Why a tube weld seam must be oriented relative to future bends and holes ART-193

Start with the part's function

There is no universal rule that the seam must always face upwards or always lie on the inside of a bend. The finished part's function determines the required position. One product prioritises the decorative side, another leak-tightness, a third the accuracy of bolt holes and a fourth bend consistency. Simply specifying a fixed angle without linking it to the drawing will quickly turn the rule into a formality.

Divide the tube surface into functional zones:

• areas containing holes, slots and contours with narrow ligaments;
• the outer and inner sides of future bends;
• locations where other parts will be welded on;
• locating planes in a fixture;
• surfaces contacting a seal, roller or bushing;
• visible decorative sides;
• neutral areas where seam position does not affect function.

For each zone, specify both the preferred and prohibited positions. For example, the designer may allow the seam within a sector between two planes but prohibit it from intersecting the edge of a precision hole. Such a sector is more realistic than demanding a single perfect angle, because both the detection system and the tube itself have some degree of error.

Why a hole near the seam needs particular attention

When a contour crosses the seam or passes close to it, the laser does not necessarily encounter the same surface as on the base metal. Height, reflectivity, local thickness or internal bead condition may change. This does not mean that cutting through the seam is prohibited. It means that feasibility must be confirmed on the specific tube, rather than assumed from a machine catalogue.

The risk depends on geometry. A large process hole with a wide tolerance may be insensitive. A small hole, narrow slot, pre-tapping hole, precision datum edge or thin ligament near the seam requires more rigorous verification. If the internal bead protrudes, a detached particle or molten material may behave differently inside the tube. After cutting, inspect not only the outside edge but also the inner surface.

Check separately whether the seam lies in an area intended for subsequent flaring, bushing insertion or fastener contact. Even a perfectly cut hole may fail to fulfil its function if the local surface profile interferes with assembly. Acceptance criteria should be based on assembly requirements, not merely a photograph of the edge.

Bending changes the priorities

During bending, one side of the tube is stretched and the other compressed, while the cross-section may become oval. The seam's position relative to these zones sometimes affects repeatability, but neither the direction nor the magnitude of the effect is universal. They depend on the material, radius-to-diameter ratio, wall thickness, tooling, mandrel, lubricant and seam properties.

The decision must therefore be based on testing. For a critical part, produce samples with several angular seam positions, bend them using the actual tooling and compare ovality, wrinkles, cracks, springback and the position of features after bending. If a broad sector is acceptable, specify that sector in the drawing and CAM. If only a narrow position is acceptable, detection and angular positioning requirements become stricter.

Do not automatically place the seam on the neutral axis simply because it sounds logical. For a specific process this may be beneficial, irrelevant or undesirable. The author of this article does not prescribe a bending process. The designer and process engineer approve the final rule after a control series.

The orientation map must form part of the data

A verbal note saying “seam at the bottom” is unreliable if it is unclear which coordinate system defines the bottom. The angle must be tied to the part datum, profile axis and machine zero position. For rectangular tube, it is convenient to specify a side or sector relative to a particular plane. For round tube, specify an angular position relative to a reference contour or the plane of the future assembly.

The part record should contain the following fields:

| Field | What is recorded | Why | |---|---|---| | Angular datum | Reference plane, face or axis | To prevent different systems from interpreting the angle differently | | Preferred sector | Permissible range of seam positions | To avoid demanding unrealistic zero error | | Prohibited zones | Holes, bends, visible surfaces | To allow CAM to detect a conflict | | Inspection method | Camera, sensor, mark or manual check | To make the rule executable | | Response to uncertainty | Stop, rescan, quarantine | To prevent the system from continuing on a guess | | Result after bending | Finished-part characteristics and tolerances | To assess function, not just the angle before cutting |

If orientation is critical, the data must pass from the design model to the programme, machine setup, batch marking and inspection report. Manually transferring an angle between paper notes creates a hidden risk of changing its sign or datum.

Detection and positioning are different operations

The system must first locate a seam feature and then rotate the tube into the required position. Detection may rely on the external or internal surface profile, optical contrast or another principle used by the specific machine. The ability to see one clean black seam does not prove operation across all suppliers, materials and surface conditions.

BLM GROUP describes Active Weld as a function that detects the seam and orients it according to the design specification; external or internal seam detection is stated separately for the LT6. For specific TruLaser Tube machines, TRUMPF describes SeamLine Tube with automatic detection and alignment. This confirms that this class of solution exists, but does not make the solutions interchangeable or guarantee coverage of a particular tube. The option, camera, lighting, permissible surface profile, cycle time and response to uncertainty must be checked in the offered configuration.

Accurate angular positioning is required after detection. Clamping, slippage, ovality, twist, regripping and the system's own error affect the result. FAT must therefore measure the actual seam angle relative to the finished contour, not merely show a green indicator on the screen.

When a manual rule is sufficient

Not every production operation needs an automatic function. If batches are small, the seam is readily visible, tube geometry is stable and orientation is required for only a few parts, manual locating with a checking template may be economically justified. However, the manual method also needs a standard: where the mark is placed, how the operator finds the seam, which gauge checks the sector and what is entered in the routing record.

Automation becomes more valuable when batches are large, several surface types are used, the seam may be internal and a single error can ruin an expensive assembly after bending. Compare the overall economics rather than the speed of a single scan: search time, stops, repeated setups, scrap after subsequent operations and traceability.

