Machine Design #33: How to Specify Chamfers, C and R

A note such as “break edges,” “C1,” or “R as required” may be interpreted very differently by the designer, machine shop, and quality team. A seemingly minor edge can directly affect assembly, operator safety, coating, fatigue strength, and inspectability.

This article distinguishes an edge of undefined shape from a geometrically defined chamfer and radius, then explains how to specify each requirement by function so the shop does not have to stop and ask for clarification.

1. Three requirements that are often confused

Edge of undefined shape

The designer only needs to limit burr or missing material and does not require the edge to be an exact bevel or arc. This is an “edge of undefined shape.”

Geometrically defined chamfer

The edge must be a bevel with a defined size and angle, for example a linear dimension combined with an angle. This may be required to guide assembly, prevent interference, or create functional geometry.

Geometrically defined radius R

The edge must be an arc with a specified radius. A radius may reduce stress concentration, provide clearance, match the cutting tool, or protect a seal.

If the function only requires the edge not to be sharp, do not automatically turn every edge into a tightly inspected chamfer or radius.

2. “Break all sharp edges” is not a complete requirement

The instruction “break all sharp edges” communicates a safety intent, but it leaves several questions unanswered:

In which direction may a burr remain?
How much missing material is acceptable?
Must the edge be chamfered, rounded, or only deburred?
Which edges contact a seal, cable, or the operator’s hand?
Are functional edges also covered by the general note?

For noncritical edges, a project-wide edge convention may be appropriate. Functional edges should be called out separately and excluded from the general note where necessary.

3. When should a chamfer be used?

A chamfer is suitable when the design must:

guide a shaft, pin, or part into a hole;
provide clearance from a mating edge;
remove a sharp edge with controllable geometry;
prepare an edge for a subsequent process;
provide a visual or assembly orientation feature.

A defined chamfer should specify at least two independent parameters, usually a size and an angle. Abbreviations such as C should only be used when the project, customer, and supplier have explicitly agreed on their meaning.

Do not assume that every organization interprets “C1” in the same way. When drawings cross countries or go to a new supplier, a complete callout is usually safer.

4. When should radius R be used?

A radius is suitable when:

stress concentration at an internal corner must be reduced;
a seal or cable passes over the edge;
the geometry must match a cutter or grinding-wheel radius;
cast, stamped, bent, or molded parts need better material flow;
a sharp edge must be removed without creating a flat bevel.

An internal radius also affects machinability. A milling cutter has a radius, so a perfectly sharp internal corner is not realistic. Too small a radius may require a small tool, increasing cycle time and reducing rigidity. Too large a radius may interfere with the mating part.

An accurate external round also has a cost. If the function only requires deburring, do not specify a tight radius without a functional reason.

5. External and internal edges have different failure modes

External edges

Typical risks include burrs, cuts to operators, scratches on adjacent parts, severed wires, and damaged coating. The requirement should control both excess material and the extent of edge break.

Internal edges

Risks include residual material that causes interference, a cutter radius that reduces assembly clearance, or an undercut that weakens a loaded area. Both the mating geometry and the tool path must be considered.

A single note covering “all edges” may therefore be unsuitable when external edges only need deburring but internal corners require a functional radius.

6. Do not forget burr direction

A burr is defined not only by its size; its direction also matters. An outward-facing burr may cut a hand or block assembly, while the same burr facing a nonfunctional area may be less critical.

When burr direction affects the product, the drawing or process requirement should state:

which side must be burr-free;
which datum or contact surface must be protected;
that deburring must not round a functional feature;
whether inspection is visual, tactile with a safe tool, by gauge, or by suitable equipment.

Bare fingers should not be used to inspect sharp edges in a production environment.

7. C and R tolerances must follow function

Not every chamfer or radius needs an individual tolerance. Before adding one, ask:

Does the chamfer guide a close-clearance component?
Does the radius determine fatigue strength or clearance?
Is the feature produced by a form tool or by manual deburring?
Will quality control use an optical comparator, CMM, gauge, or visual inspection?

If the function only requires “not sharp,” an undefined edge zone is usually more economical than precise C/R geometry. If the actual profile must be controlled, the callout and inspection method must support that requirement.

8. Relationship to surface treatment and heat treatment

An edge can differ before and after coating. Plating, anodizing, paint, or other coatings may build up at an edge, while deburring after treatment can damage the protective layer.

Heat treatment may also make a thin edge prone to distortion or oxidation. The sequence should be clear:

machine the feature;
deburr, chamfer, or round it as specified;
perform heat or surface treatment;
protect the edge during transport;
inspect the finished condition if that is the functional state.

The drawing should state whether the edge requirement applies before or after treatment whenever the difference matters.

9. Callouts that reduce shop-floor questions

For nonfunctional edges

Use one general note based on the applicable standard or agreement.
Define its scope: external edges, machined edges, or all unspecified edges.
Exclude functional edges with dedicated callouts.

For functional chamfers

Fully define the geometry: size, angle, and tolerance where needed.
Identify the feature and number of edges if ambiguity is possible.
Check that the callout does not conflict with assembly dimensions.

For functional radii

Place the R callout on the correct corner or surface.
Specify a maximum, minimum, or tolerance when clearance or stress requires it.
Confirm machinability and the inspection method.

For repeated identical edges

A general instruction or “n places” may be used if it creates no ambiguity. Avoid dimensioning the same feature in multiple views.

10. Common mistakes

Using C without agreeing whether it implies a 45° chamfer.
Writing “R as required” where assembly clearance is limited.
Applying one deburr note to sealing or press-fit edges.
Specifying a perfectly sharp internal corner on a milled part.
Using an unnecessarily small radius that increases tooling cost and cycle time.
Specifying a precise chamfer when quality control has no suitable inspection method.
Failing to state whether the requirement applies before or after coating.
Giving different values for the same edge in multiple views.
Relying on a rendered CAD image instead of a clear drawing callout.

11. Pre-release checklist

Does this edge only need to be non-sharp, or does it need defined geometry?
Is the failure mode interference, a cut hazard, seal damage, stress concentration, or a burr?
Should the requirement use an undefined edge, chamfer, or radius?
Are the size and angle sufficient to define the chamfer?
Is R compatible with the cutting tool and mating part?
Does burr direction affect function?
Does the general note unintentionally cover a functional edge?
Does the requirement apply before or after surface treatment?
Can the supplier produce it consistently?
How will quality control verify it?

Conclusion

C, R, and “break edge” are not interchangeable ways to write the same requirement. An edge of undefined shape is appropriate when only burr or missing material must be limited; chamfers and radii are appropriate when the edge geometry has a clear function.

Selecting the right requirement, fully defining it, and planning the manufacturing and inspection method reduces questions, rework, and unnecessary cost caused by a small edge.

Public references

ISO 13715:2017 — indication and dimensioning of edges of undefined shape; confirmed current in 2024.
ISO 129-1:2018, including Amendment 1:2020 — general principles for presenting dimensions, applicable when edge geometry is defined, such as a 1 × 45° chamfer.

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