TL;DR

Inconsistent springback between parts typically means the material yield strength has shifted between coils. Re-measure springback on each new coil and update the overbend compensation value in the CNC.

What you might see

final bend angle varies part to part without parameter change
parts at the start of a new coil measure consistently different from the last coil
overbend compensation set but angle still out of tolerance

Likely causes

Tap to mark as ruled out

Material yield strength variation between tube coils or lots
Overbend compensation value in the CNC outdated for current material
Bend die wear changing the effective bend radius and springback characteristics
Inconsistent clamping force from clamp die wear

Required tools

Digital angle gauge
Profile template for bend die groove inspection
Digital caliper
Scrap tube from the production lot

Safety first

Keep hands and fingers clear of the die area during test bends.
Wear eye protection when inspecting bend dies for metal fragments.

Procedure

7 steps · ~30 min

1
Bend a test sample from the current tube lot. Measure the actual angle after release with a digital angle gauge.[1]
2
Compare the measured springback to the programmed overbend value in the BLM CNC. Calculate the difference.
3
Update the springback compensation parameter in the CNC program for the current part by the measured error amount.[1]
4
Bend two more test pieces and measure to verify the correction is stable.
5
Inspect the bend die groove for wear. A worn groove increases the effective bend radius and changes springback. Measure the groove profile with a template gauge.
6
Inspect the clamp die for wear marks or scoring. Uneven clamping force causes angle variation because the tube slips or deforms differently.
7
Document the springback value for this material lot in the job setup record so it can be re-applied when the same lot is run again.

Sources

Schwarze-Robitec BLM Group e-TURN / Smart Tube / Pipe Bender general technical documentation, Schwarze-Robitec
BLM Group e-TURN / Smart tube bender programming and material setup documentation (general)

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