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The first successful operation of a laser occurred more than a decade ago. Since that time both the available laser equipment and its application have undergone extensive development. One of the main uses of Laser is able to machine difficult-to-machine materials, like ceramics, carbide and hardened steel with excellent productivity and surface quality. Laser is used for many operations like cutting, drilling, turning, milling, welding, engraving, surface treatment etc...This makes the laser to be used in the major fields of automotive, aerospace, tool and die making, medical, optical and micro electronics packaging industry etc...
The figure 1 shows the conventional method of making the mould and also depicts time consumed for each operation in making of mould or die. Since most of time was consumed in drilling and milling operations and also minor losses of set up, tool change loss. So, lead time for making mould will be high, which forces to go for an alternate process like EDM to make mould in shorter time.
The figure 2 shows the mould making by means of EDM and LBM. One main disadvantage of EDM is manufacturing of electrode which consumes time when compared to laser machining. So, laser machining is preferred for making of moulds. Now the lead time is reduced which in turns increases the productivity.
3.0 Manufacturing Method of Mould using laser
3.1 Method 1
Laser machining in die making
In this method rough operation is done by High Speed machining (HSM) Process and finishing by laser ablation process. In laser ablation process, the characteristic of beam plays an important role.
The following are the properties of the laser beam:
The beam should have high power density on small spot area.
Beam should be coherent in nature.
It should have good collimation and mono chromatic nature.
These properties make the laser to be used in variety of machining process.
3.2 Overview of Laser Machining
In Laser machining material removal is achieved by varying the intensities of laser. The intensities can be classified for various machining tasks:
At low intensity of the order of (100 w/cm2), materials is heated to the martensic state and material remains in solid state.
At higher intensity, surface of materials is heated up, which relieves thermal stresses on the materials.
Further increase in intensity of laser, material get vaporises and resulting in deep welding effect.
Further increases lead to plasma formation which shields and reduces the intensity of laser.
The laser machining requires the selection of optimal machining parameters of job.
3.3 Machine system
The machine tool used in Deckel Maho Pfronten CNC milling machine which is equipped along with ND-YAG laser of wavelength 1064nm. This laser can be operated on two modes either continuous or Q-mode depending on the machining requirements. The laser head of machine system consists of following:
Tactile measuring system is used for process control.
Camera used for positioning the work piece correctly in zero position.
The above machine system is capable of doing both milling and laser ablation process. The following are the major parameters of the machine:
3.3.1 Resonance cavity
Fig 4 Schematic design of Resonance cavity
The resonant cavity consists of hollow space casing which consist of crystal made up of yttrium-aluminium-garnet doped with neodymium ions and a krypton arc lamp. The resonant cavities facilitate the reflection of beam. Before the cavity, reflecting mirror used for reflection of laser beam radiations, diaphragm used for laser mode vibration and Q-switch to enable the mode of operation (continuous or pulse mode). After beam gets reflected from the cavity, it travels through semi reflecting mirror and beam expander which reduces beam divergence.
3.3.2 Laser scanner
The laser beam from beam expander is deflected by two galvanometric mirrors which is NC control by means of control program. This control programs controls the scanning of beam on the work piece. This control programme consists of track displacement and z axis compensation in step by step.
3.4 Machining parameters
The number of process variable to be varied for obtaining the optimum parameter for the optimum output of the process.
