Why sealed CNC housings need planned air and fluid paths
Sealed CNC housings for electronics, pneumatics, gearboxes and outdoor enclosures look complete when every seam has a gasket and every fastener lands in a boss. In service, temperature swings, washdown, condensation and pressure differentials still move air and water. Without intentional vent, drain and weep features, moisture collects in low pockets, pressure builds during thermal cycles and gasket faces see loads the designer never sized. Buyers often discover the gap after the first field return or after a leak test fails for reasons that are not porosity in the aluminum.
This guide is for RFQ teams and designers who purchase CNC-machined housings, manifolds and enclosure lids. It explains typical roles of vent holes, drain holes and weep holes, drawing callouts shops need and an RFQ checklist. Values here are orientation only; OEM sealing standards, IP rating methods and your assembly process own the final sizes. Related practice includes O-ring groove design for CNC housings, edge break chamfer and deburr callouts, and five-axis aluminum housing design tips.
Vent, drain and weep: three different jobs
Vent holes
A vent equalizes pressure between the enclosure volume and the outside (or a controlled plenum). Without equalization, heating air expands, cooling air contracts and the gasket or connector seals become temporary pumps that pull moisture in. Vents may be open holes, threaded ports for membrane breathers, sintered fittings or filtered hardware. The housing feature is usually a through hole or boss sized for a purchased vent; the membrane or filter is a separate BOM line.
Drain holes
A drain lets collected liquid leave a designed low point under gravity (or under washdown rinse). Drains are typically larger than weeps, often with a clear free path and sometimes a removable plug for shipping. Place drains at the true lowest point in the installed orientation—not the lowest point in the CAD model if the unit mounts sideways in the field.
Weep holes
Weep holes are small openings that allow slow escape of condensation or seepage without turning the enclosure into an open box. They appear in window frames, door bottoms, outdoor housings and cast-style pockets machined from billet. Weeps are easy to omit on CNC prints because they look “cosmetic”; they are functional and should carry size, location and finish notes. Do not use one word for all three on the drawing. Write the function so estimators and assemblers do not guess whether a Ø3 hole is a membrane vent, a drain or a paint weep. Illustration 1 compares typical buyer intents with the callouts shops price.
Geometry and location notes that change the quote
Orientation and gravity
State the installed attitude on the drawing or in the RFQ text (floor mount, wall mount, inverted lid). A drain on the “bottom” of a floating model is wrong if the customer mounts the box with that face vertical. Add a note: “DRAIN AT LOW POINT IN INSTALLED ORIENTATION PER VIEW …”
Bosses, countersinks and hardware interfaces
Vent ports for screw-in breathers need thread callouts, seal face flatness and sometimes a spotface. Coordinate with designing threads and tapped holes and with counterbore countersink spotface practice so the vent hardware does not fight the gasket land. Blind tapped vents that never break into the cavity are a common CAD error—confirm the hole breaks through where intended.
Wall thickness and chip traps
Small cross-drills and weeps in thick walls can leave burrs inside the cavity that no external brush reaches. Call for internal deburr or specify that weeps are drilled from the outside after pocket cleanup. Thin walls around a large drain can ovalize; give minimum wall from hole to outer surface.
Mesh, membranes and secondary ops
If a purchased vent presses or threads into the housing, list the supplier part number and whether the shop installs it. If the shop only machines the port, say so. Powder coat or paint that closes a weep is a classic scrap mode—mask weeps or re-open after coat. See powder coating vs painting metal parts and chem film vs Type II anodize when finish sequence matters.
Materials, finishes and leak-test context
Aluminum housings
Common for outdoor electronics and machine guards. Anodize and chem film change hole size slightly; membrane vent threads still need to start cleanly. State whether IP or leak testing happens before or after finish. Hardcoat inside small weeps is rarely practical—mask or machine after coat when size is critical.
