effectsinthe GrindingProcess,1991) foundthat the actual contact length depends on the elasticity
of the grind wheel and the workpiece. They did this using computer modelling and testing on
different grinding machines.
(Outwater & Shaw, 1952) looked at what the grinding temperature would be if all of the energy
generated went into the chips and from that determined that 35% of the shear energy generated
went into the workpiece. (Malkin & Cook, 1971) found that the energy generated is the sum of the
energy generated through the three stages of the chip grinding; cutting, ploughing and rubbing.
Usingthe ideathat (Jaeger,1942) came up withaboutslidingenergy, (Malkin & Cook, 1971) showed
hownearlyall slidingenergyisconductedtothe workpiece andhow asthe wheel becomesbluntthe
energybeingimpartedincreases.However very little energy is transferred due to chips cutting and
using a sharp grind-wheel, showing the importance of dressing the workpiece regularly.
The work conductedbyRowe,Pettit,Boyle etal.intheirpaper (Rowe,Pettit,Boyle,&Moruzzi,1988)
came upwitha methodof predictingthe burn threshold limit by using some of the parameters and
characteristics involved in the grinding process. This work was a break through as it used a new
concept of energy partitioning that allowed the percentage entering the workpiece to change
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dependingonthe workpiece velocity.Thisallowsforamore realisticvalue of heat flux to be applied
to the critical specific energy equation. This, along with the improvements to the critical specific
energy calculation, allowed a more accurate model to be developed. This model was the most
accurate to date when published.
Buildingonthe workof Rowe,Pettit,Boyle et al; Rowe, Morgan and Allanson wrote a paper (Rowe,
Morgan, & Allanson,AnAdvance inthe Modelling of Thermal Effects in the Grinding Process, 1991).
This paper looked at the difference between the wheel-workpiece contact zone and the average
grain contact zone; if a distinction is not made between the two then conceptual difficulties may
arise due to the difference in the relative speeds. They then used this to come up with a more
conservative method of burn prediction, which could be used in industry straight away.
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3. Methodology.
It was notpossible tocomplete the grindinglaboratoryexperimentandcollectthe requireddatadue
to the failure of external cylindrical grinding machine. The machine was out of service due to the
dynamometer being temperamental and giving out false power readings, therefore it was not
operational duringthe periodof thisproject.However,reliable data was available from established
experimental testscompletedpreviouslyonthe equipment. If it had been possible to complete the
testsinthe requiredtime,thenthe followingexperimental procedure wouldhave beenadopted and
executed.
A workpiece is picked with a known material, thermal conductivity and diffusivity and their initial
diameters are measured. A grind wheel is then carefully chosen, it has a known grade; the grind
wheel isselected byusingthe table andthe correspondinginstructionsalongsideit,as shown earlier
(Boothroyd & Knight, 1989). The workpiece is then placed in between two centres; the headstock
and the tailstock.The headstock is connected to a motor which causes the workpiece to rotate; the
tailstock is free to rotate and just keeps the workpiece perpendicular to the grind wheel. The CNC
system is then programmed with the correct velocity, in feed rate and the end diameter and the
machine isstarted.Asit begins, coolant starts to flow over the grind wheel. Just as the grind wheel
touchesthe workpiece,the powerisnoteddown;thisiscalledthe ‘noloadpower’or the ‘start grind
power’.The grind wheel isthen fed onto the workpiece and as it works away, the maximum power
achievedis recorded. No spark out or dwell should be programmed into the machine, as this could
clear off any signs of burn ever occurring. This would mean the only way of finding out if burn has
occurred would be to destroy the sample and take a metallographic etch of the sample. The grind
wheel thenretractsquicklyback to its starting position and the power is stopped being supplied to
the motors sotheycome to rest. There islittle backlashinvolvedinaCNCprogrammed machine due
to the type of lead screw used in the machine so this should not be a problem during the testing.
In industry,aspark outwouldbe programmedintothe machine; thisisdone toallow the machine to
relax andtherefore canguarantee the dimensionalaccuracyof the workpiece.
