CNC Machining Tolerances 101

Most rejected parts and surprise surcharges trace back to one thing: the tolerance. Too tight, and the part costs more than it should — and may not be makeable on the floor you picked. Too loose, and it will not fit, seal, or function. This is a practical tolerances primer for buyers specifying CNC machined and non-standard parts: what the numbers mean, what they cost, and how to write a drawing a shop can actually hold and prove.

What a tolerance actually is

A dimension like 50 ±0.05 mm is not a single target — it is a band the part must land in, every time, across the whole run. The real question is not “can the machine cut 50 mm,” but “can it cut 50 mm reliably, and can you prove it?” That proof is measurement, and it is part of the spec, not an afterthought.

The tolerance–cost curve

Cost does not rise in a straight line with tightness. Each step down in tolerance usually requires a different process, more setup, and more measurement — and the effect compounds.

Tolerance (typical)	How it is normally held	Cost signal
±0.1 mm	Standard CNC milling / turning	Baseline
±0.05 mm	Capable CNC, careful setup	Slight premium
±0.01 mm	Precision CNC, verified setup	Notable step up
±0.005 mm	Precision CNC / 5-axis, CMM-checked	Premium
±0.002 mm / surface	Surface grinding (flatness & finish)	Highest, finish-critical

A part that needs a handful of ±0.005 mm features is a different job from one that needs everything held to ±0.005 mm. Tightening only where it matters is where good buyers save money — and it is exactly what a design-for-manufacturability (DFM) review is for.

GD&T without the panic

You do not need to be a drafter to spec well. A few concepts cover most cases:

Datums — the reference surfaces everything else is measured from. Pick stable, machinable faces, not a thin edge.
Position vs. size — a position tolerance controls where a hole sits; a size tolerance controls how big it is. They are different, and both matter.
Flatness & surface finish (Ra) — when a face must seal or bear load, flatness and finish matter more than the overall length tolerance. Surface grinding is how very flat, very smooth faces are made.

If you are unsure which features are critical, say so. Mark the few dimensions that must be right and let the rest be commercial. A shop can hold a tighter band on the few that matter and relax the rest — usually for less money.

Specifying non-standard parts

Non-standard parts are where tolerance and fixturing meet. An oversized one-piece base, a long shaft, or a frame most shops would weld is defined as much by its envelope as by its tolerances:

Give the working envelope, not just overall size. A 6-meter gantry changes what is even fixturable in one setup; a 3-meter limit means welding or multiple setups. See how we handle large-format and long-part work.
Say “one-piece” if it must be one piece. That single word rules out the cheap workaround and tells the shop to plan a single setup.
Mark what is datum-critical on a long part. A shaft held to tight runout over 11 meters is a different measurement problem than the same tolerance on a 100 mm boss.

When a part is outside a standard envelope, it helps to talk to a partner who already runs oversized and long work — rather than a shop that will quietly sub it out. Our non-standard machining page covers the envelope and the questions worth asking.

Material and finish callouts that matter

Name the material grade (for example, 6061-T6 aluminum or 304 stainless). “Aluminum” alone leaves too much open.
Call out surface finish (Ra) only where it functions — sealing faces, bearing seats, decorative edges.
When finish is critical, expect a grinding operation and budget for it. Our grinding and surface finishing work covers flatness and finish control.

Inspection is part of the spec

A tolerance with no measurement plan is a hope. Ask how the critical dimensions are checked:

Setup checks at first article
In-process checks during the run
Final CMM report per shipment

When the coordinate-measuring-machine (CMM) report travels with the parts, you can verify what actually arrived — not just what was promised on a call. It is one of the reasons we treat CNC and 5-axis work as measurement-first, not just cutting.

A practical spec checklist

Before you send the RFQ, your drawing or note should state:

Material grade and any certificates needed
Overall working envelope (and “one-piece” if required)
Which dimensions are critical, and to what tolerance
Datum scheme (or “shop to propose”)
Surface finish where it functions
Finish / coating, if any
Inspection expectation (per-shipment CMM report)

Questions to ask the shop

What working envelope can you machine in one setup?
What tolerance can you hold, and on which machines is it measured?
Do you run 5-axis for complex geometry, and grinding for finish?
Is a CMM inspection report included with every shipment?
Can you deliver the part assembled and inspected, not just machined?

Bottom line

Tolerances are a cost-and-capability dial, not a quality score. Tight where it matters, relaxed where it does not, measured and reported per shipment — that is a part that gets made, fits, and scales. Over-spec everything and you pay for capability you do not need; under-spec and you pay in rejects.

Trying to spec a part that does not fit a standard envelope? Request a quote and we will come back with a clear quote, a DFM note, and the envelope check — usually within 24 hours. See the full range on our capabilities page.

Frequently asked questions

Q: What tolerance can CNC machining normally hold?
A: Standard CNC milling and turning typically hold around ±0.1 mm as a baseline. Precision CNC with careful setup reaches ±0.01 mm, and ±0.005 mm is held on precision or 5-axis machines with CMM checking. Surfaces held to ±0.002 mm or finished by grinding are the most cost-intensive.

Q: Should I specify the tightest tolerance everywhere?
A: No. Tightening tolerance only where the part actually functions usually costs less and is more reliable. Mark the few critical dimensions and relax the rest — that is exactly what a DFM review is for.

Q: How is tolerance verified, not just promised?
A: Through setup checks at first article, in-process checks during the run, and a final CMM inspection report per shipment. The report travels with the parts so you can verify what arrived.