Rapid Prototyping China: What Tolerances and Surface Finishes Are Realistic?

Every prototype drawing arrives with the same two questions buried in it: how tight can the shop actually hold this dimension, and how smooth does that surface really need to be? Suppliers answer with a tolerance band and a finish number, but the numbers only mean something when you know what the process can hold, what the measurement proves, and which callouts are doing real work versus adding cost and days for nothing. This guide is the buyer's reality check for rapid prototyping in China — the tolerance bands and finish ranges a CNC floor can deliver consistently, how geometry and material move the numbers, how a drawing should state them so the part you approve is the part that ships, and the evidence to ask for so a quoted tolerance stays honest.

What tolerances can a rapid-prototyping shop in China actually hold?

A capable CNC floor holds a repeatable band, and the band is wider than the marketing sheets suggest. For a prototype — a single part or a handful, cut to validate a design — the practical reality is a standard machining band of ±0.003 mm on milling, turning, and mill-turn work, with precision grinding reaching ±0.002 mm for the features that need a ground surface. That is not a peak capability quoted for show; it is the band the floor holds routinely, verified on CMMs against the drawing, with process capability tracked at Ppk ≥ 1.67 at the initial study on key characteristics and Cpk ≥ 1.33 on general characteristics. Capability is reported per characteristic: every report names the feature, its tolerance band, the sample size, the sigma method, and the study window.

Process	Tolerance band	Finish as-cut	Right call when
Milling / turning / mill-turn	±0.003 mm	Ra 1.6–3.2 µm	Most machined features on a prototype
ISO 2768 general block	±0.1–0.5 mm by size range	—	Non-critical dimensions left to the drawing's general note
Precision grinding	±0.002 mm	Ra 0.8 µm and finer	Sealing or seating faces, bearing fits, features that mate against another part
Outside finishing (anodize, plate, coat)	Adds a layer that changes final size	Coating-dependent	Corrosion or appearance, not a substitute for a machined finish

Read the table as a planning tool, not a promise: the tighter band is real, but it is earned per feature, not applied to the whole part. A dimension that must hold ±0.003 mm gets a slower feed, a finish pass, and a CMM check. A dimension that only needs to clear another part stays on the general block and machines in one pass. The drawing that separates those two is the drawing that quotes fast, ships fast, and fits the first time.

The honest starting point is the general note. ISO 2768-1 gives the shop a default band — for a dimension between 6 and 30 mm, for example, grade m allows ±0.2 mm and grade f allows ±0.1 mm — so the drawing states "ISO 2768-m unless noted" and then tightens only the dimensions that function demands. That is how a serious shop keeps the fast default on 90% of the part while holding the critical 10% precisely, and it is the discipline worth matching on your own drawing.

How does geometry and material move the tolerance you can hold?

The tolerance a feature can hold depends on how rigid the feature is, not just how good the machine is. A short, stiff bore in a solid block holds ±0.003 mm without drama. The same callout on a thin wall, a deep pocket, or a long slender shaft asks the material to stop deflecting under the cutting force, and material does not always cooperate. Realistic planning means tightening only what the geometry can physically support, and loosening the rest.

Material moves the same numbers in the same direction. Aluminum 6061 cuts cleanly and holds a tight band with light passes; 7075-T6 holds it almost as easily with slightly more tool wear. Stainless steel and titanium cut slower, deflect more per pass, and transfer more heat into the part, which is why a ±0.003 mm callout on a thin titanium wall is a different job than the same callout on an aluminum block. The band is the same on the drawing; the process behind it is slower, and the schedule and the price both know it.

The buyer's rule is to isolate the functional dimensions before the RFQ goes out. Mark the two or three features that mate, seal, or index against another part and hold those to the tight band. Leave the remaining dimensions on the ISO 2768 general note. A prototype exists to prove the design, and it proves the design best when the tolerances on it match the tolerances the production part will carry, not when every dimension on it is drawn tight because it felt safer.

Which surface finishes are realistic on a machined prototype?

Surface finish is stated as an Ra value, and the values that matter for machined prototypes fall in a narrow, achievable range. As-cut milling and turning land around Ra 1.6–3.2 µm — the natural finish a sharp tool leaves at normal feeds, and the right default for most prototype surfaces. Pulling a functional face down to Ra 0.8 µm takes a fine feed or a light finishing pass, still on the machining floor. Going finer than that, to Ra 0.4 µm or below, moves the feature to grinding, which is a separate process family with its own machines and its own slower cycle.

