The short answer
For CNC aluminum RFQs, choose **cut threads** when loads are light and wall is thick enough; choose **wire or key-locking inserts** (Helicoil-style / Keensert-class) when you need reworkability, higher pull-out, or thin bosses. The RFQ must say which—silent “M6 threads” leaves [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimates guessing tap vs insert labor.
This guide is for buyers packaging aluminum housings and brackets. Illustrations are **Example part** only. Broader insert theory: [thread inserts vs tapped holes in aluminum](https://3dwk.com/resources/thread-inserts-vs-tapped-holes-aluminum); here we focus on **RFQ packaging and estimate impact**.
Cut taps: when they are enough
Cut or form taps in 6061/7075 work for moderate clamp loads, low cycle counts, and bosses with enough radial wall (rule of thumb: ≥1× major diameter radial material for cut taps—confirm with your DFM, not a guarantee). Form taps can strengthen 6061 but need correct minor diameter and may raise torque on small CNC mills.
Call class of fit (e.g. 6H), depth of full thread, and whether blind holes need chip relief. Related: [designing threads and tapped holes](https://3dwk.com/resources/designing-threads-and-tapped-holes), [designing bosses and ribs for CNC housings](https://3dwk.com/resources/designing-bosses-and-ribs-for-cnc-housings), [DFM checklist before an online CNC quote](https://3dwk.com/resources/dfm-checklist-before-online-cnc-quote).
Wire inserts and key-locking inserts on the RFQ
Wire inserts need a larger STI tap drill, install tooling, and often a different inspection story. Key-locking inserts need a counterbore/spotface for the keys. If the PDF only shows a finished M5 callout, shops cannot tell whether to quote STI prep + insert or a standard tap.
Write explicitly: “M5×0.8 wire insert, install after Type II anodize” or “M6×1 cut thread, 6H, before finish.” Sequence matters—anodize growth changes minor diameters. See [how anodize choice changes CNC RFQ cost](https://3dwk.com/resources/how-anodize-choice-changes-cnc-rfq-cost) and [chem film vs Type II](https://3dwk.com/resources/chem-film-vs-type-ii-anodize-aluminum).
Illustration 1 contrasts an **Example part** boss with cut tap vs STI + wire insert prep.
Estimate bands under rates.v1.1
Insert lines add kit cost + install minutes. Under **rates.v1.1** estimates, aluminum machine-time is the baseline; insert install is a secondary adder that still widens the mid when qty is low. Soft plastics (PA66 coeff ×0.8) almost always want inserts for reusable threads—do not paste aluminum tap notes onto nylon. Hot alloys (17-4PH ×1.4, IN718/IN625 ×2.5) rarely use wire inserts the same way; if your RFQ mixes aluminum inserts language onto IN718, expect **needs_manual**.
Bar $/kg planning bands (estimates): PA66 ~5, 17-4PH ~9, IN718 ~55, IN625 ~60. Insert hardware cost is separate from bar stock—list insert commercial series or “buyer-furnished inserts” if that is the plan. Lines whose mid planning band exceeds configured thresholds (e.g. hot-alloy / CoCr rules above ~$2000) auto-route to **needs_manual** even when the thread story is clear.
Thin walls, pull-out and rework
Repeated service fasteners on thin 6061 bosses favor inserts. One-time assembly can stay cut-tapped. If you expect field rework, say so—estimators price different risk. Pull-out requirements without a test method invite clarification holds.
Blind holes with inserts need depth for the insert length + tool clearance. Call whether the insert sits flush or below flush after plating/anodize. Avoid placing inserts on sealing faces that must stay flat after install—install tools can raise burrs.
Inspection and FAIR notes
Say whether insert presence is verified by go/no-go after install, by breakaway torque sample, or by visual only. Qty 2 prototypes often imply near-full thread checks; qty 50 may sample. If the PDF says “100% CMM every thread after insert,” inspection stays heavy and the mid rises. Related: [first article inspection reports](https://3dwk.com/resources/first-article-inspection-reports), [what needs_manual means](https://3dwk.com/resources/what-needs-manual-means-preliminary-quote).
Example part walkthrough
**Example part:** 6061-T6 cover ~100×70×12 mm, four M4 corner holes. Option A: cut M4×0.7 6H ×8 mm deep before Type II. Option B: STI prep + wire insert install after Type II, flush. PDF states option, anodize sequence, and “no wire insert on mating seal face.” With STEP + clear thread block, expect a clearer automatic band on [quote-new.php](https://3dwk.com/quote-new.php) than “threads TBD.” No customer logos or invoice prices—**rates.v1.1** estimates only.
Illustration 2 shows a vague “M4 somehow” note versus an insert/tap decision block.
Mixed materials and captive hardware
If steel fasteners into aluminum see vibration, inserts reduce galling risk. Captive screws or PEM-class clinch nuts are a different family—call them by name; do not say “Helicoil” when you mean a clinch nut. Related: [press-fit pins and bushings](https://3dwk.com/resources/press-fit-pins-bushings-cnc-housings), [counterbore vs countersink vs spotface](https://3dwk.com/resources/counterbore-countersink-spotface-cnc-bolted-joints), [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote).
What to do next
Pick tap vs insert per hole group, write sequence relative to finish, then upload on [quote-new.php](https://3dwk.com/quote-new.php). Service: [CNC machining](https://3dwk.com/cnc-machining-service.php), [materials](https://3dwk.com/materials.php). Treat **rates.v1.1** bands as estimates. If **needs_manual** appears, answer insert brand/series and install timing rather than inventing a firm unit price.
Common questions
Can I leave insert vs tap to the shop?
Only if function truly allows either. Otherwise state the choice—shops will not silently upgrade every hole to inserts.
Do inserts always raise the mid estimate?
Usually at low qty, yes—kit + install. At higher qty the unit adder shrinks; see [quantity breaks](https://3dwk.com/resources/how-quantity-breaks-change-cnc-unit-cost). Still call them out.
Should inserts go in before or after anodize?
Common pattern: machine STI, anodize/mask as needed, then install. Confirm with your finish house—oxide in STI threads can interfere with install if not planned.
Are Helicoil and Keensert interchangeable on the RFQ?
No. Wire vs key-locking change hole prep. Name the family and size; “insert somehow” widens bands.