Steel and stainless
Heavier corrosion risk at stagnant drain pockets. Passivation or electropolish notes belong with stainless drain paths; see passivation vs electropolishing stainless. Carbon steel drains in washdown service often need plating or paint with masked openings.
Plastics
PEEK and POM housings still need vents for sealed electronics, but thread inserts and press fittings differ from metal. Do not copy metal NPT notes into plastic without checking boss design.
Leak and pressure tests
RFQs that require “air decay” or “IPXX” must say whether vents are plugged, open or fitted with production breathers during the test. A housing quoted open-vent will fail a sealed pressure test that assumes blanking plugs. List plugs, torque and which ports are blanked on the FAI.
Process notes shops use to price small holes
How small holes get made
Drilling, peck drilling and, for deep small diameters, gun-drilling-style approaches all appear depending on L/D. Buyers should specify diameter, depth, break-through condition and position; process choice stays with the shop unless a special finish or straightness note forces a method. Deep weeps through thick walls relate to deep hole drilling and gun drilling guidance when L/D is large.
Burrs and break edges
A sharp internal burr at a weep can wick moisture or cut a cable. Call measurable edge break on both ends when accessible, or “internal deburr to remove raised metal” when only one side is reachable. See edge break chamfer deburr callouts for CNC.
Position and cosmetic faces
Weeps on a show face need position and sometimes a countersink for a flush plug look. Interior-only drains can sit in as-machined pockets with looser position if function is gravity drain only—state that explicitly so estimators do not assume tight TP on every hole.
RFQ checklist for vents, drains and weeps
Before you send files:
1. Every opening is labeled vent, drain or weep (or hardware P/N) with diameter and depth or through condition.
2. Installed orientation and the low-point drain location are unambiguous.
3. Threaded vent ports list thread, seal method and whether hardware is customer-supplied.
4. Finish sequence notes masking or re-opening of small holes after coat.
5. Leak / IP test setup states which ports are open, plugged or fitted.
6. Internal deburr expectations for cross-holes into pockets are written.
7. Quantity of features and any shipping plugs are in the RFQ text.
8. Related service pages: CNC machining service and request a quote when you are ready to price the housing.
Side-by-side buyer language
| Intent | Prefer writing | Avoid |
|---|---|---|
| Pressure equalize | Vent port + breather P/N or membrane note | “Air hole” alone |
| Empty liquid | Drain at installed low point + size | Drain on CAD bottom only |
| Condensation escape | Weep Ø + face + deburr | Random Ø2 “vent” on lid |
| Threaded breather | Thread + spotface / seal face | “Tap for vent” without size |
| Test reality | Ports open/plugged during leak test | IP note with no blanking plan |
Illustration 2 is a decision aid from sealing goal to feature type.
Common field failures drawings prevent
- Enclosure breathes through the gasket: no vent; thermal cycling pumps moisture past the O-ring.
- Standing water in a pocket: drain missing or above the true low point after mount rotation.
- Paint-closed weeps: finish after drill with no mask or re-drill note.
- Failed leak test on a vented box: test procedure assumed sealed ports; RFQ never said so.
- Cable abrasion at an internal burr: weep broke into a wireway without deburr callout.
- Wrong hardware: NPT vs G-thread mismatch on a purchased breather boss.
How this fits a sealed-housing RFQ package
Send PDF (or PMI) plus STEP, vent/drain hardware datasheets and a short note on installed orientation and leak-test blanking. Keep O-ring grooves, fastener counterbores and vent ports as separate note families so estimators do not merge “all holes are the same.” When the housing alloy choice is still open, 6061 vs 7075 aluminum for machined parts and stainless 304 vs 316 vs 17-4PH give material context; vent and drain function still rides on location, size and finish sequence.
Summary for estimators and buyers
Vent, drain and weep features are sealing-system geometry, not afterthought drill points. Name the function, place drains in the installed attitude, size ports for real hardware, plan finish masking and say how leak tests treat each opening. That package quotes cleaner, machines with fewer buried burrs and survives the first thermal and washdown cycles without improvising holes in the field.