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| {{Generalize|date=January 2011}}
| | <br><br>Earlier than you let your youngster run off into the woods unsupervised you might want to compare him to that child we all knew growing up. Is your baby that baby? If so, then you will want to wait a handful of extra years or extra earlier than allowing them to have a pocket knife. It boils down as to whether or not [http://Remhits.warnerreprise.com/news/best-throwing-knife-kill-ever-927476 you're feeling] your youngster is mature, respectful and clever sufficient to care for a doubtlessly dangerous device. Take some time and observe your youngster, particularly how he or she reacts round other children. Peer pressure can result in unfortunate resolution making.<br><br>Some of my most frustrating experiences in presenting have been in larger institutional venues the place laws and demarcation rules stop you from being self-reliant. In the absence of the authorised particular person, your presentation can fail as a consequence of bureaucratic processes that leave everyone disempowered. On a couple of event I've ignored guidelines and made minor bodily modification of techniques to allow a presentation to proceed. Having my Leatherman available has been important. When setting up displays, particularly at outdoor venues dealing with wind and climate, my Leatherman, some blue-tack, cord and gaffer tape, have solved surprising issues on many events.<br><br>Sleeping Bag- You had better like sleeping within the nice outdoors because that is precisely what you can be doing if you end up on a wildland hearth crew. Make sure that you have a very good sleeping bag. The night's often get chilly no matter what time of year it's in case you are up within the mountains. You additionally [http://www.thebestpocketknifereviews.com/best-throwing-knives-top-recommendations/ Cheap throwing knives] need to just be sure you have a sleeping bag that covers you from head to toe. A tent will probably be extraordinarily useful also. You might not want to sleep outdoors the complete time you might be on crew. A small one or two man tent ought to work simply wonderful.<br><br>In addition to its single handed operation, what made the Christy knife so useful was its blade quality. You possibly can inform that Christy has a long historical past of making scalpels and razor blades, as a result of for me this has always been an exceedingly sharp knife. Although the Christy knife is designed for easy blade substitute, I have had the identical blade in mine for the last ten years, and I contact up the edge solely often with a ceramic hone. I love that the Christy knife remains a hundred% American made and backed by a common sense lifetime guarantee which doesn't cowl re-sharpening or abuse, however does cowl another defects.<br><br>Another Boker?! Yes, we know we're just a little keen on Boker here at BestPocketKnifeToday.com but you cannot fault their eye-catching designs. Within the Albatros we see a precision cutting device with simple strains and understated class. The VG-10 metal blade is 2 and a half inches lengthy and overall the knife is slightly below four inches lengthy when closed. The [http://karabine.com/film/groups/best-throwing-knives-158395982/ titanium deal] with blends effectively with the blade and feels solid and [http://www.gradbible.com/members/derekdanielso/activity/179688/ durable] within the hand. Sure, just a little pricier than many comparable sized choices but properly definitely worth the cash in our opinion. >> See newest pricing for the Boker Albatros. Wenger Giant.<br><br>The character of this psycho-social poison from televised violence was highlighted for me by an expertise in Tasmania in the late 1970’s that I've retold many times. I had arranged to buy the22 calibre rifle that I nonetheless personal as a “licensed weapon” from Janet Mollison (Invoice Mollison’s oldest daughter who was my own age). At the time, Janet lived in a flat above the Salvo’s Op store in Elizabeth St, North Hobart. I had organized to pick up the rifle on a Saturday night. The transaction was easy with no paper work being required for ownership of rim-fire rifles in those occasions. |
| '''Laser drilling''' is a process in which a [[laser]] is used to make holes, instead of conventional [[drilling]].
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| ==Applications==
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| Manufacturers of [[turbine engine]]s for [[aircraft propulsion]] and for [[power generation]] have benefited from the productivity of [[laser]]s for drilling small (0.3–1 mm diameter typical) cylindrical holes at 15–90° to the surface in cast, [[sheet metal]] and machined components. Their ability to drill holes at shallow angles to the surface at rates of between 0.3 to 3 holes per second has enabled new designs incorporating film-cooling holes for improved [[fuel efficiency]], reduced noise, and lower NOx and CO emissions.
