Maintenance, Inspection and Quality Basics
This domain tests maintenance types, non-destructive testing methods, gauge use and the basic quality tools and terms.
Maintenance types
Preventive maintenance is scheduled cleaning, lubrication, adjustment and part replacement before failure. Predictive or condition-based maintenance uses vibration analysis, oil analysis, thermography and noise monitoring to plan the shutdown. Breakdown or corrective maintenance happens after failure and costs the most downtime. Proactive maintenance finds and removes the root cause. Keep a machine history card and a lubrication chart.
Inspection and testing
Inspection is incoming, in-process (first-off and patrol) or final. Destructive tests include tensile, hardness and impact tests. Non-destructive tests: dye penetrant finds surface-breaking defects only; magnetic particle testing suits ferrous parts and finds surface and near-surface flaws; ultrasonic testing finds internal flaws and needs a couplant; radiography shows internal flaws but is a radiation hazard; eddy current suits conductive materials.
Gauges and quality terms
A limit gauge has a GO and a NO-GO end: the GO end must enter the hole and the NO-GO end must not. If the GO end will not enter, the hole is undersize. Quality control detects defects; quality assurance prevents them through systems. The seven QC tools include check sheet, histogram, Pareto chart, cause-and-effect diagram, control chart, scatter diagram and flowchart. SPC uses control limits at plus or minus three sigma. Also know 5S, Kaizen, poka-yoke, Six Sigma (3.4 defects per million) and ISO 9001.
Worked example
A dimension is 20.00 plus or minus 0.05 mm, so the tolerance is 0.10 mm, and the process standard deviation is 0.01 mm. Cp = tolerance / 6 sigma = 0.10 / 0.06 = 1.67. Cp above 1.33 means the process is capable, but Cp does not show whether the mean is centred, so Cpk must also be checked.
Traps that cost marks
- Mixing up quality assurance and quality control.
- Believing dye penetrant finds internal defects.
- Using a worn or uncalibrated gauge.
- Treating Cp and Cpk as the same measure.