Introduction
The useful answer is not a single number. CNC machining can produce very accurate parts, but the tolerance that is practical for a specific feature depends on the material, feature geometry, part size, workholding, cutting access, thermal behavior, surface finish, inspection method, and production route. A tight number on a drawing is therefore a manufacturing requirement to evaluate, not a general promise that should be copied from one project to another.
For many custom parts, a general tolerance framework is the efficient starting point. Tighter tolerances should be applied only to features that control fit, sealing, motion, alignment, or another defined function. This approach reduces unnecessary machining and inspection effort while protecting the dimensions that actually matter.
T-One's supplied capability information identifies ±0.01 mm as a machining reference and ±0.005 mm for selected precision features, subject to engineering review. It also lists ISO 2768-mK as a general drawing-tolerance convention. These are separate statements: ISO 2768-mK does not mean a constant ±0.01 mm across all dimensions. The final commitment must identify the feature, material, datum scheme and inspection conditions.
What Does CNC Machining Tolerance Mean?
A tolerance defines the permitted variation around a nominal dimension. A 20.00 mm diameter with a ±0.02 mm tolerance may be accepted from 19.98 mm to 20.02 mm. It does not mean every measured point will be exactly 20.00 mm, and it does not by itself define surface texture, form, position, or assembly function.
Size, form, position and surface are different controls
An engineering drawing may control several different things:
- Size controls a diameter, width, thickness, or distance.
- Form controls whether a surface is flat, round, straight, or cylindrical.
- Position controls where a hole, slot, or feature is located relative to datums.
- Surface requirement controls texture, coating allowance, cosmetic condition, or contact behavior.
These controls interact. A shaft can have the correct diameter but still create an assembly problem if a shoulder is not square enough for the bearing seat. A hole can be within size tolerance yet be in the wrong position relative to the mounting face. For this reason, an RFQ should identify critical functional features rather than listing one tight tolerance across the whole drawing.
What Determines Achievable Tolerance?
Material behavior
Materials do not react to machining in the same way. Aluminum, stainless steel, carbon steel, brass, engineering plastics, and heat-treated alloys differ in cutting force, heat transfer, stress relief, and stability after machining. Plastics in particular may move with temperature or moisture exposure. A tolerance that is sensible on a compact aluminum bracket may require a different strategy on a long plastic component.
The practical question is not simply “Can you machine this material?” It is “Can the material, shape, and measurement condition support this requirement after the part is released from fixturing and delivered into use?” That distinction is especially important for thin sections, long unsupported features, and parts that will be anodized, plated, painted, heat treated, or assembled with mating components.
Geometry and tool access
Deep pockets, narrow slots, long bores, thin walls, small internal radii, interrupted surfaces, and multi-face relationships can make a tolerance more demanding. The mechanism is usually straightforward: limited access can require a smaller or longer-reach tool; a longer tool is generally less rigid; lower rigidity can make deflection and vibration more sensitive; then the machining sequence, finishing passes, or workholding may need to become more conservative.
That does not mean a deep pocket or small radius automatically makes a part expensive or impossible. It means the feature should be reviewed with its depth, material, access direction, and functional purpose. A small internal radius at the bottom of a shallow pocket is a different manufacturing problem from the same radius at the bottom of a deep, narrow cavity.
Workholding and datum strategy
Every part must be located and clamped before it is cut. If the part is thin, flexible, irregular, or needs machining on multiple faces, the fixture and datum strategy can have a major effect on repeatability. A dimension measured from a carefully controlled datum surface is easier to manage than a chain of unreferenced dimensions across several setups.
Good drawings make this easier. Define the functional datum surfaces, show which features relate to them, and avoid tolerance stacks that ask multiple independent operations to cancel each other by chance. During installation preparation for an assembly, confirm whether the most important relationship is hole-to-hole position, hole-to-face perpendicularity, shaft runout, flatness, or another functional control.
Temperature and process sequence
Machining generates heat, and parts can change condition between roughing, finishing, deburring, washing, coating, and final measurement. For demanding work, the sequence matters: rough machining may remove bulk material; finishing may be reserved for critical surfaces; a secondary process may be placed before or after a final operation depending on the requirement.
