CNC Machining Guides
Surface Roughness Ra for CNC Parts: A Buyer’s RFQ Guide
Compare Ra values, mark critical faces and agree measurement scope before ordering CNC parts. Avoid blanket finish callouts and request a clear surface-finish quote.
Do you need Ra 3.2, 1.6 or 0.8 on a machined part—and are you paying for that requirement on the right surfaces? The useful buying decision is not to request the smoothest finish everywhere. It is to connect each requirement to the drawing, the part’s function and an agreed way to check it.
Mark the surfaces that matter before requesting a quote. Include the material, quantity, finish treatment and whether roughness is required before or after that treatment. Send your drawing for a surface-finish quotation review. Ask which requirements can be supported and what needs clarification; this guide does not state a blanket YSBK roughness capability or an inspection service included on every order.
What Ra Means—and What It Cannot Tell You
Ra describes the arithmetic mean of the absolute deviations of a roughness profile from its mean line. It is not the distance from the highest peak to the deepest valley. Other parameters, including Rz, describe different aspects of texture; there is no single exact Ra-to-Rz conversion for every surface. Mitutoyo’s measurement guide illustrates these distinctions.
For the same parameter and evaluation conditions, a smaller Ra indicates a smaller average profile deviation. It does not by itself establish flatness, dimensional accuracy, a mirror appearance, freedom from scratches or suitability for a seal. Keep those requirements distinct in the order.
Ra Values and Units: A Quick Comparison
These are numerical comparisons, not universal application recommendations or YSBK process guarantees. The approximate conversions use 1 microinch = 0.0254 micrometre. Do not replace the specified value with a rounded conversion without approval.
Drawing value | Approximate equivalent in microinches | What the buyer should confirm |
|---|---|---|
Ra 3.2 µm | 126 µin | Is this the agreed requirement for the identified surfaces? |
Ra 1.6 µm | 63.0 µin | Does the function justify the lower limit, and is the inspection scope defined? |
Ra 0.8 µm | 31.5 µin | Which faces need it, and what process or access constraints affect the quote? |
Ra 0.4 µm | 15.7 µin | Has the proposed manufacturing and verification approach been confirmed? |
Check the drawing’s units and notation before quoting. “Ra 32” in microinches is not “Ra 32” in micrometres. “Polished,” “smooth” and “as-machined” are also not interchangeable with an agreed numerical requirement.
Choose Requirements by Function, Not a Generic Chart
Sealing surfaces: obtain the requirements for the actual seal, groove, mating surface and service conditions. Parker’s O-Ring Handbook distinguishes static and dynamic sealing and explains why Ra alone can be insufficient. That is why this article does not assign Ra 1.6 to every sealing face. A gasket supplier’s specification or an approved design may call for other parameters or a texture range.
Sliding or mating surfaces: identify the interface and its approved requirements. Roughness, form, dimensional fit and surface treatment are separate considerations. Do not substitute “make it smoother” for the designer’s specification.
Visible surfaces: define appearance separately, with an agreed reference or acceptance criteria where needed. Two surfaces can meet the same Ra requirement and still look different. A photograph is useful for appearance discussion, but cannot establish a numerical roughness result.
Other surfaces: ask what the supplier means by its proposed as-machined finish. Do not assume an unspecified default is Ra 3.2, that tool marks are prohibited, or that a particular roughness is achieved at no additional cost.
Example: Quote One Critical Face Separately From the Whole Housing
Hypothetical RFQ example—not a delivered YSBK job or a sealing design recommendation: a buyer needs 20 housings in 6061-T6 aluminum, each with an envelope of 100 × 60 × 25 mm. The approved drawing sets Ra 1.6 µm maximum on the mating face identified as Face A. The designer has selected that requirement; it is not inferred merely because the face touches a seal.
Item to settle | Information for the quotation |
|---|---|
Face A | Identify its full extent and the approved roughness requirement |
Remaining machined faces | State an approved general requirement or request the proposed finish for review |
Treatment | Define anodizing scope, any masking and whether the requirement applies before or after treatment |
Other acceptance criteria | Define relevant dimensions, form tolerances, edge condition and appearance separately |
Verification | Agree measurement access, locations, method, sampling and required records |
Request two clearly separated options if you want to understand a cost difference: the approved face-specific specification, and an alternative applying Ra 1.6 to more surfaces. Any alternative requires design approval before manufacture. Do not accept a cheaper finish that changes a released requirement without authorization.
