가공 공정을 모르면 설계를 할 수 없다 — 가공 여유와 형상 제한의 실무

If a designer doesn't understand machining processes, drawings become disasters instead of blueprints. CAD can draw any shape, but machine tools have physical limits. To avoid hearing "this shape can't be machined" from the shop floor, master the following.

1. Milling — Internal Corners Must Have Radii

When milling pockets, internal corners always have a radius equal to the end mill radius. Perfectly sharp internal corners are physically impossible.

• Practical rule: Internal R ≥ tool radius (typically R1–R3). Deep pockets (>3D depth) require larger R due to tool rigidity
• Tip: If sharp corners are essential, switch to EDM or add relief cuts (dog-bone features) at corners

2. Turning — Don't Forget Center Holes

Long shafts (L/D > 4) on a lathe require center holes (KS B 1011 Type A/B) at both ends. Without specification on drawings, machinists improvise or skip centers, causing runout.

• Practical rule: Ø10–30 shaft → A2.5 center, Ø30–60 shaft → A4 center
• Tip: If center holes can't remain on finished parts, note "remove center holes after machining" or "center holes permitted" on the drawing

3. Grinding — Leave Stock Allowance

Grinding corrects post-heat-treatment distortion and achieves high precision (IT6 or better). Turning/milling stages must leave grinding stock.

• OD grinding: 0.1–0.3mm per side on diameter (0.2–0.6mm total)
• ID grinding: 0.1–0.25mm per side. Minimum bore Ø15 (small grinding wheel limit)
• Surface grinding: 0.1–0.3mm per side. Up to 0.5mm if heat treatment distortion is large

4. EDM (Electrical Discharge Machining) — Not a Silver Bullet

EDM machines regardless of hardness but is slow and expensive. Surface finish Ra 1.6 or better needs additional finishing. Electrode (copper, graphite) fabrication cost must be factored in.

• Wire EDM: Minimum corner R 0.1mm (based on Ø0.2mm wire). Ideal for through-cuts in plate
• Sinker EDM: Micro-holes, complex cavities. Account for electrode wear (copper: 1–3%)

5. Boring — Depth and Diameter Limits

Boring bars develop chatter as the L/D ratio increases.

• Practical rule: Standard boring bar L/D ≤ 4, anti-vibration boring bar L/D ≤ 7
• Tip: For deep holes, consider BTA drills or gun drills. For tight tolerances, add honing

6. Undercuts and Relief Grooves — Essential for Grinding and Threading

Grinding wheels need relief grooves to reach shoulder faces. Threading also requires undercuts to avoid incomplete threads at the end.

• Grinding relief groove: Width 3–5mm, depth 0.5–1mm (ref. JIS B 0612)
• Thread undercut: Width = pitch × 2–3, depth = at or below minor diameter