Engineering / Engineering reference
CNC tolerances
CNC tolerances communicate design intent. Define the controlled feature and reference system before reviewing the machining setup and verification plan.
Engineering reference · Sources included
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Define CNC tolerances on the drawing
CNC tolerances communicate how much variation a controlled feature may have. Start with the part's functional requirement and the drawing's reference system, then assess the machining and verification plan. The word CNC identifies a manufacturing method; it does not, by itself, demonstrate that a particular tolerance will be met.
The ASME GD&T fundamentals overview introduces dimensioning, datum reference frames and geometric controls such as position and profile. Specify the drawing standard and its edition. This introductory reference does not replace the drawing standard's definitions or detailed interpretation rules.
| Topic | Question for the design review |
|---|---|
| Dimensions | Which dimensions communicate the required size? |
| Datums | Which reference features establish the intended frame? |
| Position / profile | Which geometric relationship must the drawing control? |
A feature's size and its relationship to other features are separate review questions. Identify the required control rather than making every dimension tighter as a substitute for stating design intent. The table is a discussion aid, not a method for choosing a numeric tolerance.
Review the setup against the feature
The Sandvik groove and slot guide explains why cutter access, overhang and support matter to a milling operation. A deep slot may need a different cutter concept or setup from a shallow, closed feature. Those considerations help frame a feasibility review; the guide does not establish a universal CNC tolerance capability.
| Input | Question |
|---|---|
| Controlled feature | Which operation produces the feature governed by the requirement? |
| Tool access | Can the tool reach the feature without a conflicting fixture? |
| Support | How will the tool and workpiece be supported during that operation? |
| Verification plan | How will the required feature be checked against the drawing? |
These are editorial review prompts. Commanded coordinates, a machine specification and a conformity result answer different questions. Ask for evidence relevant to the actual feature and setup before treating a capability claim as demonstrated.
Keep the material and drawing connected
For a 7075 component, record the required temper and stock form alongside the drawing. Kaiser's rod-and-bar data gives qualified material information; it does not determine a geometric tolerance. Keep the material specification and drawing requirements identifiable in the handoff.
For an illustrative closed-slot component, a useful review package contains the controlled drawing, stock specification, proposed operation and verification plan. The designer resolves the functional requirement; manufacturing and inspection review how to produce and check it. This example assigns no acceptance threshold and claims no validated capability.
If the required geometry is unclear, clarify the drawing before selecting machining parameters. Continue with the 7075 aluminum and CNC milling entries in Related references once the requirement and stock condition are agreed.
Sources
- Y14.5 GD&T Fundamentals Learning Path — public overviewASME · Primary referenceSupports: Drawing language, datum frames and geometric controls.Locator: Public learning-path overview: Y14.5 concepts and datum reference frames.
- Rod & Bar Alloy 7075 — Technical Data, KA-RBH-7075-8.10Kaiser Aluminum · Primary referenceSupports: 7075 temper, rod/bar stock and typical data qualifications.Locator: Pages 1–2: alloy description, comparative characteristics and typical properties.
- Groove or slot millingSandvik Coromant · Primary referenceSupports: Cutter access, stability and slot setup.Locator: Groove or slot milling: cutter concepts and how to apply.