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| Incremental improvements in laser process and control technologies have led to substantial increases in the number of cooling holes used in turbine engines. Fundamental to these improvements and increased use of laser drilled holes is an understanding of the relationship between process parameters and hole quality and [[drilling]] speed.
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| ==Theory==
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| Following is a summary of technical insights about the laser drilling process and the relationship between process parameters and hole quality and drilling speed.
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| ===Physical phenomena===
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| Laser drilling of cylindrical holes generally occurs through [[melting]] and [[vaporization]] (also referred to as "[[ablation]]") of the workpiece material through absorption of energy from a focused [[laser beam]].
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| The energy required to remove material by melting is about 25% of that needed to vaporize the same volume, so a process that removes material by melting is often favored.{{cn|date=August 2012}}
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| Whether melting or vaporization is more dominant in a laser drilling process depends on many factors, with [[laser pulse]] duration and energy playing an important role. Generally speaking, ablation dominates when a Q-switched Nd:YAG laser is used.{{Citation needed|date=January 2011}} On the other hand, melt expulsion, the means by which a hole is created through melting the material, dominates when a [[flashtube]] pumped Nd:YAG laser is used.{{Citation needed|date=January 2011}} A Q-switched Nd:YAG laser normally has pulse duration in the order of [[nanosecond]]s, peak power on the order of ten to hundreds of MW/cm<sup>2</sup>, and a material removal rate of a few [[micrometre]]s per pulse. A flash lamp pumped Nd:YAG laser normally has a pulse duration on the order of hundreds of [[microsecond]]s to a [[millisecond]], peak power in the order of sub MW/cm<sup>2</sup>, and material removal rate of ten to hundreds of [[micrometer]]s per pulse. For machining processes by each laser, ablation and melt expulsion typically coexist.{{Citation needed|date=January 2011}}
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| Melt expulsion arises as a result of the rapid build-up of [[gas pressure]] (recoil force) within a cavity created by [[evaporation]]. For melt expulsion to occur, a molten layer must form and the [[pressure gradient]]s acting on the surface due to vaporization must be sufficiently large to overcome [[surface tension]] forces and expel the molten material from the hole.<ref>Basu, S. and T. DebRoy, "Liquid Metal Expulsion During Laser Irradiation." J. Appl. Phys., 1992. 72: p. 3317-3322.</ref>
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| The "best of both worlds" is a single system capable of both "fine" and "coarse" melt expulsion. "Fine" melt expulsion produces features with excellent wall definition and small [[heat-affected zone]] while "coarse" melt expulsion, such as used in [[percussion drill]]ing and [[trepanning]], removes material quickly.
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| The recoil force is a strong function of the peak [[temperature]]. The value of T<sub>cr</sub>{{Clarify|date=January 2011}} for which the recoil and surface tension forces are equal is the critical temperature for liquid expulsion. For instance, liquid expulsion from [[titanium]] can take place when the temperature at the center of the hole exceeds 3780 K.
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| In early work (Körner, et al., 1996),<ref>Körner, C., R. Mayerhofer, M. Hartmann and H. Bergmann, "Physical and Material Aspects in Using Visible Laser Pulses of Nanosecond Duration for Ablation." Appl. Phys. A, 63 (1996), pp. 123–31.</ref> the proportion of material removed by melt expulsion was found to increase as intensity increased. More recent work (Voisey, et al., 2000)<ref>Voisey, K.T., C.F. Cheng, and T.W. Clyne. "Quantification of Melt Ejection Phenomena during Laser Drilling in Laser-Solid Interactions for Materials Processing." MRS, 2000. San Francisco.</ref> shows that the fraction of the material removed by melt expulsion, referred to as melt ejection fraction (MEF), drops when laser energy further increases. The initial increase in melt expulsion on raising the beam power has been tentatively attributed to an increase in the pressure and pressure gradient generated within the hole by vaporization.
