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Difference Between LBM and PAM – Laser Beam Machining and Plasma Arc Machining

Pintu — Sat, 09 May 2020 12:31:26 +0000

Non-traditional machining (NTM) processes can directly employ various forms of energy for removing material from workpiece in order to fabricate the intended 3-D feature. EDM, LBM, EBM, and PAM are four common NTM processes that use thermal energy (heat) to selectively remove material. In these processes, material removal mostly takes place in vaporized and sometimes in molten state. The source of heat is, however, different for these four processes. Laser Beam Machining (LBM) is one thermal energy based NTM process where material is removed by melting and vaporization through the action of a high energy density laser beam. First, a monochromatic and coherent beam of laser (Light Amplification by Stimulated Emission of Radiation) is generated using suitable lasing medium (Nd-YAG, Nd-Glass, Ruby, CO₂). The beam is then constricted to increase energy density, and the constricted laser beam is focused on the working surface with the help of lenses. Intense heat generation occurs at a very narrow spot where the laser beam strikes the workpiece surface leading to an instantaneous localized temperature rise of the order of 10,000°C. Such a high temperature can rapidly melt down and vaporize any solid material. Accordingly, majority of the material removal occurs in vapour state. The workpiece is usually integrated with the CNC system to move the worktable at a specified path at pre-set feed rate for cutting a particular profile.

Plasma is an extremely hot matter consisting of ionic particles. Plasma Arc Machining (PAM), also known as Plasma Arc Cutting (PAC), utilizes a high velocity jet of plasma to supply thermal energy (heat) for melting and vaporizing the workpiece. In order to artificially generate plasma, a suitable plasma forming gas (such as air, argon, nitrogen, and hydrogen) is first introduced into a gas chamber under high pressure. An electric arc is also constituted between a tungsten electrode and copper nozzle of the chamber (or workpiece, if it is conductive). The plasma forming gas is then forced to pass surrounding the electric arc through the small nozzle opening. Owing to the intense arc heating, the gas gets converted to high temperature plasma. This plasma jet is then directed towards the workpiece, where it rapidly elevates the cutting zone temperature to as high as 20,000°C leading to melting and vaporization. Although a fraction of the workpiece material is removed in vaporized form, rest of the material in molten state is blown away from the cutting zone by the high velocity plasma jet. Thus, material removal in PAM is a combination of (i) displacing the molten metal and (ii) vaporizing the material. Plasma jet is relatively wider, and thus the process is associated with broader heat affected zone and wider kerf. PAM is not suitable for precise and miniaturized feature fabrication. Rather it is preferred for thick cutting or plasma heating regardless of the mechanical, electrical and chemical properties of the sample. Various similarities and differences between LBM and PAM are given below in table format.

Similarities between LBM and PAM

Both the LBM and PAM processes are thermal energy based non-traditional machining (NTM) processes as material removal takes place by melting and vaporization of workpiece materials. However, the source of heat is different for these two processes.
No vacuum chamber is required for operation of LBM or PAM process. This is unlike EBM and IBM (FIB) where the work chamber must have low vacuum pressure.
Both the processes can be applied for electrically conductive as well as non-conductive materials. Electrical conductivity plays no role on the performance and applicability of these two processes. However, the arcing fashion in PAM is different for electrically conductive and non-conductive materials.
Both the LBM and PAM processes can create thermal damages to the workpiece. A narrow heat-affected zone (HAZ) exists on the processed samples. However, this HAZ is usually wider for PAM as compared to that for LBM.
In both the processes, several parameters and various movements are usually controlled by CNC system.

