Internal tension mandrel for spray pipe machining and spray pipe fine machining method

The invention relates to an internal tension mandrel for spray pipe machining. A spray pipe comprises a neck part, a flaring part and a narrow opening part, wherein outer diameters of the flaring partand the narrow opening part are gradually increased from the two ends of the neck part. The internal tension mandrel comprises a connecting piece, an internal tension mandrel inner sleeve, an internal tension mandrel outer sleeve and a rear terminal disc, wherein the connecting piece comprises a fixed rod and a screw cap, the fixed rod is arranged at one end of the internal tension mandrel innersleeve, the internal tension mandrel outer sleeve is movably arranged on the internal tension mandrel inner sleeve in a sleeving manner, the fixed rod is sleeved with the rear terminal disc, the screwcap is screwed at the end, away from the internal tension mandrel inner sleeve, of the fixed rod, the fixed rod is sleeved with the spray pipe in the machining process, the far end of the flaring part is clamped to the interna