Univerzalab.fs.uni-lj.si/kolt/LastedNet/Final Report.pdf · The principle behind the process for...

Univerza ν Ljubljani Fakulteta za strojništνo

Optodynamics and Laser Applications Page 1

INDEX Acknowledgements ........................................................................................................................................................... 2

1 Introduction .............................................................................................................................................................. 3

2 Theory ....................................................................................................................................................................... 4

2.1 Process Overview .............................................................................................................................................. 4

2.2 Process Mechanisms ......................................................................................................................................... 5

2.2.1 The Temperature Gradient Mechanism (TGM) ........................................................................................ 5

2.3 The Buckling and Shortening Mechanisms ....................................................................................................... 6

2.4 Materials ........................................................................................................................................................... 7

2.5 Shape acquisition .............................................................................................................................................. 8

2.5.1 Laser Triangulation .................................................................................................................................... 8

2.6 Laser Safety ..................................................................................................................................................... 14

2.6.1 The Dangers ............................................................................................................................................ 14

2.6.2 The Safety Limits ..................................................................................................................................... 15

2.6.3 Laser Classification .................................................................................................................................. 18

2.6.4 Typical Class 4 Safety Arrangements ...................................................................................................... 18

2.6.5 Where Are the Risks in a Properly Set Up Facility? ................................................................................. 20

2.6.6 Electrical Hazards .................................................................................................................................... 20

2.6.7 Fume Hazards .......................................................................................................................................... 20

2.6.8 Conclusions ............................................................................................................................................. 21

3 Practical case ........................................................................................................................................................... 22

3.1 Laser Forming Experimental Setup ................................................................................................................. 22

3.2 Shape Measurement System .......................................................................................................................... 23

3.3 User Interface .................................................................................................................................................. 24

3.4 How to start the experience ........................................................................................................................... 25

3.5 Laser Forming Procedure ................................................................................................................................ 27

3.5.1 Selected work‐piece ................................................................................................................................ 27

3.5.2 Move work‐piece to the starting point ................................................................................................... 27

3.5.3 Processing ............................................................................................................................................... 28

3.5.4 Measurement .......................................................................................................................................... 31

4 Results ..................................................................................................................................................................... 32

5 Conclusions ............................................................................................................................................................. 37

6 Suggestions ............................................................................................................................................................. 38

7 References .............................................................................................................................................................. 39



ACKNOWLEDGEMENTS

We want to express appreciation to all staff from the Department of Optodynamics and Laser

Applications for their extended support and especially to Professor Janez Diaci and Assistant Drago Bračun

for all patience and knowledge.



1 INTRODUCTION

Laser forming has become a viable process for the shaping of metallic components, as a means of

rapid prototyping and of adjusting and aligning. The laser forming process is of significant value to

industries that previously relied on expensive stamping dies and presses for prototype evaluations. Relevant

industry sectors include aerospace, automotive, and microelectronics. In contrast with conventional forming

techniques this method requires no mechanical contact and hence offers many of the advantages of process

flexibility associated with other laser manufacturing techniques such as laser cutting and marking [1]. Laser

forming can produce metallic, predetermined shapes with minimal distortion.

The process is similar to the well established torch flame bending used on large sheet material in the

ship building industry but a great deal more control of the final product can be achieved [2]. The laser

forming process is realized by introducing thermal stresses into the surface of a work piece. These internal

stresses induce plastic strains, bending or shortening the material, or result in a local elastic plastic buckling

of the work piece depending on the mechanism active [3]. The range of metals that can be laser formed is

considerable. As there is only localized heating involved below the melting temperature the bulk properties

are not altered and good metallurgy is retained in the irradiated area [4].

Materials of particular interest are specialist high strength alloys [5]. These include titanium and

aluminium alloys. These materials are widely used in the aerospace industry where the implementation of

laser bending as a replacement of existing manufacturing processes is under investigation [6] as well as

other industry areas [7].

There are numerous application areas of the laser forming process in use today that are considered

cost effective in the design and/or manufacturing process, these include:

• Rapid prototyping of irregular shapes for turbine blade applications

• Non-contact forming for installation and adjustment of non-accessible parts

• Automotive shapes for prototype and validation testing

• Aerospace shapes for precision shaping of tanks and pressure vessels

• Unbending techniques for repairs and alignment applications

• Tube and pipe precision forming.