Differences between LBM and PAM

Laser Beam Machining (LBM)	Plasma Arc Machining (PAM)
A high intensity beam of laser (coherent photon particles) is used to supply heat for melting and vaporizing the workpiece material.	A high temperature plasma jet (highly excited ions and electrons) is used as the heat source for melting and vaporizing the workpiece material.
No separate gas is required to supply to generate laser beam as it is directly generated using the lasing medium (Nd-YAG, Nd-Glass, Ruby, CO₂) with the assistance of flash tube.	Suitable plasma forming gas (air, argon, nitrogen, hydrogen) is required to supply continuously to a closed gas chamber under high pressure, which ultimately comes out as high temperature plasma jet.
No electric arc is required to establish in LBM process.	An electric arc is essentially required to supply immense heat for converting the gas into plasma. This arc can be constituted between (i) a tungsten electrode and (ii) nozzle or workpiece.
Although melting and vaporization both occur simultaneously, most of the material is removed in the form of vapour. Only small percentage is removed in molten form.	Both melting and vaporization occur in PAM too; however, majority of the material is blown away by the high velocity plasma jet when the workpiece material remains in molten state. A comparatively smaller fraction of the material is removed in vaporized form.
The laser beam characteristics (such as beam diameter, energy density, incidence timing, incidence pattern, etc.) can be controlled effectively and easily. Such parameters can also be changed rapidly as per the desired level.	PAM is less flexible in terms of parameter controlling and their variation.
Laser beam can be allowed to strike the workpiece either continuously or intermittently (as in the case of femtosecond laser).	Plasma jet is usually allowed to continuously strike the workpiece.
In LBM, the kerf width can be maintained very small (even below 0.1 mm).	In PAM, kerf is relatively wider (3 – 10 mm).
A narrow localized heat-affected zone can be noticed in the specimens after cutting by LBM. So it causes very little thermal damage to the workpiece.	Specimens cut by PAM can have significantly wider heat-affected zone. The extent of thermal damage made in PAM is more.
LBM process capability is affected by optical properties of the workpiece surface (transparent, translucent and opaque). Highly reflective surface can reduce rate of heat input.	PAM process is not affected by the optical properties of the workpiece surface.
LBM process is not noisy and can be carried out without specific personal protections.	PAM process is very noisy. Thus proper hearing protection (like ear muffler) is needed for operators.

References

Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
Advanced Machining Processes by V. K. Jain (Allied Publishers Private Limited).
Nonconventional Machining by P. K. Mishra (Narosa Publishing House).

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Difference Between LBM and IBM – Laser Beam Machining and Ion Beam Machining

Pintu — Fri, 08 May 2020 17:44:30 +0000

Several advanced machining processes have been developed over the last few decades to cater the evergrowing demand of high quality small-scale products made of a wide variety materials with highly finished surfaces and close tolerance. Laser beam machining and ion beam machining are two such processes that follow subtractive manufacturing approach to fabricate intended features with improved accuracy and tight tolerance. However, their working principle and extent of capability are different. Laser Beam Machining (LBM) is one thermal energy based non-traditional machining process where a concentrated and coherent beam of photons (laser) is used to supply thermal energy (heat) for selectively removing material from the workpiece. When the laser beam strikes the workpiece, intense heat generation takes place in a narrow localized zone that can cause an instantaneous temperature rise of the order of 10,000°C. Such a high temperature can virtually melt and vaporize any solid material. Accordingly, the material is mostly removed in the form of vapour of the concerned workpiece material. The LBM process does not require a vacuum chamber for its operation. Though LBM offers relatively high material removal rate, it leads to the formation of a narrow heat-affected zone (HAZ). The process is also affected by re-cast layer formation and taper cutting while machining thick samples.

Ion Beam Machining (IBM) is another non-traditional machining process, but here material removal occurs due to mechanical impact. Unlike LBM, here thermal energy (heat) has no direct role in removing material. In IBM, a concentrated beam of high velocity ions is allowed to strike the workpiece surface. Upon striking, momentum transfer takes place between the incidental ions and the atoms of the workpiece surface. This results in dislodging of atoms from the surface leading to material removal. Thus material removal occurs in atomic level without any melting or vaporization. Although IBM offers very low material removal rate (MRR), it generates highly finished surface with nanometric level roughness. In fact, a variant of IBM known as Focused Ion Beam (FIB) is preferred for micro- or nano-fabrication and surface smoothing. Furthermore, the process is free from collateral thermal damages and re-cast layer formation. However, the IBM process must be carried out in a soft vacuum chamber to avoid haphazard collision between the ions and air molecules. Creating vacuum chamber is time consuming, which makes the overall process less productive. Various similarities and differences between LBM and IBM processes are given below in table format.

