Undercut per DIN 509: Forms, Dimensions, Labeling

A manufacturing drawing from the Far East shows the callout "DIN 509 – E 0.6x0.3" at a shaft shoulder. In-house, though, it's not immediately obvious what that actually specifies. If the undercut is missing entirely on the finished part, a hub or bearing often won't seat flush against the intended shoulder. The reason is an unintended radius transition getting in the way.

In brief: An undercut (Freistich) is a standardized relief groove on a shaft or bore per DIN 509. It provides tool clearance for grinding, ensures adjoining parts seat flush, and reduces stress peaks at the transition. The standard distinguishes forms E, F, G, and H. The drawing callout, such as "E 0.6x0.3", states the form, radius, and depth in millimeters.

What Is an Undercut, and What Is It Used For?

An undercut is a locally limited relief groove on a shaft or bore with a form standardized under DIN 509. It's usually placed directly in front of a shoulder or collar, as precifast.de explains in detail, and serves three purposes in practice:

• Tool clearance: Grinding or fine-turning an adjoining surface requires room for the tool without it running into the shoulder itself.

• Assembly safety: Without an undercut, the transition from the cylindrical surface to the shoulder retains a small, unavoidable tool radius. That radius keeps a hub or bearing from sitting flush.

• Stress reduction: As a defined notch, the undercut redirects the force flow and reduces stress peaks that would otherwise be considerably higher at a sharp or uncontrolled transition.

What Forms Exist Under DIN 509 (E, F, G, H)?

DIN 509 distinguishes four basic forms, as cad-markt.de confirms, which differ in geometry and intended use.

Form E is used when only one of the two adjoining surfaces needs further machining. Form F is the most commonly used version in practice, since it covers both sides at once. Form G suits lightly loaded parts where space is tight. Form H, by contrast, specifically reduces notch stress on more heavily loaded parts through a larger radius.

When designing a new part, it's worth agreeing the form with the manufacturer early, since it directly affects tool choice and machining time.

What Does the Callout "DIN 509 – E 0.6x0.3" Mean?

The drawing callout follows a fixed pattern: standard, form, and two dimension values in millimeters.

For the normal size of Form E, the standard specifies fixed values by shaft diameter:

The radius carries a tolerance of ±0.1 mm. For higher loads, DIN 509 specifies different, equally fixed values for forms G and H, which designers pull from the complete standard table.

Absent a specific callout, the undercut surface defaults to a roughness of Rz ≤ 25 µm. Other dimensions without an individual callout fall back similarly on general tolerances per ISO 2768. The current edition of the standard dates to December 2022, superseding the previous 2006 edition.

What's the Difference Between an Undercut per DIN 509 and a Thread Relief per DIN 76?

Both standards govern a relief groove on a shaft, but they cover different applications. DIN 509 applies to general undercuts on cylindrical surfaces and shoulders, regardless of whether a thread is present. DIN 76, by contrast, specifically governs the thread relief groove, meaning the run-out right at the end of a thread. It lets the thread-cutting tool run out cleanly so the last thread turn doesn't end incomplete.

Anyone reading "undercut" on a drawing should check carefully whether the callout references DIN 509 or DIN 76. Both standards use similar letter-and-number schemes, but they describe geometrically different features at different locations on the part.

In practice, this mix-up often shows up on shafts with a threaded section near a shoulder. A manufacturer who reads the callout as DIN 509 instead of DIN 76 correctly machines the general undercut in front of the shoulder. This manufacturer, however, overlooks the thread relief actually required at the end of the thread. As a result, the thread-cutting tool can't run out cleanly, and the last thread turn stays incomplete.

How Is an Undercut Labeled on a Technical Drawing?

An undercut is typically indicated with a leader line pointing to the relevant spot on the shaft or bore. It's labeled with the short callout made up of the standard, form, and the two dimensions, such as "DIN 509 – F 0.6x0.3". A complete graphical rendering with every detail isn't required, since the short callout alone fully defines form, radius, and depth. Form and position tolerances work the same way, specified through a compact symbol instead of a lengthy description. How closely an undercut relates to the fit between adjoining parts also shows up in H7 fits and tolerance tables. Together, the two specifications determine whether a shaft and hub actually mate as intended.

In our projects with Far East manufacturers, we occasionally see drawings where the undercut is sketched graphically but carried over without the exact short callout. In that case, it stays unclear which form and which dimensions are actually meant to be manufactured, and the maker ends up choosing at their own discretion. That's difficult to reconstruct later during incoming inspection, which is why the complete short callout should already be on the source drawing.

What Should Buyers Watch for on Parts With an Undercut?

A missing or incorrectly executed undercut often only shows up at assembly, when a part doesn't seat against a shoulder as intended. When sourcing, it's worth checking three things:

• the complete short callout in the inquiry

• consistency between the drawing and what's actually manufactured

• coordination with adjoining parts like bearings or hubs

When qualifying Far East suppliers for turned or ground shaft parts, we specifically check whether the manufacturer actually machines undercuts to the stated DIN 509 short callout. Alternatively, we check whether they instead apply a similar but not identical local practice. Small deviations in the radius rarely affect function in practice. During a later dimensional check, though, they quickly show up as a formal deviation from the drawing dimension, even when the part fits perfectly fine in reality.

Frequently Asked Questions

What does DIN 509 mean?

DIN 509 is the standard that defines undercuts on shafts and bores using standardized forms (E, F, G, H) and the associated radius and depth dimensions.

What is an undercut used for?

An undercut provides tool clearance for grinding or fine-turning, prevents assembly collisions at shoulders caused by an otherwise unavoidable tool radius, and reduces stress peaks at the transition.

What's the difference between DIN 509 and DIN 76?

DIN 509 governs general undercuts on cylindrical surfaces and shoulders. DIN 76 specifically governs the thread relief groove at the end of a thread, a geometrically different feature with a different function.

Does an undercut always need to be dimensioned?

For unambiguous manufacturing, the complete short callout made up of the standard, form, and the two dimensions should appear on the drawing. Without it, the manufacturer decides at their own discretion, which complicates later inspection.

Conclusion: Undercuts Are a Small but Decisive Drawing Detail

An undercut looks like a minor detail on a drawing. It directly determines whether a part can actually be ground, whether adjoining parts seat correctly, and how much load the transition can safely carry. Knowing the forms and short callout under DIN 509, and stating them completely in the inquiry, avoids misunderstandings between design and manufacturing.

Line Up also checks the correct execution of undercuts per DIN 509 when qualifying Far East manufacturers for turned and ground shaft parts. 👉 Schedule a no-obligation consultation.