63 Ra surface finish explained for machined metal parts
What a 63 Ra surface finish means
A 63 Ra surface finish usually means an arithmetic average roughness of 63 microinches, which converts to about 1.6 micrometers Ra. In machining terms, it is a controlled finish, not a mirror finish. It is smoother than a rough general machined surface, but it is less demanding than fine grinding, honing or lapping.
The important point is that Ra is an average roughness parameter. It is not a maximum scratch depth, not a flatness requirement, and not a complete prediction of how the surface will perform in service. A drawing that simply says 63 Ra may be understood in many inch-based machine shops, but a clearer specification states the unit, applicable standard, measurement method and any functional direction or lay requirement. For related finishing topics, see the surface finishes category.
Ra describes the arithmetic mean deviation of a measured roughness profile from its mean line. Public summaries from NIST list roughness average, or Ra, among the surface roughness parameters used in calibration work, while ISO 21920-2:2021 specifies terms, definitions and parameters for determining surface texture by profile methods. (nist.gov)
63 Ra in microinches and micrometers
The most common source of confusion is the unit system. On many U.S. inch drawings, a callout such as 63 Ra is normally read as 63 microinches Ra. One microinch is 0.000001 inch, so 63 microinches is 0.000063 inch. Using the exact inch-to-metric conversion, 63 microinches equals 1.6002 micrometers, usually rounded to 1.6 µm Ra.
| Common callout | Equivalent value | Typical interpretation |
|---|---|---|
| 63 µin Ra | About 1.6 µm Ra | Controlled machined finish |
| 1.6 µm Ra | About 63 µin Ra | Metric expression of the same roughness level |
| 0.000063 in Ra | 63 microinches Ra | Decimal-inch expression, less common on drawings |
The number should not be separated from the unit system. A metric drawing that says Ra 63 µm would describe a very rough surface compared with 63 µin Ra. When old prints, supplier notes or scanned drawings omit units, confirm the drawing standard and base units before quoting, machining or inspecting the part.
Where 63 Ra fits among common surface finishes
A 63 µin Ra finish sits near the middle of many machining roughness charts. It is usually smoother than 125 µin Ra, which is often treated as a general as-machined surface, and rougher than 32 µin Ra or 16 µin Ra, which may require tighter machining control or a secondary finishing operation. Industry reference charts commonly pair 63 µin with 1.6 µm and, on legacy roughness grade tables, with N7. Those charts are useful guidance, but they do not replace the controlling drawing or standard. (refmanual.com)
| Ra value | Metric equivalent | General shop meaning | Typical caution |
|---|---|---|---|
| 125 µin Ra | 3.2 µm Ra | General machined surface | May show visible tool marks |
| 63 µin Ra | 1.6 µm Ra | Controlled machined surface | Still not a sealing or mirror finish by itself |
| 32 µin Ra | 0.8 µm Ra | Fine machined or lightly finished surface | Often needs more process control |
| 16 µin Ra | 0.4 µm Ra | Fine finishing, grinding or polishing range | Cost and inspection effort usually increase |
This comparison helps with early cost and process planning. The actual requirement, however, depends on material, feature geometry, cutting direction, tool condition, fixture rigidity, coating and how the surface contacts another part. Two surfaces can both measure 63 Ra and still behave differently, because Ra averages height deviations. It does not describe the shape, spacing or direction of the peaks and valleys.
When 63 Ra is a reasonable callout
A 63 Ra surface finish is often reasonable for machined metal faces that need more control than rough stock or a heavy roughing pass, but do not need an ultra-smooth finish. Examples may include non-critical mating faces, bracket surfaces that contact other machined parts, bored or turned features with moderate functional contact, and general mechanical components where a consistent machined appearance is expected.
It may not be enough for every sealing, bearing, sliding, sanitary or fatigue-sensitive surface. A gasket face, hydraulic sealing land, press-fit surface or shaft running under a lip seal may need a different roughness value. It may also need parameters beyond Ra, such as Rz, lay direction, waviness limits or a defined process. On the other hand, specifying 32 Ra or 16 Ra everywhere can raise cost without improving function if the surface does not actually need that finish.
A practical way to decide is to connect the roughness value to the surface function:
- Cosmetic control: 63 Ra can reduce heavy tool marks, but it is not the same as a decorative polish.
- Static contact: 63 Ra is often suitable for general metal-to-metal contact if sealing is not the controlling requirement.
- Moving contact: Check wear, lubrication and material pairing before relying on Ra alone.
- Sealing: Confirm the seal supplier or engineering standard; roughness, waviness and lay may all matter.
- Coated surfaces: Specify whether the 63 Ra requirement applies before coating, after coating, or both.
How shops produce a 63 Ra finish
There is no single process that automatically creates 63 Ra. Milling, turning, boring, reaming and grinding can all produce surfaces near this range under suitable conditions. The result depends on cutting speed, feed per revolution or tooth, tool nose radius, insert condition, machine rigidity, coolant, material microstructure and vibration. A worn insert, long tool overhang or unstable setup can leave chatter marks even when the nominal feed calculation looks acceptable.
