Busbar punching and hole design

What punching does to a bar

Punching is a shearing operation. The punch pushes a plug of metal through the bar and the die supports the material around it. The hole is not a clean cylinder: it has a rounded entry, a sheared band, a fracture zone and a burr on the exit side. The quality of those zones depends on the punch and die clearance, the sharpness of the tool and the temper of the copper.

The operation also leaves stress around the hole. If the hole is close to an edge, the material between the hole and the edge can stretch or tear. If two holes are close together, the web between them can deform. If the hole is near a bend, the bend can pull the hole out of position or turn it into an oval. Good hole design is mostly about keeping enough sound material around the hole.

Hole size and punch limits

A hole smaller than the bar thickness is difficult to punch cleanly. The punch is slender, the load per unit area is high and the punch can snap. As a starting rule, keep the hole diameter at least equal to the bar thickness, and preferably 1.25 times the thickness for a production tool.

The hole size should also match the fastener. A clearance hole for an M8 bolt is usually 9 mm, and a 10 mm bar can take that without difficulty. A 6 mm hole in a 10 mm bar is below the starting rule, so it needs a smaller punch, a slower stroke or a drilled hole instead. The drawing should give the fastener, the hole diameter and the tolerance, not just the nominal size.

For a joint that has to move or adjust, an oblong hole may be better than a round one. A slot adds length in one direction and can relieve tolerance stack-up. It also removes more material, so the edge distance and the clamp area have to be checked again.

Edge distance, pitch and bend clearance

The table below gives starting rules for a punched copper bar. They are not a substitute for a joint calculation, but they catch the drawing errors that cause most punch and assembly problems.

Burr direction and the contact face

A punch leaves a burr on the side where the material exits. If that side becomes the contact face of the joint, the burr sits between the bar and the lug or the mating bar. The joint closes on the high points, the contact area falls and the resistance rises. The burr also relaxes over time, so the clamp load can drop.

The drawing should say which face is the contact face and which way the burr may run. If the punch direction cannot be controlled, a deburring step is needed. A light belt or brush deburr is usually enough, but it has to be controlled so that it does not round the hole edge or remove plating from the contact area.

Burr is also a handling hazard. A sharp burr on a large bar can cut a fitter's glove or damage insulation during installation. A burr limit is therefore both an electrical call and a safety call.

Slots, adjustment and clamp area

A slot is a hole with a length. It is useful where the joint has to accommodate tolerance in the frame or where the bar has to slide during assembly. The slot length should be the adjustment range plus the hole diameter. The width follows the same rules as a round hole, and the edge distance is measured from the slot edge, not from its centre.

A slot reduces the clamp area under the washer. If the washer is smaller than the slot length, it can bridge the opening and bend when the bolt is tightened. Use a washer that covers the slot, or add a clamp plate. The contact area should be calculated on the material that actually sits under the fastener, not on the nominal overlap of the two bars.

Slots also change the inspection. A round hole is checked with a pin gauge; a slot needs its width, length and position checked. If the slot is punched in the same tool as the holes, the tool layout has to leave enough web between the slot and the nearest hole.

Deburr, plating and the joint face

Deburring comes before plating. A burr that is plated over is still a burr, and the plating adds a brittle layer on top of it. If the part is tinned or silver plated, the contact faces should be free of deep scratches, pits and burrs before the finish is applied. The plater can only cover the surface it is given.

Masking is the other call. Threads, locating diameters and any face that has to make a low-resistance joint may need masking or a controlled thickness. A heavy tin layer on a joint face can flow under clamp load, which is not always bad, but it changes the joint thickness and the torque. State the finish and the thickness on the drawing and say which areas are contact faces.

After plating, check the hole diameter again. Plating adds thickness to the bore, so a hole that was at the top of its tolerance before plating may be undersize after it. The same applies to a slot width. If the fastener fit is critical, give the finished hole size and let the supplier allow for the coating.

What to check on the first article

The first article should check the hole diameter, the position, the edge distance, the pitch, the burr height and the condition of the contact face. Use a pin gauge for the diameter, a calliper or a CMM for the position and a profile gauge or a dial indicator for the burr. Check the hole after plating, not just after punching.

If the bar is bent after punching, check the hole position after forming as well. A hole that is correct on the flat blank can move when the bar is bent, especially if the hole is within one thickness of the bend tangent. The drawing should call the hole position from a datum that survives the bend, such as the finished end face or a hole in the flat leg.

• Hole diameter after plating, with a pin gauge.
• Position from the finished datum, not the flat blank.
• Edge distance and pitch on the actual part.
• Burr height on the contact face.
• Slot width, length and alignment.
• Contact face free of scratches, pits and plating nodules.