CNC 공차 설명: 2026 GD&T 실용 가이드

What a tolerance really is

A tolerance is the allowable variation in a dimension. If a drawing says “25.00 ±0.05 mm”, the part is acceptable when the measured dimension falls between 24.95 and 25.05 mm.

Three things determine the achievable tolerance on any given feature:

1. Machine capability. A 20-year-old manual mill holds ±0.1 mm at best. A modern Mazak 5-axis cell holds ±0.005 mm comfortably. The machine sets the floor.
2. Material behaviour. Aluminium machines stably; titanium springs back; thin-walled stainless deflects under cutter pressure. Material doubles or halves the achievable tolerance.
3. Feature geometry. Tolerances on a 5 mm feature near the chuck are easy. Tolerances on a 200 mm-long thin wall are hard. Geometry can make the same nominal dimension 10× harder to hold.

What CNC can actually hold

Tolerance stack-up — the silent cost driver

When several toleranced features have to interact, their individual tolerances add up. This is “tolerance stack-up” — and ignoring it leads to assemblies that fail QA even though every individual part is within spec.

Worked example: a stack of three identical washers, each toleranced ±0.05 mm thick:

1. 1

   Per-washer tolerance

   Each washer can be 0.05 mm too thin or 0.05 mm too thick. Range = 0.10 mm per washer.

2. 2

   Worst-case stack

   Three washers at the worst end: 3 × 0.05 = 0.15 mm thinner OR 0.15 mm thicker than nominal. Total range: 0.30 mm.

3. 3

   Statistical stack (RSS)

   In practice not every washer is at the extreme. Root-Sum-Square gives more realistic ±0.087 mm at 3-sigma.

4. 4

   Design implication

   If your assembly needs to fit in a 25 ±0.10 mm slot, 75 ±0.30 mm worst-case won’t fit. You must either tighten individual tolerances OR widen the slot OR redesign.

GD&T (Geometric Dimensioning and Tolerancing) in 5 minutes

GD&T is a symbolic language (ASME Y14.5 / ISO 1101) for specifying not just dimensions but the geometric relationships between features. It’s how aerospace, automotive and medical drawings communicate what really matters.

Two key concepts make GD&T more powerful than “plus-minus” tolerancing:

How to specify tolerances on a drawing

1. 1

   Use a title-block default

   Top right of the drawing: “General tolerance: ISO 2768-mK” (or ASME equivalent). Now most dimensions don’t need explicit tolerances.

2. 2

   Tolerance only what matters

   Mating dimensions, sealing faces, bearing seats, datums. Leave decorative or non-functional features at the default.

3. 3

   Use the tightest tolerance only where required

   A typical optimised drawing has 3–5 features at ±0.025 mm and the rest at ±0.1 mm default.

4. 4

   Specify surface finish where it matters

   Use the standard finish symbol (✓ with Ra value) on the surfaces that need it. “Ra 0.8” on a sealing face; rest defaults.

5. 5

   Add datums for GD&T-controlled features

   Pick the most-stable, most-machined surface as datum A. Usually a large flat face. Datum B and C are perpendicular to A.

How tolerances are verified

Cost impact of tightening tolerances

Indicative cost multiplier vs the same feature at default ±0.10 mm:

Common mistakes to avoid

• Tightening every dimension to ±0.005 mm. The classic rookie mistake. Triples cost for no functional benefit.
• No general-tolerance call-out. Without ISO 2768 in the title block, every dimension becomes ambiguous and the shop will quote conservatively.
• Toleranced angles smaller than ±0.5°. Most CNC mills handle ±0.5° easily. Tighter angles often require fixturing or grinding.
• Demanding mirror surface (Ra 0.05) on functional surfaces. Polishing adds significant cost and lead time. Use Ra 0.8 unless optical or sealing function actually requires better.
• Overlapping datum schemes. If A is the bottom and B is the side, don’t also reference the bottom as B somewhere else. Pick a clean A-B-C scheme and stick with it.
• Forgetting MMC modifiers. True position with no modifier is the strictest interpretation. Adding Ⓜ (MMC) gives the shop legitimate bonus tolerance — use it where appropriate.
• Tolerancing what can’t be measured. If the only inspection tool that can verify the spec costs $250k, expect to pay for that inspection.