CNC Turned Firefighting Hose Couplings and Nozzle Bodies

The challenge

Fire hose couplings are used under pressure by people wearing gloves, often in poor conditions. They must thread on quickly, seal against a gasket and not gall, and they take impacts when hoses are dropped on concrete.

The requirements for this program were:

• Coupling threads to the thread standard specified by the customer, which must still gauge correctly after hard anodizing
• Gasket groove width and depth held to ±0.03 mm so the gasket seals without being pinched or blown out
• Rocker or pin lugs milled on the outer diameter for spanner wrenches
• Smooth internal bore in the nozzle body for water flow, with no steps or burrs
• Type III hard anodize for wear and corrosion resistance

The main technical risk is the coating. Type III hard anodize is typically 25–50 µm thick, and roughly half of that grows outward from the original surface. On a thread, that growth applies to each flank, so a thread machined to the normal size will be too tight after coating. The pre-coat size has to be planned, not guessed.

Our approach

We treated the thread and groove sizes as "before coating" dimensions derived from the final requirement.

1. DFM and coating allowance. With the customer, we agreed on a target hard anodize thickness range and calculated pre-coat thread pitch diameter and groove sizes from it. These pre-coat values were added to our internal drawing.
2. Op 1, turning. From 6061-T6 bar, we turned the outer profile, bore and first-end threads on a turn-mill lathe. Threads were single-point turned so the pitch diameter could be adjusted in small steps.
3. Op 2, second end. The part was flipped into soft jaws on a finished diameter, and the gasket groove, seat face and second-end features were turned.
4. Lug milling. Spanner lugs and flats were milled with live tooling in the same clamping, keeping them in position relative to the thread start.
5. Nozzle body bore. The internal flow passage was turned with a profiled boring bar and a finishing pass for a smooth surface.
6. Deburring. Thread starts were chamfered and the first partial thread removed to prevent cross-threading and damage to gloves.
7. Pre-coat gauging. Every thread was checked against pre-coat gauges before the parts went to anodize.
8. Type III hard anodize. Parts were hard anodized, with the coating thickness specification agreed in step 1.

Quality plan

Inspection was split into before-coating and after-coating checks.

• First-article inspection of all dimensions before and after anodizing, so the actual coating growth could be confirmed against the plan
• Coating thickness measured with an eddy-current gauge on agreed locations
• Threads checked with go/no-go thread gauges after anodizing, on 100% of parts
• Gasket groove width and depth measured on every part
• Mating check of male and female couplings by hand assembly on a sample from each lot
• Visual inspection for coating defects, burrs and damaged thread starts

Material certificates for the 6061-T6 bar and the anodize thickness records were supplied with each lot.

What was delivered

We delivered prototype couplings and nozzle bodies, then production lots on the same route. Each lot came with:

• First-article reports showing pre-coat and post-coat dimensions
• Thread gauge and groove inspection records
• Coating thickness measurements
• Material certificates for the bar stock

Parts were packed with thread protectors so threads and lugs arrived undamaged. Hydrostatic pressure testing and any product certification were handled by the customer. The pre-coat dimension sheet now acts as the controlled reference for repeat orders, so thread fit stays consistent from lot to lot.

Questions about this project