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4. Results.
Table 4.1 The results table for the original test.
When calculating the experimental critical specific energy, three equations where used using the
numberscollectedfromthe previoustest.The firstequationcalculatesthe specificgrindingpowerby
finding out how much power is needed during grinding; this involves taking the maximum power
achieved during grinding and taking away the power of the machine when it’s not engaged in
grinding.Itisthendividedbythe workpiecelengthtogive the specificgrindingpowerperunitlength
which allows it to be directly compared to another specific grinding power. The unit for specific
grindingpowerisWattsper millimetre.Thisiscanbe derivedfrom the equation as it’s power minus
another power i.e. Watts minus Watts, divided by a length in millimetres therefore it’s Watts per
millimetres.
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐺𝑟𝑖𝑛𝑑𝑖𝑛𝑔 𝑃𝑜𝑤𝑒𝑟 =
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝐺𝑟𝑖𝑛𝑑𝑖𝑛𝑔 𝑃𝑜𝑤𝑒𝑟 − 𝑁𝑜 𝐿𝑜𝑎𝑑 𝑃𝑜𝑤𝑒𝑟
𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝐿𝑒𝑛𝑔𝑡ℎ
Equation 4.1 Specific Grinding Power.
Workpiece no. Vw Dressed? Pn PL Burnt? ds dW lW Vf Vs
1 5 Yes 816 7344 Yes 441 43.347 47 0.0120 33000
2 5 No 819 6108 Yes 441 43.347 47 0.0120 33000
3 6 No 816 5484 Yes 441 43.348 47 0.0120 33000
4 7 No 816 4968 No 441 43.351 47 0.0120 33000
5 8 No 813 4785 No 441 43.35 47 0.0120 33000
6 9 No 819 4590 No 441 43.35 47 0.0120 33000
7 10 No 813 4521 No 441 43.451 47 0.0120 33000
8 11 No 816 4422 No 441 43.351 47 0.0120 33000
9 12 No 819 4407 No 441 43.352 47 0.0120 33000
10 13 No 813 4344 No 441 43.352 47 0.0120 33000
11 14 No 813 4587 No 441 43.354 47 0.0120 33000
12 15 No 810 4167 No 441 43.355 47 0.0120 33000
13 17 No 810 4623 No 441 43.355 47 0.0120 33000
14 19 No 816 3936 No 441 43.356 47 0.0120 33000
15 19 Yes 810 6930 Yes 441 43.356 47 0.0120 33000
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The second equation calculates the specific metal removal rate and involves calculating half of the
diameter,thisisdue toit beinga cylindrical grinding process, and multiplying by the in feed rate to
give the specificamount of the metal takenoff the workpiece in one pass. The units for the specific
metal removal rate is mm2
*s-1
. This is derived from the in feed rate being measured in mm per
second(mm*s-1
) asit isessentiallyadistance travelledinatime period.This,whenmultiplied by the
initial workpiece diameter, the only other variable with a unit, gives mm2
*s-1
.
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑀𝑒𝑡𝑎𝑙 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝑅𝑎𝑡𝑒 =
𝑃𝑖
2
× 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 × 𝐼𝑛 𝐹𝑒𝑒𝑑 𝑅𝑎𝑡𝑒
Equation 4.2 Specific Metal Removal Rate.
The final equation uses the two previous equations and divides the first by the second to give the
critical specificenergy.Essentiallyitisthe powerusedto remove aspecificamountof the workpiece.
Takingthe unitsderivedforthe first equation and putting Watts into its derivative of J*s-1
gives the
unitsof specific grinding power as J*mm-1
*s-1
.Using the improved units for specific grinding power
and mm2
*s-1
for the specific metal removal rate, the units for the critical specific energy can be
derived. This is shown in detail in equation 4.4.
𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝐶𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐸𝑛𝑒𝑟𝑔𝑦 =
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐺𝑟𝑖𝑛𝑑𝑖𝑛𝑔 𝑃𝑜𝑤𝑒𝑟
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑀𝑒𝑡𝑎𝑙 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝑅𝑎𝑡𝑒
Equation 4.3 Experimental Critical Specific Energy.
𝐽
𝑚𝑚 × 𝑠
÷
𝑚𝑚2
𝑠
≡
𝐽
𝑚𝑚 × 𝑠
×
𝑠
𝑚𝑚2 ≡
𝐽 × 𝑠
𝑚𝑚 × 𝑠 × 𝑚𝑚2 ≡
𝐽
𝑚𝑚3
Equation 4.4 derivation of critical specific energy units.
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Table 4.2 The results table for calculating the experimental critical specific energy.
Vw Pg Zw Us
5 138.8936 0.81707 169.9895
5 112.5319 0.81707 137.7259
6 99.31915 0.81709 121.5522
7 88.34043 0.81715 108.1084
8 84.51064 0.81713 103.424
9 80.23404 0.81713 98.19027
10 78.89362 0.81903 96.32543
11 76.7234 0.81715 93.89179
12 76.34043 0.81717 93.42096
13 75.12766 0.81717 91.93685
14 80.29787 0.8172 98.25932
15 71.42553 0.81722 87.40035
17 81.12766 0.81722 99.27243
19 66.38298 0.81724 81.22812
19 130.2128 0.81724 159.3321
Table 4.2 showsthe resultsfromcalculatingeachof the equationsabove foreachvelocityof the
workpiece asshowinequation4.1,4.2 and 4.3.
To guarantee thatthere is a consistencyinthe unitsusedinthe equationsandthattheyare all SI
units,a conversionmusttake place intermsof the workpiece velocity.The workpiece velocitymust
be convertedfromm*min-1
tomm * s-1
;thisis done bydividingthe velocityinm*min-1
by60 to give
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m*s-1
andthenmultiplyingthatanswerby1000 to take the meterscomponentandconvertingitinto
mm.
𝑚
𝑚𝑖𝑛
÷ 60 ≡
𝑚
𝑠
× 1000 ≡
𝑚𝑚
𝑠
Equation 4.5 The conversion of units from meters per minute to millimetres per second.
Calculatingthe theoretical critical specificenergy usingthe thermal model firstderivedbyRowe,
Pettit,Boyle etal.andpublishedin (Rowe,Pettit,Boyle,&Moruzzi,1988). Thisthermal model uses5
equations;the firstis usedtocalculate the depthof the cut and usesthe total circumference
multipliedbythe infeedrate overthe workpiece velocity.The unitsfordepthof cutismm, as the
infeedrate andworkpiece velocityhave the same unitsandthereforecancel out.AlsoasPi has no
units,itleavesthe initialworkpiece diameterasthe onlyquantityhavingunitsleftmeaningthe units
for the depthof cut are mm.
𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐶𝑢𝑡 = 𝑃𝑖 × 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 ×
𝐼𝑛 𝐹𝑒𝑒𝑑 𝑅𝑎𝑡𝑒
𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝑆𝑝𝑒𝑒𝑑
Equation 4.6 Depth of cut.
The second equation involves finding out the equivalent diameter, which to some extent is a
measure of the length of contact. It is calculated by taking the product of the workpiece diameter
and the grindwheel diameteranddividingbythe total of the two diameters. This is done to make it
easier later on as less parameters are needed if this equation is used correctly. The equivalent
diameterhasthe unitsmm.Thiscomesfrom the fact that althoughdW multiplied by ds gives mm2
as
the units,whenitisthendividedbythe total of the two whose units is mm, it gives the overall unit
of mm.
𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 =
𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 × 𝐺𝑟𝑖𝑛𝑑𝑤ℎ𝑒𝑒𝑙 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟
𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 + 𝐺𝑟𝑖𝑛𝑑𝑤ℎ𝑒𝑒𝑙 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟
Equation 4.7 Equivalent diameter.
