ISO 2768 on CNC RFQs: what m/f/c mean

The short answer

**ISO 2768** is a general-tolerance system for linear and angular dimensions that are left untoleranced on a drawing. On CNC RFQs the class letters buyers see most often are **m** (medium), **f** (fine), and sometimes **c** (coarse)—plus, when form is also covered, the angular/form classes **H**, **K**, and **L**. Saying “ISO 2768-m” (or “2768-mK”) is not the same as putting a ±0.05 mm box on every feature, and it is not a substitute for GD&T on locating patterns, sealing faces, or press fits.

When you upload STEP and PDF on the 3DWK quote page (quote-new.php), state the ISO 2768 class once in the title block or notes, and still call out any feature that must be tighter (or deliberately looser) than that class. This article is typical buyer guidance—no invented scrap rates, Cpk numbers, shop certifications, or price tables. For a quick sanity check of common linear bands, the live CNC tolerance tool at tools/cnc-tolerance.php is a companion, not a replacement for your print.

What ISO 2768 actually covers

ISO 2768 is split into parts that buyers usually meet as one note:

• **Part 1 (linear and angular sizes)**: classes **f** (fine), **m** (medium), **c** (coarse), **v** (very coarse). Most machined-metal RFQs use **m** or **f**.
• **Part 2 (geometrical tolerances for features without individual tolerance indications)**: classes **H**, **K**, **L** for general straightness, flatness, perpendicularity, symmetry, and runout-style defaults when no GD&T frame is shown.

A common title-block line looks like **ISO 2768-mK** or **ISO 2768-fH**. The first letter is the size class; the second is the geometrical class. If only “ISO 2768-m” appears, shops treat size defaults as medium and do **not** invent a geometrical class for you—features that need flatness or perpendicularity still need explicit callouts (see flatness, parallelism and perpendicularity on machined parts).

ISO 2768 does **not** automatically cover:

• Fits with ISO 286 shaft/hole codes (e.g. H7/g6)
• Position or profile frames on hole patterns
• Surface roughness (Ra / Rz)
• Thread tolerances (they follow the thread standard)
• Coating growth after anodize or plating

GD&T basics for buyers and GD&T callouts that raise CNC quote bands sit next to this article when the part needs functional geometry, not only size defaults.

What m / f / c mean in practice for buyers

Think of the class as a **default envelope for untoleranced dimensions**, not as a quality badge:

• **f (fine)**: tighter default bands; more inspection attention on ordinary dimensions; often chosen for precision housings, optical mounts adjacent to CNC, or parts where many untoleranced edges still matter cosmetically or for assembly clearance.
• **m (medium)**: the workhorse for general CNC metal parts. Many shops quote “ordinary” milled and turned sizes against medium unless the print says otherwise.
• **c (coarse)**: wider defaults; useful for large weldments, rough blanks, or non-critical stock removal where fine defaults would over-constrain the job.
• **v (very coarse)**: rare on precision CNC RFQs; more common on heavy fabrication notes.

Exact millimeter tables live in the ISO 2768 standard (and in many shop wall charts). Buyers do not need to memorize every cell; they do need to know that **the allowed deviation grows with the nominal size**, and that a silent “±0.1 everywhere” note is **not** identical to ISO 2768-m across all size ranges. When your RFQ mixes small bores and long overall lengths, one blanket ± value can be tighter than 2768 on the long size and looser on the small one—or the reverse. State the class, then override critical sizes.

Typical buyer pattern on quote-new.php:

1. Title block: ISO 2768-mK (or m / f as appropriate)

2. Critical sizes: explicit ± or limit dimensions

3. Fits: ISO 286 codes or measured limits after finish

4. Datums and position/profile where assembly cares

General tolerances vs GD&T vs fits

Position vs profile tolerances on CNC brackets and datum schemes for multi-setup CNC parts explain why estimators widen bands when a “2768 only” print still has to assemble. Tolerances for CNC 17-4PH what buyers specify is the material-specific companion when heat treat moves size.

