Why rectangular-profile corners cut differently from flat walls ART-197

Corner geometry is not a single point

The external radius is formed by rolling or another profile-manufacturing method. The internal radius differs, and material distribution may differ from the centre of the wall. Nominal side dimensions of 100 by 50 millimetres do not reveal the actual radius, corner-zone thickness, wall convexity or weld seam position.

Programming requires the actual surface. If a contour crosses from one wall to another, its developed shape depends on the radius. A model error changes the line's position and length on the finished part. For a hole that extends partly onto a corner, simply projecting a circle onto nominal planes may not produce the shape the designer expects.

Before testing, measure external dimensions, radii, thickness at accessible points, squareness, convexity and twist. If tube suppliers provide different radii within the purchasing specification, CAM and the measuring system must have a clear response.

The surface normal rotates continuously

At the centre of a flat wall, the beam and nozzle can be oriented relative to a constant normal. On a radius, the normal continuously changes direction. Even on a 2D tube cutting machine, this requires synchronised rotary and linear axis movement. A 3D head adds tilting motions and the risk of reaching a kinematic limit.

The beam's angle relative to the surface affects cut shape, effective path thickness and gas-flow direction. If the machine slows down at the transition, the process has a different interaction time. Without parameter adaptation, the corner may overheat or fail to cut through consistently.

BLM GROUP describes Active Speed as a function that adapts parameters to actual speed, and Active Focus as automatic focus control for specific systems. This confirms the principle of coordinating process and kinematics, but does not provide a universal recipe for every profile.

Actual corner speed differs from the programmed value

A programme may contain one nominal feed rate, but axes have acceleration, jerk and synchronisation limits. Near a corner, one coordinate decelerates, another accelerates, and the rotary axis changes angle. The actual cutting-point speed drops or fluctuates. At constant power, this changes the energy per unit length.

The problem is particularly noticeable on a small radius, short contour or closely spaced sequence of corners. Examine not just the NC feed but a record of actual speed and the process controller's response. If the supplier claims automatic adaptation, ask to see it on a trace, log or another available indicator.

Do not draw conclusions from one attractive profile. Heavy, large tubes have different rotational dynamics; thin ones have a different thermal capacity; and different radii change the toolpath. Verification must cover the limits of your matrix.

Thickness and heat dissipation may change

A profile's corner zone is formed by material deformation. Actual thickness and metallurgical condition may differ from the flat portion, but the magnitude depends on the specific product. It cannot be assumed without measurement or supplier data.

In its material on differences between tube and sheet cutting, BLM GROUP notes that heat can concentrate on a tube, especially at the edges of special profiles, causing deformation or inaccurate or interrupted cuts. This is a vendor explanation of the general mechanism, not a quantitative model.

At a corner, heat dissipates into two adjacent walls, but the toolpath, deceleration and local geometry change the result. Avoid the simplistic conclusion that a corner always heats more or less. Verify this over a series by observing edge quality, geometry and temperature using a validated method.

The opposite wall changes position

When a flat face is cut, molten material and gas are directed inside the tube towards the opposite surface. As the cutting point moves onto a radius, the distance and impact angle change. This may affect internal spatter, covered by ART-195, and the risk of thermal marks.

A small rectangular profile is especially sensitive: the opposite wall is close, and the corner toolpath changes direction rapidly. If internal protection or a suction probe is used, coverage must be checked at the corners, not only at the centre of a face.

A result from round tube cannot be transferred to rectangular tube. A round surface changes smoothly around its entire perimeter; a rectangular one has long, nearly flat sections and local radii with different dynamics.

The nominal model and the actual profile

CAM usually works with idealised geometry. A real tube has tolerances, side ovality, twist and radius variation. If a contour is far from the corner, the error may be insignificant. If it crosses the radius, the difference between model and surface directly affects shape.

Measuring systems can scan a profile or individual features and correct the toolpath. BLM Active Scan is presented as geometry correction based on the actual shape of a deformed or twisted tube. This does not mean automatic reconstruction of any radius to the required accuracy. Ask what exactly is measured, how often, for which cross-sections and how the correction is transferred to the contour.

In an official story about TruLaser Tube use, TRUMPF states that TruTops Tube performs complex calculations at rectangular-profile corners. This confirms the importance of specialised software, but is not a universal guarantee for another system or any imported CAD file.

Define a contour across a corner by its function

A circle on a developed surface, a circle in a plane and a round hole for an inserted element are not always the same geometry. If a hole crosses a radius, the designer must define what should be round and relative to which coordinate system. CAM cannot guess the function.

An insert, bushing or joint needs an assembly check. A decorative cutout needs the right shape on the visible surface. A drain hole needs a clear passage. Acceptance must therefore not be limited to measuring a line on the developed surface.

Create a digital note or feature type that unambiguously describes the intent. If the programmer has to rebuild every corner contour manually, version and error risks increase.

Cutting order affects the corner

A large cutout in one wall weakens the profile before the transition to the adjacent wall. If contours are close to a corner, the local ligament may heat up and deform. Side sequence, entry points, cutting direction and pauses between contours affect the result.

The process strategy must retain sufficient stiffness until critical features are complete. This may mean another sequence, microjoints or deferred separation, where permitted. The decision is validated on the part; this article does not prescribe a specific sequence.

During a series, observe whether heat accumulates on one side. If CAM always travels around the profile in the same direction, the last corner may be in a different condition from the first. Comparing positions helps distinguish a material defect from a programming pattern.

