Key takeaways
- Inspect the lug against the function it performs: conductor fit, terminal fit, current path and crimp.
- Barrel concentricity is a wall thickness check, not a cosmetic one. An off-centre bore cracks on the thin side.
- Plating is part of the joint. Measure thickness and check adhesion before and after crimping.
- A first article covers every drawing call. Routine inspection covers the features that can drift.
Start with the drawing, then with the joint
A cable lug is a simple part with four jobs: receive the conductor, land on the terminal, carry the current and survive the crimp. Inspection should follow those jobs. A dimension that does not affect one of them can be covered by a general tolerance. A dimension that locates the conductor or the palm deserves its own check and its own record.
The drawing should show the barrel bore and length, the palm hole and spacing, the palm width and thickness, the angle between barrel and palm, the overall length and the plating. If a feature is missing, the inspector has to guess, and the guess will not be the same at the supplier and at goods-in.
Read the title block before measuring anything. The general tolerance, the material, the plating standard and the revision decide how the rest of the drawing is interpreted. A lug measured against the wrong revision can pass every dimension and still be the wrong part.
The dimensional set that matters
The barrel bore is the first call. It must accept the conductor class and strand count, not just the nominal area. Measure the bore at both ends and at the middle, because a barrel that tapers or bells unevenly changes the crimp. A bore that is too large lets the strands spread; a bore that is too small will not accept the conductor without cutting strands.
The palm calls are next: hole diameter, hole spacing, palm width, palm thickness and the angle to the barrel. Hole spacing is a functional call because it decides whether the lug fits the terminal without strain. The angle matters on an angled palm, where a few degrees of error can load the barrel when the bolt is tightened.
Overall length and barrel length close the set. A long barrel gives room for two crimps and a longer strip length. If the barrel is short, the conductor may not reach the inspection window or the second crimp position, and the joint loses margin even when every other dimension is correct.
| Feature | Method | Record | Failure it catches |
|---|---|---|---|
| Barrel bore | Pin gauge or bore micrometer at both ends | Diameter and taper | Conductor fit and strand spread |
| Barrel wall | Section and measure at four points | Minimum and maximum wall | Thin wall and crack risk |
| Concentricity | Bore offset from the outside diameter | Offset or wall difference | Uneven crimp and barrel split |
| Palm hole and spacing | Pin gauge and calliper | Hole diameter and centre distance | Terminal fit and bolt loading |
| Palm angle | Angle gauge or comparator | Angle from the barrel axis | Strain at the bolted joint |
| Plating | Thickness gauge and adhesion test | Thickness, coverage, adhesion | Corrosion and contact resistance |
| Crimp features | Visual and crimp height gauge | Bellmouth, height, cracks | Strand damage and loose joint |
The exact set follows the drawing. Add any feature that locates the conductor, the terminal or the crimp.
Barrel concentricity and wall thickness
Concentricity sounds like a rotating shaft call, but on a lug it means something practical: is the bore centred in the barrel wall? If the bore sits to one side, one wall is thick and the other is thin. The crimp die compresses the thin side first, and that is where a crack starts.
The check is a section through the barrel at a point clear of the bellmouth and clear of the palm transition. Measure the wall at four points, or measure the bore offset from the outside diameter. Record the minimum wall, not the average. An average hides the thin side, and the thin side is the one that fails.
A lug can pass a bore check and still have a concentricity problem. The bore is the right size, the outside diameter is the right size, and the wall is uneven between them. This is why a bore gauge alone is not enough for a part that will be crimped. Add the wall check to the first article and to the supplier's process control plan.
The thin side decides the crack
A barrel with an average wall inside tolerance can still have one wall below the minimum. Record the lowest reading at each section, not the mean.
Plating: thickness, adhesion and coverage
Plating protects the joint from oxidation and keeps contact resistance stable. On a crimped lug it also has to survive deformation. A coating that is thick but brittle can crack at the barrel and expose the copper underneath, while a thin cosmetic flash can wear through at the palm.
Measure thickness on the palm and on the barrel, because the two areas can differ. Check coverage inside the bore, where a brush or a masking plug may have missed the surface. For tin, check that the coating is continuous and free of blisters. For silver, check for tarnish and for a uniform colour rather than a patchy finish.