• Final configuration production parts for small quantities of parts.

• Rapid prototyping of parts prior to final production.



2 THEORY

2.1 PROCESS OVERVIEW

The laser-forming process is realized by introducing thermal stresses (without melting) into the

surface of a work-piece with a high-power laser beam. These internal stresses induce plastic strains that

bend the material or result in local elastic/plastic buckling. The laser-forming process is principally used at

the macro level to form metallic sheet material.

Current research at the University of Liverpool (UK) and other research groups has shown that it can

produce parts with 2-D bends or 3-D formed parts. Research is also ongoing on the use of the process to

remove unwanted distortion in conventionally formed parts, as well as the distortion due to welding

operations. (In addition, research is ongoing in the use of lower-powered lasers to align micro-electronic

components and actuators.)

The principle behind the process for sheetmetal uses a laser beam that's guided across the sheet

surface (shown in figure 1). The path of the laser is dependent on the desired forming result. In the simplest

case it may be a point; in other cases it may be a straight line across the whole part. And, for spatially

formed parts and extrusions, the paths may be very sophisticated radial and tangential lines.

Fig.1 – Laser beam guided across the sheet metal



2.2 PROCESS MECHANISMS

There are several distinct mechanisms of laser forming, depending on the process setup. The

significant process variables (of many) that determine the active mechanism are the traverse speed, laser

spot diameter, incident laser power and absorption coefficient. There are three main mechanisms for the

laser forming of sheet, tubes and extrusions; these are outlined in figure 2.

Fig.2 - Schematic of the Three Main Laser Forming Mechanisms [8]

2.2.1 THE TEMPERATURE GRADIENT MECHANISM (TGM)

This mechanism is the most widely reported, and can be used to bend sheet material out of plane

towards the laser. The conditions for the temperature gradient mechanism are energy parameters that lead to

a steep temperature gradient across the sheet thickness (figure 2). This results in a differential thermal

expansion through the thickness. The beam diameter is typically of the same order as the sheet thickness but

can be larger. The path feed rate has to be chosen to be large enough that a steep temperature gradient can be

maintained. The feed rate/temperature gradient has to be increased if materials are used which have a high

thermal conductivity. The laser path on the sheet surface is typically a straight line across the whole sheet.

This straight line coincides with the bending edge. Initially the sheet bends in the direction away from the

laser. This is called counter bending.



With continued heating the bending moment of the sheet opposes the counter bending and the

mechanical properties of the material are reduced. Once the thermal stress reaches the temperature

dependent yield stress any further thermal expansion is converted into plastic compression. During cooling

the material contracts again in the upper layers, and because it has been compressed, there is a local

shortening of the upper layers of the sheet and the sheet bends towards the laser beam. The yield stress and

Young’s modulus return to a much higher level during this cooling phase and little plastic re-straining

occurs. Bends of approximately one or two degrees per pass are achieved with this mechanism [8, 9].

2.3 THE BUCKLING AND SHORTENING MECHANISMS

Both of these mechanisms are activated by the use of laser parameters that do not yield a temperature

gradient in the depth of the material (figure 2). For the case of the buckling mechanism a beam diameter

much larger than the sheet thickness and a slow traverse speed is used. This results in a large amount of

thermoelastic strain that in turn results in a local thermoelastic-plastic buckling of the material. The buckle is

traversed along the length of the sample and once the buckle reaches the exiting edge of the sheet the elastic

strain dissipates and the remaining plastic strain causes a deflection. This mechanism can be used for out of

plane bending of sheet material, it may be accompanied by some in plane shrinkage as well. The part can be

made to bend in either the positive or negative directions.

The direction depends on a number factors including the pre-bending orientation of the sheet, pre-

existing residual stresses and the direction in which any other elastic stresses are applied, (for example a

forced air stream acting on the bottom of the sheet.) The buckling mechanism results typically in bending

angles between 1 and 15 degrees. This is significantly larger than observed for the temperature gradient

mechanism. This is not a result of a higher degree of performance but a result of the fact that using the

buckling mechanism more energy can be coupled into the workpiece in one step. For the shortening or

upsetting mechanism the geometry of a workpiece would prevent buckling due to the increased moment of

inertia compared to sheet material.