The third equationcalculatesthe geometriccontact length by finding the square route of the depth
of cut multipliedbythe equivalent diameter. In (Rowe, Principles of Modern Grinding Technology,
2009) it states that to get this length it uses the principle of intersecting chords which leads to this
equation. The units for lg comes from the fact that although a, whose units is mm, and de, whose
unitsisalso mm, are multiplied together to give mm2
they are then square rooted to give just mm.
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𝐺𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝐿𝑒𝑛𝑔𝑡ℎ = √𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝑐𝑢𝑡 × 𝐸𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟
Equation 4.8 Geometric contact length.
Testing done in relation to (Qi, Mills, & Rowe, 1994) who found that the actual contact length must
be used as this gives the most accurate results to testing. The actual contact length is 3 times the
geometric contact length, as first suggested in (Makino, Suto, & Fokushima, 1966) this is shown in
the fourth equation.
𝐴𝑐𝑡𝑢𝑎𝑙 𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝐿𝑒𝑛𝑔𝑡ℎ = 3 × 𝐺𝑒𝑜𝑚𝑒𝑡𝑟𝑖𝑐 𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝐿𝑒𝑛𝑔𝑡ℎ
Equation 4.9 Actual contact length.
“The partition ratio in grinding is defined as the proportion of the grinding energy conducted into the
workpiece in the contact area.” (Rowe, Black, Mills, Morgan, & Qi, 1997). This equation stems from
the work undertook by Pettit in his paper with Rowe, Pettit, Boyle et al. (Rowe, Pettit, Boyle, &
Moruzzi, 1988). As this is a ratio it has no units associated with it.
𝐸𝑛𝑒𝑟𝑔𝑦 𝑃𝑎𝑟𝑡𝑖𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 =
1
( 𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 × 𝐺𝑟𝑖𝑛𝑑 𝑊ℎ𝑒𝑒𝑙 𝑆𝑝𝑒𝑒𝑑
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝐺𝑟𝑖𝑛𝑑 𝑊ℎ𝑒𝑒𝑙 × 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝑆𝑝𝑒𝑒𝑑
)
0.5
×
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝐺𝑟𝑖𝑛𝑑𝑤ℎ𝑒𝑒𝑙
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒
+ 1
Equation 4.10 Energy Partition Ratio
“Specific grinding energy is the energy that must be expended to remove a unit volume of workpiece
material.” (Rowe, Principles of Modern Grinding Technology, 2009). Like the equation above, this
equationisfirstderivedin (Rowe,Pettit,Boyle,&Moruzzi,1988). The critical specific energy has the
units J * mm3
, the derivation of this can be found in the appendices, appendix I.
𝐶𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑆𝑝𝑒𝑐𝑖 𝑓𝑖𝑐 𝐸𝑛𝑒𝑟𝑔𝑦 = 𝐶𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒× (
𝐴𝑐𝑡𝑢𝑎𝑙 𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝐿𝑒𝑛𝑔𝑡ℎ
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐷𝑖 𝑓 𝑓𝑢𝑠𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 × 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝑆𝑝𝑒𝑒𝑑
)
0.5
×
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒
0.887 × 𝐸𝑛𝑒𝑟𝑔𝑦 𝑃𝑎𝑟𝑡𝑖𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 × 𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐶𝑢𝑡
Equation 4.11 Theoretical critical specific energy according to Rowe, Pettit, Boyle et al.
The results table is presented below (Table 4.3) and the full results table is disclosed in the
appendices as well as the table in it is equation form, appendix II. These results have then been
plottedona graph (Figure 4.1) of critical specificenergyagainstworkpiece velocity,the redpointson
the experimental critical specific energy line are the experiments at which burn occurs and the
hollow blue markers are the experiments where burn didn’t occur.
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Figure 4.1 A graph showing the theoretical & experimental critical specific energy against the workpiece velocity using the Rowe, Pettit, Boyle et al. model.
