CNC加工 公差: Standard vs Tight — What You Actually Need

重要ポイント

• Standard machining tolerance is ±0.1 mm (ISO 2768-m) — and it’s fully adequate for most features on most industrial parts. US shops quote the equivalent ±0.005″ convention.
• The tolerance-cost curve is exponential, not linear. Tightening from ±0.1 to ±0.05 mm adds modest cost; pushing to ±0.01 mm and beyond multiplies it — industry analyses put precision work at roughly 4× and ultra-precision at up to 24× the cost of standard machining.
• Every tight callout buys real work: slower feeds, extra finishing passes, additional setups, temperature control, and CMM 検査 time.
• Only fit, function, and interchangeability justify tight 公差 — bearing bores, press fits, sealing grooves, mating patterns. Cosmetic faces and clearance holes don’t.
• Loosening non-critical callouts pays instantly: relaxing ±0.01 mm features to ±0.03 mm has been shown to cut machining cost by around 22% with zero functional impact.
• Specify smart: one general ISO 2768-mK note in the title block, individual callouts only where function demands them.

Ask five engineers what 出荷前に問題を発見します。 公差 their part needs and you’ll often get one honest answer and four reflexes: “make it tight to be safe.” That reflex is expensive. Tolerance is the single biggest cost lever on a machining quote after material choice — and unlike material, it’s invisible until the invoice arrives.

This guide gives you a practical machining tolerance chart, explains what “standard” really means, shows where the cost curve bends, and ends with a specification method that keeps precision where it matters and money where it belongs. (For the full ISO 2768 class tables and GD&T symbol reference, see our companion guide to tolerance standards in metal fabrication — this article focuses on choosing, not defining.)

What “Standard Tolerance” Actually Means

When a drawing carries no individual callouts, machine shops fall back on a general tolerance convention:

• ISO 2768-m (medium) — the international default: ±0.1 mm on linear dimensions under 30 mm, scaling up with size. Most European drawings specify 「ISO 2768-mK」 in the title block.
• ±0.005″ (0.127 mm) — the long-standing US job-shop convention for metals when nothing is specified; plastics typically default to ±0.010″ (0.254 mm).
• 精密 classes (IT6–IT7) — ±0.01 to ±0.02 mm, reserved for features that locate, seal, or rotate.

The key insight: standard tolerance is not “low quality.” A modern CNC mill holds ±0.05 mm routinely without special effort — standard classes exist precisely so that the 90% of dimensions that only need to clear each other don’t get billed like the 10% that must fit.

加工公差表：各工程が保持できる公差

材料 matters too: soft aluminum can deflect under clamping and cutting pressure, while stainless and titanium hold tight limits more predictably. If your part is machined aluminum headed for hardcoat 陽極酸化, remember the coating itself grows the part — a ±0.01 mm bore callout can be blown by the finish, not the machining.

公差とコストの曲線：±0.01mmが±0.1mmの数倍のコストになる理由

The relationship between tolerance and cost is an exponential curve. Tightening from ±0.1 mm to ±0.05 mm raises cost modestly — slower finishing feeds, a fresher tool. Pushing past ±0.01 mm is a different world: industry analyses put precision-class work at roughly 4× the cost of standard machining, and ultra-precision (±0.0025 mm class) at up to 24×. Here’s what that money actually buys:

• 低速 cutting. 仕上げing passes at reduced feed and depth, often multiple spring passes to relieve tool deflection.
• More setups and better fixturing. Tight positional 公差 across faces force precision workholding or single-setup 5-axis strategies.
• Tool and thermal management. Fresh cutters dedicated to the feature, warm-up cycles, and for the tightest work, temperature-stabilized environments.
• 検査 overhead. Every tight callout must be measured — CMM 検査 time scales directly with the number of controlled features, and near-limit parts trigger reviews and potential rework.
• より高い scrap risk. The tighter the window, the more parts fall outside it — that expected loss is priced into your quote.

The reverse also works. One published analysis found that relaxing non-critical callouts from ±0.01 mm to ±0.03 mm cut machining cost by around 22% — no design change, no functional loss, just honest tolerancing.

When Tight 公差 Are Worth It — and When They’re Not

• Worth it — bearing bores and shaft seats: the fit defines runout, vibration, and bearing life.
• Worth it — press fits and locating pins: interference is a function of microns; too loose spins, too tight cracks.
• Worth it — sealing surfaces and O-ring grooves: leak paths open at the tolerance limit, especially under pressure.
• Worth it — interchangeable production parts: when any part must mate with any counterpart across batches.
• Not worth it — cosmetic and external faces: nobody measures the outside of an enclosure to ±0.01 mm.
• Not worth it — clearance holes and pockets: a bolt through a 6.5 mm hole doesn’t care about ±0.02.
• Not worth it — “uniform” blanket callouts: a title-block default of ±0.02 mm on every dimension is the single most common quote inflator we see in RFQs.

最適な用途 tight (±0.01–0.05 mm): fits, seals, locating features, rotating interfaces, certified interchangeability.
Keep standard (±0.1 mm / ISO 2768-m): external profiles, clearance holes, non-mating faces, brackets, covers — the majority of every part.

スマートな公差指定方法

1. Set one general note in the title block — “General 公差: ISO 2768-mK” covers every unlabeled dimension at standard cost.
2. Add individual callouts only to functional features. Ask of each one: what fails if this drifts 0.05 mm? No answer = no callout.
3. Account for 表面処理. 陽極酸化, plating, and 粉体塗装 change fit-critical dimensions — state whether 公差 apply before or after coating.
4. Flag your critical features in the RFQ. Telling your machinist which two dimensions truly matter gets them process attention and honest feedback — buried in twenty callouts, they get averaged. Prepare drawings per our guide to technical drawings for custom fabrication.
5. Ask for a DFM review before ordering. A good shop will tell you which callouts drive the price and propose relaxations that cost nothing functionally — that conversation regularly saves 15–25% on precision parts.

Real-world example: an industrial automation client sent us a sensor mounting plate with a blanket ±0.02 mm title-block tolerance. DFM review found exactly three functional features — two dowel holes and one bearing bore. We held those at ±0.01 mm, returned everything else to ISO 2768-m, and the quote dropped by roughly a third while the assembly fit improved, because machining effort concentrated where it mattered.

適切な公差を適切な価格で

Tolerancing well isn’t about tight or loose — it’s about intentional. Default to standard, tighten only what fits, seals, or locates, and say so clearly on the drawing. If you’re unsure which callouts on your part genuinely need precision, 送信 us the drawing: BravoFabs machines standard and tight-tolerance parts daily for industrial clients across Europe and North America, holds ±0.01 mm where it counts, and every RFQ includes a free DFM review that tells you exactly which 公差 are costing you money.