A logical step after CNC machining is inspection. With inspections, Closed-Loop Machining (CLM) can be realized to maximize the efficiency of a machining process by maintaining a tight control in a manufacturing system. CLM is normally regarded as the highest level of CNC automation. CLM however, requires a tight integration between CAD, CAPP, CAM, and CNC, in particular CAM and CNC and inspections. The questions that are to be answered are (a) what type of inspections is fit for CLM and (b) is there a good data model that one can use to bring machining and inspections together? This chapter tries to provide some possible solutions to these questions. Prior to this, a brief review of the past research work is given. Toward the end of the chapter, a conceptual framework for integrating machining with inspections is presented.
Closed-Loop Machining And On-Machine Inspection
Inspection is an essential element in any closed-loop machining system. It can gather data to achieve precise measurement and monitor machine tool’s performance during machining operations. In most CLM systems, Coordinate Measuring Machine is widely employed as a way of collecting the measurement data from a machined part. However, there are problems in this type of CLM systems. These problems relate to time, data compatibility and data modelling.
Inspection using CMM is an off-line operation. It leads to increasement in machining cycle time due to relocating workpiece between machines and CMMs.
Inspection using CMM is typically a three stage activity: programming, execution of the program and evaluation of the results. These activities are often carried out on separate systems which cause complex interface problems.
The inspection results cannot be easily incorporated into the current NC programme, which is G-code-based. This is because G-code is a low-level programming language. Geometrical information about the part being machined and inspected is not preserved.
On-machine inspection (OMI) offers an attractive alternative solution. On-machine inspection enables measurement taking, data collection as well as data feedback and process adjustment fully automated and integrated. With on-machine inspections, a part can be measured at the machine and corrected there to avoid relocation between CNCs and CMMs. Small samples can be made and then checked immediately. Problems such as overcompensation and under-compensation can be identified at an early stage.
The benefits of CLM incorporating on-machine inspection are multiple (Jesse, 2001),
It can reduce the reject rate by proofing fixture and part setups, automating offset adjustments, and monitoring the machine’s “health” through machine self-checks;
It can reduce part cost by promoting tooling standardization and reducing operator intervention;
Inspection costs can be reduced through eliminating hard gauging, increasing the flexibility of measuring methods, avoiding cost of acquiring a CMM, and overcoming the CMM bottlenecking problems; and
Data about parts, processes and equipment collected can be used for real-time, adaptive control.
The common data model for OMI is G-code-based. Using G-code, inspection and machining operations are characterized by a complex sequence of manual and automated activities. The measurement results from inspections cannot be fed back to the CAM system directly. The STEP-NC (ISO 14649-1, 2004) data model in contrast, provides higher level information for manufacturing processes including the part geometry and tolerances, hence enables a bi-directional information flow. STEP-NC (ISO 14649-10, 2004) provides the control structures for the sequence of programme execution, mainly the sequence of Workingsteps and associated machine functions. The “machining_schema” entity contains the definition of data types which are generally relevant for different technologies (e.g. milling, turning and grinding) (ISO 14649-10, 2004). Probing Workingstep is also defined in this part for inspection operations. However, the probing operations are specified in ISO 14649-16 (2004). The dimensional inspection data model is specified in ISO 10303 AP219 (2007).Top
In the past forty to fifty years, research work has been carried out in all aspects of CLM and OMI such as different ways of carrying out inspection operations, condition monitoring and machine reliability. The intention has been to prevent the equipment from breaking down during production, and facilitate robust process. However, most of the research is based on data models using G-code for data exchange with CNCs. Therefore, they fell short of achieving integration of inspection operation and machining process in a seamless, integrated process chain. With design, manufacturing and inspection consolidated on one platform, i.e. STEP (and STEP-NC), the above mentioned problems can be solved. STEP and STEP-NC can now be used to realize a true CLM with bi-directional information flow, including feedback of inspection results to process planning.