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
180.00
70.00 120.00 170.00 220.00 270.00 320.00
CriticalSpecificEnergy(J/mm3)
Workpiece Velocity(mm/s)
A graph to show the theoretical and experimental critical specific energy generated during the grinding
process against the workpiece velocity using the Rowe, Pettit Boyle et al. thermal model.
Experimental Critical Specific Energy Theoretical Critical Specific Energy @ 523K Theoretical Critical Specific Energy @ 503K
Theoretical Critical Specific Energy @ 493K Theoretical Critical Specific Energy @ 483K
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3 years after Rowe, Pettit, Boyle et al. came up with their model, Rowe, Morgan and Allanson
developed a more conservative method of predicting the burn threshold in their paper (Rowe,
Morgan, & Allanson,AnAdvance inthe Modelling of Thermal Effects in the Grinding Process, 1991).
𝐶𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝐸𝑛𝑒𝑟𝑔𝑦 = 0.89 × 𝐶𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒×(
𝐴𝑐𝑡𝑢𝑎𝑙 𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝐿𝑒𝑛𝑔𝑡ℎ
𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 𝑆𝑝𝑒𝑒𝑑
)
0.5
× (
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑖 𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒2
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ 𝑒 𝑊𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒
)
0.5
×
1
𝐸𝑛𝑒𝑟𝑔𝑦 𝑃𝑎𝑟𝑡𝑖𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜× 𝐷𝑒𝑝𝑡ℎ 𝑜𝑓 𝐶𝑢𝑡
Equation 4.12 Theoretical critical specific energy according to Rowe, Morgan and Allanson.
An abbreviationof the resultsof thisthermal model isshownintable 4.4below,the full resultstable
and a table showingthe equationsusedare displayedinthe appendices, appendix III. The results of
this model have been placed on a graph; this graph is visible in figure 4.2 below. The red points on
the experimental critical specificenergycurve are the pointswhere the workpiece was burnt during
the experiment; the hollow blue ones represent the experiments when burn did not occur.
Table 4.4 The results table showing the theoretical results calculated using the thermal model
conceived by Rowe, Pettit Boyle et al. and generated using critical temperature values of 523K, 503K,
493K and 483K.
Vw ϴm
ec* @
ϴm=573K ϴm
ec* @
ϴm=548K ϴm
ec* @
ϴm=523K
83.33333333 573 112.056 548 107.167 523 102.278
83.33333333 573 112.056 548 107.167 523 102.278
100 573 112.973 548 108.044 523 103.115
116.6666667 573 113.929 548 108.958 523 103.987
133.3333333 573 114.898 548 109.885 523 104.872
150 573 115.858 548 110.803 523 105.748
166.6666667 573 116.660 548 111.570 523 106.480
183.3333333 573 117.724 548 112.587 523 107.451
200 573 118.624 548 113.448 523 108.273
216.6666667 573 119.503 548 114.289 523 109.075
233.3333333 573 120.356 548 115.105 523 109.854
250 573 121.189 548 115.901 523 110.614
283.3333333 573 122.795 548 117.437 523 112.080
316.6666667 573 124.321 548 118.897 523 113.473
316.6666667 573 124.321 548 118.897 523 113.473
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Figure 4.2 A graph showingthe theoretical andexperimental critical specific energy against the workpiece velocity using the Rowe, Morgan and Allanson
model.
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
180.00
70.00 120.00 170.00 220.00 270.00 320.00
CriticalSpecificEnergy(J/mm3)
Workpiece Velocity(mm/s)
A graph to show the theoretical and experimental critical specific energy generated during the
grinding process againstthe workpiecevelocity using the Rowe, Morgan & Allanson thermal model.
Experimental Critical Specific Energy Theoretical Critical Specific Energy @ 573K
Theoretical Critical Specific Energy @ 523K Theoretical Critical Specific Energy @ 548K
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5. Discussion.
Using the thermal model first presented by Rowe, Petit, Boyle et al. it is possible for lines to be
plottedonthe graph to determine atwhatcritical specificenergyburnoccursat different velocities.