Adhesion is a separate test from thickness. A bend or a tape test on a sample can show whether the coating lifts. After crimping, inspect the barrel for cracks and flakes at the bend and at the crimp edge. If the plating cracks, the joint may still pass a pull test and fail later in a damp environment.
Crimp features and the finished joint
The bellmouth at the conductor entry should be visible and even. It relieves the stress concentration where the strands leave the crimp. A bellmouth that is closed flat, or one that is so large that the barrel no longer grips the insulation, is a defect. Inspect it before and after crimping, because the die can change it.
Crimp height is measured across the flats on a hex crimp, or at the indent on an indent crimp. Record it with the die number. Inspect the barrel for cracks along the bend and at the transition to the palm. On an insulated lug, check that the insulation sleeve is not trapped in the crimp and that the colour code matches the conductor size.
The finished joint should be checked for straightness. A lug that sits at an angle to the cable loads the terminal when the bolt is tightened. A slight bend is normal after crimping, but a consistent lean points to a locator or die problem rather than to the operator.
Visual, marking and material checks
Visual inspection catches the defects that gauges miss: cracks, folds, tool marks, corrosion, a missing chamfer and a damaged palm face. Use a defined light and a magnifier for the barrel, and photograph the accept and reject conditions so that the standard does not drift between shifts.
Check the marking against the drawing: size, material code, manufacturer mark and any certification mark. A lug with the correct dimensions but the wrong marking cannot be traced. Ask for the material certificate with the lot, and check that the copper grade and the temper match the drawing.
The palm face deserves a separate look. Scratches and burrs reduce the contact area, and a raised burr around the hole prevents the lug from sitting flat on the terminal. A burr that is folded over can also crack when the bolt is tightened.
First article and routine inspection
A first article inspection covers every drawing call, including the features that will not be measured again. It includes the material certificate, the plating certificate, the dimensional results and a photograph of the part. The first article is the reference for the rest of the programme, so it should be measured on calibrated equipment and signed.
Routine inspection covers the features that can drift: bore, wall, palm hole and spacing, plating thickness and crimp height. The frequency depends on the process. A turned or forged lug may need a dimensional sample per lot, while a stamped lug may need a check at each tool change. The control plan should say which feature is checked at which frequency.
Keep the records with the lot. If a joint fails in service, the first question is whether the lug was inside the drawing at the time it was made. A record that shows the bore, wall and plating for that lot answers the question quickly. A record that shows only a pass stamp does not.
Making the accept or reject call
A dimension outside the drawing is a reject, but the decision needs context. A bore at the top of the tolerance may be fine for a flexible conductor and marginal for a coarse stranded one. A wall at the minimum may be acceptable on a small lug and a crack risk on a large one. Record the reading and the conductor it is intended for.
Do not accept a lug on a visual check alone when the drawing has a functional call. A barrel that looks round can be off-centre, and a plating that looks bright can be too thin. The cost of a section and a thickness measurement is small compared with the cost of a joint that fails after installation.
When a lot is rejected, keep the samples and the readings. A supplier can only correct the process if they know which feature moved. A rejection note that says out of tolerance is less useful than one that says the barrel wall measured 0.8 mm on one side against a 1.0 mm minimum at the mid-barrel section.
Frequently asked questions
How is barrel concentricity measured on a cable lug?
Section the barrel at mid-length, away from the bellmouth, and measure the wall at four points. Alternatively, measure the bore offset from the outside diameter. Record the minimum wall, because the thin side is where a crimp crack starts.
What plating thickness should a cable lug have?
The thickness comes from the drawing or the product standard, and it differs between tin, silver and nickel. State the minimum thickness and the test method, and check the plating on both the palm and the barrel. A cosmetic flash is not a specification.
Should a cable lug be inspected before or after crimping?
Both. Before crimping, check the bore, wall, palm, plating and marking. After crimping, check the crimp height, bellmouth, cracks, insulation placement and straightness. Some plating defects only appear after the barrel deforms.
Does a general tolerance cover cable lug dimensions?
It can cover non-functional dimensions such as overall length or a chamfer. Functional calls such as bore, wall, hole spacing and palm angle should have their own tolerances. A general tolerance applied to every feature tells the supplier that nothing is critical.
Sources and further reading
- IEC 61238-1, Compression and mechanical connectors for power cables
- DIN 46235, Compression cable lugs
- ISO 2768-1:1989, General tolerances, linear and angular dimensions
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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