Guide
What does a break all sharp edges note require, and how big should the edge break be?
An edge break is the small amount of material removed from a corner where two machined or cast surfaces meet, left as a tiny chamfer or round so the edge no longer cuts, chips or holds a burr. The familiar general note “BREAK ALL SHARP EDGES” asks for one on every edge that the drawing does not dimension, but on its own it names no size, no shape and no way to check the result.
By Saola Manufacturing engineering team · Published · Updated
This guide covers why the bare note causes disputes, how ISO 13715 turns an edge requirement into a signed number, how to handle edges on die castings and coated parts, and how to write the note for a supplier abroad. It describes the standards, not the practice of any particular shop.
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What does “break all sharp edges” actually require?
Read literally, it requires only that no edge be left sharp, and it leaves the size, the shape and the acceptance test to whoever makes and inspects the part. That is why two competent shops can ship very different edges against the same drawing and both claim compliance.
The note survives because it is short and because, for a great many parts, the difference does not matter. Trouble starts on parts where the edge does something: it guides a mating part into a bore, it carries a seal, it must take an even coat of anodize, or it sits next to a hand on a finished product. On those parts the unwritten number becomes a real requirement that nobody wrote down.
The bare note also hides three separate questions. Is a burr allowed at all, and how large? Must the edge be broken, and by how much at most, so that a small feature is not lost to an oversized chamfer? And does the requirement cover every edge, including inside corners, cross holes and thread starts, or only the edges you can see? A drawing that answers those three questions no longer needs anyone to interpret the word “sharp”.
How does ISO 13715 specify edges of undefined shape?
ISO 13715:2017 (adopted in Europe as EN ISO 13715:2019) gives a drawing symbol for edges whose exact shape does not matter, together with a signed size: a plus value allows material beyond the ideal sharp corner, and a minus value requires material to be removed inside it. The shape inside that limit is left to the manufacturer, which is what “undefined shape” means.
The standard’s own scope, as summarized in the BSI catalog record for BS EN ISO 13715:2019, is the indication and dimensioning of such edges where the edge condition matters for function or safety, for example where an edge must be free of burrs or where a controlled imperfection is acceptable. It applies to both outside and inside edges.
- Outside edges. A plus value is a burr allowance: material may stand proud of the ideal corner by up to the stated amount. A minus value is an undercut: the corner must be broken, by a chamfer, a round or anything else, within the stated depth.
- Inside edges. A plus value allows material to remain in the corner, the way a fillet left by a tool nose does. A minus value asks for an undercut into the corner, such as a relief so that a mating square edge can seat fully.
- Either direction. A plus-or-minus value allows either condition up to the stated size, which is the closest the system comes to a plain “sharp edge”.
A single value sets a limit, and two values set a band, so an outside edge can be required to be broken by at least one amount and at most another. The symbol can be placed on an individual edge, or used once near the title block as a general indication that covers every edge not otherwise shown, with any exceptions listed next to it. That general form is the direct, measurable replacement for a bare “break all sharp edges” note.
One change in the 2017 edition matters for anyone copying an old drawing. Per the BSI record, the revision removed the recommended edge sizes and the definition of a sharp edge that the 2000 edition carried. The standard now tells you how to state an edge, not what size to pick, so the number has to come from the design.
Which edge callout should you use?
Use a dimensioned chamfer or radius where the edge has a job, an ISO 13715 general indication for every other edge, and the bare note only on parts where any light deburr is acceptable. The table compares the options.
| Callout | Example on the drawing | What the inspector can check | Best used for |
|---|---|---|---|
| Bare general note | BREAK ALL SHARP EDGES | Only that edges are not sharp to the touch | Internal, non-cosmetic parts where any deburr works |
| General note with a range | BREAK SHARP EDGES 0.1–0.4 mm | Size of the break, if the note says removal, not burr | Legacy ASME drawings being tightened up |
| ISO 13715 general indication | Edge symbol with −0.1 / −0.4 near the title block, exceptions listed | Direction and size of every undimensioned edge | Default for new drawings on ISO or ASME sheets |
| ISO 13715 symbol on one edge | Edge symbol with +0.05 on a sealing edge | Burr limit on that edge only | Seal lands, cutting edges, bore entries |
| Dimensioned chamfer or radius | 0.5 × 45° or R1 with tolerance | Full geometry, like any other dimension | Lead-in chamfers, stress-relief radii, visible design edges |
| Fillets modeled on a casting | All cast edges filleted in the model, parting line excepted | As-cast geometry against the model | Die-cast faces that stay unmachined |
Whichever you use, do not let a general edge break swallow a small feature. A thin wall, a narrow land or a short thread engagement can lose a meaningful share of its size to a break that would be trivial on a large face. Put such edges in the exception list or give them their own callout.
How big should an edge break be?
No current standard gives one answer, because the 2017 edition of ISO 13715 dropped its recommended sizes, so pick the size from what the edge must do. The smallest break that removes the burr and makes the edge safe to handle is usually the right default, and larger breaks belong only where there is a functional reason.
Four functions drive the size. Handling safety needs only enough to remove the cutting edge and the burr. Assembly needs a lead-in on the entry of a bore or on the end of a shaft, large enough to guide the mating part, which is usually better drawn as a dimensioned chamfer. Coating needs a round rather than a sharp chamfer corner, for the reason explained in the next section. Cosmetics needs a consistent break on visible edges, so state that the break must look uniform and agree a reference sample if appearance matters.
General tolerances can help here. ISO 2768-1 includes a separate table for external radii and chamfer heights, so a drawing that already cites that standard has a default tolerance for any dimensioned break; the ISO 2768 and NADCA tolerances guide explains how that note is read.
Why do sharp edges matter for anodizing, plating and paint?
