The short answer
On housing and plate RFQs, a **countersink (CSK)** and a **counterbore (C'BORE)** are not interchangeable fastener-seat notes. A countersink is a conical seat that lets a flat-head screw sit flush or slightly below the face—the drawing must name the **included angle** and how flush is judged. A counterbore is a cylindrical recess with a flat bottom that clears a socket-head cap screw (SHCS) or similar cap—the drawing must name **head diameter and depth**. Silence on which seat family, which angle or depth, and whether a shallow **spotface** would have been enough is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the drill minutes alone.
This guide is for buyers packaging aluminum RFQs where CAD shows mixed flat-head and cap-head seats, or a notes cloud that says “CSK/C'BORE as needed.” Illustrations use an **Example part** only—not a customer electronics enclosure. Sibling notes live in [counterbore vs countersink on a CNC RFQ](https://3dwk.com/resources/counterbore-vs-countersink-cnc-rfq) and [counterbore, countersink and spotface on bolted joints](https://3dwk.com/resources/counterbore-countersink-spotface-cnc-bolted-joints); here we focus on **callout packaging** so estimators do not guess an 82° vs 90° cutter or a missing depth.
What each seat means for quoting
**Countersink** typically means a conical cutter or a combination drill/CSK tool sized to a named included angle (commonly 82° for many Unified flat heads, 90° for many metric flat heads). Shops plan tool choice, watch breakout on thin walls, and ask whether “flush” allows a small proud or under-flush band. **Counterbore** means a flat-bottom recess large enough for the cap and a driver, with a through or tapped hole in the center. A **spotface** is a shallow counterbore used only to clean a bearing face for a washer or hex head—it is not a hidden C'BORE. Process pages for electronics-thermal aluminum ([CNC 3-axis Al 6061 electronics-thermal](https://3dwk.com/en/cnc-3axis/al-6061/electronics-thermal.html), [CNC 5-axis Al 6061 electronics-thermal](https://3dwk.com/en/cnc-5axis/al-6061/electronics-thermal.html), [CNC 3-axis Al 6061](https://3dwk.com/en/cnc-3axis/al-6061.html)) assume seat notes when lots mix open drilled holes with recessed fasteners.
Do not bury “CSK as needed” inside a notes cloud. Put **M3 × 90° CSK, FLUSH +0 / −0.2 TO FACE A** or **M4 C'BORE Ø7.0 × 4.2 DEEP FOR SHCS** 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).
Angle, depth and which fastener family
An included angle only helps when the fastener family is named. Calling “CSK” without 82° or 90° (or the actual purchased-head angle) leaves the cutter unknown. Calling “C'BORE for M4” without a recess diameter and depth leaves head-clearance unknown—purchased SHCS head heights vary by standard. The through-hole or tap in the center is a separate note; see [designing threads and tapped holes](https://3dwk.com/resources/designing-threads-and-tapped-holes) and [thread depth callouts on blind tapped holes](https://3dwk.com/resources/thread-depth-callouts-blind-tapped-holes-cnc). Position of the pattern still needs a datum scheme—[datum schemes for multi-setup CNC parts](https://3dwk.com/resources/datum-schemes-for-multi-setup-cnc-parts) and [GD&T callouts that raise CNC quote bands](https://3dwk.com/resources/gdandt-callouts-that-raise-cnc-quote-bands).
Aluminum 6061 bar/plate $/kg planning bands sit in the mid-commodity range under **rates.v1.1** material tables; material cost is rarely the seat story—tool choice, thin-wall breakout and inspection of flushness are.
When a spotface is enough
Use a named spotface when the head sits above the face and you only need a clean, locally flat bearing land for a washer or hex head. Say the spotface diameter and a minimum clean-up depth. Promoting every bearing land into a full-depth C'BORE (or every cap seat into a CSK) over-controls the part and can force extra tools. For most housing buyers, splitting the pattern into **named CSK seats** for flat heads and **named C'BORE seats** for SHCS is clearer than one “recessed as CAD” note.
Tolerance and inspection traps
Calling “CSK flush” without a proud/under band, mixing an 82° note on a metric flat-head drawing, omitting C'BORE depth so the cap sits proud into a sliding cover, or asking for 100% optical check of every open CSK without saying which seats are cosmetic, all force **needs_manual**. Default non-critical holes to ISO 2768-m and put functional seat dimensions only on the fastener families that matter. 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), [reaming vs boring for CNC hole size](https://3dwk.com/resources/reaming-vs-boring-cnc-hole-size). Hole size in the center of a CSK or C'BORE belongs next to the seat note, not in a separate unmarked cloud.
Illustration 1 shows an **Example part** plate with labeled 90° CSK seats versus flat-bottom C'BORE seats on the same face.
Example part walkthrough
**Example part:** Aluminum 6061-T6 plate ~90×55×10 mm, four M3 through holes with 90° countersinks for flat heads, two M4 tapped holes with counterbores for SHCS, ISO 2768-m general. With STEP + **M3 × 90° CSK, FLUSH +0 / −0.2 TO FACE A; M4 C'BORE Ø7.0 × 4.2 DEEP FOR SHCS; PATTERN POSITION TO DATUM A/B**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “all seats CSK or C'BORE as needed” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“CSK/C'BORE OK”) versus a labeled seat-callout pack and the clearer estimate path.
What widens bands fastest
Unlabeled mix of CSK and C'BORE on the same diameter callout, thin walls under a deep CSK with no breakout note, inch angles applied to metric heads, and 100% inspection of every cosmetic seat. Quantity breaks help unit minutes but do not erase seat-callout ambiguity—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 functional seat as **CSK** or **C'BORE** (or **SPOTFACE**), name the angle or the Ø×depth and the flush/proud band, 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 seat-callout questions rather than forcing a fake firm price.
Common questions
Is a countersink always cheaper to quote than a counterbore?
Not reliably under **rates.v1.1** estimates. A simple through CSK can be a combination drill; a deep C'BORE on a thin wall, or a CSK that must be truly flush across a cosmetic face, can take more planning. The bigger cost is silence—mixed notes without angle or depth.
Can I write “CSK all around”?
Only if every hole truly shares that seat and you name the included angle and fastener family. Applying a 90° CSK to cap-head holes and vent holes alike usually over-controls the part.
When should I call a spotface instead of a counterbore?
When the head is allowed to sit above the face and you only need a local clean bearing land. See [counterbore, countersink and spotface on bolted joints](https://3dwk.com/resources/counterbore-countersink-spotface-cnc-bolted-joints) for joint types—the seat label on the RFQ remains mandatory.
How should I call out seat finish?
Put Ra next to cosmetic CSK faces if they are visible after assembly. Leaving finish only in a general title block leaves the estimator guessing whether a drilled CSK is acceptable.