1.  Field of the Invention
The present invention relates to a V-shaped groove forming machine for forming V-shaped grooves on a surface of a plate material and its control method. 
2.  Description of the Prior Art
When a box-shaped product, for instance is manufacture by bending a plate material, conventionally V-shaped grooves are previously formed on one surface of a plate material, in order to reduce the radius of curvature at the bent portions of the plate material, before bending the plate material with the formed grooves inward.  Where V-shaped grooves are formed on one surface of a plate as described above, it is possible to use a shaper or a planomirror; however, when the size of the plate material is large, there exists a problem in that it is difficult to form V-shaped grooves.  That is, since there exists no dedicated machine for forming V-shaped grooves on one surface of a plate material, it has been difficult to freely start or end the V-shaped groove forming operation from or at any given position.  Further, there exists another problem in that the workability or machinability is not high when the number of grooves or the number of plates to be machined increases.