Hydraulic and pneumatic fittings live or die on two features: the thread that holds pressure and the sealing surface that stops leaks. A fitting that measures perfectly on the outside will still fail in the field if the flare angle is off by a degree or the O-ring boss has a burr. This guide walks through how a precision shop actually machines CNC machined hydraulic fittings and pneumatic fittings, what tolerances matter, how brass and stainless differ on the machine, and what the work typically costs.
Why fittings are a turning-first part
The vast majority of hydraulic and pneumatic fittings start as bar stock on a lathe. The body is round, the threads are turned or single-point cut, and the sealing cone or boss is a controlled diameter and angle. At Wexmar we run this family on our CNC turning cells, with live tooling for cross-holes, hex flats, and wrench pads so a fitting comes off the machine complete rather than moving to a second operation.
Volume drives the machine choice. A 3/8 in body run in the thousands moves to Swiss machining, where bar feed and guide-bushing support let us hold tight diameters on long slender bodies and run lights-out. Larger bodies, 1 in and up, stay on the fixed-headstock lathes.
Thread forms you will actually order
Getting the thread callout right is the single most common source of leaks and rejected fittings. The forms below cover most of what a fluid power buyer needs.
- NPT and NPTF: Tapered pipe threads that seal on thread interference. NPTF (dryseal) is machined to a tighter crest-to-root fit so it seals without tape. We gage these with an L1 ring or plug and log the standoff.
- SAE J1926 ORB (O-ring boss): Straight thread with a machined boss face and a spot for the O-ring. The seal is the O-ring against the boss, so the boss diameter, the spotface flatness, and the thread runout all matter.
- JIC 37 degree flare (SAE J514): The seal is metal-to-metal on a 37 degree cone. The cone angle and surface finish carry the seal, so we hold the angle tight and finish the cone to Ra 32 uin or better.
- BSPP and BSPT: British parallel and tapered. Common on imported equipment. Thread gaging differs, so confirm the standard on the print.
- ORFS (O-ring face seal, SAE J1453): Flat face with an O-ring groove. The groove depth and width and the face flatness are the controlled features.
Typical tolerances on a machined fitting
- Sealing cone or boss diameter: +/-.001 in to +/-.0005 in depending on the seal type
- Flare angle: +/- 0.5 degree, often tighter on high-pressure lines
- Overall length: +/-.005 in unless a stack-up calls for tighter
- Hex across flats: +/-.005 in so standard wrenches seat
- Sealing surface finish: Ra 16 to 32 uin on metal-to-metal seals, Ra 63 uin acceptable on thread flanks
Brass vs stainless: the machining reality
The material choice changes cost, speed, and where the fitting can be used. Below is the practical trade-off buyers weigh most often.
| Factor | Brass (C360 / C377) | Stainless (303 / 316) |
|---|---|---|
| Machinability | Excellent, near-free machining, fast cycle times | Slower, work-hardens, 303 far easier than 316 |
| Corrosion resistance | Good in most hydraulic oil and clean air | Excellent, holds up in marine, washdown, and chemical |
| Pressure rating | Fine for typical pneumatic and low to mid hydraulic | Higher, preferred for high-pressure hydraulic |
| Relative material cost | Moderate, tracks copper markets | Higher, 316 notably more than 303 |
| Typical use | Instrument air, low-pressure hydraulic, cost-sensitive | Defense, food, marine, high-pressure, DFARS jobs |
A practical rule: quote brass C360 when the application allows it because cycle times are short and the finish is clean off the tool. Move to 303 stainless when corrosion or pressure demands it, and reserve 316 for marine, chemical, or spec-driven work. For defense and government orders we run DFARS-compliant material with mill certs traced to the lot. See our government and defense machining page for how we handle certs and CAGE-registered orders.
Sealing surfaces are where quality is won
A fitting rejects at the seal, not the thread, more often than buyers expect. Three things we control on every sealing feature:
- Finish: A metal-to-metal 37 degree flare needs a clean, chatter-free cone. We hold Ra 32 uin or better and inspect with a profilometer on first article.
- Concentricity: The seal must run true to the thread. We hold sealing surface runout to the thread pitch diameter within .002 in TIR or tighter.
- Burr-free transitions: Cross-holes and the seal edge get deburred so an O-ring is not sliced on assembly.
On critical lots we run first article inspection and CMM inspection on our Brown & Sharpe CMM, then thread-gage every setup. Details are on the inspection and quality page.
What CNC machined fittings cost
Pricing depends on material, thread complexity, tolerance, and quantity. The table gives typical illustrative estimates for a mid-size fitting body, roughly 1/2 in to 1 in, not a binding quote.
| Cost driver | Prototype / low qty (1-50) | Production (500-5,000) |
|---|---|---|
| Setup and programming | $150 to $450 per part amortized | $0.30 to $1.50 per part amortized |
| Material (brass) | $1.50 to $5.00 | $1.00 to $3.50 |
| Material (316 stainless) | $3.00 to $9.00 | $2.50 to $6.50 |
| Machine time | $4.00 to $12.00 | $1.20 to $4.00 |
| Thread gaging and deburr | $1.00 to $3.00 | $0.25 to $0.90 |
| Inspection (FAI, CMM as needed) | $75 to $250 per lot | $0.10 to $0.50 per part amortized |
These are illustrative ranges. Brass at volume on Swiss machines lands at the low end; small stainless prototypes with tight FAI requirements land at the high end.
How to spec a fitting so it comes out right
- Call out the thread standard by name and size, not just a dash number.
- State the seal type: dryseal NPTF, ORB, JIC 37 degree, or ORFS.
- Give sealing surface finish and any runout requirement.
- Name the material and grade, and flag if DFARS certs are required.
- Note pressure rating so we can confirm brass vs stainless.
Send a print and quantity and we will size the right machine for the run. Start a quote through our instant quote page.