Finish target	How it is produced	What it costs the prototype
Ra 3.2 µm	Standard milling or turning feeds	Nothing extra — the as-cut default
Ra 1.6 µm	Fine feed or light pass on the machining floor	A little extra machining time, usually free of schedule impact
Ra 0.8 µm	Careful finishing pass, sometimes grinding	A finishing step with its own setup and inspection
Ra 0.4 µm and finer	Precision grinding or lapping	A grinding setup and finish pass; the feature moves to a slower process

Two finish traps cost buyers the most on prototypes. The first is calling out Ra 0.4 µm on a face that only needs to look clean — the machining floor can hold it, but the part quietly moves to grinding or a manual polishing step for no functional gain. The second is treating a coating as a finish. Anodizing and plating change the surface chemically and add a layer that shifts the final dimension; they do not reduce the Ra of the machined surface underneath, and they should be specified as a coating requirement, not as a finish number. Machined finish and applied coating are two separate lines on the drawing, and each should be stated on its own.

The realistic question to ask before adding any finish is what the surface does. A face that seals against an O-ring or mates with a bearing needs the tight band and the fine finish. A cosmetic face that sits inside an enclosure does not — it needs to look acceptable, and Ra 1.6 µm as-cut does that without moving the schedule. Every finish you can justify dropping keeps the prototype in a faster tier and the quote closer to the machining cost.

How should a prototype drawing state its tolerance and finish?

A drawing that states tolerance and finish cleanly is a drawing a shop can quote in one pass and machine without asking. The three lines that carry the intent are the general tolerance note, the finish note, and the per-feature callouts, and the discipline is to keep each line short and specific.

Start with the general note: "ISO 2768-m unless otherwise noted" tells the shop what the default is and makes every unmarked dimension unambiguous. Add the finish note on the title block or as a surface-texture symbol where the whole part needs a floor, and then call out only the exceptions — the two or three tight features with their specific tolerance, the one sealing face with its Ra value. Mark the coating separately as a process requirement with its own allowance, because the coating changes the finished size and the machinist needs to cut to the pre-coat dimension.

State the surface-finish requirement in Ra, not in adjectives. "Fine finish" and "smooth" are negotiable in a way that Ra 0.8 µm is not; the number is what the shop can measure, quote against, and prove on the inspection report. The same rule applies to tolerances: give the band, not the sentiment. A drawing that says ±0.003 mm on the mating bore and ISO 2768-m everywhere else has told the machinist exactly which dimensions matter and which are free to run, which is precisely the information a fast, accurate quote is built from.

If the production part carries the same features, the prototype should carry the same numbers. A prototype with looser tolerances than production validates the geometry but not the process, and the whole point of machining a prototype is to prove the process that will make the real parts. Match the drawing and the process to production, and the prototype becomes the first article instead of a detour.

When do tolerance and finish together force a slower process?

A single tight tolerance is a small event; a tolerance and a finish on the same feature are a process decision. A bore at ±0.003 mm with Ra 0.8 µm is a milled or turned feature followed by a careful finishing pass and a CMM check. The same bore at ±0.002 mm with Ra 0.4 µm is a ground feature — it leaves the milling family entirely, and the schedule follows it to the grinding machines and back.

The interaction compounds across the part. Two tight features are two slow steps; a tight bore inside a tight pocket with a fine finish on the floor of the pocket is a sequence of finish cuts that each need their own feed, their own tool, and their own verification. That is how a drawing that looks compact on one page turns into a long process route on the floor, and it is why the fastest thing a buyer can do is ask which features genuinely need the tight combination before committing it to the drawing.

The lead-time effect is the part buyers most often miss. The delivery tiers on this site are set by process count and complexity — a simple part with few operations and no finishing runs to roughly 3 days, standard parts to about 7 days, and a part carrying many process steps, surface finishing, or heat treatment can run to about 30 days. Every tight tolerance and every fine finish you add is a step counted in that tier; the lead-time guide on this site walks through exactly how setups, finishing, and tolerance callouts move a part between tiers. Call tight tolerances out only where the function needs them, and the part stays in the faster tier; scatter them across the drawing, and the part quietly walks itself into the longest one. The trade is real and it is visible in the schedule before it is visible in the price.

How do you verify a quoted tolerance or finish before the parts ship?

