Counterbore vs countersink on CNC RFQs

The short answer

A **counterbore** is a flat-bottom cylindrical recess that seats a socket-head (or similar) fastener flush or below the surface. A **countersink** is a conical recess that seats a flat-head or oval-head screw flush with the surface. On CNC RFQs they are not interchangeable: different tools, different depth/angle callouts, and different ways they move setups, cycle time, and quote bands.

When you upload STEP and PDF on the 3DWK quote page (quote-new.php), name the feature type, the fastener (or clear Ø / angle / depth), and whether flush is cosmetic or functional. Do not write only “C’bore for M6” or “C’sink somewhere” and assume every shop will pick the same stack. This article is typical buyer guidance—no invented cycle times, scrap rates, fastener ratings, or shop certifications.

Counterbore vs countersink in one glance

A **spotface**—a shallow flat seat without a full recess depth—is related but not the same job. The companion design guide Counterbore vs countersink vs spotface for CNC bolted joints covers that three-way compare for joint design. This article stays on the RFQ / quote-band side: how each feature shows up in tooling, setups, and what to write so min–mid–max bands stay comparable.

When a counterbore is the right call

Use a counterbore when:

• The fastener is a socket-head cap screw (or similar cylinder-head style) that must sit flush or below the top face
• You need a flat washer seat under a bolt head on an uneven cast or milled surface (sometimes a shallow spotface is enough—state which)
• Clearance above the head is limited and a conical recess would not match the head
• You want a consistent cylindrical wall for a captive washer or a spring seat

Counterbores are usually cut with a counterbore tool, a flat-bottom end mill, or a dedicated combination drill/counterbore. Depth and diameter drive tool choice more than angle. Blind vs through holes on CNC RFQ notes still apply to the pilot: a blind pilot under a deep counterbore changes chip packing and tap/ream plans if the hole is later threaded.

When a countersink is the right call

Use a countersink when:

• The fastener is a flat-head or oval-head screw that must sit flush with the outer surface
• Aerodynamic, cosmetic, or catch-hazard rules forbid a protruding head
• The mating cover or skin expects a conical seat at a standard included angle

Countersinks are cut with a countersink cutter (or a mill interpolating a cone). The **included angle** must match the fastener standard—common buyer-facing values are 82° (many Unified flat heads), 90° (many ISO metric flat heads), and 100° (some aerospace / sheet styles). Guessing the angle from a silent CAD cone is a classic first-article reject. Designing threads and tapped holes for machined parts and thread milling vs tapping for CNC buyers sit downstream when the pilot is also tapped.

How these features move the quote band

Estimators rarely add a line labeled “counterbore surcharge.” You see the effect as:

• **Extra tool changes** when many distinct C’bore Ø / C’sink angles appear on one face
• **Wider min–mid–max bands** when depths are missing and flush intent is only visual in the STEP
• **Manual review** when a deep counterbore sits next to a thin wall, a tapped blind hole with little remaining material, or a hard alloy that does not like a large spot drill / C’sink push
• **Extra setups** when counterbores or countersinks land on faces that are not the primary mill orientation—see how workholding changes a CNC quote and datum schemes for multi-setup CNC parts
• **Finish / coating notes** when Type II / Type III anodize or plating growth must still leave the head flush—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: automation often prices a simple through drill; a family of undocumented recesses is a common reason the band widens or the job parks in manual review. What drives CNC machining cost and what affects CNC machining lead time put fastener recesses in the same stack as setups, material, and inspection.

Typical drawing callouts (buyer-readable)

Write features so a second shop would quote the same intent:

1. **Counterbore**: e.g. “Ø11 counterbore × 6.5 deep; Ø6.6 through” (or “C’bore for ISO 4762 M6 — head flush”)

2. **Countersink**: e.g. “90° countersink to Ø12; Ø6.6 through” (or “C’sink for ISO 10642 M6 — flush”)

3. **Quantity and face**: which face, how many, pattern or position notes if they matter for GD&T

4. **Blind stack**: remaining material under a blind counterbore + tapped pilot—state full depth and tap depth separately

5. **Coating**: “dimensions apply after anodize” vs “machine to print, coat may grow into clearance”—pick one

GD&T callouts that raise CNC quote bands matter when a positional frame sits on a pattern of counterbored holes. Position vs profile tolerances on CNC brackets is the companion when the recess pattern locates a cover. Edge break, chamfer and deburr callouts CNC buyers should use is separate: a chamfer on a hole edge is not automatically a full countersink for a flat-head screw—state the fastener seat explicitly.

Tooling and setup notes buyers should care about

You do not need to pick the cutter brand. You do need to avoid silent geometry that forces bad assumptions:

• One face with five different counterbore diameters often means five tools or a mill program that is slower than a dedicated C’bore
• Mixing 82° and 90° countersinks on the same job without labeling which holes are which invites the wrong cutter
• A counterbore deeper than a practical stub tool on a tall wall may need a longer tool, a second setup, or a redesign of head style
• Plastic parts (POM, PA66, PEEK) still take C’bore / C’sink features, but heat and thin remaining webs under a blind recess matter—see POM vs PEEK for CNC functional prototypes and PA66 vs aluminum CNC prototypes

Helicoil vs tapped holes in CNC aluminum and thread inserts vs tapped holes in aluminum interact when the pilot under a counterbore is insert-ready: the counterbore for the insert tooling is not the same feature as the fastener-head counterbore—label both.

What to write on the RFQ (checklist)

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

• Every recess is named **counterbore**, **countersink**, or **spotface**—not a vague “flush screw”
• Fastener standard or explicit Ø / angle / depth is on the PDF (STEP alone is not enough)
• Blind vs through pilots are clear, with tap/insert depth if threaded
• Flush is defined as cosmetic vs functional (and whether coating growth is allowed)
• Patterns that need position or profile callouts say so
• Faces that are not the primary mill side are obvious so setup count is visible

What files to send for a CNC quote and the DFM checklist before an online CNC quote both apply: STEP + PDF with readable C’bore / C’sink notes beat a model full of unnamed conical and cylindrical cuts. Prototype vs production CNC quotes often splits here: a prototype may accept a milled approximation of a countersink; a locked production print may demand a standard angle and flush check on FAI.

Common buyer mistakes

• Exporting CAD with a decorative cone or cylinder and no PDF callout for angle, depth, or fastener
• Writing “C’sink for M6” with no angle (82° vs 90° vs 100°)
• Using a countersink callout for a socket-head screw (or a counterbore for a flat-head)—head geometry will not seat
• Ignoring coating growth on flush cosmetic covers
• Stacking a deep blind counterbore over a short tapped hole with almost no material left under the floor
• Treating every hole-edge chamfer as a fastener countersink
• Leaving multi-face fastener patterns undocumented so estimators under-count setups

Reaming vs boring for CNC hole size and deep-hole drilling / gun-drilling CNC sit next door when the pilot itself is tight or deep; they do not replace a clear head-seat callout.

Bottom line

Counterbore vs countersink on CNC RFQs is a geometry and tooling story, not a synonym quiz. Flat-bottom cylinder for socket-style heads; cone at a stated angle for flat heads. Missing angle, depth, or fastener identity widens quote bands, adds tool changes, and can flip the job into manual review. Name the feature, put Ø / angle / depth (or a fastener standard) on the PDF, and upload STEP plus those notes on quote-new.php so estimators quote the same flush intent. Ready to price a bolted cover or housing? Start on quote-new.php.