Manufacturing and Production Systems
What to master
Expect breadth: traditional processes (casting, forging, machining, welding, sheet forming, injection molding) with their capabilities, tolerances, and typical defects, plus modern topics - CNC, CAD/CAM, robotics, additive manufacturing, metrology (GD&T, CMM), and lean manufacturing (value-stream mapping, 5S, kanban, poka-yoke, SMED, TPM). Be able to compute machining time (t = L/(f x N), with N = 12V/(pi x D) in inch units), material removal rate, and tolerance stack-up (worst-case arithmetic vs statistical root-sum-square). Understand automation economics, group technology and cellular layouts, and the theory of constraints: identify, exploit, subordinate, elevate.
Traps
- Unit mismatches: surface speed in ft/min, feed in in/rev, spindle speed in rpm.
- Confusing feed per tooth with feed per revolution in milling.
- Tolerance stack-up: worst-case sum vs RSS - read which one is asked.
- Reporting OEE as availability alone; OEE = A x P x Q.
- Treating takt time as cycle time; takt is demand-driven, cycle time is capability.
- Assuming automation always lowers unit cost - volume matters.
Worked example
A shift runs 420 minutes, produces 900 units, with 30 minutes downtime, 950 units of ideal-cycle output, and 45 defects. Availability = 390/420 = 0.929; Performance = 900/950 = 0.947; Quality = 855/900 = 0.950. OEE = 0.929 x 0.947 x 0.950 = 0.836, or 84%.
Rule of thumb
Compare each work center's available time to its required load. The constraint is the resource with the highest utilization, and throughput improves only when that resource is relieved.