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Tolerances & surface finishes explained

Two of the biggest hidden price drivers on a machined part are how tightly it must be held and how it's finished. Understand these and you can spec a part that does its job without paying for precision it never needed.

The short version: ask for standard tolerance (around ±0.1 mm) unless a feature genuinely needs to be tighter — call out the critical dimensions on a drawing instead of tightening the whole part. Finishes like anodising or bead-blasting are separate processes that add cost and lead time.

What "tolerance" actually means

No part is made exactly to nominal — tolerance is the allowed +/- window around a dimension. A 20 mm feature at ±0.1 mm may measure anywhere from 19.9 to 20.1 mm and still pass. Standard CNC tolerance sits around ±0.1 mm, which is fine for the vast majority of parts.

When to pay for tighter

Tighter tolerances (±0.05, ±0.02 or finer) mean slower machining, more skilled setup and more inspection — so cost climbs quickly. They're worth it for mating features: bearing seats, dowel holes, sliding fits, sealing faces. The trick is to tighten only those features on a drawing and leave everything else at standard. Tightening the whole part "to be safe" can double the price for no benefit.

Tolerance	Roughly	Use for	Cost impact
Standard	±0.1 mm	Most features, general fit	Baseline
Precision	±0.05 mm	Locating features, some fits	Higher
Tight	±0.02 mm & finer	Bearings, dowels, seals	Highest — inspect every part

Surface finishes

How the part comes off the machine — and what happens to it afterwards — is a cost and lead-time decision of its own:

As machined: the default. Fine tool marks visible, edges deburred. Cheapest, no extra step.
Bead-blasted: a uniform matte texture that hides tool marks — a popular, affordable cosmetic upgrade.
Anodising (aluminium): a hard, corrosion-resistant layer that can be clear or coloured (black is common). Great protection and looks, adds cost and a few days.
Powder coat / paint: tough coloured coating, often used on steel to stop it rusting.
Plating (zinc, nickel): corrosion protection for steel, or a conductive/decorative layer.

Because most finishes are outsourced to specialist platers and anodisers, they add both money and lead time — worth knowing when you're planning a deadline.

Tapped threads & deburring

Tapped (threaded) holes are an extra operation, so more threads means a little more cost — always call out size and quantity (e.g. "4 × M5"). Deburring — knocking sharp edges off — is usually included as standard; ask for a specific edge break only if the application needs it.

Spec it right, price it instantly

On our quote page you can set the tolerance band, choose a finish and flag tapped threads — and watch the price respond so you can find the sweet spot.

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