The measurement plan matters too. A dimension evaluated immediately after a warm machining cycle may not behave exactly like the same part after it has stabilized at the agreed inspection condition. This is one reason a credible supplier should discuss both the drawing tolerance and how it will be verified.
A Practical Tolerance-Planning Method
1. Classify features by function
Start by dividing features into three groups:
Feature group | Typical purpose | Drawing approach |
|---|---|---|
Critical functional features | Bearing seats, sealing faces, locating bores, precision threads, mating interfaces | Specify the required tolerance and datum relationship. |
Controlled production features | Mounting holes, standard pockets, non-critical faces | Use a sensible drawing tolerance or general tolerance note. |
Non-critical features | Clearance areas, hidden cosmetic surfaces, non-mating profiles | Avoid over-controlling unless function requires it. |
This is not a recommendation to lower quality. It is a way to place control where it improves the finished product. When every dimension is tight, inspection effort increases everywhere and the truly critical relationships can become harder to see.
2. State the inspection requirement early
If a feature requires a CMM report, first-article inspection, material certificate, thread-gauge result, or 100% checking, include that requirement with the RFQ. T-One's documented quality flow includes incoming inspection, first-article inspection, in-process checks, patrol inspection, and final inspection. The appropriate report and sampling plan still need to be agreed for the individual project.
Early agreement avoids a common sourcing failure: the buyer expects a detailed dimensional report, while the supplier has only priced standard final inspection. Neither approach is inherently wrong; the mismatch appears when requirements are not documented before production.
3. Review finish allowance
Anodizing, plating, powder coating, painting, polishing, blasting, or heat treatment can change a surface condition or affect a finished dimension. If a coated surface mates with another component, identify whether the specified dimension applies before or after finishing. If masking is needed on threads, bores, grounding surfaces, or precision fits, mark it clearly.
For example, a press-fit diameter should not be treated like a decorative external face. The supplier needs to know whether the finish is part of the functional stack and whether a final machining step is required after the secondary process.
4. Use a representative sample or first article when risk is high
For a new part with critical interfaces, a first article creates a controlled checkpoint before full production. It can confirm not only dimensions, but also material, finish, thread engagement, assembly behavior, and inspection method. This is particularly valuable where customer requirements include an approved sample, traceable material documentation, or a special measurement format.
Factory Testing and Shipment Inspection: What Buyers Should Request
Factory testing for a machined component is usually a combination of dimensional verification, visual examination, thread or gauge checks, material-document review when requested, and confirmation of packaging or identification requirements. It is not one universal test.
Before shipment inspection, an effective checklist normally answers:
1. Which drawing revision is controlling?
2. Which dimensions are critical to function?
3. Which datums and gauges will be used?
4. Is a first-article or CMM report required?
5. Does finishing change any controlled dimension?
6. Are there customer-specific labeling, packaging, traceability, or documentation needs?
Providing this information in the quotation stage is more reliable than trying to add it after parts have been finished and packed.
Common Mistakes That Create Avoidable Cost or Risk
Applying a tight tolerance to every dimension
This may increase machining and inspection effort without improving fit or performance. Mark critical features clearly; use sensible general tolerances elsewhere.
Omitting datums
Without a clear datum structure, different parties may interpret the measurement origin differently. That can create disputes even when individual dimensions appear acceptable.
Ignoring the mating part
The tolerance of one component should be considered with its mating feature. A bore, shaft, thread, gasket face, or mounting pattern has a purpose in an assembly. Sharing a mating drawing or functional description during RFQ can reveal risks before production.
Treating one successful project as a universal benchmark
An earlier part may have been smaller, stiffer, made from another material, or inspected differently. Use prior success as useful context, not as a blanket guarantee.
Reading a Tolerance Callout Without Misinterpreting It
A useful distinction is the difference between total tolerance width and the plus-or-minus value. In the earlier 20.00 ±0.02 mm example, the full permitted range is 0.04 mm. Calling that a “0.02 mm total tolerance” would communicate a different requirement. Limit dimensions can avoid this ambiguity by giving the upper and lower acceptance values directly. Keep the units and drawing convention consistent across the package, especially when sourcing teams exchange metric and inch drawings.
