Busbar tapping and thread depth

What a tapped joint has to do

A tapped hole in a busbar turns a bolt into clamp force. The bolt carries the preload, the thread in the bar resists that preload, and the joint faces carry the current. If the thread strips, the joint loses clamp force and the contact resistance rises. If the bolt breaks first, the joint also fails, but the failure is easier to see and less likely to damage the bar.

The design aim is therefore a balanced joint. The thread should be strong enough that the bolt reaches its intended preload, and the bolt should be the part that yields first if the joint is overloaded. That balance depends on the bar material, the bar thickness, the thread size and the length of thread that actually engages.

Copper busbars are usually made from a soft or half-hard grade, and the thread cuts into a material with lower shear strength than the steel bolt. Aluminium is softer again and creeps under a sustained clamp load. Both materials can hold a thread, but the engagement rule and the service temperature need to reflect that difference.

Thread engagement and strip load

Thread engagement is the length of full thread in contact with the bolt, expressed as a multiple of the nominal diameter. A common starting rule is one diameter in copper and one and a half diameters in aluminium. The rule is a starting point, not a guarantee, because the strip load depends on the shear area, the material strength and the fit of the thread.

For illustration, an M8 coarse thread with eight millimetres of full engagement in a copper bar has a nominal shear area of roughly 100 square millimetres. At an illustrative internal thread shear strength of 150 MPa, the thread would resist about 15 kN before stripping. The same thread in aluminium would need more engagement, or a larger thread, to reach a similar figure. These numbers show the method, not a design value.

Bar thickness is the usual constraint. A ten millimetre bar cannot give twelve millimetres of engagement unless a nut, a boss or a thicker section is added. Stacked bars can share the engagement if the joint is designed as one clamped stack, but the thread only exists in the outermost bar or in an inserted nut. The drawing should show which part carries the thread and how much full thread it provides.

Drill depth, thread depth and tap type

The drill depth is always greater than the usable thread depth. A blind hole needs room for the chip and for the tap run-out, so a common call is thread depth plus two millimetres, or thread depth plus one and a half pitches. Without that room, the tap bottoms out, the last threads are incomplete and the bolt cannot reach the intended engagement.

A through hole avoids the problem. It is easier to drill, easier to tap, easier to clean and easier to gauge. If the hole breaks into a face that must stay flat, add a small chamfer or a spot face so the breakout burr does not sit under the joint. A through hole also lets a longer bolt pass, which can be useful when the joint needs a nut on the far side.

Cut tapping is the usual choice in copper. The material is gummy, so the tap needs sharp flutes, a suitable lubricant and a controlled feed. Form tapping displaces material instead of cutting it and can give a stronger thread in some ductile materials, but the hole size and the material ductility have to suit the process. The drawing does not need to choose the tap, but the note should say whether the thread is cut or formed if the choice affects the inspection.

Plating and thread fit

Plating does not sit only on the flat faces. It also builds on the thread flanks, and a coating of five to eight micrometres on each flank can close a fine clearance. Tin is soft and tends to smear, silver is softer still, and nickel is harder and thinner for the same corrosion benefit. Each changes the fit in a different way.

The usual choices are to mask the thread, to plate the thread and gauge afterwards, or to use a thread insert. Masking keeps the fit predictable but adds a process step and a mask line at the hole edge. Plating through the thread is common for tin, provided the drawing calls the thread after plating and the supplier checks it with a gauge. A thread insert costs more but gives a harder, replaceable thread in aluminium or in a thin bar. Blind holes trap solution and can bridge the last thread, so a through hole is easier to rinse and gauge.

Thread lubrication is a separate decision. A dry joint and a lubricated joint need different torque figures for the same clamp force. The drawing should say whether the thread is supplied dry, waxed or with a specified lubricant, because the assembly torque depends on it.

Inspection that proves the thread

A tapped hole is inspected with a go and no-go gauge, a depth gauge and a visual check for burrs, chips and plating build-up. The go gauge should pass through the full thread, and the no-go gauge should not enter more than the allowed number of turns. If the drawing calls the thread after plating, the gauge check belongs after plating.

Depth needs a clear definition. Measure the full thread, not the drill point, and record it from the joint face. A depth gauge with a narrow blade works on a through hole, but a blind hole needs a gauge that seats on the face and reaches the last full thread. A bolt of the correct length is a practical functional check on a sample.

• Gauge the thread after the final finish, not before plating.
• Record full thread depth from the joint face, not the drill depth.
• Check the thread axis against the face that seats the bolt.
• Inspect for burrs on both the joint face and the hole entry.
• Run a sample torque test to confirm the joint reaches its intended clamp force.
• Keep the gauge and the torque record with the first article.

What to put on the drawing

The drawing should give the thread size and pitch, the minimum full thread, the drill depth for a blind hole, the chamfer and the finish. It should also say which face is the datum for the thread axis and whether the thread is gauged before or after plating.

The RFQ should add the joint load, the bolt grade, the lubricant and the assembly torque, because those inputs decide whether the engagement is enough. If the bar is aluminium, say so, and say whether a thread insert is acceptable.