Are multi piece blanks made on a single mandrel and cut or on separate mandrels and fit together ?

Most modern blanks of multi-piece design are made on separate mandrels. Some hand fitting of the ferrules is still necessary to achieve the desired fit.

Tip-Over-Butt ferrules generally require that each section be made individually.

In years gone by, multi-piece rods were often made as a single piece blank, then cut into pieces and fit with spigot/plug ferrules. Nowadays, even those blanks with the spigot ferrules are usually made in individual sections on individual mandrels and some type of reinforcement is added at the ferrule areas.

If you choose to try and make your own multi-piece rod from a single piece blank, I would recommend the Volume 4 #3 issue of RodMaker wherein Ralph O'Quinn details how to do this with good results.

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I highly recommend the above article. Tried it out and works well.