Cause reference table
| Symptom | Common cause | How to check | What to do |
|---|---|---|---|
| Even flattening on the outside of the bend | No mandrel, or mandrel positioned wrongly | Measure diameter at the crown of the bend, compare with the straight section | Fit a mandrel; adjust the mandrel nose position |
| Flattening increases with bend angle | Mandrel nose set too far back from the tangent point | Trial bend at 30°, 60°, 90° and measure each | Advance the mandrel towards the tangent point |
| Flattening only at the start of the bend | Insufficient clamping, tube slipping | Mark the tube before bending and look for slip marks | Increase clamp force; check the clamp die surface |
| Flattening together with inner-wall wrinkling | R/D beyond what this tooling set can do | Calculate R/D | Increase the radius, or add a wiper die |
| Stainless flattens, carbon steel does not | Stainless springs back more and needs tighter conditions | Trial bend both materials at the same settings | Set separate parameters for stainless |
Two ratios to know before you bend
R/D — centreline radius divided by outside diameter. The smaller this ratio, the tighter the bend and the more the outer material is stretched.
D/t — outside diameter divided by wall thickness. The larger this ratio, the thinner the wall relative to the diameter, and the less able it is to hold its own shape.
These two ratios decide the outcome before you touch the machine. A part with a small R/D and a large D/t cannot be bent cleanly on any machine without a mandrel — that is a material limit, not a machine limit.
The point most often misunderstood
Many workshops blame the machine when a tube flattens, while the problem sits in the tooling and the mandrel.
A four-axis CNC without a mandrel still flattens thin-wall tube; an NC machine with a correctly positioned mandrel produces a good part. A mandrel is not an upgrade — for a part that needs one, it is the condition for that part existing.
The second frequent error is mandrel position. The nose must sit close to the tangent point: too far back and it supports nothing; too far forward and the tube scores or jams. This is set by trial, not by formula.
Before concluding the machine cannot do it
Work through this order, changing one thing at a time:
- Confirm a mandrel is fitted, and that its size matches the tube's inside diameter
- Adjust the mandrel nose position, trial bend at 90°, measure the diameter at the crown
- Increase clamp force and check the tube for slip marks
- If it still flattens, recalculate R/D — the part may need a larger radius or a thicker wall
Send us the part drawing. For tube bending a drawing says more than any description — and we will tell you plainly whether your part needs a mandrel, including when the answer means changing the design.