The following are the major parameters in ND-YAG laser:
Lamp current (I)
Pulse repetition rate (fP)
Scanning speed (v)
Pulse diameter (D)
3.4.1 optimisation of pulse diameter(D)
Fig 5 Overlap Behaviour at 40Âµm Beam diameter
The above graph is plotted between Degree of overlap (Ud) in % and Pulse repetition rate ( fp) in kHz by varying the scanning speed (v) from 100 to 500 mm/s. From the graph it clearly shows that the degree of overlap is high while the scanning speed of 100mm/s and pulse repetition rate of 10 kHz. The pulse overlap is zero when scanning speed is increased to 400 mm/s for the same pulse repetition rate of 10 kHz. The degree of overlap depends on the scanning speed, pulse repetition rate and pulse diameter. Degree of overlap is governed by the equation:
Ud = [1 âˆ’ vs(fp D)âˆ’1]100%
3.4.2 optimization of surface finish
Surface finish (Ra) parameter is optimized by varying the scanning speed from 100 to 500 mm/s, pulse repetition rate from 10 to 50 kHz and current densities from 24 to 30 amps. From the graph surface finish is low by increasing the current densities and also by increasing the scanning speed and decreasing the pulse repetition rate. High surface of the around of less than 2Âµm is achieved by operating on the current density of 24 amps, scanning speed of about 300 mm/s and pulse repetition rate of 50 kHz, Insufficient pulse overlap which occurs on scanning speed of 500 mm/s and pulse repetition rate of 10 kHz also leads to poor surface finish.
3.4.3 optimization of material removal rate
The optimal material removal rate (as) is achieved by varying the current density from 24 to 30 amps, scanning speed from 100 to 500 mm/s, pulse repetition rate from 10 to 50 kHz. From the graph it depicts that by increasing the current densities material removal rate is increased. But removal rate is high when the pulse repetition rate is 18 kHz.
3.5 Machining time
The machining time of making die using LBM as follow:
3 hr 55 min
The table above shows the machining time of making embossing die from two different materials with different geometrical features.
4.0 Method 2
Laser Assisted Milling (LAM)
The working principle of LAM is Preheating of the work piece area to be machined by means of laser beam and then machining it by the milling cutter.
The advantage of this method over conventional machining is increase in material removal rate, Tool life and productivity.
In this method laser can be operated in two modes like continuous mode or pulsed mode. In continuous mode, there will be deep and wide heat affected zone, which alters the micro structure of the work piece. So, pulse laser is preferred. Among pulsed laser, short pulsed micro second (Âµs), nano second (ns) lasers also exist. It is chosen based on geometry of the work piece and material removal rate.
4.1 Properties of the pulse laser beam
Laser spot size and beam quality: The size of the laser determines the quality of the machining. If it is larger than the required size it generates excessive slopes in the sidewalls.
Peak power: The peak power should be less than melting point of the material. So, optimum value of laser intensity is applied to soften the material.
Pulse Duration: Pulse duration should be less than thermal relaxation time of the material. If it is more, than heat gets dissipated to the whole bulk of material other than localized area.
Pulse repetition rate: Higher pulse rate leads to higher the cutting feed rate of the milling cutter. If it is lower than required, heat is wasted to surrounding bulk of the material. So, higher pulse rate is recommended.
4.2 Experimental setup
Fig 6 Experimental set up of pulsed laser assisted micro milling
The setup consist of pulse laser, ultra precision frequency controlled high speed spindle operated at the range of 80,000 rev/min, milling cutter with 3.175mm shank size and as small as25Âµm in diameter. micromilling blind drilling, for all the three samples the
Fig 7 Experimental set up for micro end milling
The test bed integrated with laser system which controls the movement of work piece under the milling cutter. A CCD camera for positioning of work piece and monitoring of laser assisted machining operation.
The characteristics of pulsed laser system (Nd: YAG) are as follows:
â€¢ Laser wavelength: 1064 nm
â€¢ Focused laser spot area: 1 mm2
â€¢ Total energy at exit port: 485 mJ
â€¢ Total power at exit port: 4.85 Watts
â€¢ Peak power: 100-70 MW
â€¢ Pulse duration: 5-7 ns
â€¢ Pulse repetition rate: 10 Hz
4.3 Finite element model of laser assisted micro milling
The advantage of using FEM is to predict the chip flow, cutting force, temperature distribution on tool and work piece and their respective stresses. The Simulation software used is DEFORM 2D. In this material is removed by plastic deformation. John cook model is suitable for studying the plastic deformation analysis.
