Machined parts for switchgear and connectors: threads, faces and current paths

Three jobs on one part

A machined part inside switchgear usually does three jobs at once. It conducts current, it locates another component and it holds a dielectric gap. A terminal stud conducts current into a busbar, locates a cable lug and sets the creepage distance between the live metal and the earthed enclosure. A contact stem does the same while also locating a moving contact.

That combination is why a switchgear drawing cannot be treated as a general machining drawing with a plating note added at the end. A change that helps one job can hurt another. Polishing a contact face improves resistance and reduces the grip on a joint. Shortening a stud reduces material and reduces the creepage distance. Deepening a thread increases engagement and reduces the wall left around it.

The useful drawing makes the hierarchy explicit. Mark the current path, the locating features and the insulation surfaces, then assign tolerance and finish to each group according to its duty.

The part families and what they need

Most switchgear machining falls into a small set of families: terminal studs, contact stems, busbar adapters, threaded inserts, arc runner supports and operating shafts. Each family has a different critical call, and a supplier who sees the family will price the route correctly.

A terminal stud is dominated by the thread and the contact face. A contact stem is dominated by concentricity and by the surface where the contact sits. A busbar adapter is dominated by the hole pattern and by flatness against the bar. An insert is dominated by the thread and by the wall thickness around it. None of these is difficult in isolation. They are difficult when a drawing mixes the calls without saying which one matters.

Critical calls by part family

The table gives a starting point for a drawing review. Materials and finishes depend on the current, the environment and the mating part, so confirm them against the assembly rather than a generic preference.

Threads carry more than a diameter

A thread call of M8 is not a specification. The drawing needs the tolerance class, the engagement length, the thread depth, the runout to the contact face and the condition at the thread start. A 6g external thread and a 6H internal thread are standard defaults, but a joint that is tightened often, or one that carries current, may need a different fit or a controlled plating thickness on the thread.

Engagement length matters more than most drawings suggest. A stud threaded into a soft copper block can strip at the first torque if the engagement is short. A common rule is one diameter of engagement in steel and more in a softer metal, but the real number depends on the alloy and the torque. Ask for the torque, then size the engagement to it.

Plating changes the thread fit. Silver at 5 µm per side closes the pitch diameter by roughly 10 µm, which can turn a free-running thread into an interference fit. A plated thread needs the pre-plate dimensions set so the finished thread gauges correctly. State whether the thread is plated or masked, because the two routes produce different parts.

Contact faces are a system, not a number

The contact face is where the current crosses from one part to another. Its resistance depends on the real area in contact, and the real area depends on flatness, roughness, plating and clamp force together. A flat face with a rough finish has fewer contact points. A smooth face with a convex shape touches only at the crown. A plated face with a porous deposit oxidises under the film.

That is why a face call should combine flatness, roughness and plating, and why the three should be inspected together. A flatness of 0.05 mm across a 40 mm face and a roughness of Ra 1.6 µm are reasonable starting values for a bolted copper joint. A sealing or sliding contact needs tighter values, and the assembly test should decide them.

Do not polish a contact face beyond the function. A very smooth surface can be worse than a lightly textured one if the plating cannot key to it or if the joint relies on a controlled bite. The finish should match the joint design, not a general idea of quality.

The insulation path runs through the part

Creepage and clearance are usually treated as enclosure design, but a machined part sets both. The distance from the live stud to the earthed panel depends on the stud length, the shoulder position and the radius at the base. A sharp corner at the base creates a path that a radius would lengthen, and a recess can trap contamination that shortens the effective distance.

The relevant requirements come from the insulation coordination standard for the equipment, for example IEC 60664-1 for low voltage systems or the IEC 62271 series for high voltage switchgear. The drawing does not need to repeat the standard, but it should show the surfaces that form the dielectric path and the dimensions that set the distances.

Practically, this means a machined part should not be changed for cost without a check on the gap. A shorter stud, a smaller shoulder or a deeper undercut can all reduce the distance. If the change is proposed by a supplier, ask for the before and after dimension on the insulation path, not just the price difference.

Datums and the call that links features

A switchgear part usually has one feature that locates it and one feature that carries current. The drawing should make the locating feature the primary datum and relate the current-carrying face to it. If the thread is the mounting feature and the contact face is the functional one, the runout of the face to the thread is the call that matters.

A typical call is a total runout or a position tolerance rather than a tight linear dimension. A face that is parallel within 0.03 mm to the mounting face is easier to inspect and more meaningful than a stack of dimensions from an edge. A concentricity call between a stem and a contact seat is best expressed as runout to a common datum axis.

Keep the number of datums small. Three well-chosen datums cover most parts, and each extra datum adds an inspection setup without adding function. If a feature does not locate, seal or carry current, a general tolerance is enough.

What to inspect and what to send

A first article for a switchgear part should include the thread gauge result, the contact face flatness and roughness, the plating thickness on the contact face and the thread, and the runout between the locating feature and the functional face. Add a contact resistance check on a sample if the joint is critical, and a torque test if the thread is the mounting feature.

Send the drawing with the current path and the insulation surfaces marked, the mating parts, the torque, the plating specification and the applicable equipment standard. Add the annual volume and the inspection level. A supplier who knows the part is a current path will not quote a decorative polish, and a supplier who knows the creepage distance will not shorten the stud to save bar stock.