Non-Traditional Machining and Metrology
EDM, ECM, USM, AJM, LBM and EBM principles and MRR, limits, fits and tolerances, hole and shaft basis, gauges, surface roughness, comparators and interferometry.
Non-traditional machining removes material without a sharp cutting tool, for hard materials and fine features. Metrology makes sure parts fit together: limits, fits and measurement.
Non-traditional machining
| Process | How it removes material | Typical use |
|---|---|---|
| EDM | Electric sparks in a dielectric melt and vaporise tiny craters | Dies and moulds in hardened steel (conductive work only) |
| ECM | Anodic dissolution in an electrolyte (reverse electroplating) | Turbine blades; no tool wear, no residual stress |
| USM | Abrasive slurry hammered by a tool vibrating at about 20 kHz | Brittle materials: glass, ceramics |
| AJM | High-speed jet of abrasive particles in gas | Deburring, cutting thin brittle sheets |
| LBM / EBM | Focused laser or electron beam melts and vaporises | Very small holes; EBM needs a vacuum |
| Water jet | High-pressure water jet (with abrasive for metals) | Cutting soft materials, composites |
ECM material removal rate (Faraday's law): MRR (mass per second) = A I/(F Z), where A is atomic weight, Z valency, F = 96,500 C. Volume rate = A I/(ρ F Z).
EDM: the tool also wears; MRR rises with spark energy (RC circuit energy ½CV²), but finish gets worse.
Limits, fits and tolerances
- Basic size: the nominal size. Tolerance = upper limit − lower limit.
- Allowance: the minimum clearance (or maximum interference) between mating parts.
- Fits: clearance (always a gap), interference (always tight), transition (either).
- Hole-basis system: the hole is fixed (H, lower deviation zero) and the shaft varies. Most common, because holes are made with standard tools.
- IT grades: smaller numbers mean tighter tolerances. Tolerance unit i = 0.45 ∛D + 0.001D (µm, D in mm).
Gauges: a GO gauge checks the maximum material limit (smallest hole, largest shaft); a NO-GO gauge checks the least material limit. Taylor's principle: GO gauges check all features together (full form); NO-GO gauges check one feature at a time.
Measurement
- Surface roughness: Ra (arithmetic average of deviations), Rz, Rmax.
- Comparators (mechanical, optical, pneumatic) compare a part with a standard.
- Slip gauges build up precise lengths; sine bars measure angles: sin θ = h/L.
- Interferometry with optical flats: each fringe = λ/2 change in height.
Practice
A hole is 50.000/50.025 mm and its shaft 49.950/49.975 mm. What is the maximum clearance in mm?
Show answer and explanation
Answer: 0.075Max clearance = largest hole − smallest shaft = 50.025 − 49.950 = 0.075 mm.
For the same hole and shaft, what is the minimum clearance (allowance) in mm?
Show answer and explanation
Answer: 0.025Min clearance = smallest hole − largest shaft = 50.000 − 49.975 = 0.025 mm. Both are positive, so it is a clearance fit.
A 200 mm sine bar is set with 50 mm of slip gauges. What angle (degrees) does it set? (Give 2 decimal places.)
Show answer and explanation
Answer: 14.48sin θ = 50/200 = 0.25, so θ = 14.48°.
Which process cannot machine an electrically non-conducting ceramic?
Show answer and explanation
Answer: AEDM needs a conductive workpiece for sparks to form. USM and AJM suit brittle non-conductors.
In ECM of iron (A = 56, Z = 2) at 1000 A, what is the mass removal rate in g/s? (F = 96,500 C; give 3 decimal places.)
Show answer and explanation
Answer: 0.290MRR = AI/(FZ) = 56,000/193,000 = 0.290 g/s.