Similarities between LBM and IBM

Both laser beam and ion beam machining processes are considered as non-conventional machining processes (NTM) or advanced machining processes (AMP).
In none of the processes, there exists physical contact between a solid tool and the workpiece. So these processes are free from burr formation, residual stress generation, negative influences of gradual wear, etc. Note that, residual stress generation may occur in LBM owing to thermal cycle.
Both the processes are independent of electrical, chemical, and mechanical properties of the workpiece material. So these processes can be applied on a wide variety of materials.
These processes are expensive, and thus are used for sophisticated works only.
These processes are commonly integrated with computer control system to facilitate precise control.

Differences between LBM and IBM

Laser Beam Machining (LBM)	Ion Beam Machining (IBM)
LBM is a thermal energy based non-traditional machining process.	IBM is essentially a mechanical or kinetic energy (momentum transfer) based non-traditional machining process.
A high intensity beam of laser (coherent photons) is used to remove material from workpiece.	A beam of high velocity ions are used to remove material from workpiece.
The mechanism of material removal in LBM is instantaneous melting and vaporization.	The mechanism of material removal in IBM is sputtering (or atom dislodging by ion impact).
In LBM, material is removed in the form of vapour of the concerned workpiece material. However, a small fraction of material is removed in molten state.	In IBM, individual atom (or a cluster of atoms) is removed or dislodged directly from the workpiece surface without any melting or vaporizing.
Power density of the laser beam is relatively higher (10⁴ to 10⁶ W/mm²).	Power density of the ion beam is significantly less (10^-2 to 10⁰ W/mm²).
LBM does not necessarily require a vacuum chamber. It can be carried out in open atmosphere. Sometimes shielding gas can be applied in machining zone to avoid high temperature oxidation of the machined surface.	IBM process is carried out in a soft vacuum chamber having pressure in the order of 10^-3 to 10^-5 atm.
Applicability of LBM process depends on the optical properties (reflectivity, absorptivity and transmittivity) of the workpiece surface.	IBM process is independent of optical properties of the workpiece surface.
Process capability and productivity depend on thermal properties (such as conductivity, diffusivity, etc.) of the workpiece material.	It is mostly independent of the thermal properties of the workpiece material.
It can cut features having high aspect ratio (up to 50:1). Deep hole drilling and thick cutting (even up to 20 mm thickness) can be carried out using LBM.	It is not suitable for deep cutting or fabricating features having high aspect ratio. IBM is preferred for micro-fabrication, shallow contouring or surface smoothening.
LBM process is affected by re-cast layer formation, especially during cutting thick specimens.	No such re-cast layer formation occurs in IBM.
The area of thermal damage on the workpiece is relatively larger in LBM process.	Impact damage on the workpiece is very narrow in IBM process. Often, no palpable collateral damage is noticed.
Material removal rate (MRR) of LBM process is significantly higher (5 – 50 mm³/min). So it is a productive and economic process.	Material removal rate (MRR) of IBM process is very low (in the order of 2 × 10^-4 mm³/min).

References

Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
Nonconventional Machining by P. K. Mishra (Narosa Publishing House).
Allen et al. (2009). Ion beam, focused ion beam, and plasma discharge machining. CIRP Annals. https://doi.org/10.1016/j.cirp.2009.09.007

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Difference Between EBM and LBM – Electron Beam & Laser Beam Machining

Pintu — Thu, 20 Jun 2019 04:16:25 +0000

There exist four basic non-traditional machining processes (EDM, LBM, EBM, and PAM) where thermal energy is utilized to melt and vaporize a tiny volume of material from the workpiece. Although mechanism of material removal is same in all four processes, the source of heat is different. In Electron Beam Machining (EBM), a concentrated beam of electrons is used as heat source. Here ample electrons are first generated using an electron gun, and then these are accelerated in an electric field under the presence of potential difference. High velocity electrons are then concentrated in the form a beam and the beam is directed towards the workpiece. When high velocity electrons strike the work surface, their kinetic energy gets converted into thermal energy and thus immense heat is generated. This increases localized temperature to as high as 10,000°C, which instantaneously melts down and vaporizes the material. To dislodge the striking electrons as well as to maintain electrical neutrality, the workpiece is grounded. Thus EBM process can be applied to electrically conductive materials only. Moreover, it requires a vacuum chamber to avoid collision of electrons with the air molecules.

