The tube itself becomes the sealing element.

The end of the heavy-wall tube is machined to a precise cone angle, and an external thread is cut into the tube a set distance behind it. A collar screws onto that thread and a gland nut engages the collar. As the gland nut is tightened into the fitting body, it drives the coned tube end into a matching conical seat machined in the body.

Metal contacts metal at the cone, and the contact stress at the interface exceeds the fluid pressure, so the joint seals. There is no ferrule, no O-ring and no soft seal to extrude, degrade or be attacked by the fluid.

That construction is why the connection works at pressures where elastomeric seals would fail, and why it tolerates a wide range of fluids and temperatures. It is also why the cone angle, the thread position and the tube's wall thickness all have to be exactly right.

Because the fitting's rating is calculated for a specific tube, and the tube is part of the pressure boundary.

At these pressures the tube's own capacity is determined by its outside diameter and wall thickness together, not by a nominal size. Two tubes with the same OD but different walls have very different ratings. The fitting is designed around a particular combination, and the coning and threading operations remove material from the tube wall in a way that is calculated for that specification.

So fitting a correctly-sized cone onto thinner-walled tube produces a joint that assembles perfectly and is rated for far less than the label implies - and nothing about its appearance reveals that.

Buy tube and fittings against one manufacturer's specification, confirm the wall thickness on delivery rather than assuming, and do not mix components between makers even where threads appear compatible.

A coning tool and a threading tool matched to the tube size and pressure class, plus the means to cut and deburr the tube squarely.

The cone must be formed at the correct angle and concentric with the tube bore. The thread must start the specified distance behind the cone, because that dimension controls how far the cone is driven into its seat when the gland is tightened. Both operations are done with purpose-made tools, hand-operated for smaller sizes and machine-driven for larger ones.

General workshop equipment does not substitute. A cone turned in a lathe to an approximate angle, or a thread cut with a standard die, will not reproduce the geometry the connection depends on.

Inspect every prepared end before assembly: the cone should be smooth, unmarked and free of chatter, and the bore should be clean. Any scoring across the sealing face is a leak path, and at these pressures a leak is a hazard rather than a nuisance.

In what seals, and in what the tube has to be.

An ordinary compression fitting uses a ferrule - a separate ring that bites into or grips a plain, unmodified tube end as the nut is tightened. The tube needs only to be cut square and deburred, and the ferrule does the sealing.

A coned-and-threaded fitting has no ferrule. The tube end is machined into the sealing element, and it must be heavy-wall material capable of carrying the pressure and of being coned and threaded without losing rating.

The practical differences are that the high-pressure connection takes far longer to prepare, requires specific tooling, and is not field-adjustable in the same casual way - but it operates at pressures an order beyond compression fittings, and it is remakeable many times because nothing is deformed permanently on assembly.

Use compression fittings within their range; move to coned-and-threaded when the pressure demands it, not as a general upgrade.

The range is normally divided into medium-pressure and high-pressure classes, each with its own tube specifications, cone geometry and fittings, and the classes are not interchangeable.

Medium-pressure covers the band above hydraulic and instrumentation service, used for pressure testing, chemical injection and process applications. High-pressure extends well beyond that, into waterjet cutting, gas boosting, high-pressure research and specialised testing.

Within each class the actual rating depends on the tube size, the wall thickness, the material and the temperature. Ratings fall as temperature rises and as bore increases for a given wall, so a single headline pressure figure is never the whole specification.

Select from the manufacturer's rating tables using the working pressure, the test pressure and the maximum operating temperature together. Where the system is cyclic, ask specifically about fatigue rating - as with hose, static rating and cycle life are different questions.

Predominantly stainless steel, with high-strength and corrosion-resistant alloys used where the service demands them.

Stainless is the default because it combines the strength needed at these pressures with corrosion resistance and good behaviour across a wide temperature range. Cold-worked stainless is used where a higher rating is needed from the same size.

Nickel alloys and other specialist materials are specified for aggressive chemicals, sour service, high temperature or seawater. Because the tube is part of the pressure boundary and forms the seal, the tube and fitting materials should be compatible with each other as well as with the fluid - mixing materials at a metal-to-metal seal can cause galling on assembly and galvanic problems in service.

State the fluid, its concentration, the temperature and any trace contaminants when specifying. At these pressures a material selection error does not produce a weep; it produces a failure.

The manufacturer's published procedure, followed exactly, with the joint made up dry or lubricated precisely as they specify.

The sequence generally involves fitting the collar to the threaded tube end to a defined position, seating the cone in the body by hand, then tightening the gland nut to a specified torque or to a defined number of turns past finger tight. Which of those applies varies by manufacturer and size, and using the wrong one either under-seats the cone, so it leaks, or over-stresses it, so the cone is damaged and cannot be remade.

Do not use thread sealant on the cone - the seal is the metal interface, and sealant on the sealing face prevents proper contact.

Pressure test progressively rather than going straight to full pressure, keep personnel clear during the test, and never attempt to tighten a fitting that is under pressure. Record the torque and the test result.