Cause reference table
| Location of the marks | Cause | How to check | What to do |
|---|---|---|---|
| Inner wall, along the bend | Mandrel set too far forward, or insufficient lubrication | Withdraw the mandrel and inspect its surface | Retract the mandrel slightly; add the correct lubricant |
| Outer wall, in the clamp area | Clamp die too tight or worn | Inspect the marks on the clamp die | Reduce clamp force to just short of slipping; recondition the die surface |
| Scattered, irregular marks | Metal debris on the tooling | Wipe the tooling and run again | Clean the tooling on a schedule |
| Getting worse as the run continues | Material building up on the tooling (galling) | Compare the first and last part of the shift | Clean and lubricate more frequently |
| Only when bending stainless | Stainless tends to gall onto tool steel | Compare with carbon steel at the same settings | Change to a lubricant suited to stainless; consider surface-treated tooling |
Lubrication: the right type matters far more than the amount
Tube-bending lubricant is not ordinary machine oil. The point that needs it is between the mandrel and the inner wall, where pressure is very high for a very short time.
Too little lubricant scores the tube. The wrong type can be worse than none — it is squeezed out entirely under pressure, leaving metal on metal.
Stainless makes this harder: stainless tends to gall onto the steel surface of the tooling. Once that starts, every subsequent part scores worse than the one before.
The point most often misunderstood
Many shops see the tube slipping and wind the clamp force up to maximum. The slipping stops, and now the tube carries clamp marks instead.
Clamp force should be set just short of slipping, not at maximum. To set it: mark the tube, reduce clamp force step by step until slip marks appear, then go back one step.
Second: scoring that worsens through a production run is almost always cleaning and lubrication, not settings. If the first part of the shift is good and the last is poor, do not go and change the program.
For decorative stainless parts
If the tube will be visible — handrails, display frames, kitchen equipment — plan for this from the start:
- Lubricant suited to stainless, and a shorter tooling-cleaning interval
- Possibly a separate set of surface-treated tooling for stainless
- Consider protective film on the tube during processing
Send a tube sample and the drawing. We will bend it and send the part back so you can inspect the actual surface before deciding.