# Surface Finish Ra: Specify and Verify It

> Understand Ra surface roughness, units, lay, filtering, and inspection conditions before choosing a machining or grinding finish.

Canonical page: https://appalachiantool.com/wiki/surface-finish-ra.html

Updated: 2026-11-06T01:55:48Z

A finish requirement should describe the surface your assembly needs and the way that surface will be checked. Ra is useful, but one roughness number does not describe every peak, scratch, direction, or geometric error on a machined feature.

## What Ra measures—and what it leaves out

Ra is an arithmetic average of the absolute height deviations of a roughness profile from its mean line. Peaks and valleys both contribute positive magnitudes to the average. It is a profile parameter, so the result concerns the measured trace rather than automatically representing an entire surface. KEYENCE's parameter guide explains the distinction between roughness, primary, and waviness profiles. [[1]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-1)

Different profiles can have the same average. A narrow isolated valley and a broadly textured surface need not perform the same way even if their Ra results match. Other parameters may be necessary when extremes or surface structure matter. Rz and Ra are different quantities; do not convert one into the other using a fixed multiplier. Definitions and evaluation conventions must follow the standard invoked by the drawing. [[2]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-2)

Start the engineering discussion with the interface: a seal, sliding contact, locating surface, coating substrate, or cosmetic face. Ask the designer which surface properties affect that function. The objective is an acceptance requirement appropriate to the interface, rather than choosing the lowest available number.

## Keep roughness, waviness, lay, and form separate

ASME's surface-texture guidance distinguishes finer-spaced roughness from more widely spaced waviness. Lay describes the predominant direction of the surface pattern. Form concerns still broader deviations from the nominal shape. Filtering separates portions of a measured profile for evaluation. These concepts explain why a smooth-looking face can still need separate geometric controls. [[3]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-3)

When reviewing a sealing face, for example, ask whether the drawing defines the required texture, the direction of the machining pattern where relevant, and the geometry of the face. This is a drawing-review example, not a recommendation for any specific seal design. A surface-texture callout should not be expected to answer a flatness question.

A profile graphic is also easy to misread. Instrument plots commonly exaggerate height relative to horizontal distance, making the surface look much steeper than it is. Read the axes and units before judging a process from its chart. The schematic on this page illustrates the idea of a profile; it is not an ATM inspection trace. [[3]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-3)

## State units and the governing standard

Roughness results commonly use micrometers or microinches. A numerical value without units invites an avoidable interpretation error. Approximately 39.4 microinches equals one micrometer; this is only a unit conversion, not a recommended finish or a statement of shop capability. [[4]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-4)

ASME B46.1 addresses surface roughness, waviness, lay, and parameters used to describe surface texture. ISO 21920-2:2021 defines terms and profile texture parameters; its ISO catalog record identifies earlier ISO 4287 documents among the withdrawn predecessors. A legacy drawing may still invoke a prior system. Do not mix an older acceptance convention with a newer parameter definition without an engineering decision. [[5]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-5) [[6]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-6)

For a new or revised drawing, name the governing standard and edition, identify the actual surfaces, and provide the required parameter and limit. If different faces have different functions, make those differences visible. For an existing drawing, send the complete notes rather than extracting a finish symbol into an email without its context.

## Agree the measurement conditions

Measurement setup matters. Mitutoyo's guide describes the effects of the stylus tip and profile filtering, and illustrates why trace direction relative to machining grooves is significant. The guide discusses older ISO conventions, so use it for measurement principles while applying the edition and settings required for the job. [[7]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-7)

Before placing an order, ask the supplier and receiving team to agree on an inspection approach:

* Which surface and region will be measured?
* Which parameter, units, filter settings, and evaluation length apply?
* What trace direction is appropriate under the specified method?
* Can the instrument reach the surface without interference?
* How will isolated damage or defects be evaluated separately?
* Is verification required before or after coating or another final operation?

Keep these decisions with the inspection plan. If two parties report different numbers, compare the drawing basis and setup details before deciding that the manufacturing process changed. A photograph of a shiny surface or a subjective fingernail test is not a replacement for the agreed quantitative measurement.

## Ask about a process route, not a universal finish

ATM's shop-supplied capability and equipment records list turning, milling, CNC ID/OD grinding, and honing. Those process listings do not establish a guaranteed Ra range for every material, size, or geometry. Confirm the proposed route against your actual drawing and acceptance conditions. [[8]](https://appalachiantool.com/wiki/surface-finish-ra.html#source-8)

For quoting, identify the material and condition, finished size, critical surface locations, required texture, and final processing state. Ask whether the proposed route needs additional finishing and how the surface will be verified. If a particular finish is difficult to access, resolve that constraint before selecting the acceptance method.

Separate the functional requirement from manufacturing preferences. If the design truly requires a named process, retain it. If the design permits alternative routes, ask for them to be reviewed explicitly. Document any approved change so the same surface definition reaches manufacturing and receiving inspection.

Review [ATM's machining and grinding processes](https://appalachiantool.com/capabilities.html) and [available equipment](https://appalachiantool.com/equipment.html). The [drawing-tolerance guide](https://appalachiantool.com/wiki/gdt-tolerances.html) addresses geometric requirements alongside finish; the [Machining Knowledge hub](https://appalachiantool.com/wiki/index.html) connects the rest of the planning topics.

## Questions before you quote

### Does a lower Ra always mean a better surface?

The appropriate surface depends on function. Ask the design authority to specify the needed texture and any additional parameters instead of assuming the smallest average is always desirable.

### Can Ra be converted directly to Rz?

No universal fixed conversion establishes conformity. The parameters describe different aspects of a profile, and the applicable definitions and evaluation method must be specified.

### What Ra can ATM guarantee?

A process list does not establish a blanket Ra guarantee. Send the drawing, material condition, surface locations, and inspection requirements for a part-specific review.

## Sources and technical basis

1. [KEYENCE: arithmetic mean height and profile types](https://www.keyence.com/ss/products/microscope/roughness/line/parameters.jsp)
2. [KEYENCE: profile and areal roughness parameters](https://www.keyence.com/blog/ra-surface-roughness-vs-sa-comparing-2d-roughness-parameters-with-their-3d-counterparts.jsp)
3. [ASME: B46.1 surface-texture reference poster](https://www.asme.org/getmedia/f22128e5-acad-429d-bc75-0ac4bdffdf44/b46-1-2019-11x17-poster.pdf)
4. [KEYENCE: surface roughness units and functional selection](https://www.keyence.com/blog/what-does-surface-roughness-really-tell-you-about-a-part.jsp)
5. [ASME: B46.1-2019 surface texture contents](https://www.asme.org/getmedia/e9205d79-74b8-4713-9c7c-bdaf10bf4b2f/B46-1_2019_ToC.pdf)
6. [ISO: ISO 21920-2:2021 profile texture parameters](https://www.iso.org/standard/72226.html)
7. [Mitutoyo: surface roughness measurement principles (legacy ISO guide)](https://www.mitutoyo.com/webfoo/wp-content/uploads/Surface_Roughness_Measurement.pdf)
8. [ATM equipment register (shop-supplied capabilities)](https://appalachiantool.com/equipment.html)

## Related guides

* [Gdt tolerances](https://appalachiantool.com/wiki/gdt-tolerances.html)
* [Cmm inspection](https://appalachiantool.com/wiki/cmm-inspection.html)
* [Material machinability](https://appalachiantool.com/wiki/material-machinability.html)

[Request a part-specific machining review](https://appalachiantool.com/contact.html?intent=wiki-surface-finish-ra)