Tube Bending Troubleshooting

Tube Bending Problems Quick Answer

Use this when bends collapse, wrinkle on the inside, spring open past layout, or wall thickness fails inspection after bending.

1. Wall Thinning & Ovality at the Bend

What it looks like: Outside wall of the bend thins measurably (ultrasonic or cut section), tube goes oval instead of round, or pressure rating drops below spec after forming.

Likely causes: Stretch on the extrados exceeds material limit — tight bend radius relative to OD (low D/R), thin wall (high D/t), or over-clamping without mandrel support. Aluminum and copper thin faster than steel at the same D/t.

Fixes: Increase centerline radius, step up wall thickness, or use a mandrel/wiper stack sized for the tube. Verify predicted thinning in the Tube Bending Calculator before cutting production lengths.

2. Wrinkling on the Inside Radius

What it looks like: Ripples or folds on the intrados (inside) of the bend, sometimes visible only after paint. Bend may kink instead of flowing smoothly.

Likely causes: Compressive instability on the inside — bending without mandrel on thin-wall tube, CLR too tight, or insufficient pressure-die force. Material buckles instead of flowing.

Fixes: Use a mandrel with correct ball count and linkage for the CLR, add wiper die if needed, increase bend radius. The Tube Bending Guide maps D/t to mandrel requirement.

3. Springback Miscalculation

What it looks like: Bend opens 2–15° after removing from the machine; fixtures don't fit; opposite bends don't mirror. Repeat bends drift angle batch to batch.

Likely causes: Elastic recovery depends on yield strength, bend radius, and wall thickness. Using a steel springback factor on stainless or aluminum guarantees error. Over-bend amount not validated on scrap.

Fixes: Apply material-specific springback from the calculator, trial on coupon, record machine-specific correction. Heat on some alloys changes springback — keep process consistent.

4. Wrong Mandrel or Die for Wall Thickness

What it looks like: Mandrel sticks or drags, scratches ID, leaves witness marks, or tube slips in groove — combined with wrinkling or flattening depending on which component is wrong.

Likely causes: Die groove too wide (slip) or too narrow (pinch/oval). Mandrel ball count too few for CLR — can't reach tangent point. Plug mandrel used where linked ball mandrel is required for thin wall.

Fixes: Match die groove to actual measured OD (not nominal), select mandrel type per D/t table in the guide, lubricate ID appropriately. Measure tube OD with calipers — mill tolerance affects fit.