Generalised equation for yield stress derived by Cook method is:
Where A - yield strength of material at room temperature, á¼ - normalized strain rate with respect to reference stress rate. Temperature term in J-C model reduces the stress to zero when it reaches the melting point of material. J-C work model parameters for AISI 4340 steel and thermo mechanical properties for tool and work piece are as follows: The tool is made up of tungsten carbide with 6 - 8 % cobalt binder.
Fig 9 Johnson- Cook material model constraints
Fig 9 Thermo mechanical properties of work and tool materials
The experiment is conducted on the tool under the parameters of such as cutting speed is 80 m/min, 10Âµm feed per tooth, 0.635mm diameter of the tool, 3 Âµm tool edge radius. In the FEM model, friction factor of 0.65 is considered at the tool-chip interface.
Fig 8 FEM Simulation without laser Fig 9 FEM Simulation of milling of AISI 4340 using laser
From the above simulated model, temperature in the cutting zone is 100 to 150 C with out using laser and 350-400 C with the laser assistance. Reason is that material is preheated to 350 C with the help of laser beam for softening the material. This localized temperature in the cutting zone is very low compared to the conventional milling conditions.
is sensor delay for acoustic sensing is about 10ms to 30 s which depends on the frequency interval sampled. By implementation of fully developed dedicated hardware selective sampling (smaller frequency) & fast Fourier transform is possible. Because of this time delay between successive measurements can be reduced to below 1ms. Usually sensor models requires more time for numerical iterations, so it can be avoided.
4.4 Micro Milling Experiments
A two flute micro end mill with 0.635mm diameter is used with varying feed per tooth to get the effect of feed rate on various cutting force. These cutting forces are measured by piezo electric dynamometer and charge amplifier with an uncertainty of + 0.2 N. The forces have been recorded at 2667, 4000, 5333 Hz for spindle speed of 40,000, 60,000 and 80,000 rpm.
Feed and normal forces for 40,000, 60,000 and 80,000 rpm spindle speeds at varying feed per tooth (1.27Âµm, 2.54 Âµm, 5.08 Âµm) were measured.
Fig 10 Measured mean resultant forces at varying feed rate(Î¼m/tooth) and rpm in micro-end milling of AISI 4340 steel.
From the fig it shows that cutting force for pulsed laser assisted machining is larger than the unassisted machining at low feed rates and it is lower at higher feed rates. This is due to material hardening by PLAM at low feed rates and at higher feed rates laser assistance is much effective. In the both cases of machining, edge burr formation is the minor issue. It is overcomed by hand finishing.
5.0 Discussion from the two methods - Efficiency, Effectiveness & accuracy
From method 1
For laser drilling of acrylic materials results with 18% deviation from the theoretical analysis for blind drilling and for aluminium oxide and steel the theortical estimates 300% than actual, in case of blind drilling
Similarly the theoretical estimates shows 20% of optical measured value of hole depth for acrylic in case of through hole drilling and for aluminium oxide the theortical value over estimates by 230% than actual.
In general, for laser drilling of acrylic like materials usually have theortical results of 25% within experimental results.
The discrepancy and deviations in theortical and actual values was due to effect molten metal at erosion front (increased surface roughness at hole wall) and hole taper
With use of proper control scheme the entire system can be down to halt position once the operation is completed.
The use of magnetic field in laser drilling was discussed and hole depth regulation methodologies were derived with various control schemes. The method seems accepting the industrial requirements, such as the amount of material vaporized can be determined indirectly. The proper control over the process was possible and both manual drilling of individual holes and automated drilling with microprocessor and robotic movements is possible with the subject control scheme. Similar to case#1 the entire process can be halted by microprocessor once the operation was completed. Over all the accuracy and effectiveness seems increased than case#1, unless without the experiment results from case#2 the exact accuracy, effectiveness and efficiency cannot be quoted.