In turning, a light finishing pass with the right nose radius and stable support is often enough. In milling, cutter runout, step-over, tooth marks and toolpath direction can dominate the visual pattern. In boring, bar stiffness and chip evacuation are frequent limitations. Cast iron, stainless steel, aluminum and hardened alloys can produce different roughness with the same programmed settings because each material cuts, tears or smears differently.
For purchasing and quality discussions, it is safer to describe 63 Ra as a target roughness level rather than a process name. A buyer should not assume that 63 Ra requires polishing, and a supplier should not assume that any standard CNC pass will meet it on every feature. Feature size, accessibility, material condition and inspection method all matter.
How to inspect and verify 63 Ra
A 63 Ra requirement is normally verified with a surface roughness instrument rather than by sight or touch. Contact stylus profilometers are widely used because they trace a profile across the surface and calculate roughness parameters from the measured line. NIST describes a mechanical stylus profilometer as an instrument used for surface texture and microform measurements, and its calibration material discusses traceability and uncertainty for roughness parameters including Ra. (nist.gov)
Inspection should be planned before production when the surface is functionally important. The drawing or inspection plan should clarify the following points:
- Parameter: State Ra, and add Rz, Rt or another parameter only when it has a functional reason.
- Unit: Use µin Ra or µm Ra instead of a bare number.
- Acceptance rule: State whether the value is a maximum, a range or another defined requirement.
- Measurement direction: Define direction if lay affects function, especially for sealing or sliding contact.
- Surface condition: Clarify whether inspection occurs before plating, after coating, after heat treatment or after final cleaning.
- Sampling locations: Identify critical zones when only part of a face controls performance.
Visual comparison plates can help operators understand approximate texture, but they are not a complete substitute for instrumented measurement when the drawing requires numerical verification. Dirt, burrs, isolated scratches, chatter and waviness can also complicate results. If a surface fails inspection, the correction may involve a toolpath change, sharper tooling, a different insert radius, lower feed, better fixturing or a secondary finishing step. See also: Buying Guides.
Standards and drawing notes to check
For U.S.-style drawings, ASME B46.1 is a central standard for surface texture, including surface roughness, waviness and lay. ASME lists B46.1-2019 as reaffirmed in 2026, so teams working from current ASME-based documentation should verify the exact edition referenced by their drawing or quality system. (asme.org)
For ISO-based drawings, the ISO 21920 series is important. ISO 21920-1 covers indication of profile surface texture and replaced ISO 1302:2002; ISO 21920-2 covers terms, definitions and surface texture parameters; and ISO 21920-3 covers specification operators. Public standards summaries and national metrology guidance describe this series as the successor to older ISO profile-texture documents. (iso.org)
Many active shops still see older drawings that use legacy symbols, N-grade references or title-block notes such as unless otherwise specified, 63 Ra. Those notes should be interpreted in context. If the drawing was created under an older standard, do not silently convert the requirement into a newer format without engineering approval. If a new drawing is being created, a clearer callout might read Ra 1.6 µm max under the applicable ISO standard, or 63 µin Ra max under the applicable ASME standard, with added lay or process notes only when they are functionally needed.
Common mistakes with 63 Ra
Confusing microinches and micrometers. This is the most expensive mistake. 63 µin and 63 µm are not close; 63 µm is roughly 39 times rougher than 1.6 µm.
Treating Ra as a maximum peak height. Ra is an average. A surface can have a compliant average roughness but still contain an isolated scratch or burr that affects sealing, fatigue or assembly.
Using roughness to control flatness. A surface can be smooth but not flat, or flat but rough. Flatness, parallelism and profile tolerances are geometric controls, while Ra is a texture parameter.
Over-specifying non-functional surfaces. Calling out 63 Ra on every surface may add machining time and inspection work without improving the part. Use general notes for non-critical surfaces and tighter callouts only where function requires them.
Ignoring post-processing. Blasting, tumbling, anodizing, plating, polishing, passivation and heat treatment can change measured roughness. If the final assembled condition matters, specify the inspection stage.
Frequently asked questions
Is 63 Ra smooth?
It is smooth enough for many machined metal parts, but it is not a mirror finish. A better description is a controlled machined finish. Whether it is smooth enough depends on the surface function.
Is 63 Ra the same as 1.6 Ra?
It can be, if the units are different. 63 µin Ra is approximately equal to 1.6 µm Ra. A bare value should always be confirmed against the drawing unit system and applicable standard.
Can CNC machining achieve 63 Ra?
Yes, CNC turning and milling can often achieve 63 µin Ra on suitable materials and accessible features with stable tooling and a proper finishing pass. However, tool wear, chatter, feed, geometry and material can prevent compliance, so inspection is still necessary for controlled surfaces.
Does 63 Ra control sealing performance?
Not by itself. Sealing can depend on roughness, waviness, lay direction, material, gasket type, contact pressure and surface damage. If sealing is critical, the drawing should include the full surface texture requirement recommended for that application.
Should a drawing use Ra 63 or Ra 1.6?
Use the value that matches the drawing unit system, and include the unit clearly. For inch-based drawings, 63 µin Ra is common. For metric drawings, 1.6 µm Ra is the clearer equivalent. The safest callout also identifies the applicable standard and states whether the value is a maximum.
