Machined brass fittings and inserts: threads, seals and design

Two different components

A machined brass fitting carries or controls a fluid. It has a port thread, an outlet, and a seal that has to hold at a stated pressure and temperature. A machined brass insert anchors a thread in a moulded plastic part. It has an external knurl or barb, an internal thread and a wall thin enough to sit inside a boss.

The design questions are different. A fitting is judged on leak tightness, pressure rating and corrosion resistance. An insert is judged on pull-out strength, torque resistance and the integrity of the plastic around it. Both are turned from brass rod, and both usually need a secondary operation, but the tolerances that matter are not the same.

Treating them as one category leads to errors. A fitting thread needs a sealing face and a pressure-tight joint. An insert thread only needs to accept a screw and resist torque. A drawing that specifies a pressure-tight thread on an insert is over-specified and adds cost for no benefit.

The thread standards that matter

Pipe threads are the most common source of mismatch. The three families below look similar and do not seal the same way.

How the joint seals

A parallel thread such as G 1/4 does not seal on its own. It seals on a mating face, an elastomer washer or an O-ring in a counterbore. The thread only provides the clamping force. That is why a G thread fitting has a flange or a spot face, and why the surface finish and flatness of that face matter as much as the thread.

A taper thread such as R 1/4 or NPT seals by interference between the male and female flanks. The thread deforms as it is tightened, and a sealant fills the spiral leak path. The joint is made up by turns, not by torque alone, and over-tightening can split the port. The usual rule is to hand tighten and then apply the specified number of turns, with a thread sealant approved for the fluid.

An O-ring seal is the most forgiving. The O-ring sits in a groove or a counterbore and is compressed by the mating part. The groove dimensions, the surface finish and the compression ratio do the work. A scratched groove or a sharp edge on the lead-in will cut the O-ring at assembly, and the leak will appear later.

Designing a brass insert for plastic

An insert in a moulded part can be moulded in, pressed in or heat set. Moulded-in inserts are placed in the tool before the shot, so the plastic flows around the knurl. Press-fit and heat-set inserts are installed after moulding, which is cheaper for low volume and allows the insert to be added at assembly.

The external feature does the anchoring. A diamond knurl resists torque and pull-out in most thermoplastics. A straight knurl resists rotation but pulls out more easily. A barb or a groove works in soft materials. The knurl must not be so coarse that it creates a stress concentration in the boss, and it must not be so fine that the plastic cannot flow into it.

The boss wall has to carry the hoop stress from the insert. As a working guide, the wall around a moulded-in insert should be at least half the insert diameter, and more for a brittle material. A wall that is too thin splits at installation or cracks in service. A wall that is too thick sinks on the outside face.

The pilot hole for a heat-set insert is sized to the insert's minor diameter, not its knurl diameter. Too small and the plastic shears; too large and the insert spins. The supplier's data sheet gives the pilot size for the specific insert and material, and it should be checked on the first moulding rather than assumed.

Pressure, temperature and corrosion

A brass fitting is rated for a pressure at a temperature, not for a pressure alone. The allowable stress falls as the temperature rises, and the rating has to account for the thread form, the wall thickness and the seal. A fitting rated for 10 bar at 20 degrees C may be rated lower at 80 degrees C, and the thread sealant may have its own temperature limit.

Dezincification is the corrosion risk specific to brass. In certain waters, the zinc is leached out of the alloy and leaves a porous copper sponge that has almost no strength. Dezincification-resistant brass, usually a DZR grade with added arsenic, antimony or phosphorus, is specified for potable water and for aggressive water. The resistance is verified by a standard test, and the grade should be named on the drawing.

Stress corrosion cracking is the other risk. Brass under residual stress in an ammonia-bearing environment can crack, often at a thread root or a bend. Stress relief after machining or forming reduces the risk. If the part is used near ammonia, cleaning chemicals or certain refrigerants, say so on the enquiry.

The tolerances that decide function

For a fitting, the critical relationships are the squareness of the sealing face to the thread axis, the concentricity of the bore to the thread, and the thread gauge result. A sealing face that is out of square by a small angle will leak at one side even with a new washer.

For an insert, the critical calls are the thread gauge, the knurl diameter and the length. The knurl diameter sets the interference and therefore the pull-out strength. The length sets how much thread is available and how much plastic is displaced. A short insert with a good knurl can outperform a long insert with a poor one.

General tolerances such as ISO 2768 cover the non-critical dimensions. Do not put a tight tolerance on every diameter. It raises the price and tells the supplier nothing about which features matter.

What to test before release

A pressure test is the only direct check of a fitting's function. The test pressure, the hold time and the medium should be stated, and the test should be done on a sample from the production lot, not only on the first article. A leak test at the rated pressure is more informative than a dimensional report on a sealing face.

For an insert, a pull-out test and a torque test on the actual moulded material are the useful checks. The strength depends on the plastic, the boss geometry and the installation method, so a test on a generic coupon is only indicative. Test the real assembly.

Both part types need a material certificate. For a potable water fitting, the certificate should confirm the DZR grade and the lead content. For an insert, it should confirm the alloy and the temper, because a soft insert will strip its thread before it pulls out.

What to send with the enquiry

Send the drawing with the thread standard named in full, the sealing method, the pressure and temperature rating, the fluid and the environment. Add the material grade and the corrosion requirement, and say whether the part touches potable water.

For an insert, send the plastic grade, the boss drawing, the installation method and the required pull-out and torque values. If the boss does not exist yet, send the insert data sheet and the intended wall thickness so the two can be designed together.

Finally, say how the part is assembled. A fitting that is tightened with a torque wrench to a stated value is a different design problem from one that is hand tightened in the field. The assembly method decides the wall thickness, the thread engagement and the seal.