That is the mandrel. Some are bars with a shaped end that finishes at the bend point, some are balls linked together and in this case disks linked together.
The object is to support the tube from the inside at the bend point to prevent deformation. This guy is doing a first class job, those bends are super smooth.
Look at 0:43. Those cups in the vise are lubricated to help promote slipping. As Norton pointed out, sometimes, balls are used. "Mandrel" is the operative word that tells you something rigid is inserted inside to prevent collapsing. I have used sand, melted solder, melted lead, etc. After the bending the tube is heated and the low temp metal flows out. Cleaning is always a problem. I have even filled with water and then froze copper tube with mixed results. Both ends have to be crimped shut and then cut off later. I don't think those methods are considered "mandrel" bending.
Last edited by Saltfever; Sep 5, 2021 at 11:25 PM.
NortonDommi (Sep 15, 2021)
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