How ISO 2768 notes move the CNC quote band

Estimators rarely add a line item named “ISO 2768 surcharge.” You feel it as:

• **Narrower automation confidence** when the class is **f** on a large, multi-setup aluminum housing with many untoleranced edges—shops assume more verification time
• **Wider min–mid–max bands** when the PDF says nothing (no 2768, no ± defaults) and the STEP is full of sharp edges that look “precision” in CAD
• **Manual review** when 2768-f is combined with thin walls, deep pockets, or hard alloys where holding fine defaults on long dimensions is unrealistic without process notes—see deep pockets and tool reach on CNC quotes and CNC machining cost drivers
• **Contradiction flags** when the title block says 2768-m but a dozen features also carry ±0.02 mm without datums—estimators cannot tell which rule wins
• **Finish interactions** when Type II anodize or plating is required “to print” without saying whether general tolerances apply before or after coat—see how anodize choice changes CNC RFQ cost and as-machined before Type II anodize

Reading a preliminary online quote (min / mid / max) and what needs_manual means on a preliminary quote both apply: silent tolerance strategy is a common reason a simple prismatic part parks in manual review. What affects CNC machining lead time puts inspection sampling next to cycle time when fine defaults cover dozens of non-functional edges.

What to write on the RFQ (checklist)

Before you submit on quote-new.php, confirm:

• The title block or notes state **ISO 2768** with an explicit class (**m**, **f**, or other)—not only “general tolerances apply”
• If geometrical general tolerances are intended, include the **H / K / L** letter (e.g. mK); if not, do not assume shops will pick K for you
• Every functional size, fit, flatness, or position need has its **own** callout—2768 is the default for the rest
• Units are clear (mm vs inch drawings mix poorly with 2768 tables)
• Coating / heat-treat sequence says whether dimensions are **before** or **after** finish
• Inspection expectation matches the class: FAI / CMM sampling plans for CNC lots matter more when many features inherit fine defaults
• Stock and process reality: plate vs billet stock CNC aluminum and how stock size changes a CNC quote still set how much material exists to hold a fine overall length

What files to send for a CNC quote and the DFM checklist before an online CNC quote both apply: a readable PDF title block beats a STEP with no tolerance story. Prototype vs production CNC quotes often splits here—prototypes may accept medium defaults; locked production may keep 2768-m and add GD&T only on mating features.

Common buyer mistakes

• Writing “tolerance ISO 2768” with **no class letter**
• Assuming 2768-f makes a hole pattern assemble without position frames
• Pasting ±0.05 mm on every dimension **and** 2768-m, then leaving conflicts unresolved
• Using 2768 to replace surface-roughness or thread specs
• Calling 2768-f on a large welded-and-machined frame where coarse defaults on non-machined lengths would have been honest
• Ignoring coating growth so “to print” general sizes fail after anodize
• Treating Part 2 (H/K/L) as equivalent to a full GD&T scheme on sealing and bearing faces
• Leaving inch prints with a metric 2768 note (or the reverse) without a conversion policy

Surface roughness Ra for machined parts, designing threads and tapped holes, and edge break / chamfer / deburr callouts remain separate notes—none of them are implied by ISO 2768 alone.

Using the tolerance tool without over-trusting it

The on-site CNC tolerance helper (tools/cnc-tolerance.php) is useful to visualize typical linear bands buyers discuss with shops. Use it to:

• Sense-check whether a “fine” ask on a long overall length is realistic next to your material and process
• Compare a blanket ± habit against class-style thinking before you freeze the PDF

Do **not** treat any web helper as the ISO text, as a quote, or as an inspection certificate. Final authority is your drawing plus the standard you invoke. Upload that drawing with the class stated on quote-new.php.

Bottom line

ISO 2768 on CNC RFQs is a **default size (and optionally form) envelope**, not a quality slogan and not a GD&T replacement. **m** is the usual medium default; **f** tightens ordinary untoleranced sizes; **c** widens them; **H/K/L** only help if you actually invoke Part 2. Missing class letters, conflicting ± stacks, and using 2768 to locate patterns are what widen quote bands and flip jobs into manual review. State the class once, override what matters, and send STEP plus a readable PDF on quote-new.php so estimators price the same tolerance story. Ready to quote a housing or bracket with clear general tolerances? Start on quote-new.php.