Coatings build unevenly at sharp corners, and anodic coatings in particular form voids at a sharp edge, so edges on a part that will be coated should be rounded, not just deburred. The US military anodizing specification says this outright.
MIL-A-8625F, the Department of Defense specification for anodic coatings on aluminum, warns in its design notes that because of the way the coating forms, anodic coatings develop voids at sharp corners and edges, that sharp edges and corners are difficult to anodize satisfactorily, and that all edges and inside corners should be radiused before anodizing (MIL-A-8625F, section 6.10.3). The same section ties the recommended radius to the coating thickness in its Table III, so a thick hard coat needs a larger round than a thin decorative one.
The practical reading for a drawing is simple. On any part that will be anodized, plated or painted, write the edge requirement as a round, or as an ISO 13715 minus value with a note that the break shall be radiused, and make it large enough for the coating thickness you specify. A chamfer made with a sharp tool leaves two new edges, and each one can still thin the coating.
How should edges be treated on die castings compared with machined faces?
On a die casting, round the edges in the model so the die produces them, and treat the parting line as a separate edge that is trimmed rather than broken. A general edge-break note should apply to machined edges, where the tool can actually produce it.
NADCA’s design guidance advises that every sharp corner and edge of a die-cast part be filleted when the part is modeled, down to the smallest features, and it names one exception: the edge around the perimeter where the die halves meet, which is left sharp (NADCA, fillets). The reason is that the parting line is where flash forms when metal pressure pushes the die halves apart, and the gates, overflows, vents and flash are then removed by a trim die (NADCA, parting line).
That leads to three separate edge requirements on a cast-and-machined part:
- As-cast edges. Controlled by the fillets in the model and the casting tolerance, not by a machining note. A general “break all sharp edges” note does not create a fillet that the die casting tool never had.
- The parting line and trimmed gates. State how much residual flash or trim witness is acceptable, and whether it must be removed by hand, by tumbling or by machining. This is a burr limit, which is exactly what an ISO 13715 plus value expresses.
- Machined edges. Faces and bores machined after casting get the general edge indication like any machined part. Watch for porosity opened at a machined edge, which can look like a chip and is a casting question, not an edge-break question.
Is there an official test for whether an edge is sharp?
In the US, the only federal sharp-edge test is written for toys and other articles for young children, so for industrial parts the drawing itself must define what passes. That is one more reason to write a number instead of the word “sharp”.
The Consumer Product Safety Commission rule at 16 CFR 1500.49 uses a rotating mandrel wrapped in a single layer of PTFE tape, pressed against the edge with a set force and turned one revolution; the edge counts as sharp if it cuts completely through the tape over a length of at least 1/2 inch (16 CFR 1500.49). Its scope is limited to toys and articles intended for children under eight. Toys and articles that must be sharp to work, such as toy scissors, are exempt only if they have no other sharp edges and, for toys, carry a conspicuous warning label.
How should the edge note read on a drawing sent to an overseas supplier?
Give it as a number with a direction and a list of exceptions, in the drawing’s own unit system, so it reads the same in translation. A shop in Vietnam, Mexico or anywhere else can then quote and inspect the edge as written instead of guessing what “sharp” meant to the designer.
- One general edge rule. An ISO 13715 general indication, or a note that states removal with a lower and upper limit. Say “break” means removal, not an allowed burr.
- Exceptions named. Sealing edges, datum edges, thin walls and thread starts that must stay sharp or need their own limit, listed next to the general rule.
- Coating stated. If the part is anodized, plated or painted, say that edges shall be radiused before finishing and give the coating so the round can suit its thickness.
- Castings separated. Parting line and gate witness limits for as-cast edges, and the general edge rule for machined faces only.
- Inside edges covered. Say whether cross-drilled holes and internal intersections must be deburred; they are the edges most often missed, and the hardest to check after assembly.
Words like “sharp”, “slight” and “smooth” do not survive translation well, while a signed number does. The RFQ package guide covers the rest of what to send with the drawing. When the package is ready, send it for a quote, and any edge requirement that needs discussion is raised in the reply rather than found at first-article inspection.
Frequently asked
Is “deburr” the same as “break all sharp edges”?
Not quite. Deburring removes the loose or raised material that machining leaves; an edge break goes further and removes the sharp corner itself. A part can be fully deburred and still have edges that cut, so if you want both, say both, or use an ISO 13715 minus value, which implies both.
Can the supplier choose between a chamfer and a radius?
Under an edge-of-undefined-shape callout, yes: the shape inside the limit is the manufacturer’s choice. If the part will be anodized or painted, or the edge is visible, say that a radius is required, because a chamfer leaves two new corners.
Does the note apply to inside corners and holes?
A bare note is read differently from shop to shop. ISO 13715 treats inside and outside edges separately, so state whether the general indication covers inside edges, cross-drilled holes and thread starts, and list any that must stay as machined.
How is an edge break inspected?
Match the method to the callout. A bare note gets a visual and touch check; a numeric break is measured with a magnifier, optical comparator or edge gauge on a sample of edges; a cosmetic edge is compared with an agreed reference part. Put the method in the inspection plan so both sides use the same one.
Sources
- BS EN ISO 13715:2019, Technical product documentation — Edges of undefined shape — Indication and dimensioning, BSI catalog record, 2019 (identical to ISO 13715:2017, which replaced ISO 13715:2000)
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 1993, section 6.10.3
- Fillets, die casting design guidance, NADCA, accessed September 2026
- Parting line, die casting design guidance, NADCA, accessed September 2026
- 16 CFR 1500.49, Technical requirements for determining a sharp metal or glass edge, US Consumer Product Safety Commission, eCFR, accessed September 2026
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)