Whenthese linesare placedonthe same graphas the experimental critical specific, it is possible to
determine the maximumtemperatureof the grindingwheel atacertainvelocity.The workpiece used
was made of steel withaRockwell hardnessCof 61; thistype of steel is thermally damaged at 523K.
Whenthistemperature isplotted using the Rowe, Pettit, Boyle et al. model, as illustrated in figure
4.1, it is an under-estimation of what is actually occurring. When the experiment was run at 100
mm*s-1
the workpiece was thermally damaged; however according to the thermal model this
occurred at a temperature less than 523K. Through running the thermal model at different
temperatures, it appears that the temperature at which the thermal damage occurs at is between
483 and 493K. Temper colours, the visual sign of workpiece burn, could occur at a temperature as
low as 493K. Whilst according to this model it means that the last workpiece was burnt at a
temperature no lower than 483K, it doesn’t mean that with this thermal model thermal damage
couldn’t occur at a lower temperature. Due to the lack of intermediary points between that point
and the nextpoint,burncouldstill occurat a lowercritical specificenergy.Thoughthisseems at first
to be a problem,it could quickly be overcome. The material could become thermally damaged at a
temperature lowerthan even492K. Howevermore likelyisthe factthatthismodel isn’tconservative
enough; but even this doesn’t stop the thermal model being valid or unable to be used. Through
furthertestingandfine-tuningitispossible forthe exacttemperature thatburnarisesinthisthermal
model tobe found.Thisallowsthe thermal model tobe used, whilst a hint of caution should still be
usedwhenoperatingthismodel.Itcouldbe usedinindustrybutwouldneed to be heavily modified
and undergo lots of experimental testing first to guarantee that no thermal damage would occur
whilst this model is implemented.
In contrastthe Rowe,Morganand Allansonmodel is what appears to be an extremely conservative
model when plotted against the experimental critical specific energy as observed in figure 4.2.
Accordingto figure 4.2, workpiece number 3 was thermally damaged at a temperature above 573K,
whichisat least50K higherthanwas thought.Thismeansthatthismodel would be a good model to
implement in industry as it means that if the critical specific grinding energy doesn’t go above this
line thenithasn’tbeenthermallydamaged. The down side to this is the fact that it isn’t as efficient
as some companies may like, and therefore may mean that workpieces are discarded even if they
aren’t thermally damaged. This obviously loses the company money on two fronts, as there is an
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unnecessaryscrappingof perfectlygoodworkpiecesandwasting time by not effectively grinding to
it’s full potential.
Ignoringthe twopointswhenthe wheel isdressed,the experimental data shows a steady and even
decline,howeveritdoesstart to fluctuate and lose its stability towards the end. The earlier, higher
pointscan be equated to the grind wheel still being sharp from the it recently being dressed and it
deterioratesquickly as the grind wheel erodes rapidly. The fluctuation later on can be put down to
the fact that the grind wheel will be wearing and degrading over the course of the experiments.
Betweenpoints4and10 the experimentaldata shows a good ‘trueness’. In contrast, accounting for
all of the pointsthere appears to be a lack of ‘trueness’ but that is to be expected when taking into
considerationwhatwasexplainedabove sothe experimentalresultsshow agooddegree of accuracy
with what is expected.
Takingthe temperature atwhich the onset of burn occurs for this type of metal to be 523K, the two
thermal models demonstrate poor precision. The Rowe, Pettit, Boyle et al. model states that the
temperature atwhichitoccurs is at least40K lowerthanthe one that is generallyaccepted;however
if the workpiece can be burnt at a temperature as low as 493K then the difference is less than 10K.
Thenagain,the Rowe,Morgan and Allansonmodel suggeststhatthe temperature thatburnoccursis
at least25K or more than the knowntemperature itoccursat. Throughfurthertestinganexact point
at which burn occurs could be established which would help state which model is more precise.