This mechanism is used to shorten or upset a workpiece in plane it may be used in different ways for a

wide range of forming results such as the bending of extrusions and tubes. By the careful selection of the

sequence of the sides of the geometry heated, a section can be made to step out of plane. The mechanism can

also be used for the shortening of small frames. This is useful for aligning operations in micro parts

production.



The mechanisms of laser forming can accompany each other to some extent because there is a

transition region of processing parameters and geometries where a switch from one mechanism to another

takes place. Additionally there is usually a coupling between in plane and out of plane deformation in

forming operations, for 3D forming operations the interactions become very complex indeed [8, 9].

2.4 MATERIALS

The range of metals that can be laser formed is considerable, as there's only localized heating

involved, below the melting temperature. The bulk properties aren't altered and good metallurgical

properties are retained in the irradiated area. Materials of particular interest are specialty high-strength

alloys, including titanium and aluminum alloys. These materials are widely used in the aerospace industry,

where the implementation of laser bending as a replacement for existing low-volume manufacturing

processes is under investigation, as well as in other industry areas where inexpensive prototype evaluation

parts prior to die manufacture would be useful.

Due to the progressive nature of the process, high accuracy can be achieved. In addition, small bends

can be produced in parts that wouldn't be possible with conventional techniques, due to the springback of the

material.

Two examples of laser formed parts produced at the University of Liverpool are shown in Figure 3.

The part on the left was formed from a 250 x 100 mm, 2-mm gauge aluminium blank; the other was from

400 x 200 mm, 1.5-mm gauge mild steel. Both were formed from flat sheet by a 1.5kW CO2 laser with no

additional tooling or restraint, save the attachment to the CNC bed.

Fig.3 – Laser formed parts



2.5 SHAPE ACQUISITION

2.5.1 LASER TRIANGULATION

Triangulation is a commonly used technique for determining spatial relations. It has been utilized in

areas as cartography and the Global Positioning System (GPS). Laser Triangulation is an Active1 Structured

Light (ASL) technique, in which a laser dot is observed (through a lens) by a sensor.

Fig.4 – Laser Triangulation

The position of the laser spot on the sensor is related to the position of the surface (along the beam)

by using triangulation [10, 11].

Fig.5 – Laser Triangulation

1 The system is said to be active as the geometry of the laser beam is used in the measurement and so need to be known [15].



The triangulation angle (and the lens) can be used to achieve the required balance of range and

accuracy [10]. In [13] precisely controlled mirrors are used to control the base line for the triangulation. As

the laser is positioned at a non-zero angle, relative to the sensor, it is possible for part of the surface to

obscure the camera’s view of the line (generated by the laser). This is less prevalent at lower angles [13],

however the accuracy is also reduced, which limits the distance over which triangulation can be used [14].

Triangulation is capable of providing high (sub-micron) accuracy over short ranges [10, 14].

As the triangulation can be implemented with an analogue photo-sensor extremely high data (and

hence measurement) rates are possible, 200K measurements/sec [12, 10]. The use of pixelised (array)

sensors allow errors, such as multiple reflections, to be detected [12, 10], however this does reduce speed.

Laser Triangulation, presented previously, can be easily extended to take a line of measurements in

parallel. This is achieved by replacing the laser point generator with a laser line generator, which produces a

line on the surface [13]. Multiple sensors, stacked upon each other, are then required to take readings along

the length of the line. It is often simpler, and cheaper, to use a standard off-theshelf CCD camera, the rows

of which represent the different sensors in the stack. The CCD array also acts as a pixelised sensor, which

allows multiple bright spots to be detected and the erroneous data to be discarded.

Fig.6 – Laser Line Triangulation

This suffers from the problems of occlusion [13] reduces the angle (or applies the light from the

opposite side of the camera) to overcome this, [17] uses a pair of light sources (both of which are visible in

the image), to minimize its effect. Others [20] use multiple cameras to minimize the problem, and allow

noisy readings to be removed [21], however this has added problems as the relative positions of each of the

cameras must be determined [13].