The weld seam may enter the corner zone

In rectangular tube, the longitudinal seam often lies on one wall, but its position has a tolerance. If it approaches the radius or crosses a corner contour, the process encounters another local change. ART-193 covers seam orientation relative to future holes and bends.

Include tubes with extreme permissible seam positions in the corner test. Do not set up the process solely on a sample where the seam is conveniently distant. If detection is needed for a consistent position, check the full detection and rotation cycle.

How to classify the defect

The description “bad corner” does not help correction. Separate the results:

| Symptom | Likely direction for investigation | What not to do immediately | |---|---|---| | Incomplete penetration on the radius | Focus, speed, thickness, gas, surface model | Increase power without a test | | Deposit or burr | Actual feed, gas, nozzle, contour direction | Reduce speed globally | | Displaced geometry | Actual radius, twist, datum, scan | Apply one offset to all tubes | | Overheating or deformation | Sequence, deceleration, contour density | Assess only the edge | | Spatter on the opposite wall | Molten-material trajectory, internal protection, cleaning | Promise absolute cleanliness | | Different corners on one part | Order, heat accumulation, kinematics | Blame everything on material |

This is not a diagnostic table with ready-made parameters. It shows which data to collect before changing a recipe.

A corner coupon for FAT

Prepare a test part containing straight cuts at the centre of each wall, identical contours near all four corners, a hole or slot crossing a radius and real functional geometry. Include variants with different radii, thicknesses and seam positions. The coupon must be long enough for stable clamping and representative dynamics.

During the test, record the actual profile, software model, CAM version, parameters, nozzle, gas, focus, toolpath and actual speed. Measure contour shape, burrs by an agreed method, squareness or angle where functionally important, internal marks and deformation after cooling.

Repeat at least several parts without manual adjustment. One coupon may show feasibility but not repeatability.

Compare 2D and 3D only against the task

A 3D head can change the beam angle and produce bevels, but that does not automatically mean a better conventional cut at every rectangular corner. It adds kinematics, possible access restrictions, programming and calibration. A 2D system can produce high-quality standard contours with specialised synchronisation.

Head-type selection belongs to ART-173, and bevels to ART-182. For the present topic, the requirement is simply to demonstrate that the selected system can handle your part's corner geometries with the required quality, time and stability.

Series-production control

After qualification, retain recipes by profile family: material, side dimensions, thickness, radius range, surface condition and corner-contour type. On the first part, check the critical corner and feature position. After changing the tube supplier or a significant batch change, repeat the actual-radius check.

Monitor defects by angular position. A problem that consistently appears at the programme's last corner points to sequence or heat. At the seam corner, it points to raw material or orientation. Random occurrence points to profile shape, clamping, measurement or process condition.

Do not combine geometry corrections and process-parameter changes into one opaque manual edit. Retain a record of exactly what changed and why, so the recipe can be reproduced.

Practical conclusion

A rectangular-profile corner is a distinct process zone because surface, kinematics, actual speed, thermal condition and the path of cutting products change simultaneously. Quality depends not on a single parameter but on coordinated CAD/CAM, actual-profile measurement, motion control and process control.

Proper verification starts with the actual radius and contour function, and ends with a series of corner coupons and assembly. Only then can the recipe enter production within the validated family.

Calibration after work on the head or axes

A corner contour depends on coordination between linear, rotary and, for 3D systems, tilting axes. After servicing, a collision, or replacement of a nozzle, optics or a kinematic-chain component, carry out the manufacturer's procedures and a control coupon. A flat test may not reveal an error that appears only during a synchronised transition across a radius.

Retain a reference profile and a programme with several corner features. Compare them using the same method after prescribed maintenance events. If geometry changes at every corner, check system calibration; if only at a particular position, check local tube shape, seam, sequence and thermal condition.

Do not use a manual offset as a permanent correction for an unknown cause. It may improve one corner and worsen another.

How to assess stability between batches

For each profile supplier, retain the actual ranges of radius, side dimensions, twist and seam position. Compare corner defects against these characteristics. If the result depends on one raw-material parameter, include it in the purchasing specification or automatic measurement.

A control chart may contain the feature dimension at every corner, burr height by an agreed method, the number of incomplete cuts, internal marks and rework requirements. Analyse spread and position, not only the average. Four corners of the same part provide a useful internal comparison because they are processed on the same tube.

Review the qualified envelope after changing material, thickness, radius, supplier, CAM version or process table. A similar rectangular profile does not automatically belong to the same family.

Typical task-definition mistakes

The first mistake is giving the programmer only a developed view without a functional description of the contour. The second is assuming a nominal sharp corner instead of the actual radius. The third is assessing speed from programmed feed without observing axis deceleration. The fourth is testing the centre of a face and treating the whole profile as validated. The fifth is measuring a hot part without a defined stabilisation time.

Another mistake is comparing 2D and 3D systems by name rather than by coupon. More versatile kinematics do not remove the need for process control, calibration and a correct surface model.

Agree criteria before testing: dimensions, inspection zone, permissible burr, inner-wall condition and functional assembly. Otherwise, the supplier and customer will assess different characteristics, and the corner coupon will not provide an unambiguous decision.

Retain the result together with the actual profile sample.

CTA. Send L-SEL your profiles, drawings of contours near corners, materials and tolerances. We will help develop a corner test to verify CAM, parameters, geometry and internal cleanliness on the specific configuration.

Limits of application

This material does not specify cutting parameters, a permissible radius, CAM corrections or an edge criterion. The manufacturer, process engineer, designer and metrologist define these for the specific profile, configuration and part function.

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