How it's Made? Sheet Metal Helix 2

[DistressedNerd]

Hello - I am stumped on how they make the sheet metal spiral for something like this (see image below). I can see how they would make a sheet metal helix without the room for a rod through the center - but can not wrap my head around the manufacturing process for a sheet metal helix with room down it's center to be welded on a rod. The process in the linked video is what could be used to make a helix of sheet metal - how would the flat piece of sheet metal be altered to have room for rod down it's center?
Link

 

[JStephen]

For the "skilled craftsman", those blanks were possibly cut out with a pattern torch, a fairly low-tech device for duplicating a shape like that.

 

[JohnRBaker]

Look at the edges of the blanks. They appear to be very clean, like they were punched, not burned. Now it's true that the edges where the rings were 'split' looks like it might have been burned, but the inside/outside of the rings themselves seem to be much smoother than you could possibly get by burning them without any additional and extensive finishing.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
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