CNC machining tolerances define the variation your part can accept while still doing its job. Start with the assembly requirement, identify the features that control function, and agree how those features will be measured. A tighter number on every dimension can add cost without solving the actual fit problem.
This CNC machining tolerances guide explains how to prepare a drawing for quotation, check a simple mating-part tolerance stack, and connect dimensional requirements with inspection. It does not assume that one tolerance value applies to every material, part size or machining supplier.
What a machining tolerance controls
A nominal dimension describes the intended size. Its tolerance defines the permitted variation. You can express that variation as upper and lower limits, as equal plus and minus values, or as unequal deviations from nominal. The notation must leave the acceptable range clear.
General tolerances cover dimensions within the stated scope of a drawing note or specification when no individual tolerance is given. Feature-specific tolerances identify requirements that need separate control. A drawing can use both, provided their scope and precedence are unambiguous.
Dimensional size is only one part of the specification. A hole can have an acceptable diameter and still sit in the wrong location. A mounting face can meet a thickness dimension while its shape prevents the assembly from seating properly.
| Requirement | What it addresses | What you need to define |
|---|---|---|
| Size limits | Permitted size variation | The feature and its upper and lower limits |
| Geometric control | Form, orientation, location or runout, as applicable | The correct control and any required datum references |
| Surface texture | The condition of a specified surface | The texture requirement and measurement criteria |
| General tolerance note | Dimensions covered by a shared rule | Applicable specification, edition, class and scope |
These requirements work together. A smaller size tolerance does not automatically provide all the geometric or surface controls an assembly needs. The governing drawing standard may also establish relationships between requirements, so avoid interpreting individual callouts in isolation.
Start with the assembly requirement
Before assigning values, identify what must happen when the part is installed. Does a shaft need to slide, locate, rotate or remain fixed? Does a mounting face establish alignment? Must a hole pattern match another component, or does a clearance opening only need to avoid interference?
For each important interface, record the mating feature and the consequence of variation. This makes it easier to distinguish a functional requirement from a dimension that merely describes the part's overall shape.
Choose references that represent the assembly
Datum references establish the framework for applicable geometric controls. Select physical features that represent how the part is located or supported in use, then define the controls in the chosen drawing system. The most convenient surface to measure is not always the correct functional reference.
For example, a mounting plate may require its locating holes to relate to the seating face and locating edges. Controlling only the distance between the holes could leave the pattern incorrectly positioned relative to the assembly. The appropriate drawing solution depends on the actual interface and governing standard.
Keep datum features accessible for manufacturing and verification where possible. If a critical reference will be hidden after assembly or finishing, discuss the inspection sequence before releasing the drawing.
Keep process choice separate from the requirement
Your drawing should define the result that matters. A supplier can then assess a manufacturing route against that requirement. Selecting milling or turning alone does not establish a guaranteed part tolerance. Geometry, material, workholding, finishing and inspection all affect the proposed route.
For a process comparison, see CNC milling vs turning. Use that comparison to discuss the part's shape and access requirements while keeping the drawing as the acceptance reference.
Check the full tolerance stack
Two acceptable components can still produce an unacceptable assembly if their combined variation has not been considered. Review the dimensions that contribute to the functional gap, alignment or engagement, rather than tightening each component independently.
Consider an illustrative hole and shaft. All dimensions below are in millimetres. The example uses ideal circular features at the same reference condition and considers diameter differences only. It is not a recommended fit or a designation from a fit standard.
| Item | Minimum diameter | Maximum diameter |
|---|---|---|
| Hole | 10.00 mm | 10.02 mm |
| Shaft | 9.96 mm | 9.98 mm |
The smallest diametral difference occurs with the smallest hole and largest shaft: 10.00 − 9.98 = 0.02 mm. The largest occurs with the largest hole and smallest shaft: 10.02 − 9.96 = 0.06 mm.
These are diametral differences, not radial clearances. They illustrate the arithmetic of size limits; they do not prove that a real shaft will assemble or perform correctly. Form error, alignment, surface condition, temperature and the final coating condition may change the functional result.
If your required gap falls outside the calculated range, reconsider how the tolerance is shared between the mating features. Changing the nominal sizes may be more appropriate than narrowing both tolerance bands. The decision belongs to the assembly design, followed by a supplier feasibility review.
Longer assemblies may involve several contributing dimensions. A worst-case stack combines their limiting effects. Statistical methods require justified assumptions about the processes and distributions; they should not be used simply to make an inconvenient stack appear acceptable.
Write tolerances that can be quoted
A quote is easier to compare when every supplier receives the same acceptance requirements. Provide a controlled drawing or an agreed model-based definition package, and identify which file governs when information differs.
State units and the governing rules
Show whether dimensions use millimetres or inches. Identify the drawing standard and applicable edition. Where you use a general tolerance specification, state the relevant class and scope rather than writing only “standard tolerances.”
Decimal places can carry meaning under an explicit title-block rule, but decimal formatting alone does not establish a universal tolerance. Likewise, a supplier's published default is not automatically the requirement for your purchase order. Confirm any default that will form part of the quotation.