As the triangulation angle is never zero, it is always possible for occlusion to occur. Using off-the-

shelf components it is possible to achieve sub-millimeter (0.15mm) accuracy [20]. The structured light

pattern used for the triangulation can be generated in a number of ways, [17, 18] uses a while light strip to

allow eye safe operation. [13, 16, 20] uses commercially available laser line generators. Some researchers

have extended this idea to use the edge of a shadow for triangulation, as it can be easier to detect the

transition, from light to dark, than to detect the centre of a narrow strip [18, 19].

Cyberware (www.cyberware.com), BIRIS (National Research Council of Canada) and PRIME

(California Polytechnic) are examples of commercially available systems that utilize this technique.

2.5.1.1 LIMITATIONS

There are a number of conditions under which Laser Triangulation suffers from reduced accuracy, or

even fails completely. These are explained and their effects on accuracy mentioned. Also, wherever

possible, a solution is presented.

2.5.1.1.1 ADVERSE ILLUMINATION

When the surface is illuminated strongly, particularly when it is bright in colour, the contrast of the

image is greatly reduced. This results in a bad separation between the laser line, and the background colour

of the surface, hence making detection difficult. The images below show the difference between a laser strip

shining on a dark surface, and an illuminated light coloured one. In the illuminated one, it is clear that the

laser beam becomes hard to distinguish (due to lighting) and scattered (due to light coloured surface).

Fig.7 – Adverse Illumination



This situation can present itself when working in strongly illuminated areas or with very lightly

coloured materials. The problems incurred by the light coloured surface can usually be remedied by

narrowing the aperture of the lens, hence reducing the scatter. If incident light a factor, then a filter may

alleviate the problem. The filter only allows a narrow band of wavelengths (i.e. those produced by the laser)

to pass into the camera sensor, hence reducing the ambient lighting effects. However it should be noted that

many forms of light, sunlight for instance, contains a full range of wavelengths. This could reduce the

effectiveness of the filter, or even render it ineffective.

2.5.1.1.2 OCCLUSION

A major problem in all triangulation methods is occlusion. The problem manifests itself as one of the

two following cases, see Figure 8.Either the laser light does not reach the area seen by the camera (laser

occlusion) or the camera does not see the area reached by the laser (camera occlusion). In both cases the

maximum reflection peak found in the sensor row data is the result of noise and/ or ambient light.

Fig.8 - Laser (left) and camera (right) occlusion

Both laser and camera occlusion can be minimized by careful placement of the laser and sensor.

Laser occlusion is avoided by assuring that the laser reaches all areas seen by the sensor. Ideally, the

baseline should be small so that the sensor and the laser can be considered as being in the same plane. Laser

occlusion is then avoided by ensuring that the optical center of the laser lens is further away from the scene

than the optical center of the sensor system, see Figure 9. Here the divergent sheet-of-light reaches all areas

seen by the sensor since for any occluding object edge the area not illuminated by the laser is smaller than

the area not seen by the sensor. Laser occlusion can also be avoided with multiple laser sources each

illuminating the scene a little from the side, see Figure 10.



Fig.9 – Laser Occlusion Avoidance

Sensor occlusion occurs when the baseline increases from the ideal zero value. This can only be

avoided using multiple sensors as seen in Figure 11. If two sensors are used, each viewing with a baseline B,

but from separate sides of the laser, most of the sensor occlusion is avoided. The only remaining sensor

occlusion comes from deep holes where both sensors are occluded.

Fig.10 - Laser occlusion avoidance using two lasers Fig.11 - Sensor occlusion avoidance using two sensors



2.5.1.1.3 REFLECTION

When working with surfaces, which are highly reflective or which contain pieces of highly reflective

material, it is possible for the beam to bounce off one part of the object and illuminate another part. This

causes real problems, as the secondary illumination doesn’t lie on the laser plane, which results in major

errors in the readings. In some cases it may be possible to remove the problems of reflection, by coating the

object with a temporary paint [21, 22].

Fig.12 - Reflection



2.6 LASER SAFETY

2.6.1 THE DANGERS

All energy is dangerous, even gaining potential energy walking up stairs is dangerous! The laser is

no exception, but it poses an unfamiliar hazard in the form of an optical beam. Fortunately, to date, the

accident record for lasers is very good, but there have been accidents. The risk is reduced if the danger is

perceived.

