Surface Finish Specification: Ra, Rz, and How to Apply Them on Drawings

Surface Finish: Functional Requirement, Not Aesthetic Choice

Surface roughness specification is one of the most frequently under-specified and over-specified elements of mechanical engineering drawings. Designers often copy roughness values from previous drawings without thinking about the functional requirement, or they specify the tightest possible value to be safe — without considering that tight surface finish requirements drive cost, machining time, and inspection requirements significantly. A correctly specified surface finish is one that is as coarse as the function allows and no finer than necessary.

This guide covers the parameters Ra and Rz, how they are measured and interpreted, what values to expect from common manufacturing processes, and how to apply surface finish symbols correctly to engineering drawings per ISO 1302.

Ra: Arithmetic Mean Roughness

Ra is the arithmetic mean of the absolute deviations of the surface profile from the mean line, measured over a specified evaluation length. It is the most widely used roughness parameter globally and is the default parameter in most manufacturing and quality systems.

Ra has a key limitation: it is an average value and is insensitive to the shape of the profile. A surface with occasional deep valleys (from feed marks or inclusions) and a surface with uniformly fine texture can have the same Ra value while behaving very differently in service. For sealing surfaces, bearing interfaces, and functional contact surfaces, Ra alone may not be sufficient specification.

Rz: Mean Roughness Depth

Rz (also called Rz DIN or 10-point mean roughness) is the average of the five largest peak-to-valley distances in the evaluation length. It is more sensitive to occasional deep scratches or peaks than Ra. For surfaces where occasional high peaks would damage mating components (e.g., sliding seals, precision bearing seats), specifying Rz provides additional control that Ra alone cannot give.

As a rough rule of thumb, Rz ≈ 4 to 7 × Ra for typical machined surfaces. When a drawing specifies Ra 1.6 μm, the corresponding Rz is often in the range of 6–10 μm. For critical surfaces, measure and specify both.

Rmax (also called Rt) is the maximum peak-to-valley height over the entire evaluation length — the single worst deviation. It is used in applications where even one high peak would cause damage, such as optical surfaces and precision sealing faces.

Typical Values by Manufacturing Process

ISO 1302 Surface Finish Symbol Interpretation

ISO 1302 defines the graphical symbol and notation for surface texture on engineering drawings. The basic symbol is an inverted check mark (√). Key elements:

• Material removal required: The basic symbol with a horizontal bar (⊥) indicates that material removal is required to achieve the specified roughness — machining, grinding, etc.
• No material removal: A circle added to the symbol vertex indicates the surface must be achieved without material removal — as-cast, as-forged, or already at the required finish.
• Roughness value position: The primary roughness value (e.g., Ra 1.6) is placed above the horizontal line of the symbol.
• Machining allowance: Placed in parentheses to the left of the symbol for casting/forging drawings.
• Lay direction: Indicated by a letter below the roughness value when the direction of lay affects function (= parallel, ⊥ perpendicular, × crossed, M multidirectional).

Application Rules on Drawings

Surface finish symbols are applied to the drawing using these conventions:

• Symbols are placed on the surface line in the view that shows the true surface, or on an extension line from that surface
• The symbol reads from the bottom or right of the drawing (same orientation as dimensions)
• A general note can specify the default surface finish for all surfaces not otherwise indicated: Unless otherwise specified, all surfaces √ Ra 3.2
• Surfaces with a different requirement from the general note get their own symbol
• Internal surfaces (bores, slots) should be called out in the section view where the surface is visible

Cost Implications of Tight Specifications

Every step tighter in surface finish specification typically increases machining time and cost significantly. Ground surfaces require a dedicated operation; lapped surfaces require still more time. Specifying Ra 0.4 μm on a surface that only requires Ra 1.6 μm for its function adds unnecessary cost. A good discipline is to ask: what is the functional reason for this surface finish requirement? If you cannot answer that question, the specification is probably copied from habit rather than driven by function.

FAQ

Q: Which parameter should I use as my default specification — Ra or Rz?

A: Ra is the correct default for general machined surfaces, as it is universally measured and understood. Use Rz in addition to Ra when the surface is a sealing interface, a precision sliding surface, or any application where occasional deep scratches are functionally significant. ISO 1302 and most national standards permit both; Ra is specified more frequently in practice because it is more widely measured on shop-floor instruments.

Q: My drawing says Ra 1.6 for a turned journal surface. Is this achievable with standard turning, or does it require grinding?

A: Ra 1.6 μm is achievable with finish turning using sharp tooling, correct feed rate (typically below 0.1 mm/rev), and good surface condition. However, it is at the boundary of what is reliably producible by turning — shops vary in consistency. For a bearing seat or precision journal where the dimension tolerance is also tight (IT6 or better), cylindrical grinding to Ra 0.4–0.8 μm is the more reliable and standard approach, and the grind allowance should be included in the machined dimension.

Q: How do I specify a surface that is to be left as-cast or as-forged?

A: Use the ISO 1302 symbol without a horizontal bar (basic symbol only) with the addition of the no-machining indicator (circle at the vertex). Alternatively, use a drawing note: This surface to be as-cast, no machining required. If a specific roughness is required for the as-cast surface (e.g., for cosmetic reasons), specify the required Ra value and note the process constraint. For surfaces that are to be blasted or chemically treated, note the specific process and its expected finish in the drawing notes or material specification.

Reference Standard

The exact rules behind these callouts trace back to one standard that most drawings ultimately answer to.

ASME Y14.5-2018: Dimensioning and Tolerancing

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