The short answer
On a CNC RFQ, a **blind tapped hole** with a silent thread depth is not the same quote package as a labeled **full thread / usable thread / drill depth** stack. Estimators need usable thread length, drill point clearance, and whether a thread mill or tap is acceptable. Silence on depth callouts is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the hole diameter alone.
This guide is for buyers packaging prismatic aluminum RFQs with blind threads. Illustrations use an **Example part** only—not a customer housing. Related thread notes live in [designing threads and tapped holes](https://3dwk.com/resources/designing-threads-and-tapped-holes), [thread milling vs tapping CNC buyers](https://3dwk.com/resources/thread-milling-vs-tapping-cnc-buyers), and [Helicoil vs tapped holes CNC aluminum](https://3dwk.com/resources/helicoil-vs-tapped-holes-cnc-aluminum); here we focus on **how to label blind-thread depths** so estimators do not guess.
What depth stack means for quoting
A blind tapped hole needs at least three numbers that often get collapsed into one: drill depth, full (or usable) thread depth, and remaining clearance for chip / tap lead. Process pages for aluminum robotics and automation ([CNC 3-axis Al 6061 robotics](https://3dwk.com/en/cnc-3axis/al-6061/robotics.html), [CNC 5-axis Al 6061 robotics](https://3dwk.com/en/cnc-5axis/al-6061/robotics.html), [CNC turning Al 6061 robotics](https://3dwk.com/en/cnc-turning/al-6061/robotics.html)) assume clear thread notes when lots mix blind M-threads with thin floors.
Do not write only “M6×1.0 deep.” Put **usable thread ≥… / drill to … / thread mill OK** next to each critical blind hole on the PDF and mirror it in RFQ text. Background: [blind vs through holes on CNC RFQ notes](https://3dwk.com/resources/blind-vs-through-holes-cnc-rfq-notes), [DFM checklist before an online CNC quote](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote).
Tap vs thread mill and floor clearance
Taps need lead-in and chip room; thread mills can stop closer to a floor but raise cycle time. Deep blind holes with tiny remaining wall force broken taps, plug gauges, and rework risk. Soft stock (PA66 time coeff ×0.8 under **rates.v1.1** estimates) still chips poorly in deep blind holes. Harder alloys (17-4PH ×1.4, IN718/IN625 ×2.5, CoCr ×2.2) amplify heat and tool risk; aggressive “full thread to floor” asks on hot alloys frequently auto-route toward **needs_manual**, especially when mid-line bands exceed planning thresholds.
If the mating screw only needs 1.5× diameter engagement, say the **usable** depth—not a max theoretical thread that fights the drill point.
Bar $/kg planning bands (estimates only): PA66 ~5, 17-4PH ~9, IN718 ~55, IN625 ~60. Material cost is rarely the blind-thread story—depth stack and broken-tool risk are.
Inserts, Helicoil and repair intent
If you expect Helicoil / insert after tap, say so before quoting base metal threads. Silent “aluminum M5 somehow” next to a thin boss often becomes insert + re-tap later. Related: [thread inserts vs tapped holes aluminum](https://3dwk.com/resources/thread-inserts-vs-tapped-holes-aluminum), [Helicoil vs tapped holes CNC aluminum](https://3dwk.com/resources/helicoil-vs-tapped-holes-cnc-aluminum).
Tolerance and inspection traps
Class 6H / 2B notes without go/no-go sampling plans, or 100% thread gauges on every hole in a 200-hole plate, widen QA bands. Default non-critical threads to standard class and put functional gauging only on structural fasteners. Related: [CMM sampling plans for CNC lots](https://3dwk.com/resources/cmm-sampling-plans-for-cnc-lots), [GD&T callouts that raise CNC quote bands](https://3dwk.com/resources/gdandt-callouts-that-raise-cnc-quote-bands), [surface roughness Ra for machined parts](https://3dwk.com/resources/surface-roughness-ra-for-machined-parts).
Illustration 1 shows an **Example part** blind hole with labeled usable thread, drill depth, and clearance below the last full thread.
Example part walkthrough
**Example part:** 6061-T6 plate ~100×70×18 mm with four blind M5×0.8 holes, usable thread ≥10 mm, drill depth 14 mm, thread mill OK, remaining floor ≥3 mm, ISO 2768-m general. With STEP + clear depth stack, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with “M5 deep” only and silent CAD often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“tap all holes deep”) versus a labeled depth-stack block and the clearer estimate path.
Chip exit, coolant and secondary ops
Blind taps trap chips. Say whether the shop may peck, flood, or use form taps on aluminum. Form taps need different drill sizes than cut taps—silent “any tap OK” next to a tight minor-diameter note often forces clarification. If a secondary facing op shortens the boss after tapping, call that order out so usable thread is not cut away. Related process and DFM notes: [deep pockets and tool reach CNC quotes](https://3dwk.com/resources/deep-pockets-and-tool-reach-cnc-quotes), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote).
What widens bands fastest
Single-number “deep” callouts, full thread to floor with no clearance, conflicting insert notes, and 100% gauging of every blind hole. Quantity breaks help unit cost but do not erase tap-break risk—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 **usable thread / drill depth / mill-or-tap**, state floor clearance and insert intent, 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 depth questions rather than forcing a fake firm price.
Common questions
Is “M6×12 deep” enough on a blind hole?
Usually not. Say whether 12 mm is usable thread or overall drill depth, and leave clearance for the tap point or mill stop.
Do thread mills always cost more than taps?
Often yes on cycle time under **rates.v1.1** estimates, but they reduce broken-tap risk in blind aluminum and hard alloys. Name which methods are allowed.
How do thin floors change blind taps?
Thin floors force shallower usable thread or inserts. Silent full-depth asks next to a 2 mm floor commonly trigger **needs_manual**.
Should through holes follow the same habit?
Through holes are simpler—see [blind vs through holes on CNC RFQ notes](https://3dwk.com/resources/blind-vs-through-holes-cnc-rfq-notes)—but usable thread length still belongs on the drawing when engagement matters.