The main dangers from a laser are:

1. Damage to the eye.

2. Damage to the skin.

3. Electrical hazards.

4. Hazards from fume.

These risks can be made safe by following standards which have been laid down by various

authorities. The principle standards at present are:

1. American National Standard Institute (ANSI z136.1 (1986))

2. Federal Laser Product Performance Standard (FLPPS CFR 50 (161) 33682-33702 (1985))

3. British Standards Institute (BSI 76/31221 DC)

4. British Standards Institute (BSI 4803 (1983))

5. International European Convention (IEC 825 (1984))

These standards give guidance and rules concerning: engineering controls, advice on personal protective

equipment, administrative and procedural controls and special controls. Class 4 laser installations, which is

nearly all material processing systems, should also have a laser safety officer (LSO) who should see that

these guidelines are observed.

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the eye can tolerate. For example a 1 mW He/Ne laser with a 3 mm diameter beam would have a power

density in the beam of (0,001 x 4)/(3,14 x 0,3 x 0,3) = 0,014 W/cm2. On the retina this would be 0,014 x 105

W/cm2.

TABLE 1 - Basic Laser Biological Hazards

Laser Type Wavelength [ ] Biological Effects Skin Cornea Lens Retina

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KrF 0,254 photochem X X

** Insufficient power

A blink reflex at this level would only allow a 0.25s exposure, which is the MPE level for a class 2 laser.

Notice that the calculation assumes that all the radiation can enter the pupil of the eye. Thus it is common

practice to ensure that working areas around lasers are painted with light colors and are brightly illuminated

- not so with holographic laboratories and others involved with photography, of course.

The hazard zone around a laser is that in which radiant intensities exceed the MPE level. These zones

are known as the Nominal Hazard Zone (1). The size of the zone can be calculated based upon the beam

expansion from the cavity, or lens, or fibre, or from diffuse or specular reflection from a workpiece. For

example, consider a 2 kW CO2 laser beam with a 1 mrad divergence. The MPE level for safe direct

continuous viewing of the beam (not that much would be seen with IR radiation) is when the level falls to

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2.6.3 LASER CLASSIFICATION

Lasers are classified in BSI4803 1983 and IEC 825 1984 according to their relative hazard. All lasers

of interest to material processing will be classified as class 4 except some which are totally built into a

machine in which there is no human access possible without the machine being switched off. Table 2 is a

summary of the classification:

TABLE 2 – Classification of Lasers

Class Definition

1 Intrinsically safe

< 0,2 J in 1ns pulse or < 0,7 mJ in a 1s pulse

2 Eye protection archived by blink reflex (0,25s)

< 1 mW CW laser

3A Protection by blink and beam size

< 5 mW with 25W/m2 (e.g. an 16 mm beam diameter from a 5 mW laser)

3B Possible view diffuse reflection

< 2,4 mJ for 1ns pulse or < 0,5W CW visible

4 All lasers of higher power

Unsafe to view directly, or diffuse reflection

May cause fire

Standard safety precautions must be observed

2.6.4 TYPICAL CLASS 4 SAFETY ARRANGEMENTS

The following precautions are advised:

• All beam paths must be terminated with material capable of withstanding the beam for several

minutes.

• Stray specular reflections must be contained.

• All personnel in Nominal Hazard Zone must wear safety goggles.

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2.6.5 WHERE ARE THE RISKS IN A PROPERLY SET UP FACILITY?

If the facility is properly designed then the beam is enclosed and all beam paths terminated so that

the beam cannot escape to do damage. A standard set up would have the beam focussed and pointing

downward.

As the beam expands after the lens the NHZ is considerably reduced. For robotic beam steering

systems this NHZ is the distance to which the operators can approach the robot. This system is thus safe

except in certain unlikely events. These events can be classified in a risk analysis tree. They would include

breaking of the lens and total removal with loss of the nozzle, failure of a mirror mount and the mirror

swinging free etc. The essence of such an analysis is to devise a system for rapidly identifying an errant

beam. This can be achieved by beam monitoring and/or enclosure monitoring. If the beam is monitored as

leaving the laser but not arriving at the expected target then the system should immediately shut down. If a

hot spot appears within the enclosure then again the system could shut down.

