UNIT II REVERSE ENGINEERING AND CAD MODELING: 10

Basic concept- Digitization techniques – Model Reconstruction – Data Processing for Rapid Prototyping: CAD model preparation, Data Requirements – geometric modeling techniques: Wire frame, surface and solid modeling – data formats - Data interfacing, Part orientation and support generation, Support structure design, Model Slicing and contour data organization, direct and adaptive slicing, Tool path generation.

Basic concept

• Reverse Engineering (RE) is the process of generating a Computer Aided Design (CAD) model from an existing physical part.

• It enables the reconstruction of an object by capturing the component's physical dimensions and geometrical features .

Reverse engineering, widely noted as an effective cost saving tool, is a

systematic approach used to analyze the dimensions, contours, and design of an existing device(product)

Today reverse engineering is used not as a tool to be employed to an existing problem, but as practical methodology to new challenges of unique parts (with no design or developing computer drawings/molds of parts and tools where none exist).

Need for Reverse Engineering

Manufacture of service components after discontinuation of the product line.

Redesign of an existing design which lacks adequate product data documentation

Corrupt data file or loss of CAD design for a product

Generation of cheaper alternate products as a substitute for monopoly products

In order to accurately recreate the existing part, a computerized (CAD)model of the part's geometry must be developed in some manner.

This CAD file provides the coordinates of multiple points on the product surface,used to develop the drawing of the product for redesign or manufacturing.

This data may then be analyzed and improved within the CAD program. The improved CAD design, along withother manufacturing information, is then used to create the manufacturingprocess plan and the Computer Numeric Control (CNC) tool path.

Digitization techniques

Non-contact reverse engineering digitization Techniques

Contact reverse engineering digitization Techniques

Contact methods, the more traditional manner of collecting data that has been utilized for several years, requires contact between the surface and a measuring device, usually a probe or stylus.

Contact methods generally measure the surface of the object using a contact probe, a highly sensitive pressure-sensing device that is activated by any contact with an object.

The linear distances from three axes to the position of the probe are find out, thus giving the x, y, and z coordinates of the surface.

Non-contactActive Technique - Laser ScanningPassive Technique -3D PhotogrammetryMedical Imaging

1. Magnetic Resonance Imaging2. Computed Tomography3. Ultrasound Scanning4. Medical Image Data File5. Three-Dimensional Reconstruction

Non-contact methods typically use light, or laser beams, as the main tool for deducing surface information.

Active1. structured lighting and2. spot ranging

Passive techniques1. stereo scanning, 2. range from texture, and 3. range from focus

Both methods cast a beam onto the object surface and then inspect the reflected beam using a sensor that is placed coaxial to the source.

The location of the source gives the two coordinate measurements of the surface point while the analysis of the reflected beam gives the third dimension. The third coordinate is determined either by

calculating the phase difference between the incident and reflected light or

by the time taken for the light to reflect back from the surface of the part.

Active1. structured lighting and2. spot ranging

Structured lighting methods are classified according to the pattern of light that is used to illuminate the object, such as single light beam, single stripe of light, and patterned lighting. Surface information is determined using triangulation procedures.

Spot ranging techniques are generalized based on the source used, either optical-based or ultrasonic.

These methods involve the projection of a beam onto the object surface and the inspection of the reflected beam using a sensor

Passive techniques, work with ambient light, and are divided into three categories:

1.stereo scanning, 2.range from texture, and 3.range from focus:

Stereo scanning is accomplished by acquiring two or more images of theobject from different perspectives.

The corresponding surface points onthe two images are identified, and a triangulation procedure is used toidentify the location of points

Range from texture processes are based on the fact that the further away from an object one is, the smoother its surface will appear to be. So if the texture is known, its distance from a known viewpoint can be estimated by inspecting the perceived texture at that distance. .

Range from focus techniques are based on using the focal length of a lens to estimate the part's distance.

The software for non-contact reverse engineering systems greatly simplify the data gathering.

Most parts are composed of standard shapes such as arcs, circles, spheres, and rods.

A menu of standard shapes is generally available so that a user can specify one of these shapes, measure several points, then have the software complete the shape.

Those shapes that are not standard can be digitized using one of several scan techniques in which the part is traced and the software is instructed to section the trace as parallel planes, radial sections, or concentric circles.

The best approach to digitizing a complex shape is to divide it into simple zones and use the appropriate scanner technique.

