206 Getting the Part to Fit the Fixture
Three things happen during the bending process and as the clamp and pressure die release the tube that cause changes to the part that will require adjustments:
- Material stretch - The actual straight length required to make a bend is slightly less than what an arc length calculation would indicate. Unless a lot of boost or pressure die assist is applied, a tube is in tension during the bending process and will stretch.
- Spring back - Because bending occurs after the elastic deformation zone (see 196 Cold Working and Material Hardness) at the beginning of the bend, the material also goes through the elastic deformation at the end of the bend. The effect of this is a slight reduction in degree of bend. If a tube bender was set to make 90 degrees of bend on a mild steel tube, after the dies open up the bend will measure only about 87 degrees (less in SS, more in aluminum).
- Radial growth - Going through the elastic deformation at the end of the bend also affects the centerline radius. If a bend die was machined to a centerline radius of 150mm, on a mild steel tube after the dies open up, the centerline radius will measure 153mm or more.
The effects of material stretch and radial growth cause the straight lengths between bends to be too long. Slight reductions are usually required in the DBB offset values to compensate. The effect of spring back causes a slight reduction in degree of bend. Slight increases are usually required in the DOB offset values to compensate. If the weld seam is not aligned with the inside or outside of a bend, it will cause the bend to twist out of plane or cork screw. Slight changes to the POB offset values will compensate. See 105 Basic CNC bender Programming.
Checking fixtures and
CMMs
Conventional tube bender:
- Program the first straight to be short by 2mm. Make a part. Start with bend one and adjust the offsets to correct the DOB for bend one, the DBB and POB for bend two. Make another part.
- Adjust the offsets to correct the DOB for bend two, the DBB and POB for bend three. Make another part.
- Repeat for all subsequent bends.
- How much too long is the last straight? If the first straight is 2mm short and the last straight is 6mm long, then the offset for the first bend should be changed to 2mm and the cut length of the tube should be reduced by 4mm.
- If using a tube stop in the collet, adjust the carriage position for the first bend forward by 4mm.
Twin head bender:
start with the last bends, closest to the center of the part, and work towards the ends.
Back to set up
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