2.6.6 ELECTRICAL HAZARDS

Nearly all the serious or fatal accidents with lasers have been to do with the electric supply. A typical

CO2 laser may have a power supply capable of firing the tubes with 30000 Volts with 400mA. This is a

dangerous power supply and when working on it the standard procedures for electric supplies should be

followed. The smoothing circuits contain large capacitors and so even when the power is switched off a fatal

charge is still available and proper precautions to earth the system before working on it are essential. Panic

buttons must be available at the laser and at the main exit. Access to the high tension circuit should be

protected by interlocks.

As a general rule: “Do not enter the high voltage supplies without first carefully earthing the system.”

2.6.7 FUME HAZARDS

The very high temperatures associated with laser processing are able to volatilize most materials and

thus form a fine fume, some of which can be poisonous. With organic materials, in particular, the plasma

acts as a sort of dice shaker and a wide variety of radical groups may reform into new chemicals. Some of

these chemicals are highly dangerous such as the cyanides and some are potential carcinogens.



It is necessary, as a general rule, to have a well ventilated area around the laser processing position as

for standard welding. Some of the problems with cutting non metallic materials have been identified by Ball

et al (24).

These are shown in Table 3 but it should be remembered that the volumes/s are not very large and

represent a hazard only if much work is being done over an extended time.

As a general rule: “The laser processing zone should be adequately ventilated".

2.6.8 CONCLUSIONS

The laser is as safe as any other high energy tool and should be properly handled. It is the

responsibility of the user to learn how to handle it correctly.

TABLE 3 – Main Decomposition Products from Laser Cut Non-Metallic Materials

Decomposition Products

Material

Polyester Leather PVC Kevlar Kevlar/Epoxy

Acetylene 0,3-0,9 4,0 0,1-0,2 0,5 1,0

Carbon monoxide 1,4-4,8 6,7 0,5-0,6 3,7 5,0

Hydrogen chloride 9,7-10,9

Hydrogen cyanide 1,0 1,3

Benzene 3,0-7,2 2,2 1,0-1,5 4,8 1,8

Nitride dioxide 0,6 0,5

Phenyl acetylene 0,2-0,4 0,1

Styrene 0,1-1,1 0,3 0,05 0,3

Toluene 0,3-0,9 0,1 0,06 0,2 0,2



3 PRACTICAL CASE

3.1 LASER FORMING EXPERIMENTAL SETUP

In the experimental setup it was integrated two subsystems: a processing system for LF and a

measurement system for laser based shape acquisition. This particular design allows an interactive study of

the LF process: can vary process parameters and monitor the process outcome (the change of the work-piece

shape).

LF experiments are carried out on thin metallic sheet samples. (20x50mm, thickness 0.1-

0.5mm) The laser beam is focused on the sample surface by a redesigned laser marking scan head. The

sample is moved relative to the laser beam by use of the linear translation stages.

The LF system employs a diode pumped Nd-YAG laser (Photon Energy DL8 Aries 1) operating at

1064nm wavelength and a maximum average power of 7W. The laser can operate in CW or in pulsed mode.

The minimum spot size of the single mode laser beam is 40 micrometers. The spot size is manually

adjustable.



3.2 SHAPE MEASUREMENT SYSTEM

Contactless measurement of three-dimensional (3D) work-piece shape bases on laser triangulation. It

is a form of active optical 3D shape measurement where the measured object is illuminated with structured

laser light [25-29]. The image of the illuminated surface is acquired at the triangulation angle with respect to

the direction of illumination. 3D shape is determined from a distorted light pattern on the image. Application

of structured laser light in active triangulation systems offers several advantages.

Because of physical properties of the laser light, such as small divergence and monochromaticity, the

laser beam can be focused onto small areas, offering the possibility of high-resolution measurements. In

addition, high contrast between illuminated and non-illuminated areas of the measured surface can be

achieved by using band pass filters. The time required for measurement (not for image processing) could be

as short as the image acquisition time, which can be less than 1/100 s.

The measuring field of the triangulation sensor is 32 mm wide (along light profile on the sample

surface) and 10 mm height (perpendicular to the sample surface). Each measured profile consist of 640

measured points. The accuracy of the point measurement is 0.03 mm (2σ). The minimal distance

between two parallely measured profiles is 0.01mm (minimal displacement of the translation stages).