A tolerance is a claim until a measurement backs it, and the measurement equipment is the part of the story worth checking before you commit. Dimensional callouts on critical features should be verified on coordinate measuring machines, and finish should be read on a surface-measuring instrument — a profilometer that reports an actual Ra value, not an eyeball judgment. The inspection report is what turns a quoted band into evidence you can file with the prototype and carry into production.

Ask what measures the parts and how recently the instrument was calibrated, because a measurement is only as good as the instrument behind it. On the floor behind this site, the Hexagon GLOBAL S CMMs handle the dimensional checks, a SURFCOM 1800D reads the surface finish and profile values, and the calibration program covers every instrument on a schedule and against a traceable standard, so an Ra 0.8 µm reading means the surface, not the profilometer's drift. The quality page on this site names the inspection stages and the instruments behind them. That is the standard of evidence a buyer can plan a release around.

Ask also for the process number, not just the inspection number. Capability — Cpk — tells you whether the process holds the band consistently or ships parts that happen to pass. A key characteristic held at Ppk ≥ 1.67 initial is a process under control; a part sorted after machining is a process hoping. When a supplier quotes a tight tolerance, ask for the SPC record behind it: on a representative length characteristic of 76.1–76.5 mm, the record here shows Cpk = 1.851, which is what fast-and-right looks like when it is measured instead of claimed. A number with a chart behind it is a tolerance you can trust; a number without one is a hope with a decimal point.

Read the tolerance band before you read the index. Cpk is a ratio of the tolerance band to the process spread, not an absolute measure of accuracy. The figure above was recorded on a length characteristic with a 0.4 mm band (76.1–76.5 mm) — wide by design. Hold the same process spread against a ±0.005 mm feature and it fails outright. Ask for the band, the subgroup count and the sampling window on every capability number a supplier quotes.

The surface-finish question follows the same discipline. A finish quote should come with the reading method stated — contact profilometer, laser microscope, or the process data from the finishing pass — so the Ra number on the quote means the same thing to both sides. Machined finishes from a capable floor are repeatable enough to quote against; the evidence is the reading, and the reading is the deliverable a buyer should ask to see before the prototype ships across the border.

Frequently Asked Questions

What tolerance can a rapid-prototyping shop in China realistically hold?

Milling, turning, and mill-turn work hold ±0.003 mm on critical features, and precision grinding reaches ±0.002 mm for surfaces that need a ground finish. The bands are verified on CMMs against the drawing, with process capability tracked at Ppk ≥ 1.67 at the initial study on key characteristics and Cpk ≥ 1.33 on general characteristics. Capability is reported per characteristic: every report names the feature, its tolerance band, the sample size, the sigma method, and the study window.

Should every dimension on my prototype drawing be tight?

No — tighten only the two or three features that mate, seal, or index against another part, and leave the rest on an ISO 2768 general note. Every tight callout adds a slower feed and a verification step, which is time and cost the function does not need if the dimension does not actually matter.

What surface finish can I expect as-cut on a machined prototype?

Standard milling and turning leave around Ra 1.6–3.2 µm. Ra 0.8 µm takes a fine feed or a light finishing pass; Ra 0.4 µm and finer moves the feature to precision grinding or lapping, which is a slower process family.

Is anodizing or plating a way to improve surface finish?

No. Anodizing and plating are coatings — they change the surface chemically and add a layer that shifts the final dimension, but they do not reduce the Ra of the machined surface underneath. Specify the machined finish and the coating as two separate requirements.

When does a fine finish push my prototype into a slower delivery tier?

A finish that needs grinding adds a process step and a setup, and the delivery tiers on this site are set by process count. A machined-only Ra 1.6 µm finish stays in a faster tier; a feature moved to grinding, or a part sent outside for coating, carries the extra step and the longer schedule with it.

What evidence should I ask for to verify a quoted tolerance or finish?

Ask for the measurement method and the record: CMM reports for critical dimensions, a profilometer reading for surface finish, and the SPC data behind any Cpk claim. A key characteristic held at Ppk ≥ 1.67 initial with a chart behind it is a process under control; a number without a record is a claim.

Should the prototype carry the same tolerances as the production part?

Yes — the prototype exists to validate the process that will make the real parts. Match the drawing, the tolerances, and the finish to production, and the prototype becomes the first article instead of a detour that has to be re-proven at volume.

What should I send to get an accurate tolerance and finish quote?

Send the STEP or IGES model with a toleranced 2D drawing that states the ISO 2768 general note, the Ra values on the surfaces that need them, and the coating requirement with its allowance. The more precisely the drawing states its intent, the closer the quote comes to the part you actually need.