Process control
A process is a series of steps that turns materials into a product. Process control means managing these steps so that the work happens in the right order, at the right time, with the right people and machines. Managers use a work order (what to make), a routing (the path through steps) and a progress chart (what is done and what is late). They track each job, and if a step is delayed, they speed it up or change the plan.
Good process control cuts waiting, stops lost jobs and keeps delivery dates.
Quality control: checking results
Quality means the product does what it should and meets the standard. No process makes every part exactly the same, so we set a target size and limits, for example 30.0 mm with limits 29.8 to 30.2 mm. Inspection measures parts. A gauge or a measuring tool such as a caliper checks the size. Parts inside the limits pass. Parts outside fail and are fixed or thrown away.
Checking every part takes time and money, so often we check a sample, for example 5 parts from every 100.
Control charts: seeing a problem early
A control chart plots measured values in time order, with a centre line and an upper and lower limit. If dots stay between the limits and bounce around the centre, the process is steady. If dots climb in one direction or leave the limits, something has changed, for example a worn tool. We stop, find the cause and fix it. This is better than only throwing away bad parts after they are made, because it prevents defects.
Try it
In the 3D, slide tool wear from 0 to 10. At which value does the first part turn red? At which value do 3 or more parts fail? At home, measure 8 pencils with a ruler and see how much their lengths differ. What limits would you set?
Key formulas and definitions
- Allowed size = target +/- tolerance
- Pass if lower limit <= size <= upper limit
- Defect rate = defective parts / parts checked x 100
- Process control = right steps, right order, right time
- Quality control = check the result against limits
Worked examples
1. A rod must be 50 mm with limits 49.8 to 50.2 mm. Does a 50.15 mm rod pass? Does a 50.3 mm rod pass?
50.15 is between 49.8 and 50.2, so it passes. 50.3 is above 50.2, so it fails.
2. A supervisor checks 40 parts and finds 3 defective. What is the defect rate?
Defect rate = 3 / 40 x 100 = 7.5 percent.
3. On a control chart the last 6 dots rise steadily but are still inside the limits. What should the operator do?
Check the machine now. A steady rise means something like tool wear is changing the size. Fixing it early stops bad parts before the dots cross the limit.
Common mistakes
- Thinking quality means perfect parts. Real parts always vary a little; limits say how much is allowed.
- Checking only at the end and never watching the process.
- Ignoring a steady drift because the dots are still inside the limits.
- Mixing up process control (steps and time) with quality control (checking the result).