What Tolerances Can CNC Machined Parts Typically Achieve?
What Tolerances Can CNC Machined Parts Typically Achieve?
CNC machined parts commonly use ±0.02 mm to ±0.05 mm as an RFQ screening range for accessible, rigid size features. Selected bores or diameters may be planned at ±0.005 mm to ±0.01 mm only when geometry, material state, setup, finishing, and measurement capability support the requirement. These are not blanket part or supplier guarantees. The drawing should define each critical size, datum-related geometric control, final condition, surface texture, and acceptance method before capability is confirmed.
Surface roughness is a separate requirement. Ra 1.6 μm to 3.2 μm can be an initial as-machined planning range, while Ra 0.8 μm to 1.6 μm may require a controlled finishing pass or another process on selected surfaces. CNC grinding can improve size, form, and texture when qualified for a specific feature. polishing can lower visible roughness but may round edges or remove stock, so it cannot substitute for a dimensional or geometric acceptance rule.
1. Separate Size Tolerance from Geometric Acceptance
Typical CNC tolerance cannot be reduced to one number because size, location, orientation, form, and surface texture control different failure modes. A plus-or-minus size band limits a measured dimension; position or profile controls a feature relative to datums; flatness controls form without a datum. The drawing must identify which control protects fit, sealing, alignment, motion, or interchangeability.
Use a general tolerance only for truly noncritical unmarked dimensions. If ISO 2768-1 or another general-tolerance standard is required, state its edition, class, and application. It does not define supplier capability. Critical features need explicit callouts, datums, modifiers, and an inspection method that matches the ASME Y14.5 or other specified drawing language.
Control Type | RFQ Screening Boundary | Qualification and Acceptance Action |
|---|---|---|
General accessible size | ±0.02 mm to ±0.05 mm on rigid features | Confirm nominal range, setup, final condition, and gauge method |
Selected fit bore or diameter | ±0.005 mm to ±0.01 mm only after capability review | Check process evidence, thermal state, form needs, and measurement uncertainty |
Hole position or feature relationship | No responsible generic number without a datum scheme | Define basic dimensions, datums, modifiers, and the inspection plan |
Ground precision surface | Size, roundness, runout, and Ra remain separate | Qualify stock allowance, grinding datum, burn risk, and final inspection |
2. Surface Roughness Does Not Define Size or Sealing Alone
Surface roughness describes texture over a specified evaluation length; it does not prove diameter, flatness, waviness, sealing, friction, or cosmetic appearance. An Ra value can be correct while a bore is out of size or a sealing face has unsuitable directional lay. The drawing should name the parameter, value, functional location, lay when relevant, and governing surface-texture standard.
An as-machined Ra range must be confirmed against material, toolpath, cutter condition, and measurement method. grinding may improve cylindrical size and texture together, but acceptance still checks each callout. polishing can reduce peaks and alter appearance while also softening edges or changing size; use it only where those effects are allowed.
3. Rigidity, Datum Transfer, and Release Drive Variation
Thin walls, long shafts, deep pockets, interrupted cuts, and weak workholding can move under cutting force or heat. A feature measured while clamped may shift after release. The relevant result is the part in the drawing's stated free-state or restrained condition, after it has stabilized enough for the agreed measurement.
Multiple setups add a datum-transfer chain. A bore and face made in one setup may maintain their relationship better than features re-established from secondary fixtures, but this is not automatic. The supplier should map machining datums to inspection datums, control locating-surface condition, and verify the final relationship rather than quote machine positioning accuracy.
4. Material State and Temperature Change the Measured Result
Material affects tolerance through thermal expansion, residual stress, hardness, burr behavior, and distortion during stock removal. Thin aluminum may relax after roughing; heat-treated steel may move before finish grinding; plastics may absorb moisture or creep under clamping. The material grade, temper, heat-treatment sequence, stock condition, and inspection state must be fixed before quoting a tight band.
Under ISO 1:2022, dimensional and geometrical product specifications use 20 °C as their reference temperature. That does not mean every shop reading is automatically corrected or stable. The control plan should state stabilization conditions, measurement temperature where relevant, post-coating or post-heat-treatment status, and whether burrs or surface films are present.
Variation Source | Likely Failure Mode | Required Control and Final Check |
|---|---|---|
Part rigidity | Feature springs after unclamping or wall deflects under probing | Define free or restrained state; release, stabilize, then inspect |
Material and thermal state | Size drifts after heat treatment, coating, or temperature change | Fix the process sequence and inspect in the final specified condition |
Setup and datum transfer | Hole, face, or axis relationship shifts between operations | Map process datums to drawing datums and verify the relationship |
Measurement system | Gauge resolution looks adequate but pass/fail results disagree | Specify method, traceability, uncertainty, decision rule, and sampling |
5. Grinding Helps Selected Features but Changes the Control Plan
Standard milling or turning may establish the main geometry, while CNC grinding is reserved for a fit diameter, journal, bore, face, or other feature needing tighter size, form, or texture control. Grinding needs stock allowance, a stable reference, process control, thermal management, and defined final inspection. It is not evidence that every dimension on the part can use the same tolerance.
Grinding also introduces distinct risks, including burn, taper, lobing, datum mismatch, and size change during cooling. A buyer should identify the functional feature and acceptance characteristics instead of asking for a generically “ground part.” If only one interface needs the tighter result, isolate it on the drawing and keep unrelated dimensions at suitable general tolerances.
6. Tighter Tolerances Need a Measurement Decision Rule
A tight tolerance is credible only when the measurement system can resolve and reproduce the required decision under agreed conditions. Instrument resolution alone is insufficient. Probe strategy, fixturing, datum simulation, calibration status, surface condition, temperature, operator method, and measurement uncertainty can all change the reported value.
Metrological traceability links a result to a reference through a documented, unbroken calibration chain; it does not prove that the selected method is suitable. The RFQ should name the characteristic, inspection equipment or method, sampling or full-inspection requirement, report format, and conformity decision rule. For a narrow band, resolve guard banding or uncertainty before production rather than after a disputed rejection.
7. Build the Drawing Around Function and Final-State Acceptance
Apply tight tolerances to features that control assembly, sealing, bearing fit, alignment, motion, or load transfer. A size limit on a hole does not locate the hole; a surface-finish callout does not establish flatness; a position tolerance without a coherent datum reference may not protect assembly. Functional drawing logic matters more than a larger number of decimal places.
State whether acceptance occurs before or after anodizing, plating, heat treatment, passivation, coating, deburring, or assembly. Finishes can change size and edges, and post-process fixturing can distort thin parts. Include mating-part requirements and functional gauges or test conditions when they define success more directly than isolated coordinate measurements.
8. Send a Tolerance RFQ That Can Be Qualified
A tolerance-ready RFQ identifies part revision, material and condition, quantity stage, critical sizes, datum scheme, GD&T standard, surface texture, final finish state, and inspection/reporting requirements. Mark features expected to use grinding or surface finishing, but do not assume those processes automatically deliver the desired size, form, or function. Ask the supplier to flag unsupported callouts and propose a qualification route before price and lead time are fixed.
Treat ±0.02 mm to ±0.05 mm and ±0.005 mm to ±0.01 mm as conditional screening ranges, not acceptance language. Release the order only after the supplier confirms each critical feature, process sequence, measurement method, and final-state decision rule. That closes the gap between a nominal CNC capability statement and a part that assembles and performs as the drawing requires.