The dimensions above do not define the cavity, holes, threads or tolerances. Supply the complete drawing and matching CAD revision. For a housing project, see custom machined housings.
What Can Change the Quoted Cost?
Ask whether the requested finish changes cutting strategy, tooling, setups, a secondary operation, inspection access or record preparation. The effect depends on material, geometry, surface location, treatment, quantity and the existing process. A smaller Ra does not always require polishing, and not every tighter callout adds a new operation.
Compare like-for-like quotations: the same faces, parameter, limit, surface condition and verification scope. Ask for the cost and timing of a requested alternative to be identified separately. There is no defensible universal “Ra 0.8 adds 20%” rule for every part.
If a sample or first-part approval would help, agree what it will establish, the charges and whether production waits for approval. One sample’s result does not prove every later surface in the batch. See the quotation comparison guide.
Make the Drawing and Inspection Agreement Match
ISO identifies ISO 21920-1:2021 for indicating profile surface texture and ISO 21920-2:2021 for terms and parameters. Use the standard and edition governing the drawing; do not silently reinterpret a legacy callout under a different system.
For discussion, a plain-language requirement might be: “Face A: Ra 1.6 µm maximum, before anodizing.” This is not a complete standardized drawing symbol or a universal inspection instruction. The drawing authority must resolve the applicable notation and evaluation requirements.
For numerical acceptance, agree the measuring method, measurement direction, filtering/evaluation settings, locations and sampling under the applicable specification. Access on a narrow groove or deep bore should be reviewed before quoting. Mitutoyo’s guide shows how measurement conditions affect the evaluated profile; a visual or fingernail check is not a numerical acceptance result.
Do not mix roughness with a scratch, burr or flatness requirement and expect one number to cover all of them. Specify the needed evidence before ordering. A full roughness report, specialist equipment or third-party measurement is not automatically included just because a value appears on a drawing.
If anodizing or another treatment follows machining, define the required surface condition and any masking explicitly. Do not assume treatment will correct tool marks or that a pre-treatment measurement verifies a post-treatment requirement. Use the anodizing drawing-callout guide to coordinate those instructions.
What Different Buyers Should Ask
First-time buyers: explain the function and mark the mating or visible surfaces. If the correct value is undecided, identify it as an open design question. Ask what information is missing rather than copying a low Ra value onto every face.
Experienced purchasing teams: compare the finished condition, measurement scope and exclusions as carefully as the unit price. Resolve inconsistent units or drawing revisions before placing the order.
Traders and buyers working for an end customer: carry the end customer’s actual surface requirements into the RFQ. Confirm who may approve an alternative and what records are needed for their receiving inspection. Do not promise a finish or test report upstream before the manufacturing scope is agreed.
Copy This Into Your Surface-Finish RFQ
Please quote the attached drawing and CAD revision at [quantity] in [material]. The marked surfaces require [parameter, units and limit] in the [before/after treatment] condition. Please identify unresolved callouts, confirm the proposed manufacturing and measurement approach, and state included inspection and records. Quote any proposed alternative separately with its cost and timing effect. Do not change the approved finish without written authorization.
For a smaller batch, review low-volume CNC machining. Request a quote with critical surfaces marked, including the intended interface, treatment, destination and required delivery timing. Clear surface requirements make it easier to compare the part you need, rather than an ambiguous promise of a “premium finish.”
Questions Buyers Ask About Ra
Is Ra 1.6 suitable for every sealing face?
No. Use the approved requirements for the particular sealing system and application. Ra alone may not describe all the texture features that matter.
Does as-machined mean Ra 3.2 µm?
Not automatically. Ask for the proposed finish and acceptance basis to be confirmed for the part and surfaces in your quotation.
Does Ra 0.8 mean a polished mirror finish?
No. Ra is a profile parameter, not a complete appearance specification or manufacturing-process instruction.
Can a supplier confirm Ra from a photograph?
A photograph can support an appearance discussion. Numerical conformity needs an agreed measurement approach and applicable evaluation conditions.
Will tighter roughness always make the part better?
No. The requirement should serve the approved function. Evaluate cost, geometry, treatment and acceptance together, and authorize any design change before production.
