PURPOSE: To easily machine and correct a damaged safety valve tuyere sheet in a short time by aligning a portable vertical lathe with the axes of a valve body and a tuyere via a guide, screwing the lathe to the valve body, and lathing the tuyere sheet.

CONSTITUTION: A vertical lathe assembly is attached to a safety valve body 5 by a screw 4, and a screw 2 is loosened to move the assembly 13 downward until a knife 6 approaches a sheet 7, and then tightened to secure the body 3 to a flange 1. When a head 9 is rotated clockwise for the feed of the knife 6, a dial 10 indicates the movement value of a knife holder 14, and an internal crown gear 20 transmits the rotation of a motor 12 to a main spindle 17 by means of a pinion 21. A knife-holder assembly formed by the parts 6, 14, 18, 22, 23 is replaceable with assemblies of different dimensions, the part 22 having three millimeter indentations and being able to place the knife 6 at the accurate point to be lathed.