Fig.17 - Measured profile (in mm) of the metallic sheet sample after laser forming.

3.3 USER INTERFACE

Trainees direct and monitor the process through a computer interface, which enables variation of

process parameters, acquisition of work-piece shape and visual monitoring of the processing area through

video camera. Experiment runs on LabView platform.

Fig.18 - Visual monitoring of the processing area through the video camera



3.4 HOW TO START THE EXPERIENCE

• First step is contact the laboratory staff to prepare experiment, say witch type of experiment it will be

done, the parameters chosed for the Nd-YAG laser and the measures of the samples. They can be

contacted by the followed email address: [email protected]

• Then ensure that you have installed Java Runtime environment, it can be download at:

http://www.java.com/en/download/index.jsp

• Use Microsoft Internet Explorer version at least 5.0.

• Visual inspection of the experiment is available by IP camera (http://193.2.79.235/), Camera

username is: Guest; Password is: guest.



After you see the user interface on the screen, click with the right button of the mouse over it and select “Request Control of VI”.

If all Ok, you get the message “Control Granted”

At the end, experiment MUST be stopped by pressing button STOP.



3.5 LASER FORMING PROCEDURE

3.5.1 SELECTED WORKPIECE

In this experiment it was used two types of samples, for study the influence of the path feed-rate

in the bending angle the samples have 20x50x0.15 mm. For study the influence of the feed number in

the bending angle the samples have 20x50x0.12 mm.

3.5.2 MOVE WORKPIECE TO THE STARTING POINT



3.5.3 PROCESSING

3.5.3.1 CHOSE THE STARTING POINT FOR PROCESSING

3.5.3.2 SETUP THE PROCESSING PARAMETERS

3.5.3.2.1 STAGE SHIFT



3.5.3.2.2 STAGE SPEED

For the first study it was used four values for the speed (path feed-rate), this values were 1, 2, 3 and 4

mm/s for each group of 3 samples. In the second study the speed was constant for all samples with the value

of 1 mm/s. These values were changed in the user interface (next figures).



3.5.3.2.3 NUMBER OF TRANSITIONS

In the first study the feed number was constant for all samples with the value of 8. For the second

study it was used four values for number of transitions (feed number), this values were 2, 4, 6 and 8 for each

group of 3 samples. These values were changed in the user interface (next figures).



3.5.4 MEASUREMENT

3.5.4.1 MEASURE 2D PROFILE

3.5.4.2 SAVE PROFILE



4 RESULTS

For the analysis of the results it was made a code in Matlab, the code is listed above.

clf clear [fn,fp]=uigetfile('*.*','Select profile'); if isequal(fn,0) || isequal(fp,0) disp('User pressed cancel') else disp(['User selected ', fullfile(fp, fn)]) end tfilename = strcat(fp, fn); sample = load(tfilename, '-ascii'); x=sample(:,1); y=sample(:,2); m=length(x); x1=zeros(m,1); y1=zeros(m,1); poz=1; for i=1:m if( (x(i)~= 0) && (y(i)~=0)) x1(poz)=x(i); y1(poz)=y(i); poz=poz+1; end end x=x1; y=y1; plot(x,y) hold a=1; b=fir1(30,0.5); yf=filtfilt(b,a,y); plot(x,yf,'m') y=yf; axis([-15,15,280,315]) npts=4; [xs,ys]=ginput(npts); index=zeros(1,npts);



for i=1:npts dx=x-xs(i); %dx=vector [minv,index(i)]=min(abs(dx)); end L1=polyfit(x(index(1):index(2)),y(index(1):index(2)),1); L2=polyfit(x(index(3):index(4)),y(index(3):index(4)),1); vx=[min(x),max(x)]; y1=polyval(L1,vx); y2=polyval(L2,vx); plot(vx,y1,'r'); plot(vx,y2,'g'); angle=(abs(atan( L1(1)-L2(1) ) ) ) *180/pi; title(['angle=',num2str(angle)]) hold off

This code is useful to determinate the bending angle of the samples. The code will import the files

created in the LabView platform to Matlab and will reduce the noise (with a FIR low pass filter) of the

values obtained in the measurement.



Then select four points in the image that will be created, with the four points the code will create two

lines. The angle is calculated between these two lines.