Avoid mixing geometric notation or default assumptions from different drawing systems. If an inherited drawing has conflicting notes, resolve them before asking suppliers to commit to the part.
Make exceptions easy to find
Call out the critical features clearly and identify the dimensions governed by general requirements. Use a consistent feature or inspection identifier when you need a report, so the drawing and measured results can be matched without interpretation.
Avoid redundant dimensions that create conflicting acceptance criteria. If you provide a reference dimension for convenience, identify it appropriately under the chosen drawing system. Resolve differences between the CAD model, drawing and purchase specification before production.
Specify the finished condition
State whether a critical dimension applies before or after coating or another finishing operation. Material addition or removal can change the fit, and masking may be needed on selected surfaces. A finish name alone does not explain how every interface must be protected.
Keep detailed finish selection in the CNC machining surface finish guide. In your tolerance package, identify the affected features and the condition in which they will be accepted. Ask the supplier to flag any conflict between the proposed finish and the drawing limits.
Match inspection to the requirement
A tolerance is useful only when the feature can be assessed with a suitable method. Discuss inspection while preparing the quote, especially for deep bores, inaccessible surfaces, flexible features or relationships between several faces.
A caliper reading, a bore measurement and a coordinate measurement do not necessarily answer the same question. The method must address the specified characteristic and reference system. A measurement of local diameter, for example, does not by itself verify every aspect of a bore's geometry.
Distinguish resolution from measurement confidence
The number of digits on an instrument display does not establish the uncertainty of the result. Calibration, setup, contact conditions, alignment, the part and the environment can all contribute to the measurement.
For a demanding characteristic, agree how the inspection method supports the tolerance and how results close to a specification limit will be evaluated. The acceptance decision rule should be clear between buyer and supplier. Avoid assuming that any displayed value inside the limits settles the question regardless of uncertainty.
Agree the part condition during measurement
Temperature affects dimensions. A recently machined part and its measuring equipment may not be at the same temperature, and dissimilar materials can respond differently to temperature changes. Define relevant reference conditions and allow an appropriate measurement procedure to account for them.
Also identify whether the part is inspected free of restraint or in a specified setup when this matters to function. Clamping can affect a flexible feature's measured shape. The inspection arrangement should reflect the drawing requirement, rather than whichever setup produces the most convenient result.
Define the report you need
Specify the characteristics to report, the drawing revision and the required evidence. If you need first-article results, sampling or inspection of every part for selected features, agree that scope before placing the order.
An inspection report records results under its stated method and sampling plan. It does not automatically establish process capability across future batches. Requests for capability evidence need a separate agreement about the relevant process, data and evaluation.
Control cost without weakening function
Review tight tolerances feature by feature. Ask what would fail if a particular limit were relaxed, and check the answer against the assembly analysis. Retain the requirements that protect function; reconsider inherited precision that has no identified purpose.
Tighter limits can require changes in machining, workholding or inspection. The effect depends on the part, so a generic percentage cost premium is rarely a useful basis for your decision. Ask for a quote against the released drawing and, where appropriate, a separately identified alternative for a proposed design change.
Keep the alternatives distinct. A lower price based on relaxed limits is not directly comparable with a quotation that accepts the original specification. Record any agreed change in the controlled drawing or purchasing package before production.
Prepare your CNC tolerance RFQ
Before sending your request, check that the package includes:
- The current drawing and CAD revision, units and file precedence.
- Material grade and condition, with the required finishing operations.
- General tolerance rules and clearly identified feature-specific requirements.
- Functional interfaces, mating-part information and relevant datum references.
- The finished state in which critical dimensions must be accepted.
- Inspection characteristics, report scope and any agreed sampling requirements.
- Order quantity and any separate prototype and production requirements.
- Open questions that need a written feasibility response before the order is accepted.
You do not need to prescribe every manufacturing step. You do need enough information for the supplier to identify exceptions, propose a route and explain how the important requirements will be verified.
Frequently asked questions
What is the standard tolerance for CNC machining
There is no single value that automatically governs every CNC part. Use an explicit drawing requirement or an agreed supplier default with a clear scope. Material, feature geometry, part size and the proposed process must be considered before a supplier confirms feasibility.
Do more decimal places mean a tighter tolerance
Only when the drawing's stated rules assign that meaning. A title block may define tolerances by decimal places, but that convention must be explicit. Check the governing notes and individual callouts instead of inferring acceptance limits from formatting alone.
Is a size tolerance enough for a mating hole
It may not be. Size limits address diameter, while the assembly may also need control of location, orientation, form or surface condition. Review the mating function and the governing drawing rules before deciding which additional requirements are necessary.
Does a tight tolerance require inspection of every part
The tolerance defines an acceptance requirement; it does not by itself specify a sampling plan. Inspection frequency and reporting should be agreed based on the application, risk and purchasing requirements. Measuring every part also does not remove the need for a suitable method.
Discuss your drawing with Jinquan Seiko
When you request CNC machining services, include the drawing revision, material, quantity and the features that control your assembly. Identify the inspection evidence you need and any dimensions affected by finishing.
Send your drawing to Jinquan Seiko to discuss the requirements for your part. Ask for confirmation of the proposed scope and any exceptions before approving the order.
