What tolerance can CNC machining actually hold in production?

On a standard machining centre, ±0.015 mm is reliable in production. High-speed machines hold ±0.005 mm. Jig grinding reaches ±0.002 mm. The number a shop quotes on a sample is not the number it holds across a production run on a warm machine at 2am — always ask which one you are being given.

When should I stop machining from solid and switch to casting plus machining?

Roughly between 500 and 2,000 units per year for a typical housing, depending on how much material the cutter removes. Machining from solid has near-zero setup cost and a high unit cost. A die cast tool costs money once and then produces at a fraction of the unit cost, with only the critical faces machined afterwards. Where those two lines cross is the decision, and it is worth calculating before committing to either.

Why do two CNC quotes for the same drawing differ so much?

Usually four things: which tolerances they intend to actually hold versus nominally accept, whether inspection is on a CMM or a caliper, how many setups they have planned, and whether the price assumes your material or a substitute. A quote that did not ask a single question about the drawing has made assumptions you have not seen.

Does a tighter tolerance always cost more?

Yes, and the step is not linear. Moving from ±0.05 mm to ±0.015 mm is modest. Moving from ±0.015 mm to ±0.005 mm changes the machine class, the fixturing, the inspection method and the scrap rate. The expensive part is rarely the cutting — it is the measuring and the rejects.

CNC Machining: Real Tolerances and What They Cost

42CNC machines — 29 standard, 13 high-speed

≤0.0005 mmCMM verification, on six machines

3×typical spread between quotes for one drawing

Achievable tolerances, by machine class

These are the numbers from my own equipment list. They are what the machines hold in production — not the best single part you can coax out of them for a sample submission.

Machine class	Tolerance	Typical use	Count
Standard machining centre	≤0.015 mm	General features, housings, brackets	29
High-speed CNC	≤0.005 mm	Fine features, mould cores and cavities	13
CNC turning	≤0.005 mm	Round features, shafts, bores	3
Jig / surface grinding	≤0.002 mm	Flatness, parallelism, gauge surfaces	30
Sinker EDM	≤0.015 mm	Features a cutter cannot reach	48
Mirror EDM	≤0.005 mm	Polished cavity surfaces	8
Wire EDM (slow wire)	≤0.005 mm	Profiles, inserts, tight corners	6
CMM verification	≤0.0005 mm	The measurement, not the cutting	6

158 machines across CNC, EDM, milling and drilling. 15 CNC programmers. Key equipment includes Mori Seiki high-speed machining centres, Mitsubishi mirror EDM and AgieCharmilles slow-wire.

CNC cutting with coolant on a fixtured part — The tolerance on the drawing is decided here — but whether you hold it is decided by fixturing, thermal drift and how often the machine is measured.

The cost step is not where people expect

Tightening a tolerance does not raise cost smoothly. It steps — and the steps are in odd places.

Tightening from ±0.05 to ±0.015 is nearly free. The jump to ±0.005 changes the machine, the fixture, the inspection method and the scrap rate all at once — which is why it is the one worth arguing about.

The expensive part is almost never the cutting. It is the measuring and the rejects.

A shop that quotes ±0.005 mm across a whole drawing, cheerfully and immediately, is either not going to hold it or has not read it.

Which is why the single most useful edit to most drawings is not tightening anything — it is loosening everything that does not need to be tight, so the two or three features that genuinely matter get the attention and the budget.

⭐ When CNC stops being the right answer

I own 42 CNC machines. Here is the advice that costs me machining hours.

Machining a housing from solid has almost no setup cost and a high cost per part. Casting has a real tool cost once, then a low cost per part, with only the critical faces machined afterwards. Those are two straight lines on a chart, and they cross.

For a typical aluminium housing, the crossover sits somewhere around 500 to 2,000 units a year — the exact point depends on how much metal the cutter has to remove. A part that is mostly air when you are finished is a part you are paying to turn into swarf.

Two straight lines. Below the band, machining wins. Above it, the tool has already paid for itself and keeps paying. The band is wide because it moves with how much material the cutter removes — a solid block becoming a thin-walled housing sits at the left edge; a near-net shape sits at the right.

A die cast part being CNC machined on a fixture — This is the answer most of the time at volume: cast the shape, machine only the faces that need precision. The casting carries the geometry, the CNC carries the tolerance.

Two things make this decision go wrong in practice:

You ask a CNC shop. They will quote CNC. Not dishonestly — it is what they have. A shop with only one process will always find that its process is the answer.
You compare a per-part price to a per-part price, and the tooling cost sits outside the comparison in a separate email. It has to be amortised into the unit cost across a realistic annual volume, or the comparison is meaningless.

I have mould making, die casting and CNC under the same roof, which is the only reason I have no stake in which one you choose. How die casting compares →

Why two quotes for the same drawing differ

When a founder shows me three CNC quotes with a 3× spread, it is almost always these four things — none of which appear on the quote itself:

1 · Which tolerances they plan to hold

A drawing has a general tolerance block and specific callouts. The cheap quote priced the general block. The expensive one priced the callouts. Both are technically quoting your drawing.

2 · How the parts get measured

CMM on a defined sample plan, or a caliper by the operator? This is a real cost difference and it is the one that decides whether problems are found in the factory or in your warehouse.

3 · Number of setups

Every re-fixture adds cost and stacks tolerance. A quote that assumes three setups where the part really needs five is not cheaper — it is wrong, and you find out at first article.

4 · Material

6061 versus 6063 versus "equivalent". The substitution is usually legitimate and occasionally not. Ask for the mill certificate before the price discussion, not after.

A quote that came back within an hour and asked nothing is the one to be careful with. In machining, as in casting, the fastest quote is the one that did not ask any questions — and it is very often the most expensive outcome.

The 42-check supplier audit checklist →

The floor this is written from

CNC machining hall — The CNC hall — part of a 32,000 m² site

CNC operator at a machining centre — 15 CNC programmers, 900 people on site

Coordinate measuring machine inspecting a machined part — One of six CMMs. Where ≤0.0005 mm stops being a spec-sheet number

Certification note, stated plainly: the group holds ISO 9001, ISO 14001 and IATF 16949. The ISO scope covers mould making and injection moulding — it does not cover die casting. I would rather you learned that here than during due diligence. The full equipment and certification detail →

Where to go from here

You have a drawing

Send it. You get back the two or three tolerances that are driving the price, and whether the volume says machine it or cast it.

Project services & rates →

You are a brand producing at volume

If machining is one line in a bill of materials with fifty suppliers behind it, the machining is not really your problem.

The managed department →

You are building your first product

Different conversation, still open, still free — and the templates below are free with no email required.

Free templates & checklists →