What to do when the seam is not found

The most important scenario is low confidence, not successful detection. The system must not silently classify a random scratch as a seam or continue cutting at an unknown angle. For critical parts, define permitted responses: rescanning, rotating and searching again, calling the operator, separating the workpiece or blocking the programme.

Record the reason for failure against the metal batch. If problems are concentrated around one supplier, surface condition or internal bead type, this is a purchasing signal. Separate statistics are needed for false positives, where the system confidently identifies something other than the seam. These cases are more dangerous than an explicit failure because the part may proceed downstream.

FAT must cover different tube batches

Five identical pieces preselected by the machine supplier are not sufficient for acceptance. Build a set from actual purchasing sources: minimum and maximum cross-sections, light and dark surfaces, external and internal manifestations of the seam, permissible ovality, and long and short workpieces. If protective film or a coating is used, include it in the test.

For each sample, record:

1. whether the seam was found on the first attempt; 2. detection time and number of retries; 3. the actual angle after positioning; 4. the distance from the seam to the critical contour; 5. cut quality in the base metal and near the seam; 6. behaviour after the actual bend; 7. the assembly and functional inspection result.

Repeat the series after changing the diameter, cleaning the optics, restarting the programme and warming up the machine. What is needed is process repeatability, not one successful image.

Do not confuse machine accuracy with raw-material quality

A tube may have a seam that wanders along its length, changes its local surface profile or occurs together with profile twist. The machine may correctly identify a feature at one point, but this does not guarantee the same angular position along the entire length. For long parts, check the beginning, middle and end.

If the deviation falls outside the accepted sector, several distinct solutions are possible: tighten requirements for the tube supplier, modify part geometry, increase the safe clearance, use additional scanning or change the process route. Responsibility cannot automatically be placed entirely on the tube cutting machine's servo drive.

Inspection after cutting and bending

Before cutting, the seam angle can be checked relative to the datum. After cutting, its position can be checked relative to the contours. After bending, the finished-part geometry can be checked. These are three different inspection points. Measuring only the first leaves the cause of defects at the next operation unknown.

A simple record is useful for series production: tube batch number, programme, specified sector, detection result, angle measured on a sample, bending programme number and final status. Collecting excessive amounts of data is unnecessary, but the connection between raw material, cutting and the finished part must be maintained.

How to formulate a requirement for the machine supplier

Instead of saying “we need a seam camera”, provide an application matrix. It should include tube types, the external or internal manifestation of the seam, surface condition, required sector, critical contours, batch time and response to uncertainty. The supplier must name the specific option, coverage limits, calibration method and method of recording the result.

Ask separately for a programming demonstration. BLM's ArTube pages describe the ability to define seam position during design to avoid conflicts with cutting geometry. Check exactly how this rule passes from CAD/CAM to the machine, whether the programmer can see the prohibited zone and what happens after mirroring the part or changing its datum.

A practical decision sequence

First determine whether the seam affects the function of at least one part. Then specify an acceptable sector relative to an unambiguous datum. Check raw material from several batches and decide whether manual positioning is sufficient. If automation is required, test detection, rotation, the response to uncertainty and the actual angle. Then carry out cutting, bending and assembly, and set the acceptance criterion on the finished part.

This approach turns the seam from a hidden variable into a controlled process-route parameter. The aim is not always to hide it in one place, but to give every part a predictable position validated against its function.

How to assess the economic case for automation

A detection function has value only if it reduces real losses or enables a required part. To assess it, calculate manual search and locating time, the frequency of resetting, the number of seam-critical parts, the cost of scrap after bending and traceability costs. Add automatic scanning time, unsuccessful attempts, maintenance, calibration and licences.

Compare three scenarios: manual positioning with a template, automatic detection on every tube and selective use only for critical programmes. The third scenario often provides a better balance: simple products are not slowed down, while critical ones receive documented inspection. The decision depends on the product portfolio, not the machine's maximum functionality.

Calculate defect cost up to the point where the defect is detected. If an incorrect position is found immediately after cutting, the loss is limited to the tube and time. If the part has already been bent, welded, painted or installed in an assembly, the consequence is much more expensive. This risk may justify automation even with only a small number of critical items.

Changes that require the rule to be requalified

Validation is not indefinite. Reverification is required after a change in tube supplier or standard, material, surface condition, permissible seam, detection algorithm, camera, lighting, CAM version, jaws, regripping arrangement or bending programme. Even a drawing change that brings a hole closer to the seam can put the part outside the scope of the previous evidence.

Create a list of parameters defining the qualified family. If a change remains within that family, first-part inspection is sufficient. If it goes beyond the limits, a repeat test is required. This guards against silently applying a recipe to a similar but actually different tube.

Typical implementation mistakes

The first mistake is purchasing the option without including the angle in the digital data. The second is trusting the detection indicator without measuring the finished contour. The third is testing one polished tube without checking purchasing variability. The fourth is setting an unnecessarily narrow sector that the function does not require, causing avoidable stops. The fifth is failing to define the response to uncertainty.

Another mistake is inspecting cutting alone. If the seam matters because of bending or subsequent welding, those operations must be part of qualification. Otherwise, the process optimises an intermediate characteristic without evidence of the final result.

CTA. Send L-SEL the part drawing, tube data, future bends and visible-surface requirements. We will help prepare a seam orientation map and a verification programme using a control batch.

Limits of application

This material does not establish an acceptable seam position for a specific design and is not a bending instruction. The designer, process engineer and process owner determine the final sector, inspection method and acceptance limits based on the drawing, certified raw material and testing.

Need to identify the right component?