The values of the bending angles obtained on the first part of the experience are showed in the next table and

figure:

TABLE 4 - Influence of the path feed-rate on the bending angle

Sample Number

Velocity /

1,0 2,0 3,0 4,0

Angle Degrees °

1

2

3

26,87

27,32

25,72

13,42

15,00

14,14

10,14

10,72

11,05

6,15

7,63

5,63

Mean 26,6 14,2 10,6 6,47

Fig.19 – Changes in the bending angle with the path feed-rate 50 , 20 , 0.15 ,

8, 4.7 68% , 30 , 30

0

5

10

15

20

25

30

1 2 3 4

Angle α

(º)

Velocity (mm/s)



The values of the bending angles obtained on the second part of the experience are showed in the next table

and figure:

TABLE 4 - Influence of the feed number on the bending angle

Sample Number

Fee Number

2 4 6 8

Angle Degrees °

1

2

3

18,78

19,34

19,46

32,90

33,04

32,22

42,73

42,63

41,43

61,54

61,65

61,43

Mean 19,19 32,72 42,26 61,54

Fig.20 – Changes in the bending angle with the path feed-rate 50 , 20 , 0.12 ,

1 / , 4.8 67% , 30 , 30

0

10

20

30

40

50

60

70

2 4 6 8

Angle α

(º)

n



5 CONCLUSIONS

Effect of path feed-rate

The figure 19 shows the relationship between the path feed-rate and the bending angle for a given set

of conditions. It can be seen that the bending angle decreases with increase in the path feed-rate. The

relationship also appears to approximate to a straight line.

Effect of feed number

The figure 20 shows the bending angle by the repetition of laser feed, from which it can be seen that

the bending angle is approximately in direct proportion to the laser feed number.

The results of the influence of these parameters in the bending angle here the expected when

compared with the ones in literature [30], so it can be said that this was a successful experiment and that this

parameters (path feed-rate and number of feed) are very important in the laser forming of materials.

Other experiments can be made with this long range interactive system, like for example study the

influence of others laser energy parameters (laser power and beam spot diameter); material parameters; and

sheet geometric parameters (sheet length, sheet thickness, and sheet with).



6 SUGGESTIONS

The user interface provide a good and easy interaction with the laser mechanism and the camera

proved that can give a good image and movement almost in real time, this is very important for who’s

making the processing of the experiment in a long distance. It’s recommended to use two monitors, one for

the user see and interacts with the interface and other to put the image of live camera in full screen. This will

permit a good view of what is happening in the experiment processing.

This is a long range method, you can use it in any part of the world so it’s recommended the use of a

good internet connection that can provide always the control of the VI. However if for some reason the

connection fails it’s always possible and easy to gain the control of the VI again and continue with the

experiment processing.

The results of the measurement can be saved on your own computer and then they can be analyzed

using the Matlab code, this code was only tested in this study so it´s probable that it will need some

improvements to give more accuracy to the results.



7 REFERENCES

[1] Magee, J.; Watkins, K. G.; Steen, W. M.: Advances in Laser Forming. In: Journal of Laser Applications,

December 1998, Vol. 10 No. 6, 235-246

[2] Moshaiov, A.; Vorus, W.: The Mechanics of the Flame Bending Process: Theory and Applications. In:

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and Active 3-D Vision Systems. Institute for Information Technology, National Research Council, Ottawwa,

Ontario, Canada K1A 0R6, June 1995.

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Imaging Systems for Remote Sites. Institute for Information Technology, National Research Council,

Ottawa, Ontario, Canada K1A 0R6.

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to Effect the Automatic Recovery of Facial Shape. Irish Machine Vision and Image Processing Joint

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[22] E. Trucco, R. B. Fiser, and A.W. Fitzgibbon, Data Calibration and Data Consistencyin 3-D Laser

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[24] Ball et al. Industrial Laser Annual Handbook 1989 publ Penwell books, Tulsa, Oklahoma,USA p 23.

[25] Drago Bračun, Matija Jezeršek and Janez Diaci: Triangulation model taking into account light sheet

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[29] Donges A., Noll R.: Lasermeßtechnik, Grundlagen und Anwendungen, Heidelberg: Hüthig, (1993).

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Materials Processing Technology, 110 (2001) 160-163.

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