The short answer
Call out undercuts on a CNC RFQ when a cutter cannot reach a feature from the primary approach—side mills, T-slots, dovetails, or hidden grooves. Silence on undercuts is what flips many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands into **needs_manual**, not the undercut geometry itself.
This guide is for buyers packaging prismatic RFQs. Illustrations use an **Example part** only—not a customer fixture. Related geometry rules live in [CNC pockets, thin walls and undercuts](https://3dwk.com/resources/cnc-pockets-thin-walls-undercuts); here we focus on **when and how to label** undercuts so estimators do not guess.
What counts as an undercut for quoting
An undercut is any feature whose wall or floor is shadowed from the tool spindle axis used in that setup. Classic cases: T-slot, dovetail, O-ring groove under a lip, side keyway cut after a flip is already planned, or a pocket that wraps under a flange. If a standard end mill from +Z cannot finish the surface without colliding, mark it.
Do not bury the note inside a long GD&T paragraph. Put an **UNDERCUT / SIDE ACCESS** callout on the PDF next to the feature and mirror it in the 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), [3-axis vs 5-axis for buyers](https://3dwk.com/resources/3-axis-vs-5-axis-cnc-for-buyers).
Tool access vs machine class assumptions
Online engines often assume 3-axis with limited side milling unless you say otherwise. Continuous 5-axis or dedicated side-mill setups change machine-time assumptions. Soft materials (PA66 time coeff ×0.8 under **rates.v1.1** estimates) still need reach notes—deflection and tool stick-out matter. Harder alloys (17-4PH ×1.4, IN718/IN625 ×2.5, CoCr ×2.2) amplify stick-out risk; long-reach undercuts on hot alloys frequently auto-route to **needs_manual**, especially when the line mid exceeds configured USD thresholds (e.g. above ~$2000 planning bands for hot alloys/CoCr).
Bar $/kg planning bands (estimates only): PA66 ~5, 17-4PH ~9, IN718 ~55, IN625 ~60. Material cost is rarely the undercut story—access time is.
When you can omit the undercut label
If the feature is reachable after a documented flip with a short tool, and your datum scheme already shows that setup, a simple “machine in Setup 2” note may suffice. If the CAD clearly shows an open channel with no shadowed faces, labeling every radius as an undercut creates noise. Prefer precise labels over blanket “all pockets undercut.”
Side milling, T-slots and specialty cutters
T-slot and dovetail cutters are specialty tools. Call the slot width, depth under the lip, and whether the floor must be flat. If a commercial cutter catalog number is acceptable, say so; if you need a custom form tool, expect **needs_manual**. Related: [milled and turned features on one CNC part](https://3dwk.com/resources/milled-and-turned-features-on-one-cnc-part), [datum schemes for multi-setup CNC parts](https://3dwk.com/resources/datum-schemes-for-multi-setup-cnc-parts).
Illustration 1 shows an **Example part** with a side-access groove marked UNDERCUT versus a deep but open pocket that is not.
Tolerance and finish traps under lips
Tight true position under a lip forces probing or custom gauging. Cosmetic bead blast that must reach under a flange needs process notes—media may not wash into shadowed zones evenly. Anodize or plating under lips can pool; mask maps matter. See [how anodize choice changes CNC RFQ cost](https://3dwk.com/resources/how-anodize-choice-changes-cnc-rfq-cost) and [surface roughness Ra](https://3dwk.com/resources/surface-roughness-ra-for-machined-parts).
Default non-critical undercut walls to ISO 2768-m. Put functional tolerances only on sealing or mating undercut faces.
Example part walkthrough
**Example part:** 6061-T6 plate ~120×80×25 mm with an open 12 mm deep pocket (not an undercut) plus a 4 mm high × 6 mm deep side groove under a 3 mm lip (true undercut). PDF marks UNDERCUT + SIDE MILL on the groove, Setup 1 for the open pocket, Setup 2/side approach for the groove, ISO 2768-m general, groove width ±0.05. With STEP + clear labels, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “machine complete somehow” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“hidden features OK”) versus a labeled undercut block and the clearer estimate path.
What widens bands fastest
Unlabeled shadowed grooves, conflicting “must be 5-axis” stickers without feature maps, long-reach undercuts in IN718/IN625 without stick-out limits, and 100% CMM of every undercut wall. Quantity breaks help unit cost but do not erase access risk—see [how quantity breaks change CNC unit cost](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost).
What to do next
Mark every shadowed feature, state preferred access (side mill / flip / 5-axis index), 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 undercut questions rather than forcing a fake firm price.
Common questions
Is every deep pocket an undercut?
No. Depth alone is tool-reach risk. An undercut is a shadowed face relative to the spindle approach. Deep open pockets belong in a reach/DFM note; shadowed lips need the UNDERCUT label.
Will calling every feature an undercut tighten the mid?
No—it creates noise and often **needs_manual**. Label only shadowed features and keep open pockets in the depth/reach story.
Do undercuts always require 5-axis?
Often a side-mill setup or indexed 3+2 pass is enough. Reserve continuous 5-axis notes for simultaneous contour undercuts you truly need.
How do hot alloys change undercut quotes?
Long stick-out in IN718/IN625/CoCr raises machine-time and breakage risk under **rates.v1.1** coeffs (×2.5 / ×2.2 estimates). Expect wider bands or manual review when reach is aggressive.