The short answer
A **D-flat** on a turned shaft is not the same as a **double-D** (two opposed flats), and neither is a silent “add a flat for the hub.” A D-flat is one milled land that leaves a remaining height on the diameter—often to match a D-bore or to give a set-screw a seat. A double-D names the **distance across two parallel flats**. Silence on which family you actually need, on across-flats versus flat depth, and on keep-outs next to a journal or thread, is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the extra minutes of a mill pass alone.
This guide is for buyers packaging CNC shafts where an anti-rotation or mating-bore feature is modeled as a shaved cylinder or labeled only “D-cut.” Illustrations use an **Example part** only—not a customer shaft. This is **not** a [wrench-flat](https://3dwk.com/resources/wrench-flat-notes-cnc-turned-shafts) note: wrench pads are for a tool, and they have their own width-across-flats and length rules. Sibling notes also live in [keyway callouts on CNC turned shafts](https://3dwk.com/resources/keyway-callouts-on-cnc-turned-shafts); here we focus on **D-family packaging** so estimators do not have to guess whether a “D on Ø12” is one flat, two opposed flats, or a hex.
What D-flat versus double-D means for quoting
A **D-flat callout** names one flat, the remaining height (or the flat depth from the parent diameter), the flat length, and where it starts relative to a shoulder or datum. A **double-D callout** names the distance across two opposed flats, the length, and the clocking. Writing “D-cut for hub, size as needed” is not enough. Process pages for stainless turning and secondary milling ([CNC turning SS 304 automation](https://3dwk.com/en/cnc-turning/ss-304/automation.html), [CNC 3-axis SS 304 automation](https://3dwk.com/en/cnc-3axis/ss-304/automation.html), [CNC turning SS 304](https://3dwk.com/en/cnc-turning/ss-304.html)) assume the flat family is visible when lots mix D-shafts, [keyways](https://3dwk.com/resources/keyway-callouts-on-cnc-turned-shafts), and wrench pads on the same rotor.
Do not bury “standard D-shaft” inside a notes cloud. Put **D-FLAT: REMAINING HEIGHT 10.8 ON Ø12, LENGTH 20, START 2 FROM SHOULDER A, CLOCK TO KEYWAY; or DOUBLE-D: 10.8 ACROSS FLATS × L20** on the PDF and mirror it in RFQ text. Background: [turned stainless shaft drawing guide](https://3dwk.com/resources/turned-stainless-shaft-drawing-guide), [milled and turned features on one CNC part](https://3dwk.com/resources/milled-and-turned-features-on-one-cnc-part), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote).
One flat, two flats, and what they are not
A single D-flat leaves most of the cylinder; the mating hole is a D, or a set-screw bears on the land. A double-D is a pair of parallel lands, and the across-flats dimension is the one that usually locates. Neither feature is a [wrench-flat](https://3dwk.com/resources/wrench-flat-notes-cnc-turned-shafts) or a hex—do not reuse a “standard hex” note to mean a D-shaft. Clock the flat to any [keyway](https://3dwk.com/resources/keyway-callouts-on-cnc-turned-shafts) so the shop does not pick an arbitrary angle. Keep the flat off journals and full-form threads; [shoulder fillet versus undercut](https://3dwk.com/resources/shoulder-fillet-vs-undercut-cnc-turned-shafts) notes belong on the same sheet when the flat starts at a shoulder. Stainless 304 bar $/kg planning bands sit in the common CNC stainless range under **rates.v1.1** material tables; material cost is rarely the D-family story—mill setup, remaining height, and keep-outs are.
Length, blend, and interrupted diameters
Name the flat length and whether the ends may blend or must be sharp. If the D-feature must not break [journal](https://3dwk.com/resources/circularity-vs-cylindricity-cnc-shafts) B or an M10 thread, write the keep-out. [Runout callouts](https://3dwk.com/resources/concentricity-vs-runout-callouts-cnc-shafts) on the remaining cylinder should say whether they apply after the flat is cut.
Tolerance and inspection traps
Calling “D-cut on Ø12” with no remaining height, mixing a single flat with a double-D on the same note, or asking for a tight across-flats tolerance without a measurement method, all force **needs_manual**. Default non-critical hub flats to one named family and a length; reserve tight across-flats for the mating bore you actually have. Related: [ISO 2768 general tolerances on CNC RFQs](https://3dwk.com/resources/iso-2768-general-tolerances-cnc-rfq), [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
Illustration 1 shows an **Example part** shaft with a labeled single D-flat remaining height 10.8 on Ø12 versus a double-D 10.8 across flats, both clear of journal B and an M10 thread.
Example part walkthrough
**Example part:** Stainless Steel 304 shaft Ø12 × ~80 mm, single D-flat remaining height 10.8 (flat depth 1.2) × length 20 starting 2 mm from shoulder A, or the alternate pack of double-D 10.8 across flats × L20, keep-outs on journal B and an M10 thread, ISO 2768-m general. With STEP + **D-FLAT REMAINING 10.8 ON Ø12 × L20 START 2 FROM A, KEEP JOURNAL B / M10**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent “D-cut for hub, size as needed” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“standard D-shaft”) versus a labeled single-versus-double pack and the clearer estimate path.
What widens bands fastest
Unlabeled mix of one flat and two flats, CAD that shaves a diameter without a remaining-height dimension, missing clocking to a keyway, and a flat that eats a journal or full-form thread. Quantity breaks help unit minutes but do not erase D-family ambiguity—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost) and [turned stainless shaft drawing guide](https://3dwk.com/resources/turned-stainless-shaft-drawing-guide).
What to do next
Mark each shaft with a **flat family, remaining height or across-flats, length, clocking, and keep-outs**, then upload on [quote-new.php](https://3dwk.com/quote-new.php). Packaging checklist: D-flat versus double-D, start from a shoulder, and any journal or thread that must stay full form. Service pages: [CNC machining](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** min/mid/max as estimates, not invoices. If the UI shows **needs_manual**, answer the D-family questions rather than forcing a fake price.
Common questions
Is a D-flat the same as a wrench flat?
No. A wrench flat is there so a tool can hold or turn the shaft; a D-flat is usually there to clock a hub or to seat a set-screw. Use the [wrench-flat notes](https://3dwk.com/resources/wrench-flat-notes-cnc-turned-shafts) when the feature is for a wrench, and this guide when it is a D or double-D mating feature.
How do I choose remaining height versus across-flats?
For one flat, remaining height (or depth from the parent diameter) is the clear dimension. For two opposed flats, across-flats is the clear dimension. Do not mix the two on one note, and do not assume a catalog hub will match a silent “D-cut.”
Can the flat run into a thread or a journal?
Only if you say so. A flat that breaks a full-form thread or a bearing diameter is an interrupted feature and often needs a written acceptance. The default should be a keep-out.
Do I need to clock the flat to a keyway?
If both exist, yes—write the angle or a same-side note. If the shop must pick an arbitrary orientation, two lots will not assemble the same way.