As shownfromthe firstand lasttimesthe experimentwasundertaken,dressingthe grind wheel has
affected the critical specific energy needed to grind in comparison to the same workpiece velocity
whennotrecentlydressed.Thisisasurprisingresultbut canbe easilyexplained; with sharper grains
and unclogged pores, a newly dressed grind wheel is a much more effective grinding mechanism
than one with dull clogged up pores. However no change is made in the CNC programming to
account forthis newsharpnessandease of grinding.Sowhenthe freshlydressedgrind wheel is first
employed,itcutsthroughthe workpiece tooquickly;thisdrivesthe critical specificenergyupcausing
the workpiece tobe thermallydamaged.Thiscouldbe overcomeby better programming of the CNC
to allow for the new ease of grinding. If the in feed rate is increased this could overcome this
problemasit wouldincrease the amountof the workpiece ground per second thus giving it more to
cut through.Howeverinpractice thismaynot work, it will more than likely also increase the power
consumed and thus raising the critical specific energy by more than the in feed rate lowers it.
Anotherideawouldbe toincrease the speedof the grind wheel, although this wouldn’t do much to
the experimental critical specificenergy;it would increase the energy needed to thermally damage
the workpiece usingbothmodels.The problemwiththismethodisthatitdoesn’tincrease it enough
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to overtake the energy generated during a pass with the newly dressed grind wheel. So if both
strategieswere combinedthenthisproblemcouldbe easilyovercome;bycleverly programming the
computer,a buttoncouldbe installedtoeasilyswitchbetween the normal grinding parameters and
those needed to grind a freshly dressed grind wheel without burning the workpiece.
Neither thermal model takes into account how properly the partition of thermal energy into each
heat sync. Although coolant was used and sparks were created, very little of this, if any at all, is
accountedforin the equation. Even a simple calculation now could establish that around 5% of the
total energyaccumulatesin the swarf; another 5% enters the coolant and between 10 to 20% flows
intothe grind wheel. Even at a conservative estimate, around 70% of the total energy generated is
passed onto the workpiece, yet this isn’t the case in either model. Advances recently have
established ways of calculating how much energy is imparted into the chips generated during the
process. By using the fact that the swarf is hot enough to cause an exothermic reaction that turns
the chips into sparks, it is possible to work backwards from the known temperature at which these
sparks are generated to calculate how much energy has been inputted into the swarf and can be
subtracted from the total energy calculated. The same sort of principle could be applied to the
coolant if the temperature of the fluid is measured before it flows into the grinding zone and as it
leave it too. Then the change in temperature and the specific heat capacity as well as the mass the
flow the total energy gained by the fluid could be calculated and subtracted from the total energy
generated.
Althoughthe experimental datawasfroma previousexperiment, this data was recorded by a highly
reliable and experienced source in Andy Pettit. Having written and contributed to many academic
paperson the subjectof grinding,AndyPettithaslotsof knowledge whenitcomes to recording data
from grinding experiments. However just because he is a reliable source when it comes to the
recordingthe data,it doesnotstop there being systematic and random errors featuring in the data.
Systematicerrorinthisexperimentcouldstemfromthe equipmentnotbeingcalibratedproperly,for
example the dynamometer fitted to the machine or the distance between the edge of the grind
wheel andthe centre of the headstock. This error would be constant throughout the experiment if
not alteredandwouldbe amplifiedthroughthe calculations carried out, giving a slightly noticeable
problematthe end.There will alsobe a quantitative systematicerrorinvolvedinthe experimentthat
couldstemfromthe temperaturesencounteredwhilstgrinding.Thesetemperatures, if they get too
high, could affect some of the recording apparatus and cause them to give out readings that are
slightly out. If the machine were run for a long time, it would increase the likelihood of those
temperatures being generated and consequently reached. Random errors are also present in this
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experiment;thiscouldbe fromthe dynamometernotbeingable to keep up with the fluctuations in
the powerchanges.Thismay leadtothe maximumpowerbeingmissedbythe investigatorasit’snot
a linear,constantrecording,insteaditlagsbehindandjumps around sporadically making it tough to
get an exact reading.
