In-process inspection is the process of investigating a produced feature within the same process flow, directly after the feature has been machined. In-process inspection is carried out on the same machine and in the same setup which is used for producing the feature. This process consists of a visual inspection to check the surface finish as well as a dimensional measurement of the feature. The visual inspection can be carried out by means of the operator inspecting the produced feature visually or by feel as appropriate. This would be to check for any marks, indentation or similar defects if any are produced on the surface. If the feature is critical, surface measuring instruments are used to check the surface finish accurately within the process. For dimensional measurement, the operator checks the critical dimensions of the feature produced using measuring instruments such as vernier gauges and micrometers.
In advanced machining, which support probing measurements, a probing cycle is completed on the machine and then the feature dimensions are measured and stored to be compared with the original drawing or CAD model dimensions. At times such advanced machines inherit a feedback loop and the next course of action would be defined by the measured dimension. In a machining example for such cases, the sequence of operation consists of a measure cut to rough machine the surface followed by probing which includes the in-process measurement of the produced rough feature using probes. The values obtained from the probing cycle are used by the feedback loop to decide the depth of cut of the finishing pass to produce the finished feature on the component. This ensures early detection of errors on the parts if any and thus reduces cost by allowing non-scrap parts to move to the next stage of machining. In some cases where the depth-of-cut is less than the critical value, the part will be scrapped or quarantined as appropriate. The feedback loop also helps to account for part distortion, tool deflection and thermal effects. This allows for an update of the co-ordinate systems, parameters, offsets, and logical program flow depending on actual material condition. Another major advantage of using in-process inspection is the check and analysis of the part on a given setup. This reduces the re-setup time required between measuring and manufacturing.
State of the Art/Maturity
By means of in-process inspection, we can measure a produced feature either on each part produced or each part after a certain number of parts are produced. This data can be used for a Statistical Process Control study which allows us to understand how much the feature creation is in control. A feature historic data or record of that particular feature over different parts over a period of time is generated and this helps to produce control charts such as uncertainty charts, Xbar and Range chart and Standard Deviation. There after we can use this data to modify the process parameters to bring the feature generation process either within control or tighten the tolerance band further for better control of the feature.
There is state-of-the-art software available on the market which aid in this process of in-process inspection. The software assesses the capability of the machine, the initial state of the workpiece and the state of the cutting tool. This data is used to generate appropriate allowance factors which are used either during machining or while inspecting the part. At appropriate intervals the part is probed to ensure the accuracy of the generated features. Distortions or temperature variations if any found during machining are fed back to the software which in turn changes the value of the allowances in real time. The ultimate aim is to ensure a conforming part is produced after taking into account the initial state and dynamic changes during machining.
Practical applications for Machining
- In-process measurement of machining of long shafts which are susceptible to buckling under its own weight during turning by a probing cycle in CNC machine.
- In-process visual inspection for chatter and scratch marks on the surface of the semi-finished parts in between machining stages.
- In-Process measurement by probing of critical dimensions after machining of fir tree roots of turbine blades on CNC machine
- Using specialised software to measure and collect feature data by probing method to carry out downstream activities.
- In-process measurement of parts to understand distortion on the part during metal removal which can be influenced by residual stresses on the part from the forming process.