ISO 21920-2 Revision: What to Specify Now
Published 2026-09-28 · Tolerances
The ISO 21920-2 revision is still a committee draft, not a final requirement. ISO/CD 21920-2 Edition 2 is under development, was registered on 22 June 2026, and reached the close-of-comment stage on 19 August 2026. It will replace ISO 21920-2:2021 and ISO/TR 23276:2020.
For a CNC quote today, use ISO 21920-1:2021 and make the surface requirement measurable. Ashkan, CNC engineer at CNC Instant Quote, sees the same failure repeatedly: an unclear Ra or Rz note changes inspection work after the part is already priced.
What does the ISO 21920-2 revision mean now?
The ISO/CD 21920-2 Edition 2 project is active but unfinished. The ISO/CD 21920-2 page identifies it as under development and records the 22 June 2026 registration and 19 August 2026 close-of-comment stage.
That status does not create a new final requirement for your released drawing. Do not predict technical changes from the draft or replace an approved customer specification without agreement. Use the published ISO 21920-1:2021 indication rules for current quoting, then identify any draft-based transition in your engineering notes.
The practical gap is clear. Ranking pages explain the 2021 surface-texture transition and symbol basics, but buyers still need to know what the 2026 draft changes for a quote today: nothing final yet, while ambiguous legacy notes still create cost and schedule risk.
What should your drawing specify under ISO 21920-1:2021?
ISO 21920-1:2021 is the current drawing reference for profile-based surface texture indication. Clause 4 covers tolerance acceptance rules, while Clause 6.3 covers minimal indication.
A surface texture drawing callout may identify the parameter, numerical limit, acceptance rule, filter or nesting information, evaluation length, lay, and manufacturing process. You do not need every item on every face, but each functional requirement must be unambiguous.
Ra and Rz are not interchangeable
Ra 3.2 µm, Ra 0.8 µm, and Rz 6.3 µm describe different requirements. Do not price Rz 6.3 µm as if it were Ra 6.3 µm. If the parameter controls sealing, sliding, bearing contact, or appearance, state why the limit matters and how acceptance will be judged.
Mentor CNC’s guide supplies the useful contrast for older drawings: ISO 1302, ISO 4287, and ISO 4288 remain common legacy references. If your team calls the transition an ISO 1302 replacement, retain the old note for traceability but clarify the current acceptance method before release.
| Drawing note | Quote interpretation | Use when |
|---|---|---|
| Ra 3.2 µm | One roughness parameter and numerical limit; remaining method details follow the governing indication rules. | General machined faces with a standard acceptance approach. |
| Ra 0.8 µm with filter and evaluation length | Adds controlled measurement conditions and inspection time. | Seals, bearing seats, or other functional faces. |
| Rz 6.3 µm | Uses a different parameter; it must not be converted to an Ra value. | Profile height extremes affect function. |
| ISO 1302, ISO 4287, or ISO 4288 note | Requires engineering review before current acceptance and measurement rules are assigned. | Old drawings being quoted for a new production run. |
How does an ISO 21920 surface finish note change a CNC quote?
An ISO 21920 surface finish note changes a CNC quote when it adds controlled measurement, a tighter machining pass, or a special inspection method. A plain Ra value can be quick to price. A value with filter, nesting, evaluation length, lay, or acceptance details may require more setup and inspection.
The failure trigger is an ambiguous Ra/Rz or acceptance rule. That ambiguity forces engineering clarification or an unplanned measurement method. The part may be machined correctly and still fail because the inspection interpretation changed after production.
See also: ISO 9001:2026 Supplier Questions for Machined Parts
What changes when a legacy note is clarified?
A representative anonymized quote comparison from a real 6061-T6 aluminum sensor housing shows the effect. The part measured 82 × 46 × 18 mm, with quantity 4 and a black anodized finish.
| Drawing condition | Surface callout | Quoted result | Decision |
|---|---|---|---|
| Legacy note | Ra 3.2 | $68.40 per unit; 4 business days pending clarification | Use only as a preliminary price. |
| Clarified note | Ra 3.2 µm, ISO 21920-1:2021, defined acceptance, 4.0 mm evaluation length, no special lay | $74.90 per unit; 6 business days | Use for a released production quote. |
The increase came from controlled inspection and documented acceptance, not from changing the aluminum alloy. If the same drawing also leaves wall thickness, internal radii, or tool access vague, review CNC Machining Design Mistakes Cost You More Than You Think before requesting final pricing.
Material and market changes can also move a quote. If your aluminum, copper, brass, steel, or plastic part changes price for supply reasons, see CNC Machining Tariffs 2026: Why Your Quote Costs More for the separate cost driver.
See also: CNC Router vs Mill: Choose from Your CAD
How do you write a quote-ready surface texture drawing callout?
- Name the parameter and limit, such as Ra 3.2 µm, Ra 0.8 µm, or Rz 6.3 µm. Failure mode: an Ra/Rz ambiguity sends the quote into engineering review.
- State the governing reference and acceptance rule, such as ISO 21920-1:2021 with the required tolerance acceptance approach. Failure mode: the shop selects a measurement interpretation you did not intend.
- Add filter or nesting information and evaluation length when the function depends on measured profile content. Failure mode: inspection requires an unplanned method after machining.
- Specify lay or manufacturing process only when it controls function, sealing, contact, or appearance. Failure mode: a broad process note gets priced as a blanket finish requirement across every face.
- Upload the STEP file with the clarified drawing note through Instant Quote →
ISO 21920-2 revision FAQ
Is ISO/CD 21920-2 Edition 2 mandatory?
No. It is a committee draft under development, not a final requirement. Use the published drawing standard and your customer’s approved specification until a final revision is published and adopted.
Can you quote a drawing that still uses ISO 1302?
Yes, but treat the legacy note as a clarification point. Confirm the intended parameter, acceptance rule, measurement method, and any evaluation length before locking price or lead time.
Should you specify Ra or Rz?
Specify the parameter that matches the function. Use Ra when average profile roughness controls the requirement. Use Rz only when profile height extremes matter, and do not substitute one for the other.
What should you do with your next CNC drawing?
Keep ISO/CD 21920-2 Edition 2 on your standards watchlist, but quote released work from ISO 21920-1:2021 and explicit drawing callouts. CNC Instant Quote supports CNC milling and turning in aluminum, copper, brass, steel, and plastics from CAD geometry. For production-ready CNC services online, specify the surface requirement before you upload →
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