Alsocertainassumptionshave beenmade withaspectsof the experiment, some of theseaspectswill
be acceptable with no impact, however some assumptions have a slight impact but cannot be
helped. It is assumed that steady state conditions are applicable as well as constant atmospheric
conditions i.e. continuous room temperature and air pressure. These assumptions have very little
impacton the equations,itjustallowsall of the total energytoflow into the 4 heatsyncs and notthe
atmosphere. With respect to the equipment used, some of the assumptions made are constant
density through the workpiece and the grind wheel, this guarantees that the thermal conductivity
and diffusivityisconstantthroughout. Inthe real life thisisn’tthe case, this could have an impact on
the calculationsasbothparametersare crucial to bothmodels.Thenagain, a big enough fluctuation
couldn’t occur as there would be something drastically wrong and the machine wouldn’t function
properly. So although this may not be the most precise assumption, it is precise enough to work in
these models. Another set of assumptions are to do with the cooling fluid, one that all of the fluid
entersthe contact zone andtwo isthat it doesn’tgethotenoughforfluidfilmboilingtooccur. These
twoare crucial,the firstone as thiscouldhave an impactif provedwrongby changingthe quantityof
energyflowingintoeachheatsyncand the second,as discussed earlier, causes the workpiece to be
burnt a lot easier. This also leads to the fact that if the energy entering the grinding swarf was
accounted for properly, an assumption made would be that the energy entering it is at the right
levels to cause the grinding swarf to exothermically react and that temperature is a constant and
known.
Using all that has been learnt, an adaptive control system can be incorporated into the grinding
machine usingthe thermal modelstoguarantee thatthermal damage isavoidedas well as speeding
up and making the process more efficient, thus reducing costs. This strategy can be easily
programmed into the CNC system and could cost very little if implemented correctly. If a
dynamometerwasfittedandthenlinkeduptothe control system, itcouldmeasure the powerbeing
consumed at set intervals. This power could then be fed through the equations to calculate the
critical specificenergy,if thatenergygetstooclose tothe agreedline onthe pre-setthermal model it
couldalterthe grindingparameterstolowerthe powerbeingconsumed.This allows for the optimal
conditions to be exercised as well as guaranteeing the workpiece isn’t burnt, thus improving the
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speedandtime ittakesto grinda workpiece leadingto the lowering the cost of manufacturing each
unit.
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6. Conclusion.
This work has introduced the importance of burn detection in the grinding process and prevention
using thermal models to create safe grinding parameters. Whilst both models are not what was
expected in terms of precision to the real life data recorded, one of the models could be used
straight away in industry with very little modification. The Rowe, Morgan and Allanson model is a
moderate model that is cautious in its application; this works well when placed into industry, as it
means there is a very small chance that a work piece would be burnt if this model was used. This
reducesthe riskof the workpiece failinginservice;which,ultimately, is the primary objective when
machiningsuchan importantworkpiece.The Rowe,Pettit,Boyle et al.model is an aggressive model
which has the advantage of being much more efficient compared to the other model. This model
wouldbe harderto implement inindustry,howeverwithfurthertesting andimprovements made to
the energy partitioning ratio, it could be implemented safely using in process control and
measurement system.
The results are appropriate forwhat wasexpectedfromthemand especially the middle few results
showa strong correlationand expected pattern.Itillustratedhow wheel wearcan affectthe grinding
processand how itis of vital importance tomodifythe grinding setupif the grindwheel has recently
beendressed. Several assumptions have been made though these are needed and don’t affect the
outcome enough to be examined further.
Further research that could be carried out is research into the energy partition ratio and the effect
that that could have on the critical specific energy and the thermal models. Also looking into
improving the efficiency of the grinding process as well as the practical application of the thermal
modelsinindustryandimprovingthem. Additionalworkwillalsolookintomore thermal modelsand
carry out testingtogather the correct data to be inputtedintothese models.These models can then
be compared with the ones already found and critically appraised and improvements and practical
applications can then be explored.
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