The short answer
On housing and cover RFQs, a **spigot or rabbet fit** is not the same note as an [O-ring groove](https://3dwk.com/resources/o-ring-groove-callouts-cnc-housings), a [press-fit bore](https://3dwk.com/resources/press-fit-vs-slip-fit-cnc-bores), or a general “lids must sit flush” cloud. A spigot is a male shoulder that enters a female bore or recess; a rabbet (step) is a matching ledge around a lid or base so the two parts locate before fasteners are tightened. Silence on fit class, shoulder height versus recess depth, chamfer for assembly, and whether anodize or chem film is included in the diameters, is what widens many [quote-new.php](https://3dwk.com/quote-new.php) **rates.v1.1** estimate bands—not the extra finish pass alone.
This guide is for buyers packaging aluminum housing RFQs where CAD shows a locating shoulder or lid step with no fit or stock-after-finish note. Illustrations use an **Example part** only—not a customer enclosure. Sibling notes live in [five-axis aluminum housing design tips](https://3dwk.com/resources/five-axis-aluminum-housing-design-tips) and [O-ring groove callouts](https://3dwk.com/resources/o-ring-groove-callouts-cnc-housings); here we focus on **spigot and rabbet packaging** so estimators do not have to guess whether a 0.02 mm diametral clearance is a slip fit, a press, or a model leftover.
What spigot and rabbet fits mean for quoting
A **spigot fit** locates two parts on a diameter; a **rabbet fit** locates them on a step that also often sets a flush or slightly proud lid. Neither is automatically a sealing diameter. Process pages for electronics-thermal aluminum ([CNC 5-axis Al 6061 electronics-thermal](https://3dwk.com/en/cnc-5axis/al-6061/electronics-thermal.html), [CNC 3-axis Al 6061 electronics-thermal](https://3dwk.com/en/cnc-3axis/al-6061/electronics-thermal.html), [CNC 5-axis Al 6061](https://3dwk.com/en/cnc-5axis/al-6061.html)) assume fit, datum, and finish-build notes when lots mix a locating shoulder with a cosmetic lid.
Do not bury “make it assemble smoothly” inside a notes cloud. Put **SPIGOT Ø40 H7/g6 AFTER TYPE II, HEIGHT 3.0; RABBET DEPTH 3.2 SO FACES MEET; 0.3 CHAMFER BOTH** on the PDF and mirror it in RFQ text. Background: [what files to send for a CNC quote](https://3dwk.com/resources/what-files-to-send-for-cnc-quote), [datum schemes for multi-setup CNC parts](https://3dwk.com/resources/datum-schemes-for-multi-setup-cnc-parts), [how anodize choice changes CNC RFQ cost](https://3dwk.com/resources/how-anodize-choice-changes-cnc-rfq-cost).
Height, depth, and finish build-up
If the male shoulder is as tall as the female recess is deep, paint, anodize, or a small radius can stop the faces from meeting. Call the **after-finish** diameters when [Type II anodize](https://3dwk.com/resources/as-machined-before-type-ii-anodize) or chem film is on the fit, or mask the diameters and say so. A 0.2 mm extra recess depth (or a slightly shorter spigot) is a common way to let faces seat; write it, do not leave it as “shop practice.” Chamfers or small lead-ins on both parts matter for assembly and should not be a generic edge-break in the title block only.
Aluminum 6061 plate and billet $/kg planning bands sit in the mid-commodity range under **rates.v1.1** material tables; material cost is rarely the fit story—tolerance, masking, and inspection of the locating diameters are.
Datums, lids, and fasteners
Say which face is the seating face and which diameter is the locator, so a [position or profile](https://3dwk.com/resources/position-vs-profile-tolerances-cnc-brackets) callout has a clear datum. Fastener clearance should not fight the spigot; if the lid must drop straight on, say that the screws are started only after the fit is seated. A rabbet that is also a cosmetic A-side needs a face-class note so clamp witnesses are not left to chance.
Tolerance and inspection traps
Calling “slide fit” or “press slightly” without a class or a bilateral diameter, mixing as-machined CAD with after-anodize inspection, or asking for 100% CMM of a non-functional decorative step, all force **needs_manual**. Default non-critical ledges to ISO 2768-m and put a named fit only on the locating diameters. Related: [ISO 2768 general tolerances](https://3dwk.com/resources/iso-2768-general-tolerances-cnc-rfq), [flatness vs parallelism](https://3dwk.com/resources/flatness-vs-parallelism-callouts-cnc-plates).
Illustration 1 shows an **Example part** housing with a labeled after-finish spigot on the base versus a slightly deeper rabbet on the lid so the faces can meet.
Example part walkthrough
**Example part:** Aluminum 6061-T6 housing ~120×80×28 mm, lid with a rabbet, base spigot Ø40 H7/g6 after Type II, spigot height 3.0 mm, rabbet depth 3.2 mm, 0.3 mm lead-in chamfers, ISO 2768-m elsewhere. With STEP + **SPIGOT Ø40 H7/g6 AFTER TYPE II, H=3.0; RABBET DEPTH 3.2, FACES MEET; CHAMFER 0.3 BOTH; MASK THREADS**, expect a usable automatic planning band on [quote-new.php](https://3dwk.com/quote-new.php). Same geometry with silent CAD and “lid must sit flush, fit as needed” often lands DFM notes or **needs_manual**—without inventing any customer PO.
Illustration 2 contrasts a vague RFQ (“make the cover locate and sit flush”) versus a labeled spigot-and-rabbet pack and the clearer estimate path.
What widens bands fastest
Unlabeled mix of press and slip language on the same diameter, finish build-up left unspoken, shoulder and recess the same height so faces cannot meet, and 100% cosmetic inspection of a decorative step. Quantity breaks help unit minutes but do not erase fit 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 locating **spigot or rabbet** with after-finish diameters, height versus depth, and a lead-in, 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 fit-class questions rather than forcing a fake price.
Common questions
Do I need an H7/g6 class on every lid step?
Not as a silent default under **rates.v1.1** estimates. A generous clearance rabbet is enough when the fasteners and a flat face do the real locating. Use a named fit on the diameters that actually locate the assembly, and leave decorative steps at general tolerance.
Should I size the spigot before or after anodize?
Write which condition is inspected. If Type II is on the diameter, either size the machine dimensions for after-finish limits or mask the fit and say so. Inspecting an as-machined model against an anodized part is a common source of **needs_manual**.
Why make the rabbet deeper than the spigot is tall?
So the two faces can meet after coatings, radii, or a small chip at the inside corner. If you truly want the shoulder to bottom first, write that instead—do not leave both heights equal and hope the shop “knows.”
How is this different from an O-ring groove?
An O-ring groove is a sealing feature with a gland width and depth; a spigot or rabbet is a locator and often a face-seat. They can sit on the same housing, but one note does not replace the other.