A DIN 928 welding nut should not be accepted solely because its square body looks correct or a bolt can be screwed into it. Acceptance depends on the specified dimensions and tolerances, thread conformity, welding-projection geometry, material and surface condition, and traceability to the ordered requirements. A nut can pass these incoming checks and still produce an inadequate weld if the sheet, electrodes, or welding parameters are unsuitable.

The essential distinction is between conformity of the supplied nut and performance of the welded assembly. DIN 928 identifies a square weld-nut design; it does not, by its designation alone, establish an acceptable welded joint for every application. Incoming release and weld-process approval therefore need separate, documented acceptance criteria.

Establish the acceptance reference before measuring

The inspection reference should be the purchase specification, the applicable edition of DIN 928, and any approved part drawing or supplementary technical requirements. A supplier catalogue is useful for identification, but a catalogue’s nominal dimensions are not a substitute for the tolerances governing acceptance.

Confirm that the order identifies the thread size and pitch, material requirements, surface condition, and any agreed deviations from the standard design. If a drawing modifies the projection geometry or adds a special coating, the inspection plan must reflect that modification. Conflicting requirements need engineering resolution rather than an inspector choosing whichever limit permits acceptance.

A thread designation does not define the complete nut geometry. For example, M8 with a 1.25 mm pitch identifies a thread combination, not the allowable square-body width, body height, or projection height. Those dimensions must come from the relevant size row and drawing notes in the controlling specification.

Before releasing a new source, translate the drawing into a characteristic-by-characteristic inspection record. Each entry should identify the requirement, its tolerance, the measurement method, and the disposition rule. This prevents nominal values copied from different supplier tables from becoming an unofficial acceptance standard.

Dimensions that affect fit, location, and welding contact

DIN 928 nuts combine a threaded square body with welding projections. Their functional geometry must be assessed as a connected system: the body fits the feeder and fixture, the thread aligns with the assembly hole, and the projections establish the initial welding contact.

Do not treat body height and overall height as interchangeable. A measurement taken over the projection tips includes geometry that a body-thickness requirement may exclude. Record the actual drawing symbol and measurement definition rather than using a general label such as “thickness.”

Calipers may be suitable for screening accessible body dimensions when their uncertainty is appropriate to the tolerance. They are less suitable for establishing small projection-height differences or the position of a thread axis. Depending on the requirement, an optical measuring system, height-measuring arrangement, dedicated fixture, or coordinate measurement may be necessary.

The measuring setup matters as much as instrument resolution. Supporting a nut on its projections can create an unstable reference; clamping it excessively can alter contact conditions. The inspection method should define the datum, contact points, and orientation so that different inspectors measure the same feature.

Do not create an additional rejection criterion merely because a feature can be measured. If thread position is functionally important but the drawing does not establish a limit, obtain an approved requirement. An undocumented expectation cannot provide a consistent basis for supplier acceptance.

Thread inspection requires more than a mating bolt

A production bolt is not a calibrated thread gauge. It can enter an oversized or otherwise nonconforming internal thread, while its coating, wear, or own dimensional variation can make a conforming nut appear defective.

Use calibrated GO and NOT GO thread plug gauges matched to the specified nominal diameter, pitch, and internal-thread tolerance class. ISO 1502 provides a recognized framework for metric screw-thread gauging where applicable. Gauge use and acceptance must follow the relevant requirements rather than a locally improvised rule.

• Confirm the gauge identification, tolerance designation, calibration status, and condition.
• Remove loose contamination without reworking or concealing thread defects.
• Apply the gauges using the prescribed technique without forcing engagement.
• Evaluate GO engagement and NOT GO entry against the applicable acceptance rules.
• Record failures by characteristic, not simply as “bolt does not fit.”

For a short nut, the permitted NOT GO engagement must be interpreted under the applicable gauging rules. Do not rely on a remembered turn-count rule without checking its conditions and the available thread length. Chamfers and incomplete end threads also need to be distinguished from the full-form thread.

Thread gauges do not verify every assembly requirement. A nut can pass thread gauging while its thread axis is incorrectly located relative to the square body. Conversely, an entry burr can obstruct assembly even where the underlying thread dimensions are acceptable. Visual entry inspection, dimensional location checks, and functional gauging address different failure mechanisms.

Projection condition is a release characteristic, not a cosmetic detail

The corner projections concentrate the initial electrical contact during resistance projection welding. Their height, shape, and relative contact condition affect how force and current are distributed before they collapse. A flattened or damaged projection can change that sequence even when the nut’s body dimensions remain acceptable.