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| A better finish can be achieved if the melt is ejected in fine droplets.{{cn|date=August 2012}} Generally speaking, droplet size decreases with increasing pulse intensity. This is due to the increased vaporization rate and thus a thinner molten layer. For the longer pulse duration, the greater total energy input helps form a thicker molten layer and results in the expulsion of correspondingly larger droplets.<ref>Voisey, K.T., J.A. Thompson, T.W. Clyne, Damage caused during laser drilling of thermal spray TBC superalloy substrates." ICALEO 2001, Jacksonville FL, 14-18th October 2001.</ref>
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| ===Previous models===
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| Chan and Mazumder (1987)<ref>Chan, C. and J. Mazumder, "One-Dimensional Steady State Model for Damage by Vaporization and Liquid Expulsion Due to Laser-material Interaction." J. Appl. Phys., 1987. 62(11): p. 4579 -86.</ref> developed a 1-D steady state model to incorporate liquid expulsion consideration but the 1-D assumption is not suited for high [[aspect ratio]] hole drilling and the drilling process is transient. Kar and Mazumder (1990)<ref>Kar, A., and Mazumder, J., 1990, "Two-Dimensional Model for Material Damage due to Melting and Vaporisation during Laser Irradiation." J. Appl. Phys., 68, pp. 3884–3891.</ref> extended the model to 2-D, but melt expulsion was not explicitly considered. A more rigorous treatment of melt expulsion has been presented by Ganesh, et al. (1997),<ref>Ganesh, R. K., A. Faghri and Y. Hahn, "A Generalized Thermal Modeling for Laser Drilling Process: 1. Mathematical Modeling and Numerical Methodology." Int. J. Heat Mass Transfer, 40 (1997), pp. 3351–3360.</ref> which is a 2-D transient generalized model to incorporate solid, fluid, temperature, and pressure during laser drilling, but it is computationally demanding. Yao, et al. (2001)<ref>Yao, Y. L., Zhang, W.W., Chen, K., 2001, "Modeling and Analysis of UV Laser Micromachining of Copper." Int. J. Adv. Manuf. Technol., Vol. 18, pp. 323–331.</ref> developed a 2-D transient model, in which a Knudsen layer is considered at the melt-vapor front, and the model is suited for shorter pulse and high peak power laser ablation.
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| ====Laser energy absorption and melt-vapor front====
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| At the melt-vapor front, the Stefan boundary condition is normally applied to describe the laser energy absorption (Kar and Mazumda, 1990; Yao, et al., 2001).
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| :<math>I_{abs} + k \left( \frac{\partial T}{\partial z} + r \frac{\partial T}{\partial r} \right) + \rho_l \nu_i L_v - \rho_v \nu_v (c_p T_i + E_v) = 0</math> '''(1)'''
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| where <math>I_{abs} = I(t)^{-\beta z}</math> is the absorbed laser intensity, ''β'' is the laser absorption coefficient depending on laser [[wavelength]] and target material, and ''I(t)'' describes temporal input laser intensity including pulse width, repetition rate, and pulse temporal shape. ''k'' is the [[Thermal conductivity|heat conductivity]], ''T'' is the temperature, ''z'' and ''r'' are distances along axial and radial directions, ''p'' is [[density]], ''v'' the [[velocity]], ''L<sub>v</sub>'' the latent heat of vaporization. The subscripts ''l'', ''v'' and ''i'' denote liquid phase, vapor phase and vapor-liquid interface, respectively.
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| If the laser intensity is high and pulse duration is short, the so-called [[Knudsen layer]] is assumed to exist at the melt-vapor front where the state variables undergo discontinuous changes across the layer. By considering the discontinuity across the Knudsen layer, Yao, et al. (2001) simulated the surface recess velocity V<sub>v</sub> distribution, along the radial direction at different times, which indicates the material ablation rate is changing significantly across the Knudsen layer.{{Citation needed|date=January 2011}}
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| ====Melt expulsion====
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| After obtaining the [[vapor pressure]] ''p<sub>v</sub>'', the melt layer flow and melt expulsion can be modeled using hydrodynamic equations (Ganesh et al.,1997). Melt expulsion occurs when the vapor pressure is applied on the liquid free surface which in turn pushes the melt away in the radial direction. In order to achieve fine melt expulsion, the melt flow pattern needs to be predicted very precisely, especially the melt flow velocity at the hole’s edge. Thus, a 2-D [[axisymmetric]] transient model is used and accordingly the [[momentum]] and continuity equations used.
