Tolerances: precision costs money
A tolerance is how much a dimension is allowed to vary and still be acceptable. Tighter tolerances mean slower cuts, better tooling, more inspection and sometimes scrap — so every decimal place you tighten adds cost. The skill is tightening only the dimensions that matter and leaving the rest loose.
Standard machining tolerance
Most CNC shops hold roughly ±0.1mm as standard without any special effort or cost. For a great many parts — brackets, enclosures, mounts — this is perfectly adequate. If you do not call out a tolerance, this is broadly what you get.
Tight tolerance
±0.025mm (about one thousandth of an inch) is achievable on good machines but costs more in time and care. Reserve it for mating surfaces, bearing bores, shafts and sealing faces — features where fit genuinely matters.
Precision tolerance
±0.01mm or finer pushes into climate-controlled measurement, specialist tooling and CMM inspection reports. It is the right call for precision instruments and certain medical or aerospace features, and the wrong call for a mounting hole.
| Tolerance | Typical use | Relative cost |
|---|---|---|
| ±0.1mm (standard) | General features, brackets, housings | Baseline |
| ±0.05mm | Moderate fits, locating features | Slightly higher |
| ±0.025mm (tight) | Bearing bores, shafts, seal faces | Noticeably higher |
| ±0.01mm (precision) | Instrument, medical, aerospace features | Significantly higher |
The rule: default everything to standard, then tighten only the handful of dimensions that are functionally critical. A drawing where every dimension is ±0.01mm tells the shop you do not know which features matter — and you will pay for all of them.
Surface finishes for machined parts
The finish you specify changes both appearance and function — corrosion resistance, hardness, electrical properties — and adds lead time, usually because it goes to a specialist after machining.
As-machined
The default. Visible tool marks, typically around Ra 3.2µm. Cheapest and fastest. Fine for internal or non-cosmetic parts.
Bead blasting
A uniform matte texture that hides tool marks. Cosmetic, inexpensive, common on aluminium. Often done before anodising.
Anodising (aluminium)
An electrochemical oxide layer that boosts corrosion and wear resistance. Type II is the standard decorative/protective anodise and can be dyed almost any colour, including black. Type III (hard anodise) is thicker and far harder, for wear surfaces. Anodising is one of the most common finishes for Irish-made aluminium parts.
Powder coating
A durable coloured polymer coat, baked on. Tough and weather-resistant, good for enclosures and outdoor parts, but thicker than anodising so it can affect tight tolerances.
Passivation (stainless)
A chemical treatment that removes free iron and restores the corrosion-resistant chromium oxide layer on stainless steel. Often specified for medical, food and marine parts.
Plating
Zinc, nickel or chrome plating for corrosion resistance, conductivity or appearance. Adds a controlled thickness and a specialist step.
Spec it right the first time
On your drawing, call out: the critical tolerances (and only those), the finish and any colour, and any standards that apply (e.g. medical, food-contact). If a finish must not affect a tight-tolerance feature, mask it or machine after finishing — tell the shop. Get the geometry clean first with the CAD prep guide, then see how finishing and tolerances move your quote.
Frequently asked questions
What is the standard tolerance for CNC machining?
Most shops hold roughly ±0.1mm as standard at no extra cost. Tighter tolerances such as ±0.025mm or ±0.01mm are achievable but add time, tooling and inspection cost, so reserve them for functionally critical features.
How tight a tolerance can CNC machining achieve?
Good machines reach ±0.01mm or finer on critical features, but this requires specialist tooling, controlled measurement and often a CMM inspection report. Only specify it where fit genuinely demands it.
What surface finishes are available for CNC parts?
Common options include as-machined, bead blasting, anodising (Type II decorative or Type III hard) for aluminium, powder coating, passivation for stainless, and zinc/nickel/chrome plating. Each adds cost and lead time.
What is the difference between Type II and Type III anodising?
Type II is the standard decorative and protective anodise and can be dyed in colours including black. Type III (hard anodise) is thicker and much harder, used for wear surfaces.
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