Every metal part that ships from China carries the drawing's tolerance and finish callouts as the contract between design and function, and the distance makes the specification more important, not less: a dimension that was left ambiguous is a dimension the supplier's default will decide, and the correction loop for a wrong default is measured in weeks and freight legs. This guide lays out the realistic envelope for metal parts machined in China — what tolerances and surface finishes normal production holds, where the tight bands belong, what each callout costs, and how to write the RFQ so quotes are comparable and the parts are verifiable when they arrive.
What tolerance can metal parts machined in China realistically hold?
The realistic production envelope is the same in Dongguan as anywhere else serious machining is done. Milling and turning hold fit and alignment features to ±0.003 mm in normal production, with process capability at Cpk 1.67 or higher demonstrated on key characteristics. Where a surface demands more — a bearing seat, a seal diameter, a roundness-critical journal — precision grinding reaches ±0.002 mm as a separate operation. Features inside that envelope are routine for a serious floor; features tighter than the envelope do not exist on a milling process and must move to a different process class, which the quote should say rather than hope away.
Two conditions make the numbers honest. First, capability is proven per feature, from measurement data across the run, not asserted in a quotation — a first article at tolerance proves the setup, not the process. Second, the tolerance claim means nothing until the drawing fixes the datum scheme, because the same feature measured from different locating surfaces yields different, internally consistent readings. A ±0.003 mm bore is only meaningful against the datum the part actually locates from, and the geometric callouts — true position, runout, perpendicularity — carry their own bands that need CMM-level verification.
Where do the tight tolerances belong on a drawing?
Put the tight band where the function earns it. Bores that receive a mating part, sealing faces, bearing seats, and features that locate other components carry the critical callouts; everything else belongs on a general tolerance block such as ISO 2768. A drawing that scatters tight callouts across every dimension prices every surface as if it controlled function, and it does so twice: once in the machining, where each tight feature demands slower finishing passes, and once in the inspection, where each one demands measurement. The discipline of reserving the tight band is what keeps a metal part affordable without changing what it does.
The features that drive geometry across a part deserve the same discipline. True position and datum callouts that span several faces are exactly the callouts that define how the part assembles, and they are exactly the callouts that need a measurement plan — the datum scheme, the instrument, the report format — agreed before the quote. On an offshore order the plan matters more, because the report arriving with the parts is what your receiving inspection compares against; a report built on a different datum interpretation than your drawing is a report that starts an argument at the dock instead of closing one.
What surface finish is realistic for a machined metal part?
Surface finish on machined metal parts follows the same logic as tolerance: specify it by function, and understand what each band costs. Finish-milled and turned faces typically land between Ra 0.8 and Ra 3.2 µm. Ra 3.2 µm is a standard machined finish for non-contact faces; Ra 1.6 µm suits functional and sealing faces and is a common default; Ra 0.8 µm suits precision and sealing surfaces. Below Ra 0.8 µm, the honest process is grinding or polishing, which adds an operation, a fixture, and its own cost. The table below gives the working bands:
| Surface finish | Typical process | Where it is used |
|---|---|---|
| Ra 3.2 µm | Standard milling or turning | Non-contact faces, general machined surfaces |
| Ra 1.6 µm | Controlled milling or turning | Functional faces, light sealing faces, assembly surfaces |
| Ra 0.8 µm | Precision milling, then finishing passes or light grinding | Sealing faces, precision fits, fatigue-sensitive areas |
| Below Ra 0.8 µm | Grinding or polishing | Bearing seats, seal journals, optical-adjacent surfaces |
State the Ra value by feature group and state whether it applies before or after any coating or finishing step, because anodizing, plating, and passivation all change the surface after the tool leaves it. A face that is called Ra 0.8 µm after anodizing is a different job from one that expects it straight from the machine, and the difference belongs in the quote.
How much do a tight tolerance and a fine finish actually cost?