REVERSE ENGINEERING

Active Technique - Laser Scanning

. The laser scanner casts a beam onto the object surface and then inspects the reflected beam using a sensor that is placed coaxial to the source.

The location of the source gives the x and y coordinatemeasurements of the surface point while the analysis of the reflected beam by a triangulation procedure gives the third dimension

Horizontal distance (or "a) between the source and the sensor is known, and all three angles of the triangle are known, the depth measurement (or "b") is easily obtained.

The location of the source provides the horizontal and vertical coordinates of the illuminated point. thus creating the three-dimensional surface data.

Passive Technique - Three-DimensionalPhotogrammetryStereo scanning, a passive reverse engineering technique, is the

process of acquiring geometric data of an object by integrating multiple photographs of the object from different perspectives.

The primary challenge of this method is the identification of the matching image pixels that correspond to the same point on the object

Traditionally, this process has been used from large distances, for example to develop terrain maps. This is accomplished by taking multiple photographs of a terrain while flying in an airplane

Close range photogrammetry has taken hold in recent years as processingpower and digital imaging has improved. In this method, images are takenfrom positions all around the object Mathematical models have been devised to produce dimensional coordinates of discrete points on the object

Medical imagingMagnetic Resonance Imaging(MRI) creates images of

the body using the principles of nuclear magnetic resonance. MRI can generate thins section images of any part of the body-including organs, bones, and ligaments-from any angle and direction, without surgical invasion and in a relatively short period of time.

The principles of MRI take advantage of the random distribution of protons, the nuclei of abundant hydrogen atoms, which possess fundamental magnetic properties

Once the patient is placed in the cylindrical magnet, the diagnostic process follows three basic steps.

First, MRI creates a steady state within the body by placing the body in a steady magnetic field that can be over 30,000 times stronger than the earth's magnetic field. Then the MRI scanner stimulates the body with radio waves to change the steady-state orientation of the protons.

After terminating the radio signal, a short-wave radio antenna detects, at a pre-selected frequency, the electromagnetic transmissions emitted by the hydrogen nuclei. The transmitted signal is used to construct internal images of the body using principles similar to those developed by computed tomography, or CT scanners

Hydrogen proton nuclei are used because they are the most abundant nuclei in the body, with large concentration in water and lipid molecules.

After each radiofrequency pulse has been applied, the hydrogen atoms re-emit this energy as a magnetic resonance signal that induces a small voltage in a receiver coil. The hydrogen atoms then return to a relaxed state

Computed Tomography (CT) scanner

The Computed Tomography (CT) scanner, an intricate extension of the conventional X-ray device, offers clear views of any part of the anatomy, including soft organ tissues. The full body scanner rotates 180' around a patient's body, sending out a thin Xray beam at hundreds of different points.

Detectors, usually solid-state crystal or Xenon gas, positioned at the opposite points of the beam detect and record the absorption rates of the varying densities of tissue and bone. The density of the scanned object determines the quantity of the emitted X-ray beam that passes through the object, and thus the intensity of the grayscale image at that location.

Ultrasound Scanning (US)

Ultrasound is a sound wave having a frequency higher than the upperlimit of the human audible range, 20 kHz''.

Ultrasound Scanning (US) uses these high frequency sound waves to form an image of body tissues that can be viewed for medical diagnosis.

The sound waves are recorded and can be displayed as a real-time visual image or simply as a still image.

As the sound waves pass through the body, echoes are produced that can be used to identify how far away an object is, its size, and its uniformity

The ultrasound transducer functions as both a generator of sound and a detector. When the transducer is pressed against the skin, it directs the inaudible, high frequency sound waves into the body.

As the sound echoes from the body's fluids and tissues, the transducer records tiny changes in the pitch and direction of the sound. These echoes are instantly measured and interpreted by a computer, which in turn creates a real-time picture on the monitor.

CONTACT BASED Contact based methods of reverse engineering

have been available for nearly forty years. The first (and still the most popular) method of

reverse engineering, to be introduced was the coordinate measuring machine(CMM).

A Coordinate Measuring Machine gives physical representation of a three-dimensional Cartesian coordinate system.

The CMM measures the surface of the object using a contact probe, a highly sensitive pressure sensing device that is activated by any contact with an object.

The linear distances from three axes to the position of the probe are ascertained, thusgiving the x, y, and z coordinates of the surface.

Other contact methods of reverse engineering include electromagnetic digitizing and sonic digitizing'.

