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Unit 11. Inspection Technologies Sections: 1. Unit Introduction 2. Unit Objectives 3. Contact vs. Non-contact Inspection Techniques 4. Conventional Measuring and Gauging Techniques 5. Co-ordinate Measuring Machines 6. Machine Vision 7. Other Optical Inspection Techniques 8. Non-contact Non-optical Inspection Technologies
Unit 11. Inspection Technologies Inspection is the means by which poor quality is detected and good quality is assured in products that are produced in a production process. The major steps in inspection include: Presentation of the item for inspection Examination of the item for non-conformance on certain product attributes Decision-making, based on the results of the examination Action
Definition Inspection is the process of presenting, examining, deciding-upon, and acting upon an item to ensure that poor quality is detected in product attributes, and good quality is assured.
Manual technos Various technologies support the inspection procedure, enabled by various sensors, instruments, and gauges Manually operated devices such as micrometers, callipers, protractors, and go/no-go gauges
CMM machines Various technologies support the inspection procedure, enabled by various sensors, instruments, and gauges Other techniques are based upon modern technologies such as co-ordinating measuring machines (CMM) and machine vision, which use computer-controlled systems that allow the inspection procedure to be automated
11.3.1 Contact Inspection Techniques In contact inspection, physical contact is made between the object to be inspected, and the measurement device. Typically contact is achieved using a mechanical probe or other device that touches the item, and allows the inspection procedure to occur § Conventional measuring and gauging instruments § Co-ordinate measuring machines (CMMs) and related techniques to measure mechanical dimensions § Stylus type surface texture measuring machines to measure surface characteristics such as roughness and waviness § Electrical contact probes for testing integrated circuits and printed circuit boards
11.3.1 Contact Inspection Techniques Physical contact occurs between the object to be inspected and the measurement device in contact inspection; this is typically done by means of a mechanical probe or other device that touches the item, which allows the inspection procedure to occur.
11.3.2 Non-contact Inspection Techniques Non-contact inspection techniques use sensors instead of a mechanized probe favoured by contact inspection methodologies. The sensor is located at a certain distance from the object to be inspected, to measure or gauge the desired features of the object. There are two categories of non-contact inspection technologies: Optical inspection technologies Non-optical inspection technologies: electrical fields, radiation, and ultra-sonics.
11.3.2 Non-contact Inspection Techniques The advantages of non-contact inspection techniques over contact inspection techniques include: They avoid possible surface damage that can be caused upon contact Inspection cycle times are faster as the contact probe must be re-positioned for each new part inspected, while the non-contact sensor remains stationary Parts handling is lower with non-contact inspection than with contact inspection, as parts in the latter methodology usually require special handling and adjustments so that inspection can occur It allows for the possibility of 100% automated inspection, for the above reasons
11.4 Conventional Measuring and Gauging Techniques Conventional measuring and gauging techniques use manually-operated devices to assess a host of linear dimensions, such as length, depth, and diameter, as well as features such as angles, straightness, roundness and so on Measuring devices are typically used for sampling inspection. Gauges are used for sampling and for 100% inspection. They are more portable than measuring devices, and can be used on the production line itself. Certain techniques of measuring and gauging can be automated to permit feedback control of the process, or for statistical process control.
11.4 Conventional Measuring and Gauging Techniques Electronic gauges deploy transducers capable of converting linear displacement into a proportional electrical signal, which in turn is amplified and transformed into a suitable data format such as a digital read-out. Advantages of electronic gauges include: good sensitivity, accuracy, precision, repeatability, and speed of response; ability to sense very small dimensions; ease of operation; reduced human error; ability to display electrical signal in various formats; and capability to be interfaced with computer systems for data processing.
11.5 Co-ordinate Measuring Machines In co-ordinate metrology the actual shape and dimensions of an item are measured, and compared against desired shape and dimensions, as might be specified on a part drawing. Co- ordinate measuring machines (CMM) is an electromechanical system that has been designed to evaluate relevant dimensions of an item against a required standard A co-ordinate measuring machine (CMM) is an electromechanical system that evaluates relevant dimensions of an item against a required standard. Probe head and probe to contact workpart surface Mechanical structure to provide motion of the probe in the three Cartesian axes, and displacement transducers to measure the co-ordinate values of each axis
11.5.1 CMM Construction The CMM construction consists of a worktable upon which the item to be inspected is placed, and, overhead, is the mechanical structure that supports the probe head and probe. Associated equipment such as the computer system are located remotely.
11.5.2 CMM Probe The tip of the probe in the CMM is usually a ruby ball. Ruby is a form of corundum (aluminium oxide) with high hardness for wear resistance, and low density for minimum inertia, thus making it ideal for probing applications. Probes can be single or multiple tip Probes with touch-trigger mechanisms are the most common probe type: these operate in a variety of ways.
11.5.4 CMM Operation and Programming In the case of CMMs using direct computer control with computer assisted data processing capabilities, two principle methods of programming may be outlined: Manual lead-through—where the operator leads the CMM probe through the various motions that are required of it for a particular workpart, while the points and locations are recorded into programme memory Off-line programming—where the programme is prepared based on the workpart drawing, and then downloaded to the CMM controller for execution
11.5.5 Other CMM Software In addition to CMM part programming software used for programming direct computer control CMMs, other CMM software may be used. These include software in the following categories: core software (other than programming software for direct computer control) post-inspection software reverse engineering and application-specific software.
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