Laser Beam Machining (LBM) process utilizes a coherent beam of laser (photons having same frequency, wavelength and phase) as the heat source for material removal purpose. LASER (light amplification by stimulated emission of radiation) is basically a coherent and amplified beam of electromagnetic radiation. Photon particles are first generated using a laser medium (such as Nd-YAG) with the assistance of excitation source (a flash tube). Ample photon particles are then concentrated and allowed to eject through a small opening in the form of a beam. This high energy density laser beam is then directed towards the workpiece surface. The thermal energy of the photon particles instantaneously melts down and vaporizes the workpiece material. It eliminates the material restriction imposed by EBM process based on conductivity. Thus LBM can be applied to a wide variety of materials (both conductive and non-conducive) as long as the surface is not highly reflective. Moreover, no vacuum chamber is required in LBM process. Various similarities and differences between EBM and LBM are given below in table format.

Similarities between EBM and LBM

Both EBM and LBM are considered as thermal energy based non-traditional machining process.
Mechanism of material removal (i.e. localized melting and vaporization) is same in both the processes.
Both the processes utilize high intensity (i.e. high energy density) beam for material removal. However, the beam constituent is different.
No physical contact between tool and workpiece exists in any of these processes. So the processes are free from burr and mechanically induced residual stress.
Both the processes are affected by narrow heat affected zone (HAZ) and recast layer.
For both the processes, the kerf width (actual width of the machined feature) is larger than the beam diameter.
Wide variety of pulse modes and durations are also available in both the cases.
Computer control system can be integrated with both the processes to facilitate precise control.
In both the cases, beam intensity and cross-sectional area can be manipulated multifariously to harness desired performance.

Differences between EBM and LBM

Electron Beam Machining (EBM)	Laser Beam Machining (LBM)
A high intensity beam of focused electrons is used to supply heat for material removal.	A high intensity beam of laser (coherent photons) is used to supply heat for material removal.
EBM process is always carried out in vacuum (very low pressure) chamber to avoid collision of electrons with air molecules. Such collision can undesirably reduce kinetic energy of electrons and spread them.	LBM does not require vacuum chamber. It can be carried out in open atmosphere. Sometimes shielding gas can be applied in machining zone to avoid high temperature oxidation of machined surface.
It is suitable for small components as workpiece is required to keep within the vacuum chamber.	Workpiece size does not pose any restriction as it can be carried out in open environment.
EBM process is time consuming mainly due to creating a low pressure vacuum chamber.	LBM process is time efficient.
X-Ray is generated during EBM process. This possesses serious health concern to the operator.	No X-Ray is generated in LBM process.
EBM is applicable to electrically conductive workpiece materials only as workpiece must be grounded to stay neutral by transmitting striking electrons to the ground.	LBM is independent of electrical conductivity of workpiece material. So it can be used for both conductive and non-conductive materials.
Optical properties of workpiece surface (such as reflectivity, absorptivity, etc.) don’t influence EBM process capability.	LBM process capability relies on the optical properties of workpiece surface. High reflectivity can severely affect process capability.

References

Unconventional Machining Processes by T. Jagadeesha (I. K. International Publishing House Pvt. Ltd.).
Nonconventional Machining by P. K. Mishra (Narosa Publishing House).
Dubey et al. (2008), Laser beam machining—A review, International Journal of Machine Tools and Manufacture, 48, 609-628. https://doi.org/10.1016/j.ijmachtools.2007.10.017

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