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| Ganesh’s model for melt ejection is comprehensive and can be used for different stages of the hole drilling process. However, the calculation is very time consuming and Solana, et al. (2001),<ref>Solana, P., P. Kapadia, J. Dowden, W.S.O. Rodden, S.S. Kudesia, D.P. Hand and J.D.C. Jones, "Time Dependent Ablation and Liquid Ejection Process During the Laser Drilling of Metals." Optics Communications, 2001. 191: p. 97-112.</ref> presented a simplified time dependent model that assumes that the melt expulsion velocity is only along the hole wall, and can give results with a minimum computational effort.
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| The liquid will move upwards with velocity u as a consequence of the pressure gradient along the vertical walls, which is given in turn by the difference between the ablation pressure and the surface tension divided by the penetration depth ''x''.
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| Assuming that the drilling front is moving at a constant velocity, the following [[linear equation]] of liquid motion on the vertical wall is a good approximation to model the melt expulsion after the initial stage of drilling.
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| :<math>\rho \frac{\partial u(r,t)}{\partial t} = P(t) + \mu \frac{\partial^2 u(r,t)}{\partial r^2}</math> '''(2)'''
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| where ''p'' is the melt density, ''μ'' is the [[viscosity]] of the liquid, ''P(t)=(ΔP(t)/x(t))'' is the pressure gradient along the liquid layer, ''ΔP(t)'' is the difference between the vapor pressure ''P<sub>v</sub>'' and the surface tension <math>2 \sigma \over \bar{\delta}</math>.
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| ==Pulse shape effect==
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| Roos (1980)<ref>Roos, S. O., "Laser drilling with different pulse shapes." J Appl Phys 1980; 51:5061.</ref> showed that a 200 µs train consisting of 0.5 µs pulses produced superior results for drilling metals than a 200 µs flat shaped pulse. Anisimov, et al. (1984)<ref>Anisimov, V.N., et al., Appl. Opt. 23 1984 18.</ref> discovered that process efficiency improved by accelerating the melt during the pulse.
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| Grad and Mozina (1998)<ref>Grad, L., and J. Mozina, "Laser Pulse Shape influence on Optically Induced Dynamic Processes." Appl. Surf. Sci., 127-129 (1998), pp. 999–1004.</ref> further demonstrated the effect of pulse shapes. A 12 ns spike was added at the beginning, middle, and the end of a 5 ms pulse. When the 12 ns spike was added to the beginning of the long laser pulse, where no melt had been produced, no significant effect on removal was observed. On the other hand, when the spike was added at the middle and the end of the long pulse, the improvement of the drilling [[efficiency]] was 80 and 90%, respectively. The effect of inter-pulse shaping has also been investigated. Low and Li (2001)<ref>Low, D. K. Y., L. Li and P. J. Byrd, "The Influence of Temporal Pulse Train Modulation during Laser Percussion Drilling, Optics and Lasers in Engineering." 35 (2001), pp. 149–164.</ref> showed that a [[pulse train]] of linearly increasing magnitude had a significant effect on expulsion processes.
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| ==Conclusion==
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| Manufacturers are applying results of [[process modeling]] and experimental methods to better understand and control the laser drilling process. The result is higher quality and more productive processes that in turn lead to better end products such as more fuel efficient and cleaner [[aircraft]] and power generating turbine engines.
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| ==See also==
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| * [[Laser cutting]]
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| * [[List of laser articles]]
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| ==References==
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| {{Reflist}}
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| ==External links==
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| {{Commons category}}
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| [[Category:Hole making]]
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