Machining cost is nonlinear in tolerance and finish, and the two compound. A feature moved from a general band to ±0.003 mm changes the feeds, the finishing passes, the tooling, and the inspection method at once. A surface pulled below what milling holds adds a grinding operation with its own setup and its own inspection. The same drawing with the same material can quote at very different numbers depending on how many features sit at the tight end of the envelope, which is why the tolerance block is the cheapest cost-control lever in the entire drawing.
The material interacts with both callouts. A tight feature in aluminum is routine, because the material cuts fast and holds fine finishes; the same feature in a work-hardening stainless or a thin-walled section is a process-development exercise that carries its own cost. When a buyer compares quotes for a metal part, the comparison should hold the tolerance and finish callouts constant — two quotes for the same drawing at the same bands are comparable, while a quote that quietly assumes a looser finish is a quote for a different part at a number that will grow. The landed cost adds the same warning: a part quoted at a tighter finish than production can hold is a part that will be reworked at your dock or shipped late.
Why do geometric callouts deserve their own care on a drawing?
Dimensional tolerances describe size; geometric tolerances describe how features relate to each other, and on a machined metal part the relationships are where assembly and function live. A bore can measure perfectly within its ±0.003 mm band and still be eccentric to the datum it must align with, or a face can be flat within its size tolerance and still sit at an angle to the surface it seals against. That is why drawings built on GD&T carry callouts for true position, runout, perpendicularity, flatness, and parallelism, each with its own band, its own datum reference, and its own measurement method.
The cost and verification story follows the callouts. A geometric tolerance against a datum scheme needs a CMM with the datum structure programmed into the inspection routine, and a report that shows the measured relationship rather than a size reading. When a buyer names the datum reference frame on the drawing and asks the supplier to measure position and form to the standard the drawing was built on — ASME Y14.5 for most export work — the acceptance test stops being a matter of opinion. On a part machined in China, that agreement has to happen before the quote, because the report that arrives with the parts is the evidence your receiving inspection will compare against; a report built on a different datum interpretation than your drawing is a report that starts an argument at the dock instead of closing one. The quote should state that it will measure the geometric callouts, and the inspection plan should name which instrument verifies each one.
How should a critical tolerance and finish be verified?
Verification is where a tolerance or finish callout becomes a fact instead of a hope, and on an offshore part the report is the only verification you see before the lot ships. A ±0.003 mm feature belongs on a CMM against the drawing's datum scheme, not on a hand gauge at the shipping bench; a fine finish belongs on a profilometer; a roundness-critical journal belongs on a roundness instrument. The drawing or the purchase order should name the measurement method for each critical characteristic, and the report should show measured values against the callouts, with the instrument and date identified.
Agree the measurement reference as well as the instrument. Dimensional measurement references 20 °C, and a long aluminum feature measured on a warm floor will disagree with the same feature measured in a metrology lab, so decide where the critical features are measured and which result wins when the numbers differ. The quality page shows how CMM, surface, and roundness measurement are structured on lots shipped from this floor, which is the benchmark the evidence should meet. And ask for the capability data once the process is stable, not just the first-article readings — a single measured value proves the setup was right that day; a capability number at Cpk 1.67 or higher on a key characteristic proves the process holds the band across the run.
How do tolerances and finishes interact with secondary operations?
Heat treatment and surface treatment change the tolerance and finish story in ways buyers should plan for. A part that is hardened after machining moves during the heat-treatment cycle, so a critical journal or seal diameter is usually ground after heat treatment — the process chain becomes machine, heat treat, grind, and each step needs its own inspection record. Anodizing grows the dimension, so the finish allowance must be machined in before the coating, and the coating thickness on the certificate is what the print calls out. Passivation removes embedded iron from machined stainless but does not change the dimension; it restores corrosion resistance after the tool has done its work.
The schedule and cost follow the same chain, and on an offshore order the treatment step carries its own logistics. A heat-treated part with ground journals routes through a treatment partner with its own queue, and the freight plan has to include the treatment leg. When you compare quotes for a metal part, compare the process routes as well as the prices: a quote that omits the post-treatment grinding operation is a quote that will grow when the part cannot hold its final callout without it. The services overview explains how machining, finishing, and inspection combine into one order.