General tolerance notes require similar care. ISO 2768 is a system of classes and nominal-size ranges; it should not be interpreted as one constant ± value for every dimension. An individually toleranced feature and a general drawing note serve different roles. Make the applicable standard, class, edition and priority of drawing requirements clear so the supplier does not have to guess which instruction controls.
Measurement uncertainty and acceptance
A measuring instrument displaying extra decimal places does not necessarily establish that it can verify an equally small tolerance. Resolution, calibration, setup, contact strategy and environmental conditions all affect the usefulness of a result. For a demanding feature, ask how the proposed method supports the acceptance decision. A CMM is versatile, but the correct measurement method still depends on the feature and the required characteristic.
Agree how a result close to an acceptance limit will be handled. The buyer and supplier should not discover different acceptance rules after a shipment arrives. The quotation stage is an appropriate time to identify special measurement conditions, customer gauges or an agreed reference component.
Worked Planning Example: A Machined Housing
Consider an illustrative housing with a bearing bore, a mounting pattern and a clearance pocket. This is a design example, not a customer case or a claim about a completed T-One project. The bearing bore controls a fit; the mounting pattern controls assembly position; the clearance pocket simply prevents interference.
The sensible drawing strategy is to define the bearing interface and its relationship to the locating face, then control the mounting holes from the assembly datums. The clearance pocket can follow an appropriate general requirement unless its depth or wall thickness has another function. If anodizing is planned, the drawing must state the required finished condition of the bore and any masking.
A quotation based only on the outside dimensions misses the important engineering work. A quotation based on the complete functional requirements can include the correct finishing sequence and inspection scope. This is where a DFM discussion creates value: it resolves how the feature will be produced and accepted before production starts.
Send Your Requirements: Share the drawing and highlight the features that control assembly. Request a quotation that identifies the tolerance and inspection assumptions for those features.
Comparing Tolerance Quotations Fairly
Ask each supplier to quote the same revision, material condition, finish and acceptance package. One quotation may include only standard checks while another includes a complete first-article report and controlled gauging. Price differences are difficult to interpret until those scopes are aligned.
A supplier should also distinguish a proposed design change from an accepted deviation. If a non-critical dimension could be relaxed, the engineering owner should approve the change and release a revised drawing or documented concession. A casual email comment should not silently replace the controlled specification.
For repeat orders, retain the approved drawing, finish reference, inspection plan and any accepted deviations together. This gives later batches a clear baseline even if purchasing staff or production scheduling changes. Consistency depends on maintaining the agreed requirement as well as on the machine that cuts the part.
RFQ Checklist for Tolerance-Critical Parts
Provide the controlled drawing and model with matching revisions, the material grade and condition, quantities for sample and production batches, functional datums, critical feature callouts, finish and masking instructions, inspection reports, packaging requirements and intended assembly context. Where the model and drawing differ, identify which document controls. A concise package containing these items is usually more useful than a long message requesting “maximum precision” without defining what must fit.
FAQ
Can CNC machining achieve ±0.01 mm?
It can be a practical reference for reviewed features on suitable parts, but it is not a universal promise. Material, geometry, part size, workholding, tool access, finishing, and inspection method all affect feasibility. T-One's supplied machining reference is ±0.01 mm; ISO 2768-mK is a separate drawing convention. The drawing review should define the project-specific commitment.
What is the tightest CNC tolerance available?
There is no responsible single answer without a part drawing. T-One's record includes precision work down to ±0.005 mm for reviewed features, but a tighter number should be applied only after evaluating the exact feature, material, measurement method, and acceptance criteria.
Does anodizing affect tolerance?
It can. Any secondary finish should be considered when a surface is a fit, seal, thread, electrical contact, or other functional interface. State whether the controlled dimension applies before or after finish and identify any masking requirement.
Should I request 100% inspection?
Request it when the risk, criticality, customer requirement, or agreed control plan calls for it. For other work, an agreed sampling or report plan may be more appropriate. The requirement should be included before quotation.
Conclusion
CNC tolerance is best treated as a function of the part, not a number to copy from a generic capability list. The most reliable drawings identify functional features, define datums, account for finishing, and connect inspection requirements to the assembly risk. That gives the manufacturing team a clear basis for toolpath planning, workholding, measurement, and shipment inspection.