Electromagnetic digitizers determine the surface data of non-metallic objects (placed in a magnetic field) by tracing a hand-held stylus containing a magnetic field sensor across the surface of the object. The magnetic field sensor, in conjunction with an electronic unit, detects the position and orientation of the stylus.

The sonic digitizer uses sound waves to calculate the position of a point relative to a reference point.

Again, using a hand-held stylus tracing the surface, an ultrasonic impulse is emitted by the stylus and is detected by four microphones. The times taken to reach each of the four microphones are recorded and the computer calculates the x, y, and z coordinates from these time differences

Types of CMM ConfigurationsThe type of configuration for a CMM determines its

measuring parameters such as accuracy, flexibility, time or throughput of measuring process, maximum measurable work piece dimension and cost.

Most of these configurations can be controlled manually by an operator or by a program. An operator driven CMM is called a Manual CMM and the one that runs by a program is called a CNC CMM. CNC stands for Computer Numerical Control.

Types of CMM Configurations

1. Bridge Gantry type2. Cantilever Type3. Horizontal Arm Type4. Articulated Arm Type

Bridge Gantry type CMM

• In bridge style machines, the arm is suspended vertically from a horizontal beam that is supported by two vertical posts in a bridge arrangement. The machine x-axis carries the bridge, which spans the object to be measured.

• Given the rugged construction of this machine, it has higher natural frequencies which improve the dynamic response of the machine as compared to the overhanging cantilever type CMMs. This type of CMM can have a smaller footprint which is suitable for clear room or design laboratory type of facilities

A p p l i c a t i o n s

• Mechanical parts inspection• Digitalization and inspection of complex

mechanical components (gears, cams, airfoils of turbine blades)

• Free form surfaces inspection (dies, models, sheet metal, plastic,moulds)

• Point to point inspection• Continuous scanning inspection

GANTRY TYPE CMMThis class of machines is used for large part sizes which can span 4 meters or more

Gantry style machines employ a frame structure raised onside supports so as to span over the object to be measured or scanned. A horizontal beam traverses the length of the measured object

A measuring arm is mounted on this horizontal beam that moves along the width of the object being measured.

This type of machine has the best measuring volume to overall dimension ratio within CMMs.

• Applications• Inspection of large components (such as pipes,

pressure vessels,automobile frames)• Measurement of gages, and fixturing systems for heavy

and largecomplex parts• Shop-floor inspection equipment with high operational

safety

Cantilever Type CMM• In cantilever style machines, a vertical arm is

supported by a cantilevered support structure. This type of open configuration allows for easy operator access to the object being measured. Heavy parts can be measured by placing them on the fixed table.

• However, due to the overhanging cantilever structure it has a lower 'system natural frequency affecting the speed of measurement. This type of system is suitable for longitudinal parts that fit along the length of the table and have smaller dimension in the other two axes.

Applications• Marking-out on models, casts and sheets• Light milling operations• Copying of free form surfaces

HORIZONTAL ARM TYPE CMMHorizontal arm machines

are widely used in the automotive industry.

In this configuration the arm that supports the measuring probe is horizontally cantilevered from a movable vertical support

• As a result, this style is sometimes referred to as cantilever design. It is also available in dual arm configuration as shown in Figure . The overhanging arms limit the dynamic stiffness of the machine affecting speeds of measurements.

However, this error can be compensated with software correction

• Applications• Inspection of large dimension components in

one axis• Measurement of prismatic elements within

auto subassemblies• Verification of free form body contour

(automobile styling and aircraft aerofoil shape)

• An articulated arm configuration is used for portable, or tripod mounted Style machines. The articulating arm allows the probe to be placed in different directions with respect to the object being measured. These systems contain a series of counterbalanced six-degrees-of-freedom linkage arms as shown in Figure

Articulated Arm Type

Each of the arms is provided with precision rotary transducers that encode the rotary motion of the linkages and calculate coordinates in 3D space.

The measuring envelope of this type of system is spherical, enabling measurement of hard to reach locations within components.

Accuracy of measurements is largely affected by operator skill and is lower as compared to bridge style systems

Applications• Suitable for field use for wide range of

applications• Measurement of subassemblies within very

large systems (engine component within aircraft

• On-the-fly inspection of basic dimensions on hard to reach features on the part

• Continuous measurement of free form surfaces (auto styling, aero-wing aerofoil contour, etc.)

CMM Measurement process