What should go in the RFQ to make quotes comparable?
Everything above condenses into a short list that should travel with the drawing. Name the tolerance block the part runs to — the general block, plus the critical callouts with their bands. Name the surface finish by feature group, with the value and whether it applies before or after any treatment. Name the measurement method for each critical characteristic — which instrument, against which datum scheme — and the document set you expect with the lot. Then ask each candidate to state the tolerance it will hold per critical characteristic, which instruments will verify them, and what the report will contain.
A supplier that answers with named instruments and named processes is engineering your part; one that confirms everything with no questions has quoted the drawing without reading it. The parts guide shows how tolerance and finish callouts land on a quoted metal part, and the contact page is where a model and a tolerance block become an itemized quote with the measurement plan named.
Keep one rule in front of the whole exercise: the drawing is the contract, and the callouts are the specification. When the tolerance block, the finish-by-feature callouts, and the measurement methods are written down and agreed before the quote, the parts arrive verifiable and the argument never starts; when they are left to defaults and habit, the first article becomes the argument and the supplier's interpretation wins by default — a problem made worse by the distance, because the correction loop is measured in weeks and freight legs. Specifying the drawing with care is the cheapest way to make a machined metal part function as designed: it costs little more than the hour it takes to write the callouts properly, and it saves the rework, the re-inspection, and the schedule slip that an underspecified drawing always finds.
Frequently Asked Questions
What tolerance can metal parts machined in China realistically hold?
Milling and turning hold fit and alignment features to ±0.003 mm in normal production, with process capability at Cpk 1.67 or higher 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.; precision grinding reaches ±0.002 mm for bearing seats and sealing surfaces. Features inside that envelope are routine; tighter bands need a different process and a different cost.
Where should the tight tolerances go on a drawing?
On the features that mate, seal, rotate, or locate — bores that receive a mating part, sealing faces, bearing seats, datum-controlled surfaces. Everything else belongs on a general tolerance block such as ISO 2768, because a wall of tight callouts multiplies inspection cost and rejects risk without adding function.
What is a realistic surface finish for a machined metal part?
Finish-milled and turned faces typically land between Ra 0.8 and Ra 3.2 µm, with Ra 1.6 µm a common default for functional faces and Ra 0.8 µm for sealing and precision faces. Below Ra 0.8 µm, the honest process is grinding or polishing, which adds an operation and its cost.
How much does a tight tolerance or fine finish add to the price?
Machining cost is nonlinear in tolerance and finish. A tight band changes the feeds, the finishing passes, the tooling, and the inspection method at once, and a surface below what milling holds adds a grinding operation. Every feature pulled to the tight end adds time and measurement, so tighten only what function earns.
How should a critical tolerance be verified on a China-sourced part?
On the instrument the band requires — a ±0.003 mm feature belongs on a CMM against the drawing's datum scheme, a fine finish on a profilometer, a roundness-critical journal on a roundness instrument. The drawing or the PO should name the measurement method, and the report should show measured values, not pass marks.
How do tolerances and finishes interact with the material?
The material decides how hard the band is to hold. Aluminum holds fine finishes readily; stainless work-hardens and needs rigid setups; thin walls in any material deflect under the tool. A tight feature in an easy material is routine; the same feature in a tough alloy or a thin section is a process-development exercise.
What should I put in the RFQ to make quotes comparable?
Name the tolerance block, the critical callouts with their bands, the finish by feature group, and the measurement method, all on one drawing revision. Ask each quote to state the tolerance it will hold per critical characteristic, which instruments will verify them, and what the report will contain — identical inputs make the outputs comparable.
Need a quote that prices your tolerance and finish callouts honestly?
Send the drawing with the tolerance block and the finish by feature group to [email protected] or call +86 13126536955 — the quote will name the instruments, the process route, and the measurement plan in writing.