Inspect for missing or incompletely formed projections, crushed tips, cracks, heavy burrs, embedded debris, and inconsistent geometry. Distinguish the intended manufactured form from damage caused by handling or transport. A projection does not need to look sharp to be correct; conformity depends on the specified profile, not a visual preference.

Relative projection height deserves particular attention. If one projection contacts substantially before the others, initial force and current distribution can become uneven. However, the allowable difference must come from the drawing or an approved engineering requirement. A generic statement that all projections must be “equal” is not a measurable acceptance criterion.

Where a contact-check fixture or optical method is used, define what it demonstrates. Rocking on a reference surface may identify a seating irregularity, but it does not quantify every projection or prove weldability. Likewise, a photograph can document visible damage but cannot establish a small dimensional deviation without a suitable measurement reference.

Material and surface condition can change welding behavior

Correct dimensions do not establish correct material. Verify the material designation and any mechanical-property requirements against the purchase specification. Do not infer strength or hardness from appearance, or assume that every nut sold under the DIN 928 designation has the same supplied condition.

Surface requirements should be explicit. Oil, rust, scale, plating, and conversion coatings can affect electrical contact, electrode condition, and the welding process. A finish suitable for a conventional bolted fastener is not automatically suitable for a projection-welding application.

If a coating is specified, confirm its identity and any required thickness or condition. Substituting a different finish should trigger technical review, even when thread gauges still pass. The relevant question is whether the supplied surface matches the condition used to establish the approved weld process.

Loose corrosion, contamination, cracks, and damaged threads require disposition under the agreed defect criteria. Avoid authorizing cleaning, chasing, grinding, or projection reshaping as informal receiving operations. Such actions can change the part and invalidate the original inspection evidence.

What the paperwork proves—and what it does not

A statement of “DIN 928 compliant” is incomplete evidence unless it can be linked to the supplied product and the requirements being claimed. Review the supplier’s part designation, lot identification, quantity, specified material and finish, and any requested inspection results.

Where the contract requires material or inspection documentation, verify the document type and its connection to the delivered lot. EN 10204 document designations may be relevant when specified, but a particular certificate type should not be assumed mandatory simply because the product is a weld nut.

Management-system certification is not product acceptance evidence. An ISO 9001 certificate does not demonstrate that a particular shipment meets dimensional tolerances, passes thread gauging, or achieves the required welded-joint strength. Similarly, the DIN designation is not itself an application-specific safety approval.

Traceability must remain usable after receiving. If packages are split or lots are mixed in a feeder, the link between inspection results and production parts can be lost. Preserve identification through storage and issue, especially where a later weld failure could require containment of affected assemblies.

Set sampling and disposition rules before the lot arrives

Sampling should reflect the approved quality plan and the consequences of escape. ISO 3269 addresses acceptance inspection of fasteners, while ISO 2859-1 provides attribute-sampling schemes where contractually applicable. Neither should be invoked without defining the relevant lot, inspection level, acceptance criteria, and treatment of nonconformities.

An acceptable quality limit is not a guarantee that every accepted part conforms, nor is it permission to use a known defective component. Characteristics with serious safety consequences may require stronger controls, including validated automated inspection or agreed screening, rather than reliance on a general sampling plan.

Draw samples across the lot rather than from the easiest package to open. For variable measurements, retain actual readings where they support process evaluation. A simple pass mark can conceal dimensions trending toward a limit or differences between packages.

When a sample fails, place the affected lot on hold and follow the agreed disposition process. Repeatedly drawing new samples until one passes does not resolve the original failure. Sorting, rework, concession, or return should be authorized, documented, and followed by the required verification.

Keep incoming acceptance separate from weld validation

A conforming nut still requires a suitable sheet material and thickness, hole geometry, electrode arrangement, force, current, and weld time. Incoming inspection cannot demonstrate that this combination will produce an adequate joint.

Validate weld performance on the intended assembly configuration using specified tests and acceptance limits. Depending on the application, these may include torque-out, push-out, pull-out, section examination, or other approved assessments. These tests are not interchangeable: they load the joint differently and can reveal different weaknesses.

Thread usability may also need verification after welding because distortion or weld spatter can create defects absent at receiving. Changes to nut material, projection geometry, coating, sheet condition, or welding equipment should be reviewed for their effect on the approved process.

Release the incoming lot only against defined nut requirements, and release the welding process against defined assembly-performance requirements. Keeping those decisions distinct makes failures easier to trace and prevents a dimensional inspection report from being mistaken for proof of a safe welded joint.