The short answer
On turned-shaft RFQs, a **shoulder fillet** is not the same note as a **shoulder undercut / relief groove**, and neither is covered by a generic “break corners” cloud. A fillet leaves a radius where the diameter steps; that is often better for fatigue, but it can keep a mating face from seating flush against the shoulder. A relief (a narrow undercut or neck at the step, sometimes compared with a shop-standard relief rather than a named standard) lets the finishing tool run out and lets a hub, bearing, or collar sit flat. Silence on which of those two the shoulder actually needs, and on whether a leftover radius is allowed on a seating face, is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the turning minutes alone.
This guide is for buyers packaging stainless turned-shaft RFQs where CAD shows a sharp or vaguely filleted step and a notes cloud that says “undercut as needed.” Illustrations use an **Example part** only—not a customer shaft. Sibling notes live in [when to call out undercuts on a CNC RFQ](https://3dwk.com/resources/when-to-call-out-undercuts-on-cnc-rfq) (general reliefs, not this shoulder-seating choice) and [turned stainless shaft drawing guide](https://3dwk.com/resources/turned-stainless-shaft-drawing-guide); here we focus on **fillet versus shoulder relief** so estimators do not grind a seating face the model still shows as a radius.
What the shoulder choice means for quoting
A **fillet** is a concave radius at the diameter step. An **undercut / relief** is a deliberate groove or neck that clears the inside corner so a mating part can seat and a finishing tool can exit. They are not interchangeable with a [chamfer vs fillet edge break](https://3dwk.com/resources/chamfer-vs-fillet-edge-break-cnc-rfq) on an outside corner, and they are not the same problem as [sharp internal corners in milled pockets](https://3dwk.com/resources/fillet-vs-sharp-internal-corners-cnc-rfq). Process pages for turned stainless ([CNC turning SS 316 robotics](https://3dwk.com/en/cnc-turning/ss-316/robotics.html), [CNC turning SS 316](https://3dwk.com/en/cnc-turning/ss-316.html), [CNC turning automation](https://3dwk.com/en/cnc-turning/automation.html)) assume a named shoulder treatment when lots mix seating faces with blended steps.
Do not bury “fillet or undercut as needed” inside a notes cloud. Put **Ø16 SHOULDER, FACE A IS SEATING, RELIEF 1.5 WIDE × 0.5 DEEP, NO FILLET ON FACE A** on the PDF and mirror it in RFQ text. Background: [DFM checklist before an online CNC quote](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote).
Seating faces versus fatigue radii
Use a named fillet when the step is a stress riser and no part has to sit flush against the shoulder. Use a named relief when a hub, pulley, bearing, or collar must bear on the face: the fillet would leave a ring of incomplete contact. If both matters, a small relief plus a generous radius inboard of the seating land is a valid pack—but only if both features are drawn, not left as “blend.” [Keyway callouts](https://3dwk.com/resources/keyway-callouts-on-cnc-turned-shafts) and [knurling](https://3dwk.com/resources/knurling-on-cnc-turned-shafts) can sit on the same shaft; they do not decide the shoulder treatment.
Stainless 316 bar $/kg planning bands sit in the mid-to-upper commodity range under **rates.v1.1** material tables; material cost is rarely the shoulder story—tool-nose access and an extra groove pass are.
Tool nose, grind stock, and runout
A sharp corner in CAD is not a promise of a zero-radius tool nose on a finishing pass. If you need a true sharp inside corner for seating, the practical callout is a relief, not “sharp as possible.” If you will later grind the diameter, say whether the relief must survive the grind and whether grind stock exists on the shoulder face. Form notes such as [circularity vs cylindricity](https://3dwk.com/resources/circularity-vs-cylindricity-cnc-shafts) and [concentricity vs runout](https://3dwk.com/resources/concentricity-vs-runout-callouts-cnc-shafts) should apply to the named land length, not silently across a fillet that was never a functional diameter.
Tolerance and inspection traps
Calling “break corners” on a seating shoulder, mixing a fillet in CAD with an undercut in the notes, omitting relief width and depth, or asking for 100% visual “no radius allowed” on every step without saying which faces seat, all force **needs_manual**. Default non-seating steps to a named fillet or a general edge break and put relief only where a face must sit flat. Related: [ISO 2768 general tolerances on CNC RFQs](https://3dwk.com/resources/iso-2768-general-tolerances-cnc-rfq), [surface roughness Ra callouts that raise quotes](https://3dwk.com/resources/surface-roughness-ra-callouts-that-raise-quotes), [when to specify passivation after CNC stainless](https://3dwk.com/resources/when-to-specify-passivation-after-cnc-stainless).
Illustration 1 shows an **Example part** shaft with a labeled relief on a seating shoulder versus a fillet on a non-seating step of the same bar.
Example part walkthrough
**Example part:** Stainless Steel 316 turned shaft ~Ø16/Ø12 × 90 mm, one seating shoulder on face A with a 1.5 × 0.5 mm relief, one non-seating step with R0.4 fillet, ISO 2768-m general. With STEP + **FACE A SEATING: RELIEF 1.5 WIDE × 0.5 DEEP, NO FILLET; STEP B: R0.4 FILLET, NOT A SEATING FACE**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “undercut or fillet as needed” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“break the shoulder as needed”) versus a labeled fillet-or-relief pack and the clearer estimate path.
What widens bands fastest
Unlabeled mix of seating and blended shoulders on the same shaft, a fillet left in CAD while the email asks for a flat face, relief size only in chat, and 100% inspection of every inside corner. Quantity breaks help unit minutes but do not erase the shoulder choice—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost) and [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
What to do next
Mark each diameter step as a **seating face with relief** or a **non-seating fillet**, then upload on [quote-new.php](https://3dwk.com/quote-new.php). 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 shoulder questions rather than forcing a fake price.
Common questions
Is a fillet always safer than an undercut on a shaft shoulder?
Not as a silent default under **rates.v1.1** estimates. A fillet helps stress at a non-seating step, but it can keep a hub from sitting flat. Choose by function, then write the feature you actually want.
Can I write “sharp corner, no undercut”?
Only if you accept that a real tool nose still leaves a small radius, or you plan a secondary process. If the mating face must seat fully, a small named relief is usually clearer than “sharp as possible.”
When should I put both a relief and a radius on the same step?
When you need a short flat seating land and still want a generous radius inboard of that land for fatigue. Draw both. Do not leave the pack as “blend to suit.”
How should I call out finish on the seating face?
Put the Ra (and any grind stock) next to the shoulder face if that face is functional. Leaving finish only in a general title block leaves the estimator guessing whether a turned finish is enough after the relief is cut.